3
Laser-Plasma Interactions: Compact Particle Accelerators, New Optics, and Brilliant X-Ray Sources
LEVERAGING AND CONTROLLING THE MOST INTENSE LIGHT ON EARTH
An ordinary lightbulb produces only a few to tens of watts of optical power. The Watts-per-unit area is a measure of its intensity, and for an ordinary lightbulb, the intensity even at the bulb’s surface is at best tenths of a W/cm2. The sun is an extremely powerful source of radiation. However, at the surface of Earth, 93 million miles from the Sun, the intensity of solar radiation is only 0.1 W/cm2. Lasers (light amplification by stimulated emission of radiation) are devices that are capable of producing intensities perhaps beyond any source found in nature. An ordinary laser-pointer has a continuous intensity of about 1 W/cm2. Industrial lasers used for cutting and welding may produce intensities of more than 100 kW/cm2 in the desired spot size (cross sectional area of the laser beam) of only a mm in radius.
The greatest intensities are obtained from pulsed lasers or particle beams. These devices take a given amount of energy, and from that produce a very large power by putting all that energy into a very short pulse (power is energy/time). A 10 Joule laser having a pulse length of 50 fs (a fs, or femtosecond, is 10−15 seconds, a millionth of a billionth of a second) produces 200 TW of power (1 TW = 1012 Watts). Focusing that laser pulse to a small spot, say 50 μm in diameter, produces an intensity of more than 2 × 1018 W/cm2. Such intensities immediately ionize any material, converting it to a plasma. When, subsequently, lasers with such high intensity are focused inside the plasma, an entirely new realm of physics opens up. This realm corresponds to the physics of the largest electric fields ever produced by humankind. This is the field of laser-plasma interactions (LPIs). In this field,
there are many challenges, in controlling this intense light, in using this intense light (e.g., for acceleration of charged particles), and in generating light of even greater brightness (brightness controls the ability to focus the light to a given intensity).
Since high-intensity laser fields immediately turn anything into a plasma, plasmas are the only way to control and harness such laser fields. This has led to the new field of plasma optics, in which the plasma is configured to act as an optical element. By controlling the properties of the lasers and plasmas, the plasma can be used to scatter, absorb, reflect, focus and refract the laser light—much like conventional mirrors and lenses are used to manipulate ordinary light. Plasma optics also encompasses waveguiding, creation of atmospheric filaments, plasma compression, laser beam combiners and transfer of energy between laser beams. It additionally includes forming light with a structure that enhances or suppresses some particular effect. The plasma can be used to amplify and polarize lasers. Since the plasma can switch the state of light and can control unprecedented intensities, this field is often known as plasma photonics or the nonlinear optics of plasmas (NLOP). (See Figure 3.1.) In the last 10 years, new methods of controlling sequences of
laser pulses and laser bandwidth control, together with a new understanding of multibeam interactions in plasmas, have transformed the field. NLOP offers the potential to mitigate instabilities that occur in laser driven inertial confinement fusion (ICF, covered in Chapter 4), and to shape and control light. The fundamental challenge is: How can plasma optics shape and be shaped to manipulate controllably ultra-intense radiation, to enable laser conditions physics regimes, and applications unobtainable by conventional optics?
Harnessing the extreme fields of plasmas offers a new generation of compact particle acceleration methods. In the absence of a waveguide, an electromagnetic wave cannot continuously accelerate charged particles because the electric field is transverse to the direction of wave propagation, so that the accelerated particles leave the region of the laser field. However, the interaction of a laser pulse or particle beam with a plasma can generate longitudinal electric fields, so called wakefields, for which the generated electric field is oriented along the direction of propagation of the particle. The accelerated charged particles can move with the accelerating field for long distances, and consequently be accelerated to very high energies. (See Figure 3.2.) The rate of acceleration can be enormous, approaching 100 GeV/m. (1 eV is the energy gained by an electron falling through 1 V of electrical potential.) For comparison, conventional accelerators like the linear accelerator at the SLAC National Accelerator Laboratory accelerated electrons to 50 GeV over 3.2 km, or 16 MeV/m—that is, smaller by more than three orders of magnitude. There is a practical limit of around 100 MeV/m with conventional acceleration since for an electric field of that strength, electrons are pulled out of ordinary material. Now with the ability to control plasmas, wakefield acceleration is under consideration for the development of future high energy particle physics (HEP) colliders to extend our understanding of the basic laws of the universe. (A collider is a pair of accelerators that produce particle beams moving in opposite directions, that then collide head-on. At the current energy frontier, electron and positron colliders will need to be linear to reduce radiation losses from these light particles.) In another method, Direct Laser Acceleration, the particles interact with the laser fields nonlinearly in order to experience continuous acceleration. The existence of multiple acceleration mechanisms attests to the richness of this field. A community roadmap, the DOE Advanced Accelerator Concepts Research Roadmap,1 is guiding the field toward developing applications in the near term and addressing the challenging requirements of future colliders. A fundamental question is: How can we control the interactions of ultra-intense (relativistic) lasers and particle
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1 U.S. Department of Energy, Advanced Accelerator Development Strategy Report: DOE Advanced Accelerator Concepts Research Roadmap Workshop, doi:10.2172/1358081, https://www.osti.gov/servlets/purl/1358081.
beams with plasmas through shaping the laser fields and plasma profiles to efficiently generate ultra-bright, high energy charged particle beams?
Ion acceleration by the extreme electric fields produced in plasmas uses a different mechanism, principally acceleration by the plasma sheath, a region of strong electric field that forms at the edge of a plasma. In this method, the laser
ejects electrons beyond the plasma, and the associated sheath accelerates (drags) the ions along to high energies. The goal is to produce compact, ultrafast sources of energetic ions. Optimizing this process requires a higher energy density in the plasma than is typical for wakefield acceleration of electrons. In the last 10 years, high gradient acceleration of ions by the sheath electric fields has been refined and new physics regimes have been developed. Radiation pressure and magnetic vortices have been investigated to provide high performance compact ion beams, which may be useful in medical therapy and for electric and magnetic field probes in high energy density (HED) plasmas. A fundamental question, in analogy with that of electron acceleration, is: How can mechanisms of high density laser coupling and ion acceleration be understood and controlled to efficiently produce quasi-monoenergetic high energy ion beams?
The electrons accelerated by LPI are a further source of very bright X-ray beams. The electrons oscillate transversely in the laser-induced plasma fields, and this transverse oscillation (called betatron oscillation) causes the generation of X rays, just like the oscillating current in an antenna produces radio waves. This light has very high brightness, which means that it can be focused down to greater intensities using less powerful lenses. (Brightness is angular dependent intensity: optical power/angular divergence/area of illumination—with units of W/steradian/m2.) Another mechanism to produce X rays is to collide the accelerated electrons with yet another laser. In either case, one may generate bright X ray beams in a device much smaller than current approaches that utilize large laboratory light sources. The resulting photons may reduce radiation dose and increase resolution for nuclear security, medicine and industry, as well as being diagnostics for HED science. The fundamental question is: How can we understand, develop and control novel plasma-based radiation X-ray sources to enable new capabilities in imaging for medicine, biology, national security and physics diagnostics?
The combination of high intensity lasers and particle beams in plasmas will open up new physics. One example is nonlinear high field Quantum ElectroDynamics (nQED) with collective plasma interactions. nQED describes how light and matter interact while incorporating both quantum mechanics and special relativity. Under conditions of very strong light intensity, it has been predicted that the light will spontaneously convert to matter and antimatter, in keeping with Einstein’s famous equation E = mc2. Over the past 10 years theoretical frameworks have been developed that predict specific signatures, show how newly developed laser and beam capabilities can be used for initial tests, and these can advance capabilities presently on the horizon. Work has been started to develop the required models and to integrate these into plasma simulations. Note that related opportunities in quantum and exotic states of matter are described in Chapter 4. The fundamental question is: How to develop more complete theoretical models, with the computational
capabilities to capture those models, and to design experiments for current and new laser capabilities to open up new physics frontiers?
High-level science challenges for the field of LPI have been described in reports over the past decade, including the following: Basic Research Needs for High Energy Density Laboratory Physics (2009),2Frontiers in High Energy Density Physics: The X-Games of Contemporary Science (2003),3 and Workshop on Opportunities, Challenges, and Best Practices for Basic Plasma Science User Facilities (2019),4 as well as the preceding decadal Plasma Science: Advancing Knowledge in the National Interest5 (“Plasma 2010 report”). These reports, and an ongoing 2-year effort started in 2018 by the Department of Energy Office of Fusion Energy Sciences (DOE-FES) to establish a long-term strategy for the field, have provided key insights to this study. The areas addressing fundamental science challenges are intimately tied to rapidly developing advanced capabilities in high-intensity ultrafast laser facilities. These capabilities include increasing pulse energies, repetition rates and control, all enabling new areas of research. For example, chirped pulse amplification, the subject of the 2018 Nobel Prize in Physics, is the basic technique used to produce short (<100 fs), high energy (0.1-10 J) laser pulses, and has had particular impact in enabling higher laser powers. High-level laser development challenges have been described in recent reports, including Workshop on Laser Technology for Accelerators (2013),6Workshop on Laser Technology for k-BELLA and Beyond (2017),7 and Opportunities in Intense Ultrafast Lasers: Reaching for the Brightest Light (2018).8
Funding for laser-plasma optics, particle acceleration and laser sources is spread over several federal agencies. This funding is often mission or result focused rather than oriented toward investigating the basic science that underlies the applications. For example, DOE-HEP funds development of laser-plasma wakefield acceleration (LWFA) and particle beam driven plasma wakefield acceleration (PWFA) as a path
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2 See https://science.osti.gov/-/media/fes/pdf/workshop-reports/Hedlp_brn_workshop_report_oct_2010.pdf.
3 National Research Council, 2003, Frontiers in High Energy Density Physics: The X-Games of Contemporary Science, The National Academies Press, Washington, DC, https://doi.org/10.17226/10544.
4 Workshop on Opportunities, Challenges, and Best Practices for Basic Plasma Science User Facilities,” 2019, https://arxiv.org/abs/1910.09084.
5 National Research Council, 2007, Plasma Science: Advancing Knowledge in the National Interest, The National Academies Press, Washington, DC, https://doi.org/10.17226/11960.
6 S.M. Hooker, S. Mangles, and R. Pattathil, 2014, Laser and Plasma Accelerator Workshop 2013, Plasma Physics and Controlled Fusion 56:080301.
7 L.A. Gizzi, P. Koester, L. Labate, F. Mathieu, Z. Mazzotta, G. Toci, and M. Vannini, 2019, Lasers for novel accelerators, Journal of Physics: Conference Series 1340:012157.
