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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities 6 X-RAY REGION: 100 to 1 Å PRESENT CAPABILITIES Synchrotron Sources X-ray radiation in the region from 1 to 100 Å is covered primarily by synchrotron sources such as the Advanced Light Source (ALS) at Lawrence Berkeley Laboratory, the Advanced Photon Source (APS) being built at Argonne National Laboratory, the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory, the Stanford Synchrotron Radiation Laboratory (SSRL) at Stanford University, the Cornell High Energy Synchrotron Source (CHESS) at Cornell University, the Synchrotron Radiation Center (SRC) at the University of Wisconsin at Madison, the Synchrotron Ultraviolet Radiation Facility (SURF) at the National Institute of Standards and Technology, and the Center for Advanced Microstructures and Devices (CAMD) at Louisiana State University. These sources provide broadly tunable radiation to wavelengths as short as 0.3 Å as well as to wavelengths of more than 1000 Å. With suitable frequency filtering, one can achieve Δλ/λ ≤ 10−4. With the new sources (such as the APS), brightness will increase by approximately two to three orders of magnitude and transverse coherence will increase by three orders of magnitude beyond the corresponding capabilities of previous synchrotron sources. The United States has invested heavily in these new sources. For example, the ALS construction costs were $154M (Total Project Cost), and an additional $70M is being requested to develop and equip beamlines, including a crystallography facility with dedicated beamlines. The total project construction cost of the APS is $812M (Total Project Cost), with perhaps an additional $300M required to build and equip beamlines. One of the major advantages cited in justifying the development of this generation of light sources was that many experiments could be run simultaneously from a large number of beamlines. Laser Pumped X-ray Sources Tabletop terawatt lasers, costing $0.5M to $0.8M each, can generate radiation in the spectral range between 5 and 1000 nm by high-order harmonic generation in vapor targets with as much as 10 nJ per harmonic in each pulse. Intense, picosecond bursts of x-rays have also been generated by focusing these lasers onto solid targets. The x-ray user community has not yet embraced laser-produced x-ray sources for a number of reasons. Radiation from laser-driven plasmas is a mixture of
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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities incoherent broadband and line emission, and monochromators or filters are needed to produce a clean, well-characterized source of radiation. X-ray production from solid targets also produces a considerable amount of material debris that would have to be filtered out. The low duty cycle of these high-peak-power laser sources does not interface well with most x-ray applications. Currently most of the experiments with these sources have been done mainly by those who built the laser. Soft x-ray lasers pumped with very high intensity lasers have also been demonstrated. The low repetition rate and the fact that the x-ray emission is at fixed wavelengths may prevent these lasers from becoming general-purpose x-ray sources. Compton Backscattering Tunable radiation in the x-ray region can also be produced by the interaction of relativistic electrons with intense laser fields via Compton backscattering. In this scheme for producing x-rays, the periodic magnetic field of an FEL is replaced by an electromagnetic wave propagating opposite the motion of the electrons. Like synchrotron radiation, the radiation generated by this technique will have limited coherence. At present, however, this method of generating x-rays is not competitive with synchrotron sources. Additional research must be done before we will know if this idea can be turned into a practical, sufficiently intense source of x-rays. Similarly, channel radiation (x-ray radiation emitted by relativistic electrons propagating through the interatomic space in a crystal) and plasma-wave undulators (electric fields generated by plasma oscillations replacing a magnetic undulator) for a high-energy electron beam are still in a highly speculative, exploratory stage of development and cannot be counted on as a viable source of x-rays for users. WHAT IS ENVISIONED Proposals have been developed at SLAC for x-ray FELs at 40 