8 National Academies of Sciences, Engineering, and Medicine, 2018, Opportunities in Intense Ultrafast Lasers: Reaching for the Brightest Light, The National Academies Press, Washington, DC, https://doi.org/10.17226/24939.
to future lepton colliders and, under its Accelerator Stewardship program, also addresses mid-term applications of high-intensity lasers. DOE-FES funds ion acceleration and plasma optics as part of its stewardship Discovery Plasma Sciences (DPS). DOE-NNSA funds LPIs with emphasis on coupling for ICF and HED science experiments. NSF funds broad basic research in all of these areas, including as part of the NSF/DOE Partnership in Basic Plasma Science and Engineering. The Naval Research Laboratory (NRL) has a broad internal program in LPI. The Air Force Office of Scientific Research (AFOSR) funds internal and university projects on LPI and formation of plasma filaments. Other offices within DOE, such as Basic Energy Science (BES), have occasionally funded projects relevant to LPI, such a free electron laser (FEL) development. There are many opportunities for these agencies to coordinate and collaborate on supporting both the fundamental science and translating that science to applications. This coordination and collaboration will be to the betterment of the science and the application.
There are strong linkages of LPI across plasma science in the underlying physics, applications, enabling capabilities, and methods (computations, diagnostics). Plasma physics linkages include resonant excitation processes and control of the particle-wave interactions that underlie accelerators and plasma optics. LPI can be accomplished with high precision and at high repetition rate at relatively modest cost compared to other HED systems. This provides the ability to flexibly manipulate and precisely measure states in plasma-wave-radiation interactions, and space-time control of such interactions, that can benefit other areas of plasma science. Access to high field physics, including nQED, has broad impact across science. Newly developed precision, ultrafast photon and particle sources can serve as high resolution probes and diagnostics for other plasma experiments (HED sciences, ICF), and potentially other areas such as Fusion Materials and Technology (FM&T), and magnetized experiments. Computational methods being developed for LPI have strong overlap across other areas of plasma science. These methods include particle-in-cell (PIC), direct-simulation Monte Carlo (DSMC), fluid and Vlasov methods for simulating acceleration; and 3D magnetohydrodynamics (MHD), fluid, and PIC-DSMC codes for analysis of irradiated targets. (See Figure 3.3.) Advances in computations needed to investigate the new LPI regimes include reduced and integrated models, and the ability to capture comprehensive physics including wall and boundary effects. A further important need is to reduce barriers to entry for use of codes by nonexperts. There are many opportunities for collaborations between programs focused on LPI and those science areas that would benefit from LPI capabilities. This coordination and collaboration will improve both the science and the application.
In the following sections, we discuss accomplishments, opportunities and challenges in the application areas of plasma optics, plasma acceleration of light particles, plasma acceleration of heavy particles (ions), bright X-ray generation and
nonlinear QED. The committee describes needs for diagnostics, computations, and enabling technologies. The underlying science challenges to these applications areas include:
- What are the fundamental processes that occur during the interaction of ultra-intense (relativistic) laser pulses and beams with plasmas?
- How can we use these interactions to generate and control energetic particle beams, and enable new physics regimes, including addressing frontier HEP and nQED questions?
- How can LPI research be translated into compact photon and ion sources that offer revolutionary performance for medicine, national security and industry?
PLASMA OPTICS
Plasma optical techniques have been developed in the last decade that can control and improve LPI performance in regimes beyond those accessible by conventional means. Plasma-based optical components, already consisting of ionized
gas, have substantially higher damage thresholds than solid state components and can be inexpensively and rapidly replaced, for instance, at the repetition rate of a gas jet or capillary. Similar to conventional optics, the time and spatially dependent phase of a laser pulse propagating in plasma is determined by the refractive index—and that refractive index is dominantly determined by the plasma density. By controlling the spatial variation, evolution, or nonlinearity of the plasma density, the plasma can produce dispersion, refraction, or frequency conversion. In principle, a plasma can be made to mimic any solid-state optical component.
Plasma optical components include plasma gratings; plasma waveguides that combat diffraction, extending the interaction length in LWFAs; and plasma mirrors, along with optical pulse shaping to enhance or suppress specific dynamics. These components can increase intensity contrast by orders of magnitude, allowing for impulsive laser-matter interactions nearly free of premature heating; and redirect laser pulses in multistage LWFAs without degrading electron beam emittance. This advance resulted from improved understanding of and ability to shape LPIs. As such, control of high intensity laser pulses is enabled in ways not otherwise possible. In turn, these new methods allow excitation and control of plasma states both for investigation of fundamental physics and applications.
Progress and Achievements
Inertial confinement fusion (ICF) was one of the earliest applications to harness high-power lasers. The field has implemented innovative optical techniques that produced step-changes in performance. These techniques include efficient optical frequency tripling, spatial coherence control (phase-plates), and induced temporal incoherence (e.g., Induced Spatial Incoherence, ISI, and Smoothing by Spectral Dispersion, SSD). Techniques that take advantage of the bandwidth available on current laser systems have been developed that can inhibit low-frequency laser-plasma instabilities that are detrimental to ICF. For example, stimulated Brillouin scattering (SBS) can be avoided by detuning the interaction between multiple laser beams or to move speckles before the instability can grow.
ICF experiments must still navigate around laser-plasma instabilities, the consequences of nonuniform laser illumination and damage by scattering laser light into unwanted directions. Such instabilities can generate super-thermal electrons that preheat the fusion fuel, reducing its compressibility, and seeding instabilities. Plasma optical manipulation of plasma states, including use of bandwidth or STUD (spike trains of uneven duration and delay) pulses, provide solutions to these issues. The emergence of high-power, high-repetition-rate (>kHz) ultrashort pulse lasers enables creation and investigation of nonlinear propagation and material interactions governed by a combination of nonthermal and thermal modifications to matter.
Shaping of the optical drive of the plasma has been extended to techniques that control the apparent velocity. This opens new regimes in exciting Raman amplification, photon acceleration, wakefield acceleration, and THz generation. Spatiotemporal shaping can produce laser pulses that appear to violate special relativity. The peak intensity of a self-accelerating light beam can follow a curved trajectory in space, while the peak intensity of a “flying focus” pulse can travel at an arbitrary velocity, surpassing even the vacuum speed of light (see Figure 3.4). These arbitrary velocity intensity peaks result from the chromatic focusing of a chirped laser pulse. (A chirped laser pulse has a frequency that is a function of time or space.) The chromatic aberration and chirp determine the location and time at which each frequency component within the pulse comes to focus and reaches its peak intensity. By adjusting the chirp, the velocity of the intensity peak driving the many plasma processes can be tuned to nearly any value, either co- or counterpropagating along the laser axis.
Propagation of an intense laser pulse in air balances self-focusing and ionization of the air to create extended plasma filaments. Plasma filaments are long, narrow strings of plasma whose nonlinear properties (e.g., self-focusing) create
more plasma having similar string-like properties. Experiments have shown that a train of laser pulses can heat air through a combination of thermal and nonthermal effects, such as thermal blooming, ionization and Raman excitation processes, leaving behind a long-lasting low neutral density channel that can guide subsequent laser pulses. This delicate multiphysics process has numerous applications, including enhancing the collection efficiency of photons for remote detection. In high-repetition rate laser-material interactions, a laser pulse will interact with matter than has been strongly modified by the nonthermal heating of previous pulses. This heating can create periodic surface structures, change the reflectivity and absorption, or alter the molecular composition altogether.
Orbital angular momentum (OAM) laser pulses can impart angular momentum to a plasma. This transfer of momentum can modify the topology and dispersion of the plasma waves produced by the laser and the phase space of the charged particles they accelerate. For example, a laser pulse with a helical intensity profile, or “light spring,” can nonlinearly excite a wakefield that traps and accelerates a vortex electron beam—a beam that rotates around the optical axis. OAM can also modify the nonlinear propagation and interaction of high-power pulses with transparent media, resulting in helical plasma filaments or high harmonic radiation with vortex phase structure. Special phase plates are used to impart angular momentum to laser pulses.
Plasma laser amplifiers use multiwavelength interactions to transfer energy from a long laser pulse seed to amplify a short laser pulse. Progress has been made in the amount of energy transferred and in the gain of the short pulse. The technique could eventually provide a final power-amplification stage for high energy applications or to operate in novel wavelength regimes.
Current and Future Science Challenges and Opportunities
Plasma-based optical components could provide the disruptive technology needed to usher in the next frontier of laser-plasma research. Plasma optics also has the potential to enable new capabilities in broader high-energy and high-average-power laser applications; and to enable guiding/steering and controlling X rays in ways unattainable today. The unexpected features of the combination of structured light and LPI are relatively unexplored due to the technological challenges of creating such structured pulses. The further development of ultrafast pulse shaping techniques to manipulate the spatiotemporal optical properties of plasmas would bring about novel LPIs.
Laser, target and diagnostic capabilities, together with simulation, are enabling understanding and control of plasma states that will enable plasma optical components to have increasing impact. Already, plasma waveguides are core parts of advanced particle accelerators. Other plasma optics components, while still in the early stages of development, have been successfully demonstrated in experiments
that include: lenses, waveplates and polarizers, q-plates, beam-combiners, compressors, and amplifiers. Advancing these methods will provide access to new laser and plasma parameter regimes. A critical need to achieve these goals is the development of novel diagnostics that measure not only the bulk hydrodynamic properties of the plasmas but also the underlying particle distribution functions. Diagnostic accomplishments and needs are discussed below.
Concepts have been developed for further control of instabilities using broad bandwidth lasers. Broad bandwidth lasers for ICF could deliver pulses with the temporal incoherence necessary to suppress high frequency instabilities like two-plasmon decay and stimulated Raman scattering, while also providing smoothing to mitigate disruption of the capsule. Generally speaking, the broad-bandwidth mitigates laser plasma instabilities by detuning the interaction between multiple waves or incoherently drives many small instabilities instead of a single coherent instability. Bandwidth can also be used to coherently create controlled trains of shorter pulses (in the ps range) within the longer pulse envelope (in the ns range). If these manipulations can be performed faster than the growth of electron plasma waves, the train of pulses can be used to control the excited plasma state. This STUD concept, introduced above, opens the possibility of coherent laser control of plasma states and hence of laser coupling and transport. STUD and other sculpted time dependent wave structures could further be harnessed to drive far more controlled nonlinear responses in plasmas. Experiments using high repetition laser pulses aided by machine learning could identify time sequences and profiles of laser pulses that could tame the highly nonlinear kinetic and chaotic responses exhibited by plasmas driven strongly by laser beams over long periods of time. Such techniques could mitigate laser plasma instabilities by modulating the intensity to shrink individual hot spots and disperse hot spot patterns. This well controlled laser illumination could further help steer the flow of energy in plasmas to self-organized and self-sustaining states far from equilibrium or make and control plasma structures on demand to direct photons as in plasma optics.
Spatiotemporally structured ionizing laser pulses enable control over the velocity of an ionization front, thereby allowing the tailoring of the target plasma for acceleration. Controlling the plasma rise and fall and its width as a function of longitudinal distance allows improved matching of the pump laser pulse that creates the accelerating electric field, and it controls the transverse focusing of the electron beam in beam-driven systems. There are numerous opportunities here for finding more optimal systems or new systems altogether.
Control of plasma optics is likely to advance several other plasma-based applications, including Raman amplification, photon acceleration, relativistic mirrors, and THz generation. Orbital angular momentum may offer an additional degree of freedom through which laser-plasma instabilities can be mitigated or controlled. As in cross-beam energy transfer, multiple interacting beams could drive a spectrum
of plasma waves each with a different value of orbital angular momenta instead of a single coherent plasma wave with no angular momentum. A next-generation high-power laser with plasma optics could deliver extremely high intensity pulses with unprecedented control of plasma states and laser propagation. These methods could transform the landscape of LPIs, from acceleration to fusion to applications such as remote sensing and X-ray sources.
PLASMA ACCELERATION OF LIGHT PARTICLES
Over the past decade, compact plasma-based accelerators have progressed from first demonstrations to a well-developed field. (See Figures 3.5-3.7.) These devices utilize the strong longitudinal electric fields present in laser or particle beam driven plasma waves to provide accelerating gradients (energy gain per unit length) that are orders of magnitude greater than those sustainable in conventional metallic accelerating structures. Electron bunches that are loaded (or injected) into the plasma wake of the driving pulse can gain energy at rates of 1-100 GeV/m. Milestones that were achieved prior to the previous Plasma 2010 decadal survey9 included the injection and acceleration of plasma electrons in a laser-driven plasma wakefield accelerator (LWFA) in 2004. The electrons were accelerated up to 100 MeV over a distance of millimeters, producing a mono-energetic beam with tens of pico-Coulomb’s (pC) of charge. In 2006, particle-driven plasma wakefield accelerators (PWFA) provided up to 30 GeV gain in energy to the most highly accelerated particles (within a broad spread of energies) of a wake-driven electron beam over a distance of one meter. These experimental results were the outcome of decades of work since the inception of the field in the late 1970s, and demonstrated clearly the promised large accelerating gradients. In the ensuing decade, the field has grown significantly and great progress has been made toward realizing the first application-ready plasma accelerators. There have been multiple demonstrations that achieve nearly 10 GeV of energy gain in a single LWFA and PWFA. Several sophisticated techniques have been developed to improve the quality of the outgoing electron bunches. Based on the promise shown by plasma accelerators, the DOE roadmap Advanced Accelerator Development Strategy Report10 was developed in 2016 that provides a plan for realizing plasma-based high-intensity X-ray photon sources and ultimately, a plasma-based high energy physics collider. A TeV-class electron-positron collider based on plasmas would potentially reduce the size of the machine from tens of kilometers to hundreds of meters, and with lower cost.
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9 National Research Council, 2007, Plasma Science: Advancing Knowledge in the National Interest, The National Academies Press, Washington, DC, https://doi.org/10.17226/11960.
10 U.S. Department of Energy, Advanced Accelerator Development Strategy Report: DOE Advanced Accelerator Concepts Research Roadmap Workshop, doi:10.2172/1358081, https://www.osti.gov/servlets/purl/1358081.
Progress and Achievements
Plasma-based accelerators are now able to regularly provide multi-GeV energy gain to mono-energetic beams of 10 pC to 1 nC in a single plasma accelerator. (An electron bunch of 1 pC of charge contains 6 × 106 electrons. A 1 nC bunch contains 6 × 109 electrons.) LWFAs have utilized high peak-power laser systems
(100 TW to 1 PW) fired into plasma sources that are centimeters in length with plasma densities of ~1017 to 1019 cm–3. PWFAs used up to ~20 GeV electron drive beams with ~1 nC of charge in plasma sources roughly 1 meter in length and with plasma densities of ~1016 to 1017 cm−3.
Several laboratories are now able to regularly accelerate 10-100 pC of charge to energies of 10-100 MeV in multi-millimeter-length, high-density gas jet plasma sources using laser pulses with a more modest peak power (10-100 TW). These LWFAs often rely on the relativistic self-focusing of the laser pulse in the high-density gas jet to achieve laser intensities that would be inaccessible by other means. Such gas jet LWFAs, though more modest in terms of energy and beam charge, have led to vibrant and growing plasma accelerator research at university laboratories the world over. While national laboratories have continued to carry out most of the research at the energy frontier, much of the fundamental physics research has been conducted at smaller university laboratories. This synergistic research has, in turn, rapidly increased the pace at which the field is advancing.
The record for energy gain in a LWFA was set at the BELLA (Berkeley Lab Laser Accelerator) facility at Lawrence Berkeley National Laboratory (LBNL) in 2019, accelerating about 10 pC of electrons from rest to 8 GeV in a 20 cm-long capillary discharge plasma source, driven by a ~0.85 PW laser pulse. This result
was achievable thanks in large part to the development of an advanced capillary discharge plasma source that provided a guiding channel for the wake-driving laser pulse. (See Figure 3.5.) The plasma waveguide was generated by preheating the discharge plasma column with a separate nanosecond laser pulse ahead of the femtosecond wake-driving pulse, deepening the channel that is formed naturally through hydrodynamic expansion of the initial plasma filament. This structure enabled guiding of the wake-driving pulse over a longer distance, thereby increasing the total energy gain of the electron beam.
The highest energy gain achieved for a low energy spread PWFA bunch was at the Facility for Advanced Accelerator Experimental Tests (FACET) at the SLAC National Accelerator Laboratory in 2015. This demonstration provided 9 GeV of energy gain to ~100 pC electron bunch with an initial energy of 20 GeV in a 130 cm-long lithium heat pipe oven plasma source driven by a 20 GeV, ~1 nC electron bunch. The energy transfer efficiency from the driver to the accelerated beam was as high as 30 percent in the FACET PWFA experiments. This efficiency is a critical parameter in determining the power cost for future high energy, high repetition rate applications.
Coupling of two high-energy LWFAs (known as “staging”) was demonstrated for the first time at BELLA in 2018 by refocusing the electron beam between the two plasma stages with an active plasma lens. (See Box 3.1.) This provided roughly double the energy gain to the electron beam compared to the acceleration received
in a single plasma stage. The ability to perform efficient staging is also critical to future high energy applications in order to reach the final target energy. Overall, great advances have been made in the past decade in providing high energy gain with low energy spread for a significant level of charge in both laser-driven and particle beam-driven plasma accelerators.
Significant strides have also been made in improving the quality and stability of the accelerated beams. (See Figure 3.7.) The quality of the accelerated electron beam in a LWFA or PWFA is determined by the trapping and focusing dynamics in the plasma wake. There are multiple methods of controlling the electron beam injection process to produce output electron beams with minimal energy spread and emittance. (Minimizing emittance means containing electrons in the smallest volume in the 6-D phase space of spatial coordinates and velocity.) In these techniques, background electrons move from untrapped orbits that are part of the fluid motion sustaining the plasma wave to trapped orbits comprising the accelerated electron bunch. Laser-based and density-gradient-based triggering of particle trapping has been a topic of active research and has enabled a continuous improvement in beam quality. Energy spreads of less than 1 percent in the accelerated beams and emittances of less than 1 mm-mrad have been regularly achieved, the latter comparing favorably with conventional accelerators.
SLAC’s FACET facility was able to provide high energy (20 GeV), high charge (~1 nC) positron beams in the past decade and used them to explore the acceleration of positrons in beam-driven PWFAs. Positron bunches were successfully accelerated in a hollow channel PWFA, producing a few hundred MeV gain in energy. (See Figure 3.6.) To achieve this outcome, custom diffractive optics focused a ~1 TW laser pulse into a nondiffracting, high-order Bessel intensity pattern that was sustained over tens of centimeters to ionize a tube of neutral gas in a flooded vacuum chamber, thereby forming the hollow channel plasma source. The transverse wakefield theory was studied and confirmed by inducing transverse kicks to the beam prior to entering the hollow channel PWFA. In another experiment at FACET, a single, high energy (20 GeV), high charge (~1 nC), short (~30 μm) positron bunch was launched into a 30 cm-long lithium heat pipe oven plasma source. The results surprisingly produced a mono-energetic positron beam that gained several GeV of energy, revealing a previously unpredicted regime of nonlinear positron PWFA physics. With the assistance of particle-in-cell (PIC) computer simulations using the code QuickPIC, the dynamics of this interaction were understood. The mechanism was shown to rely on a longitudinal and transverse self-loading of the wake by the positron beam.
At CERN (European Organization for Nuclear Research), the Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) experiment demonstrated the ability to accelerate electrons in a high-energy proton beam-driven
plasma wakefield accelerator (PDWFA). The motivation for building such a machine is to take advantage of existing infrastructure to produce high energy, high charge proton beams and to transfer that energy to an electron beam over a relatively short distance. This approach could produce lepton collisions at high energy without the need to build a new, 30 km long linear accelerator that would be needed if using conventional technologies. Another attractive feature of the PDWFA is that it requires only one (albeit long) plasma stage, avoiding the complications associated with staging. A challenge of PDWFA is that the drive beam needed is significantly longer than the wake period. The problem is mitigated by inducing a laser-triggered beam-plasma instability that segments the proton beam into “bunchlets” that are spaced at the plasma period, thereby creating a wakefield train that can then be loaded with a train of electron bunches to be accelerated. Another challenge presented by PDWFA is the efficient coupling of an electron beam into the plasma accelerator. In spite of these challenges, a proof-of-principle experimental demonstration has been accomplished, with the acceleration of ~10 pC of electrons by 2 GeV over a distance of 10 m.
A key to successful plasma accelerator applications is the ability to produce bright (i.e., high charge, high energy, low emittance, low energy spread, ultrashort) electron beams. The outsized longitudinal electric fields in plasma accelerators can serve to achieve this goal through the process of controlled electron beam injection. The main principle of injection in a plasma accelerator is to trap a dense, small cluster of electrons in the plasma wake and to accelerate them to ultra-relativistic speeds as quickly as possible. Plasma electrons can be directly trapped in the breaking plasma wave at the rear of the wake bubble, a region where plasma electrons are completely evacuated. However, the stochastic nature of this process often leads to significant fluctuations in the properties of the beam that is produced. Researchers have therefore devised and begun experimenting with various forms of controlled injection. One such method is to produce a sharp “density downramp”—that is, a rapid longitudinal decrease in plasma density near the start of the plasma source. This results in a rapid elongation of the plasma wake, permitting local plasma electrons to be preferentially trapped and suppressing the subsequent trapping of additional electrons further downstream. Another method utilizes a plasma source generated by a lower ionization threshold (LIT) gas, such as hydrogen, to form the plasma accelerator in the presence of a background of neutral high ionization threshold (HIT) gas, such as helium. A wake driver is sent into the LIT plasma source, and a small volume of the HIT gas is ionized directly inside the blowout wake following the driver. (The blowout regime refers to plasma electrons being completely excluded from a bubble-like region behind the wake driver.) The ionized HIT gas electrons are then trapped and rapidly accelerated to produce an ultra-high brightness beam. Ionization mechanisms that have been
used in such experiments include a dedicated laser pulse, combined with a wake-driving electron beam.
There are now many university laser labs capable of carrying out fundamental LWFA research, and a growing number of national laboratories throughout the world with facilities appropriate for—and in many cases solely dedicated to—energy-frontier plasma accelerator research. Petawatt laser systems to be used for high-energy LWFA research in the next decade, include BELLA at LBNL, ZEUS/Hercules at University of Michigan, DIOCLES at the University of Nebraska, and the Texas Petawatt Laser System at the University of Texas at Austin, as well as many facilities in Europe and Asia. High energy PWFA research facilities in the next decade will include FACET-II at SLAC, the Advanced Test Facility (ATF) at Brookhaven National Laboratory, FLASHForward at DESY (German Electron Synchrotron), and AWAKE-II at CERN. There is ongoing research at various 100 TW-class LWFA laboratories and lower energy accelerator facilities. This increases access for universities, to facilitate a more widely distributed base of fundamental research on LWFA, PWFA and PWFAs driven by LWFA electron beams.
Sophisticated diagnostics have been developed to more directly probe the plasma accelerator itself as discussed in the diagnostics section. One of the more commonly used techniques, typically referred to as “shadowgraphy,” analyzes the phase pattern imprinted on a low energy, ultra-short laser pulse by the plasma. This can be used to study the plasma source profile prior to the arrival of the wake driver, though more advanced co-propagating schemes have been able to study the structure of plasma wake itself. (See Figure 3.15.) The latter application currently represents the most direct method of observing these speed-of-light wakes, and their use will likely expand in the coming decade.
Due to the great challenge of developing and applying experimental diagnostics for plasma accelerators, theory and computer simulations have been a cornerstone in the research portfolio over the past decade as discussed in the computations section. Various PIC and related codes, including OSIRIS, VSim, QuickPIC, WARP, and EPOCH, have provided deeper understanding into the physical processes than could have been gained otherwise. These tools have been used both to design experiments and to interpret experimental results. Computer simulations have also played a major role in studying the limits of the capabilities of plasma accelerators, helping to shape the research roadmaps and community consensus on where to prioritize research efforts. As the power of parallel and device-based (GPU) computing has increased over the years, so too has the speed of the major codes. The concurrent improvements in computing hardware and in advanced algorithms that are able to take advantage of the new hardware have permitted simulations of exceeding high detail and scope. The quality and scale of these simulations would have been difficult to predict in the prior decade.
Current and Future Science Challenges and Opportunities
The major milestones that are likely to motivate and guide the next decade of plasma accelerator research include the production of high quality electron beams that can be utilized for X-ray free electron lasers (X-FELs) and other photon sources. These would be electron beams with sufficient control and reproducibility that they can be used in production-level applications, requiring elimination of instabilities and kilohertz-level repetition rates. Meeting these goals will require the mastery of controlled injection techniques, improved loading of the plasma wake to minimize beam energy spread, beam emittance preservation at the plasma-vacuum interface, and high energy transfer efficiency. Alongside research motivated by these goals will be continued research into more novel applications, such as positron acceleration and proton-driven plasma accelerators. In addition, improvements in staging efficiency will need to be studied and perfected. These will drive the continued development of plasma accelerators for particle colliders to address the needs of the high energy physics (HEP) community.
A community roadmap has been developed to guide efforts toward enabling photon source applications in the nearer term and the challenging requirements of future colliders. (See Figure 3.8.) If the goals of the roadmap are realized, during the next 10 years we will achieve improved control of particle injection and laser guiding to enable phase space shaping and efficiency, 10 GeV laser particle accelerator modules, staging of multi-GeV modules, controlled emittance, and demonstration of positron acceleration. To facilitate these science advancements, lasers having repetition rates of kHz and average powers of kW will need to be developed to enable higher precision experiments by leveraging active feedback control. Several different methods of initiating electron trapping have been proposed to improve the beam quality (e.g., beam emittance and brightness) significantly beyond the current state-of-the-art, and it is likely that others will be discovered. Research is ongoing on how to efficiently excite large amplitude nonlinear electron plasma waves over long distances. Achieving this goal requires greater understanding of the nonlinear laser-plasma interaction, including energy deposition and laser propagation physics. The latter depends on many processes, including relativistic self-focusing, pondermotive self-channeling, interaction with preformed plasma channels and laser-excited plasma wakefields, and short-pulse laser-plasma instabilities, such as laser self-modulation and hosing (defined below). Plasma source development will be a major topic of research. Multiple techniques are being considered for improved plasma sources, including laser ionized guides, discharge and helicon plasmas, using gas jets, capillary tubes, clustered targets, and alkali heat pipe ovens.
The topic of hosing has garnered much theoretical interest over the past decade but has yet to be thoroughly studied experimentally. Hosing is an instability driven by head-to-tail transverse wakefield effects acting on the accelerated electron beam that can lead to catastrophic beam break up. As LWFA and PWFA beam quality continues to improve, the beams will become more sensitive to this instability. Theoretical solutions have recently been proposed and are currently under study using simulations. One such solution utilizes a modest degree of ion motion within the blowout cavity, which can suppress the resonance in the beam’s transverse motion that leads to hosing. Further work in this area is needed.
A goal of a future high energy physics lepton collider is to collide counter propagating electron and positron beams with center of mass energies of and beyond 1 TeV. Plasma accelerators are able to accelerate positron beams, although it poses significant challenges. In the highly nonlinear blowout regime used in most high energy plasma accelerators, the plasma electrons are completely evacuated in a bubble-like region behind the wake driver. This leaves behind a column of positive ions that can provide transverse focusing to a negative electron beam due to the
Coulomb forces. Unfortunately, positively charged positron beams are defocused by the positive ion column. Nonetheless, significant progress has been made toward high gradient acceleration of positron beams in the past decade, both in theory and experiment. One theoretical proposal is to accelerate positrons in the linear wake regime, which, unlike the blowout regime, responds symmetrically to electrons and positrons. Here challenges include overcoming scattering in the plasma and the natural tendency for the beam to focus in the plasma until it drives a nonlinear wake and degrades the structure. Another proposal is to use hollow channel plasma accelerators, which avoid the production of an ion column altogether. Challenges for this scheme include that minor transverse asymmetries in the beam profile or position with respect to the plasma channel can lead to the buildup of strong transverse wakefields that induce a beam break up (BBU) instability, which could degrade the beam.
To address future high energy physics collider applications, staging of multiple plasma accelerators in series will provide the highest system gradient and is desirable in managing the charge and phasing of the bunches of charge that are accelerated. This challenging process has been demonstrated by accelerating electrons to the 0.1 GeV level using a 100 TW laser driver. Multi-GeV experiments are in preparation (See Box 3.1.)
Major efforts are needed to develop more advanced plasma sources over the next decade in order to achieve most of the aforementioned goals. (See Table 3.1.) It is likely that there will be no “one size fits all” solution, but rather there will be specialized plasma sources that serve particular goals. For example, a laser-ionized gas plasma source may be well suited for electron beam-driven PWFA purposes, whereas a helicon plasma source may be better for a proton beam-driven PDWFA. Continuous evolution of gas jet plasma sources for LWFA are likely to lead to improved injection control and beam extraction capability for lower energy beam
TABLE 3.1 Primary Accelerator Laser Driver Parameter Ranges of Interest for Plasma Accelerators
| Ee | EL | TauL | Rep | Example/Science Applications |
|---|---|---|---|---|
| ~20 MeV | 5-20 mJ | 4 fs | kHz | UED, keV Thomson medical . . . |
| ~1 GeV | 2-4 J | 30 fs | 1-50 kHz | Light sources, precision LWFA |
| ~10 GeV | 10-80 Ja | 100 fs | 50 kHz | 10 GeV collider stages |
| ~50 GeV | 0.2-1 kJ | 25-250 fs | Low | High E or Q, high field physics, HEDS |
a Range corresponds to linear or nonlinear regimes.
NOTE: Parameters shown are for λ = 1 μm drive. Other wavelengths are workable. Note that additional beams are desired for injection and guiding control but are typically at lower energy and do not hence drive overall laser development.
SOURCE: Courtesy of Cameron Geddes, Lawrence Berkeley National Laboratory.
sources, while advancement in semi-hollow plasma waveguides will lead to greater beam energy and quality for high energy systems.
Progress in plasma acceleration and X-ray sources has been driven by parallel and mutually reinforcing advances in laser (and particle beam) driver technologies and deeper understanding of the laser-plasma and beam-plasma interaction physics. In this regard, the maintenance and upgrade of existing facilities and facility networks is essential. However, improving the repeatability, reliability, and repetition rate of plasma accelerators depends on the development of new laser technologies for increased precision, control and repetition rate. (See Table 3.1.) There are three areas with distinct needs for laser development. For colliders and many photon sources, the main stages must operate at high efficiency with charge and laser energy requirements set by the interaction point physics. The properties of these lasers may differ from those of the injector stage that may benefit from long wavelength drivers. For HEP applications, stages of tens of GeV acceleration are likely required, while for HED probes, electron bunches having high charge are of interest. Both applications motivate higher powers but perhaps different architectures. For low energy applications such as medical and ultrafast electron diffraction, laser systems with pulse energies of a tens of mJ and pulse durations of a few-fs are needed. Laser technology is discussed in the section on Facilities.
It is important for theoretical models and computer simulations to continue improving in terms of physical accuracy (e.g., suppression of artificial numerical instabilities), to reduce computational cost, to provide scalability, and to include more physics to help solve the challenging problems facing the community in the next decade. One area that needs to be addressed is in the modeling of plasma sources, which has received less attention compared with the acceleration process. These improved models will help in the development of plasma targets with precisely shaped density profiles, using methods that have strong overlap with low temperature plasmas to, for example, match the laser and particle pulses into the plasma. End-to-end simulations of plasma accelerator systems that are capable of resolving detailed dynamics within the electron beam and (where appropriate) laser pulse will be required. In addition, there is a call for modestly simplified simulations that can be run with sufficient speed so as to provide rapid feedback to running accelerator systems in real-time. All of these modeling goals are extremely challenging and will require deliberate effort and investment to achieve. Deliberate investments in this area would produce a much needed improvement in the ability to predict and understand the behavior of actual, physical plasma accelerator systems.
Particle acceleration using plasmas is rapidly progressing in terms of beam quality, stability, and diagnostics, and in the physical understanding that is the basis for improving performance. The next steps in advancing the field include the transition to kHz repetition rates which will enable stabilization and precision, and application relevant average powers. Taking these steps will require new
laser technologies. Progress is being made toward meeting the challenge of phase space shaped particle beam generation and manipulation for ultra-bright beams, and the applications these capabilities would enable. Advanced plasma accelerators and photon sources will enable advances in fundamental science as well as society benefiting applications, from compact TeV class high energy physics colliders to medical imaging technologies.
LPI ACCELERATION OF HEAVY PARTICLES (IONS)
Laser-driven acceleration of ions has a long history, starting with the acceleration scheme proposed by Veksler in 1957, and has become a very active area of research worldwide. (See Figure 3.9.) In particular, high-intensity ion beams driven by short pulse lasers have emerged as an important area of plasma research. Ion acceleration mechanisms are different from those for light particles because of the different dynamics. A light particle at 100 MeV is ultra-relativistic, whereas that energy is very subrelativistic for a heavy particle. One cannot easily trap subrelativistic particles in the near relativistic plasma waves produced by LPIs, and so a different mechanism is necessary.
The methods of ion acceleration include target normal sheath acceleration (TNSA), shock wave acceleration, and mechanisms that rely on volumetric laser plasma interaction due to relativistically induced transparency and radiation pressure. The most studied mechanism, TNSA, works by using the plasma sheath, which forms at a plasma boundary, in this case a narrow plasma that comes from hitting a foil with a laser. The radiation pressure pushes the electrons out the far side; the charge imbalance creates a plasma sheath that accelerates the ions. In the last 10 years, high gradient acceleration by the sheath fields created in dense plasma targets has been refined into a controllable technique. Motivating this research are numerous potential applications discussed below.
Laser-driven ion accelerators are projected to have multiple applications. Biomedical applications include cancer therapy and production of isotopes for Positron Emission Tomography (PET). Laser-driven ion beams can also be a path for producing fusion energy through fast ignition. They could enable compact neutron sources for security and industry, advanced proton radiography and isochoric heating for high energy density science, and ultrafast beams for radiation damage and single event effects. Once the ions are accelerated by the laser-irradiated target, the beam has to be transported and delivered based on the requirements of a specific application. A major goal is to design a system whose ion beams can be used for applications, the challenges for which are similar to those faced in accelerator physics and significantly more research is required in this area.
Progress and Achievements
The ion acceleration field recently made major advances due to the availability of ultrahigh power lasers with focused intensity up to 1022 W/cm2, and laser technologies that allow a temporal intensity contrast of 14 orders of magnitude. Maximum proton energies approaching 100 MeV have been achieved experimentally using ultra-thin targets. These experiments have produced proton beams from a variety of targets, ranging from nanometer to micron scale foils of solid density, to near-critical plasma density targets. (See Figure 3.10.) Recent results from large laser facilities (National Ignition Facility-NIF, Advanced Radiographic Capability-ARC and the Omega EP laser at the University of Rochester) have shown that a dramatic increase of proton energies can be achieved by increasing the laser pulse duration.
A technology advance that has had a transformational impact on ion acceleration is the use of plasma mirrors (see section on plasma optics). Plasma mirrors made it possible to dramatically reduce the laser prepulse, which allowed ultra-thin and structured targets to remain intact without losing their structural integrity prior to the arrival of the main part of the laser pulse. Plasma mirrors enabled the
experimental demonstration of mechanisms relying on relativistic transparency in ultra-thin targets. Another breakthrough is the development of novel target designs to better couple laser energy to the target. These include nano-structured targets that enable one to reduce the average target density. There are also targets with a structure whose size is comparable to the laser wavelength. Such features have been utilized to increase coupling and performance of ion acceleration regimes. Recently developed liquid crystal thin-film targets, together with liquid and cryogenic jets, can enable higher repetition rate experiments, which are important as PW lasers have made the transition from shots per day to shots per second. (1 PW = 1015 W.) The same approach can be used for plasma mirrors.
The results obtained in the last 10 years for ion acceleration demonstrate a high level of synergy between theory, computer simulations, and experiments, which will be a prerequisite for future advancements in the field. The rapid development of high-performance computing resources that occurred over the last decade enabled fully 3D kinetic simulations in many regimes of LPIs relevant to ion acceleration. This has had a tremendous impact since 2D simulations were unable to capture the ion energy gain by quasi-static plasma electric fields. 3D simulations made it possible to provide quantitative predictions and meaningful comparisons with experimental results. It has also been shown that kinetic
simulations might have to be coupled to hydrodynamic simulations in order to reproduce the physics that takes place at lower intensity prior to the arrival of the main laser pulse.
Current and Future Science Challenges and Opportunities
There are many challenges and opportunities in the field of heavy particle acceleration. To be useful for applications, energies have to be increased, and systems have to become more stable. Multiple new regimes have been proposed for the next generation of laser systems with higher power and/or intensity and better contrast. These regimes are expected to enable a significant increase in ion energies. They are also predicted to deliver mono-energetic beams. This is an opportune time to address these challenges, with increased access through LaserNetUS, new high-power lasers in the European Extreme Light Infrastructure and Asia, and the opportunity for a next generation of domestic facilities.
Application challenges include stable ion acceleration with well-controlled and predictable ion beam parameters. Two particular challenges are increasing ion energy gains well beyond 100 MeV (this is the key to many applications since the stopping distance is directly correlated to the energy) and the generation of mono-energetic ion bunches with charge in the nC range. Even though multiple theoretical and computational models predict such parameters, it has not been possible to experimentally demonstrate these values with currently available laser technology. While experimental results exhibit a promising trend toward higher ion energy, further improvements in technology are necessary to achieve the predicted ion acceleration performance. This will require a multifaceted approach that involves further developments in both laser technology and targets to fully exploit effects such as relativistic transparency, radiation pressure, and magnetic vortices. At the same time, these topics advance the fundamental plasma science of laser-plasma coupling, heating, and acceleration.
Technological challenges include progress in targets, in laser contrast and intensity. Advanced targets, such as those with structure, have shown great promise in terms of improved and well-controlled laser energy coupling to the target. However, these targets remain expensive and the cost may become prohibitive for experiments with high-repetition rate laser systems, and certainly for applications. A challenge is to significantly reduce cost while improving target control. In the absence of reusable targets, technologies must be developed for replacing the targets at an appropriate rate, which may include evolution of liquid crystal or liquid jet targets. Another important technological challenge is to increase on-target laser intensities with very high contrast. Multiple ion acceleration regimes with unique properties have been predicted at intensities that are yet to be reached experimentally. Hence, the upcoming development of multi-PW laser systems in
a variety of pulse duration regimes is important, as are methods for laser contrast and pulse shaping in space and in time.
Theory and simulations predict that a PW-class laser system can generate ion beams with a maximum energy of several hundred MeV. Further scaling to systems at the tens of PW level is of great interest and an opportunity for future facilities. A breakthrough in laser-driven ion acceleration is expected once the high-power high-intensity laser facilities at the European Extreme Light Infrastructure (ELI NP) and ELI Beamlines become available to users, delivering previously inaccessible laser parameters. These laser systems will enable experimental access to new regimes that have been explored only via numerical simulations and analytical theory. Significant increases in ion energies and charge will likely open up opportunities in biomedical research, fast ICF ignition, compact neutron sources for security and industry, advanced proton radiography and isochoric heating for high energy density science, hadron cancer therapy, ultrafast beams for radiation damage and study of single event failure effects in electronics, and drivers and probes for the studies of warm dense matter and HED physics. To enable these opportunities, major prerequisites are not only high performance but also reliable and reproducible beam characteristics.
The ion acceleration community has identified a path forward to achieve many of these goals. A road map toward an ion accelerator based on laser-driven plasmas is shown in Figure 3.11.
BRIGHT X-RAY GENERATION
A bright beam is one with a large amount of energy in a small region of phase space—that is, short duration, small dimensions, and with small angular spread. Because of phase-space volume preservation, a brighter beam can be manipulated more easily to place a large amount of energy into a small physical volume. Said another way, less powerful lenses are needed to focus the energy into a particular volume. As a result, brighter beams are more effective for diagnostics, because they provide greater sensitivity. Light generated through LPIs is naturally very bright in part because it is produced in a small volume, so that with directionality it occupies a small phase space volume.
Compact photon sources with narrow divergence, producing femtosecond bursts, are enabled by plasma acceleration of electrons. Broadband betatron X-ray emission results from the betatron oscillations of particles in the plasma and/or laser fields and is a diagnostic of the beam properties inside the accelerator. Nearly monoenergetic Compton, or Thomson, scattering of a laser beam from the particle beam produces tunable, narrow bandwidth X rays of higher energy and flux than betatron radiation, and can extract detailed beam evolution information for studying beam interactions with plasma waves. Coherent free-electron lasers are
being developed using conventional magnetic undulators (or wigglers), and there are future concepts for compact plasma undulators. Coherent X-ray emission can also be created in plasmas via population inversion and from harmonics of laser field generated in plasma interactions. In these cases, plasma optics is an important ingredient. Control of these mechanisms and photon sources requires precision shaping of the laser pulses. Additionally, one must have sensitive control of the plasma profile, preparation, and pumping by the laser pulse.
High performance X-ray light sources have the potential to enable precision measurement in materials science, industry, security, nuclear nonproliferation, and medicine. Plasma-based X-ray and EUV radiation sources are both important frontiers in plasma physics with broad application, and an enabling technology to provide diagnostics (and potentially pumps) for high energy density plasma (HED) science, and high resolution and low radiation dose imaging. (See Figure 3.12.)
Progress and Achievements
Coherent soft X-ray sources have been demonstrated using solid and gas targets for High Harmonic Generation (HHG). These demonstrations include sources in the attosecond domain, thereby permitting attosecond temporal resolution, which is the scale of atomic processes, hence allowing one to observe atomic dynamics. X-ray lasers have been demonstrated with mJ/pulse energies down to wavelengths of 46 nm, and from energies of μJ down to 6.8 nm using multipulse ps lasers. Population inversions can be driven by a variety of mechanisms including electron impact excitation, collisional recombination, and photoionization. Plasma target shaping including nano-wire arrays has greatly improved coupling of laser energy into the X-ray producing plasma. Seeding (initiating) coherent X-ray amplifiers with HHG has been exhibited and allows full coherence. Power scaling is being explored to produce sources that could meet the needs for lithography in the semiconductor industry and other applications.
Several brilliant, ultrafast, synchronized X-ray sources based on plasma accelerators have been demonstrated. These sources enable advanced X-ray capabilities in compact laboratory setups with broad-reaching impact while also improving our understanding of the plasma accelerators. Experiments have shown the unique properties and advantages of these sources. Betatron emission (from the transverse oscillations in the generated plasma wave) produces keV broadband radiation sources with micron emission spot sizes that have been used to enable sensitive phase contrast imaging and ultrafast diagnostics of high-energy density experiments. These experiments demonstrate that the quality of X rays produced by plasma accelerators can in many cases exceed those obtained in conventional X-ray sources. Thomson scattering of a laser pulse from the electron beam has been used to produce quasi-monoenergetic X rays at selectable energies from keV to multi-MeV. Experiments have verified that these low-energy spread beams could simultaneously reduce radiation dose and increase sensitivity in X-ray applications ranging from medicine to security. Several programs are investigating plasma driven FELs to enable compact, ultrafast coherent sources.
Current and Future Science Challenges and Opportunities
Developments over the past 10 years have established many of the single shot parameters needed for X-ray applications, including keV to MeV mono-energetic sources produced by Thomson Scattering, keV broadband betatron sources and plasma based soft X-ray lasers. Experiments have begun to demonstrate the benefits of these sources at the few-Hz repetition rates allowed by current laser systems, primarily for fundamental physics studies. However, applications require repetition rates at kHz and beyond.
Current photon sources including Thomson and betatron, and the ongoing development of FELs, are attractive for investigating near-term applications. These sources typically use GeV-class LWFAs, which map to laser pulses of few Joules in tens of fs. The energy spread of the accelerated beams needs to be in the range of 0.1 to 10 percent, and emittances from nm to μm. These capabilities, with improvement in beam transport and beam disposal, will enable experiments now and in the next few years using state of the art LWFAs. Technology improvements for photon sources will also benefit long term research on advanced colliders. Repetition rates of kHz are needed for applications and developing such sources are now realistic at the required few-Joule-per-pulse energies. The first PWFA applications are likely to be a single-stage afterburner (e.g., the use of plasma acceleration to double the energy of an already substantially accelerated charged particle beam) for a Free Electron Laser (FEL), or high brightness, broadband betatron X-ray or gamma-ray sources. A brightness transformer for FEL operation may also be within reach.
Plasmas offer the potential for photon sources which are both more compact and more advanced in performance than conventional systems. For example, LWFA-based photon sources have the ability to decelerate the electron beam after photon production. (See Box 3.1.) For Thomson scattering, the plasma can also guide the laser to reduce required electron current per photon produced. Both of these capabilities reduce undesired radiation generation. The Thomson process can also provide details on electron beam evolution for studying beam interactions with plasma waves. A LWFA produced electron beam could be used in an FEL with appropriate transport and phase-space manipulation (for example, by decompressing the ultra-short beam). Such a compact FEL would be an enabling capability for many scientific disciplines and several projects are in progress.
Particle accelerators and the X-ray photon sources they power are fundamental technologies supporting basic science, medicine, industry, and national security. Advanced X-ray light sources enable precision measurements that have revolutionized a broad range of basic and applied sciences at large user facilities. However, the size of these sources means that they are not accessible to many applications. Compact plasma based sources could enable broadly accessible mono-energetic hard X rays from keV to MeV energies with smaller emission spots, and coherent X-ray free-electron lasers at venues outside of km-scale facilities. Realizing this vision would produce enormous benefits to industry, security, nuclear nonproliferation, and medicine, or for high energy density science.
NONLINEAR QED
Extremely intense lasers under development and being proposed will give rise to a new field of strong-field quantum electrodynamics (SF-QED). This is the study of electron-positron pairs and plasmas directly produced from the intense
laser fields. In essence, with strong laser fields, one can create a plasma from the vacuum.
A large body of mostly theoretical effort has indicated that SF-QED can fundamentally change the nature of the interaction of charged particles and plasmas with strong electromagnetic fields. These interactions include significant transfer of beam energy to radiation, prolific production of electron-positron pairs and radiation dominated regimes, where particle motion is mainly determined by the radiative processes. These theoretical efforts have greatly advanced our understanding of charged particle interactions with intense electromagnetic (EM) fields and their linkage to collective plasma effects. The next generation high intensity laser and accelerator facilities will access a regime dominated by SF-QED effects to test these theories and enable new applications. These facilities will provide new sources of particle beams for material and nuclear science studies. (See Figure 3.13.)
Progress and Achievements
In the last 10 years a large body of mostly theoretical effort was devoted to the study of multiphoton Compton and Breit-Wheeler (BW) processes and subsequently, the EM cascade, was identified as a new phenomenon. (Compton processes are the scattering of photons from high energy particles that change the wavelength of the photon. Breit-Wheeler processes produce positron-electron pairs from the collision of two high energy photons.) Workshops and reviews have outlined the principal schemes, and the methods needed to study these effects experimentally. See, for example, Workshop on Opportunities, Challenges, and Best Practices for Basic Plasma Science User Facilities (2019)11 and Summary of Strong-Field QED Workshop (2019).12
Electron beam collisions with high intensity laser pulses produce SF-QED effects at the lowest laser intensities. Two experiments demonstrated the depletion of beam energy due to photon emission in multiphoton Compton process and initiated a new era of experimental exploration of SF-QED effects using current PW-class lasers, advances possible by rapid progress in laser technology and by LWFA of electrons.
Current and Future Science Challenges and Opportunities
In the course of the study of multiphoton Compton and Breit-Wheeler processes, it was understood that they cannot be treated analytically in either vacuum or plasma. Several numerical approaches have been developed, from simple reduced order estimates for Compton and Breit-Wheeler effects, to massive 3-dimensional simulations. QED-PIC codes are extensively used to study the interactions of high intensity EM fields with energetic beams of charged particles and photons, and plasmas of different composition and density. Computer simulations of multiple shower-like photon emissions and pair production relied on the separation of scales. The characteristic scale of the SF-QED emission process is much smaller than that of EM Field or plasma phenomena scales. Apart from either enhancing or suppressing acceleration, the strong fields were found to modify the trajectories of charged particles in the radiation dominated regime. In this regime positrons and electrons are trapped on stable or quasi-stable trajectories inside the EM fields. Development of such models continues to be an active and important topic.
Current models are not capable of describing new regimes of collective high field interaction. One example is the Local Constant Field Approximation (LCFA),
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11 H.M. Milchberg and E.E. Scime, 2019, “Workshop on Opportunities, Challenges, and Best Practices for Basic Plasma Science User Facilities,” arxiv.org/abs/1910.09084.
12 M. Altarelli, R. Assmann, F. Burkart, B. Heinemann, T. Heinzl, T. Koffas, A.R. Maier, D. Reis, A. Ringwald, and M. Wing, 2019, “Summary of Strong-Field QED Workshop,” arXiv:1905.00059v1.
which is the backbone of almost all numerical tools now being employed. Recent studies point out the parameter regimes where LCFA predictions are significantly different from full QED calculations. A number of solutions to this problem were proposed by modifying the LCFA for plane Waves. However, there are processes that cannot be described by the plane-wave model and a self-consistent treatment is needed. Furthering our understanding of these phenomena will require well-orchestrated collaborations between development of new computational capabilities and new facilities and diagnostics. (See Figure 3.14.)
Another open question is the consequence of back reactions, either pair production or photon emission, on the intense electromagnetic field. Usually these processes are considered using an external field approximation. However, it has been pointed out that the creation of new particles can lead to the depletion of the electromagnetic field energy, which invalidates the approximation of the external field. Theoretical and simulation studies of the cascades up to now have relied on the formation length and time being much smaller than the spatial and time
inhomogeneities of the electromagnetic field. However, a full QED treatment of these processes has yet to be achieved. Another example of the scientific and computational challenge is the interaction of charged particles with super strong EM fields. For these conditions, strong field perturbation theory is no longer applicable, since the contribution of the second order process becomes comparable with the first order ones. Proper treatment of spin and polarization effects on plasma dynamics is also of interest.
Further theoretical and numerical studies of SF-QED will not only advance our understanding of the EM field interaction with plasmas at highest intensities but will also provide critical insights into allied scientific fields, such as accelerator research and high energy physics. Advancing our understanding of SF-QED also requires a concentrated experimental effort, which requires a collaboration between these allied scientific fields, to validate the findings, test theoretical and numerical models, paving the way for future applications and exploring new phenomena. The study of SF-QED effects requires high power laser facilities and sources of high energy (multi-GeV) electron beams, either from LWFA or conventional accelerators. In this sense SF-QED has strong connection with laser and accelerator technologies. Moreover, different regimes of charged particles interacting with high intensity EM pulses might lead to the development of high brightness sources of X rays and gamma rays.
As laser intensities continue to increase SF-QED effects will become increasingly important, entering the regime of QED-plasma, where the number of produced particles is so large that they begin to demonstrate collective behavior, other plasma-based processes start to be affected by them. These processes include laser driven ion and electron acceleration and high harmonics generation. A series of experimental regimes will become accessible with laser technologies that will result from following the roadmap. These new regimes start with advanced experiments in collisions of beams with lasers accessible on near-term PW laser facility extensions. Over the longer term, tens of PW multibeam laser facilities could enable new regimes strongly affected or even dominated by SF-QED-plasma interactions. (See Figure 3.14.)
The study of SF-QED phenomena opens new physics across many fields. With SF QED effects entering the regime of QED-plasma, where the number of produced particles is so large that they begin to demonstrate collective behavior, other plasma-based processes start to be affected. These processes include, for example, laser driven ion and electron acceleration, and high harmonic generation. The field shares a common analytical basis, as well as common plasma accelerator technology needs, with high energy physics. SF-QED plasma studies can potentially relate to studies of high-energy hadron interactions and the creation of quark-gluon plasmas as well as the interaction point of future lepton colliders. Future light sources built on the SF-QED effects will provide photon beams that could be used
in nuclear and material science as drivers and as probes. The construction of second beamlines, targets, and diagnostics at high-repetition-rate (1 Hz) PW facilities as well as support for theory and simulation programs should be a near term priority in order to achieve rapid progress in SF-QED studies in the United States. In the longer term a many-PW facility will be needed to fully exploit this regime.
DIAGNOSTICS
Diagnostic and computational advances have enabled deeper understanding of the plasma state. For example, collective Thomson scattering has proven to be a valuable tool for diagnosing the hot plasmas typical of inertial confinement fusion, providing either spatially or temporally resolved measurements of the plasma density, electron and ion temperatures, ionization state, and flow velocity. Recent experiments have even exploited the Faraday rotation of the Thomson scattering probe laser to measure the dynamo amplification of magnetic fields in a turbulent plasma. In the past decade, optical interferometry has provided insight into the spatiotemporal dynamics of plasmas from the spatial structure of laser driven wakefields to the ultrafast transition of matter from a gaseous to plasma state. Development of new diagnostics to measure the plasma distribution function, wave amplitudes and distributions, and particle phase space, continue to be required. (See Figure 3.15.)
A critical component is the development of novel diagnostics that can measure not only the bulk hydrodynamic properties of the plasma, but also the underlying electron velocity distribution functions (eVDFs). Experiments have begun using Thomson scattering to measure the shape of the eVDF. These experiments have demonstrated that processes such as cross-beam energy transfer (an interaction in ICF and many plasma optics) and Raman amplification cannot be predicted without considering the shape of the eVDF. Fundamental processes such as inverse Bremsstrahlung absorption, collisional ionization and atomic processes, and heat transport can all modify the eVDF in ways that effect LPIs. For example, the flattening of the eVDF at low velocities due to collisional absorption can reduce the energy transfer between crossing laser beams—an effect that could be misinterpreted as a nonlinear saturation process. A similar effect could arise in Raman amplification due to electron heat flux. The heat flux can alter the Landau damping of the plasma waves leading to erroneous predictions for the length of the amplifier required to reach saturated gain (i.e., pump depletion).
Virtually all of the needed technological advances described above and science advances that will be enabled by those technologies require increasingly more capable diagnostics to characterize the state of the plasma. This is true in all subfields of plasma science and particularly challenging in LPI due to the small volumes and short durations of the laser- or particle beam-plasma beam interactions. The HED
community has been quite successful in leveraging diagnostics in a synergistic way to improve laser and pulsed power technologies while advancing our fundamental understanding of the underlying plasma physics. The LPI field would greatly benefit from a similar collaborative development of diagnostics.
COMPUTATION
Computation (or numerical modeling) of plasma acceleration, radiation generation, and the associated LPIs has been essential to the development of the field of LPI. Computations are synergistically supported by advances in theory, which enables the improved fundamental understanding required to develop the numerical algorithms. Computation enables one to investigate laser or beam interaction with plasmas that are otherwise unobservable with current diagnostics due to the fact that they occur in very short times in small regions of space. Computation enables the exploration of scenarios, such as electron injection by ionization in
mixed gases, prior to incurring the expense of experimental development of such systems, and enables one to consider regimes (such as much higher laser powers) that are not currently available. These computational investigations are essential to defining the experimental path forward. Once that path has been determined, computation enables improved design of experiments by, for example, optimizing the shape and materials of plasma targets.
Computation is intensive and challenging in this field due to the presence of multiple time scales and the need to resolve the long distance of propagation of lasers and beams. The longest time scale is the plasma channel formation time, ~ns, and this physics is typically modeled by magneto-hydrodynamic (MHD) codes. In the low plasma density regime, as needed for high-gain, single-stages, Direct Simulation Monte Carlo, Particle In Cell (DSMC-PIC) codes are now being used. This work is in its early stages. The next longest time scale is that of laser propagation through the channel, hundreds of ps, followed by the pulse duration, which is approximately the plasma period, ~10-100 fs. The shortest scale is typically the laser period, ~0.3 fs. The laser pulse propagation and plasma wave excitation are typically modeled using particle-in-cell (PIC) codes. However, this is sufficiently challenging that many physics problems can be addressed only with reduced models. Major progress has been made in both computational performance and in reduced models that take advantage of special properties of the interaction, for example by relativistically boosting to a frame that moves with the laser pulse or by averaging over its period. These methods have enabled modeling of meter-scale, multi-GeV experiments. Simulations are now routinely performed in coordination with experiments and theory to design concepts and interpret results. These improve confidence in our understanding of the physics and improve confidence in simulations for designing more advanced systems.
In spite of progress in the United States, we have seen a movement of the center of computations to non-U.S. venues. At the beginning of the decade, computation in LPI at U.S. institutions was dominant. For example, in 2004 all of the experiments for the Dream Beam issue of Nature (Volume 431, Issue 7008, September 2004) showing narrow electron beam spreads with accelerations to near GeV were accompanied by simulations performed with U.S.-developed codes. However, that dominance has steadily eroded. In at least one case, the center of development of codes originating in the United States has moved to non-U.S. institutions. In other cases, codes that were originally open source moved to a closed-source model to obtain sufficient resources for development. One outcome is that a large number of researchers have moved to using codes developed internationally. The resource sharing aspect of these internationally developed codes is good. However, it also means that U.S. researchers will have less influence over the development of features for those codes. The end result is that implementing features in these cases that are specific to U.S. research needs will have lower priority, if implemented at all.
Another result is that in the United States will not have the innovation that comes from healthy competition in algorithms and approaches. With university efforts in code development for this field having been reduced there is also a reduced production of new computational researchers in the United States capable of code development. Although difficult to quantify, it appears that hiring at national laboratories in computations is shifting toward foreign nationals who have this skill set. The lack of intellectual diversity in code development in the United States is in stark contrast to the situation in facilities, where there is a large range of facilities at national laboratories and universities investigating different drivers (beams versus lasers), plasma targets, diagnostics, and scales. Finally, we mention that there is little development in the modeling of plasma sources.
ENABLING TECHNOLOGY AND FACILITIES
Progress in LPI, plasma optics, and plasma acceleration will continue to be driven by advances in laser technology. A healthy ecosystem of facilities, programs and capabilities, having diverse and complementary capabilities and instrument size, is required to enable advances in accelerator, photon source, nonlinear optics, high field, and ion sources. The current ecosystem has supported the impressive progress in the field, and includes laser and beam drivers at multiple scales and LaserNetUS, a newly emerging network for coordination and user experiments. Rapidly advancing laser technology creates opportunities to advance the field, in several forms. Existing laser facilities should be upgraded by adding beams to support injection, shock drive and other capabilities. They should be equipped with advanced beam shaping technologies (spatial and temporal) to enable control of the interaction processes. New facilities will be needed to drive the field forward.
Collaborations have been and continue to be important to the development of the LPI field. Discussions on access between LaserNetUS and similar networks and facilities in Europe have started. International collaborations broaden access to unique facilities and techniques, the combination of which is stronger than any one effort. Roadmap exercises are also an important connection, recent examples being U.S., European and UK accelerator strategies. There are excellent opportunities for student training in LPI due to the growth of this exciting and relatively new field, which attracts students, international and domestic collaborators, users and new researchers. International collaborations will take on increasing importance to exploit state-of-the-art facilities being commissioned in Europe (in particular the Extreme Light Infrastructure -ELI) and Asia, which will in the near term will lead the multi-PW frontier. (See Figure 3.16.) The U.S. community is large but fragmented across programs sponsored by the DOE, NSF, and the defense agencies. Little cross-agency coordination exists.
Much of the progress in PWFA research over the past decade has been driven by increasingly precise drivers at national facilities, coupled with a research community at both universities and the laboratories. Major Facilities include the Advanced Accelerator Experimental Tests (FACET) at SLAC National Accelerator Laboratory, the Advanced Test Facility (ATF) at Brookhaven National Laboratory, and the Advanced WAKEfield Experiment (AWAKE) at CERN.
Over the past 10 years compact sources of high-power (up to PW) ultrashort (sub-100 fs) lasers have become available at a few Hz repetition rates. These laser capabilities have enabled broad progress in LPI and LWFA. The availability of laser capability has enabled studies with sufficient statistics to scale resonantly driven plasma acceleration up to the 10 GeV level. This is the energy scale that is viable for stages in HEP colliders and X-ray FELs (XFEL). (Other applications such as Thomson sources, medical accelerators, and ultrafast diffraction can use lower electron energies and laser powers.) These low-and-high energy examples emphasize that progress in LPA and photon sources has been enabled by a broad array of facilities at varying scales, from small single investigator university systems to large national laboratories. These many scales have enabled access to the field, training and the ability to take risks in addressing science challenges. In this regard, in 2018, the LaserNetUS facility network (Box 3.2) was established to provide broad access to midscale laser facilities.
To address the science challenges discussed in this chapter will require a range of new facilities in the next decade. In this regard, there are several research and technology frontiers with opportunities to produce new capabilities and science, and provide opportunities for U.S. leadership. Stable, high-repetition-rate lasers accompanied by precise control and diagnosis of target properties are required to push the boundaries of experimental precision. In the near term, modification of existing facilities will be required to develop plasma optics and secondary particle sources. In the longer term, co-location of multiple beams will be an enabling strategy, for example, by combining high energy drivers and short-pulse probes. Temporal control at multi-THz bandwidths (for high-energy and high-peak-power facilities) and high-order spatial phase control are necessary innovations. Specifically, the system requirements include (1) multi-kJ ns laser pulses with shaped or stochastic pulse trains co-located with other short pulse beams such as 100 J/100 fs and 30 J/30 fs; (2) laser intensities exceeding 1021 W/cm2 for high field physics and new wavelength regimes; and (3) kHz-class repetition rates for precision control and machine learning to drive performance, and for accelerator and photon source applications. These needs have been detailed in a community report, (Brightest Light Initiative Workshop Report13).
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13 R. Falcone, F. Albert, F. Beg, S. Glenzer, T. Ditmire, T. Spinka, and J. Zuegel, 2019, “Workshop Report: Brightest Light Initiative,” https://arxiv.org/abs/2002.09712.
Two technology frontiers define the path beyond current facilities: repetition rate and intensity. High repetition rate, efficiency and control define the core electron/positron accelerator and X-ray source laser requirements. Lasers with repetitions rates of kHz and higher with greater efficiency and control are required for LWFA. These lasers will enable stability and reproducibility of the accelerated electron beams with repetition rates of kHz through feedback control systems. Beam properties are strongly affected by fluctuations in facility properties—ground motion and air motion. The impact of these fluctuations falls off above hundreds of Hz. Operating at higher repetition rate enables more beam fluence in a given time, but also enables active laser feedback control where the pulse frequency (>kHz) significantly exceeds the fluctuation frequency (<kHz) of the facility. Feedback control has been demonstrated on low energy laser systems (mJ-class) and needs to be extended to systems that drive LPAs. This need defines a core accelerator development track—a few-joule-per-pulse, kHz system (with stabilization and shaping) enabling precision. The range of needs motivates a range of facilities addressing different capabilities and approaches. A near term priority is a few-Joule per pulse kHz system that will enable light sources and precision LPA through real-time-control stabilization. In addition to enabling science advances, this system will provide the learning required to develop a 10 J/50 kHz collider stage driver. In the near term many of the control techniques required for high repetition rate lasers can be prototyped on existing lasers or extensions of them. Additional beams for injection and guiding control that are at lower energy do not drive overall laser development, but they do drive facility configurations.
Other lasers are also of interest for accelerators. In particular, these include long wavelength drivers for injectors and large bubble generation. (A bubble is a cavity free of cold plasma electrons.) High energy and high intensity systems are the path to next generation HED science probes, ion acceleration, and high field drivers. These include multi-PW facilities reaching to kilojoules and beyond at tens of femtoseconds to hundreds of femtoseconds depending on application. Correspondingly, repetition rates will be lower than Joule class systems, with rates of one to a few shots/hour being accessible now, and tens of shots/hour in the near future. Such lasers are of particular interest when coupled to high energy ns drivers or additional high intensity beams for heating and target shaping (HED) or particle beam generation (high field science), and for advanced probe capabilities. It is important to have a balance between user facilities where new ideas can be tried at moderate cost, and dedicated engineered beamlines where high performance and control can be advanced.
Plasma acceleration, X-ray sources, and optics have connections to industry both through laser development and through potential applications. A recent DOE-HEP Basic Research Needs Workshop (Compact Accelerators in Security and
Medicine, 2020)14 with participation from multiple agencies and industry, discusses the importance that compact accelerators, and electrons and ions, and X-ray sources will have for applications in industry, medicine and security. These applications include nondestructive characterization (security and industrial) and medical imaging with improved resolution and lower radiation dose, new medical therapies via FLASH (high-fluence, short duration) rapid irradiation or endoscopic techniques, and improved security screening. The potential for orders of magnitude improvement in resolution and reduction in dose potentially enabled by laser-plasma sources would have discipline-changing impacts. Correspondingly, progress in plasma acceleration has been driven in large part by laser development, and this investment has been reinforcing. Lasers which enable greater capability in LPI produce scientific results that define the need for more capable lasers. The recent heavy investment in Europe and Asia in laser facilities and science advances has shifted the center of high-power laser development to those areas, with follow-on industrial benefits to areas such as shock peening for metallic surface hardening and laser machining. Future investment should combine opportunities in laser development, applications and plasma science, areas that synergistically feed on each other and that drive progress. The improved accelerators resulting from this synergy will similarly advance scientific and technical disciplines that rely on accelerators as scientific tools.
An example of the synergy between technology development and science advances is the development of short-pulse, high-intensity lasers based on chirped-pulse amplification (CPA) by Strickland and Mourou for which the 2018 Nobel Prize in physics was awarded. CPA has made compact sources of high-power (up to PW) ultrashort (sub-100-fs) lasers readily available at Hz-class repetition rates. Although higher laser power and intensities are needed for some investigations (e.g., ion acceleration and SF-QED), many of the fundamental physics challenges described here do not necessarily require higher intensities. Instead, they require laser technology advancements in precision, control, repetition rate, efficiency, and access to flexible regimes of operation (e.g., multiple beams, various wavelengths, coupled external accelerators). Presently, high-peak power, short-pulse systems are based on Ti-sapphire laser technology and operate at 1-10 Hz. This low repetition rate limits feedback control and machine learning that, with higher repetition rates, would enable submicroradian pointing stability needed for combining stages of LPA. With higher repetition rate enabled control, laser shaping would not be limited by facility fluctuations. This would in turn enable precision control of injection and acceleration. Such control has been demonstrated on kHz lasers with small
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14 U.S. Department of Energy, 2019, Basic Research Needs Workshop on Compact Accelerators for Security and Medicine: Tools for the 21st Century, https://www.osti.gov/servlets/purl/1631121.
pulse energies, and achieving the energies needed for LWFA at such repetition rates is a key technical need.
Summary
The study of plasmas driven by intense lasers and particle beams is opening new fields in plasma optics, high field physics, particle acceleration, and radiation sources and enabling new applications across science and society. These include the following:
- Transforming our understanding of the fundamental mechanisms of laser-plasma coupling and particle acceleration, and developing predictive models across experimentally available parameter spaces;
- Implementing plasma optics in laser systems to both support current experiments and access new plasma regimes;
- Advancing control of particle and radiation sources with good reproducibility, detailed on-shot characterization, and high fidelity predictions by simulations/models;
- Developing secondary beams for probes and pumps in HED physics and for applications in security, industry and medicine.
Laser and Beam Plasma Wakefield Acceleration
Many important firsts have been demonstrated in wakefield and underdense acceleration, and brilliant X-ray sources. These accomplishments have established a path to meeting needs in applications ranging from extending the reach of high energy physics to transforming the performance and dose for X-ray imaging. The rich physics of resonant control needs to be explored and exploited to achieve advances in accelerator performance. Next steps in technology development include kHz repetition rates enabling stabilization and active feedback for precision shaping of the plasma state, and new regimes in driver intensity and wavelength.
Ion Acceleration
New mechanisms for ion acceleration are now accessible, including via radiation pressure and magnetic vortices. New laser, diagnostic and target capabilities are now emerging to enable higher repetition rate and control. Although existing facilities are being used to demonstrate important fundament concepts, new facilities with increased intensity and contrast are needed to achieve the energies predicted by theory and computations. These compact ultrafast ion and neutron
sources could enable more effective cancer therapies, advanced HED science pump and probe sources, and neutron sources for industry and security. (Additional important opportunities in lasers co-located with coherent X-ray light sources are discussed in Chapter 4.)
Strong Field Science
Laser-plasma interaction studies have opened a new regime whereby the physics of a relativistic plasma is strongly affected by strong-field quantum electrodynamics (SF-QED). These opportunities include laboratory analogies of Hawking radiation in electric fields and Unruh radiation, processes important to cosmology and astrophysics. Advances in strong-field science open the possibility of investigating the basics of astrophysical objects, including black holes, pulsars, and magnetars, in the laboratory; and uncovering the dynamic interaction of inner shell electrons with highly ionized, heavy nuclei. Extensions of existing facilities will enable early experiments of laser-electron beam interactions. Future facilities at and beyond the 10 PW power level are important to fully exploit these opportunities.
Plasma Optics
Beyond adjusting parameters like intensity and frequency, the spatiotemporal structure of light offers additional degrees of freedom for controlling the interaction of intense laser pulses with plasma. Broad-bandwidth lasers could revolutionize ICF by providing unprecedented spatiotemporal control over LPIs. Magnetization has recently emerged as a new capability to modify collective behavior and manipulate plasma optics.
Across the LPI area, the Chapter 1 findings and recommendations on workforce, demographics and academic representation apply. Importantly, the state of the academic and national laboratory workforce is nearing a critical point and addressing these concerns is crucial to ensure continued progress in LPI. The U.S. workforce issues are particularly critical to LPI. There is unprecedented international growth and competition. The U.S. LPI field is strongly reliant on international graduate students and post-doctoral researchers. As international facilities take the lead over U.S. facilities, the international workforce will be attracted away from the United States, which could place the LPI field in the United States at risk.
Compact plasma accelerators, plasma X-ray sources, and plasma optical methods were in large part invented in the United States, and the United States has held a leadership position in most of the field in past decades. However, as reported in the National Academies study Opportunities in Intense Ultrafast Lasers: Reaching
for the Brightest Light (2017)15 the United States has recently lost dominance in high-intensity laser research, which is impeding progress in the essential areas of plasma science outlined in this chapter. The loss of leadership in large part is due to large investments in new laser facilities and corresponding research programs made and being made in Europe and Asia. (See Figure 3.15.) These international investments have in particular emphasized development of multi-PW systems, and these facilities are driving forward capabilities in all the research areas discussed here. RBL concluded that the research performed on non-U.S. facilities does have great value for the nation. However, without additional U.S. investments in laser facilities, leadership in LPI will drift away from the United States.
FINDINGS AND RECOMMENDATIONS
Finding: Compact plasma accelerators, X-ray sources, and optics were invented in the United States. However, as reported in Opportunities in Intense Ultrafast Lasers: Reaching for the Brightest Light,16 the United States has lost dominance in high-intensity laser research and related research that is essential to plasma science, accelerators, and their applications.
Finding: There are strategic opportunities in the next 10 years to build scientific facilities that can leap-frog international competition and enable the United States to maintain a leadership position in LPIs.
Recommendation: To restore U.S. leadership, DOE and other agencies should formulate a national strategy to develop and build new classes of high-intensity lasers that enable now inaccessible parameter regimes.
Facilities constructed through the strategy above would produce the technologically highest intensities to open up new regimes in high field physics and ion acceleration, having repetition rates at and beyond 1 kHz, with shaped pulses enabling precision control, and with active feedback and machine learning for acceleration and plasma optics.
Finding: Plasma acceleration and controlled laser-plasma optics are rapidly advancing, driven by newly available capabilities in short pulse/broad bandwidth lasers.
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15 National Academies of Sciences, Engineering, and Medicine, 2018, Opportunities in Intense Ultrafast Lasers: Reaching for the Brightest Light, The National Academies Press, Washington, DC, https://doi.org/10.17226/24939.
16 Ibid.
On the horizon are kHz lasers with active feedback together with pulse shaping and bandwidth control capabilities that together will drive progress. This will enable new capabilities ranging from X-ray characterization (medicine, industry, and nuclear nonproliferation as well as transformative HED diagnostics) and inertial fusion to medical therapy and future particle colliders. Long term translational and applied laser-plasma driven source development and high repetition laser driver development are required to realize applications. However, there are currently few efforts to achieve these goals.
Finding: Applications require robust, compact drivers. A long-term plan and resources for developing technologies that can leverage science advances into society benefiting applications are needed.
Recommendation: DOE and NSF should lead a collaborative effort with other agencies to develop an extended stewardship program for long-term, application-oriented research to enable the development of revolutionary laser-plasma driven sources that translate to applications.
Very high repetition rate, precision-controlled lasers and plasma methods will need to be included as part of the stewardship program.
Finding: Collaboration between agencies focused on source development (DOE, NSF) and potential user agencies (e.g., NIH, DoD) is needed to ensure that advanced laser capabilities are developed.
Examples of such potential collaborations are given in Table 1.1.
Rapid research progress relies on access to the latest laser and related technology, and it is important that access be available beyond the few institutions with large programs. LaserNetUS provides important access opportunities to midscale facilities.
Finding: There is need for multiple programs and approaches in experiment, theory and computation, ranging in scale from single investigator experiments to user facilities and dedicated mission focused facilities or centers.
Recommendation: Agencies focused on the fundamentals of laser-plasma-interactions (NSF-MPS, DOE-FES, DOE-NNSA) should collaboratively augment and create programs in plasma acceleration and optics that support a range of scales and multiple efforts and that coordinate research, user access, and educational support.
Table 1.1 in Chapter 1 lists opportunities for cross-agency collaborations.
Finding: A blend of science innovation (e.g., development of new physics regimes in high field science) and long-term engineering efforts to develop new facilities has been essential to progress in LPIs.
Finding: Together with support from other agencies and DOE support concentrated at the National Laboratories, NSF support devoted to LPI at universities is essential to the field.
Recommendation: NSF-MPS, DOE-SC, and DOE-NNSA should strongly support research in the fundamental physics of plasma optics, high field acceleration, and laser sources in collaboration with other agencies. This includes research, centers, and midscale infrastructure.
Finding: Computation and theory has been essential to the development of the field of LPI, providing insights and crucial input into experiment design. U.S. computation, once dominant, has lost that advantage.
Leadership in computations has transitioned to non-U.S. institutions, particularly in Europe where multiple code development efforts are being supported. In contrast, funding in the United States no longer provides sufficient support for multiple, competing code efforts.
Finding: A range of needed computational tools, both fluid-based and DSMC-PIC, is also needed for modeling plasma sources.
Finding: The innovation that comes from healthy competition would help restore U.S. leadership in computations for LPI.
Recommendation: NSF-MPS, NSF-CISE, and DOE-SC should support a diversity of computational and theoretical efforts to help restore U.S. leadership in computations for laser-plasma interactions.
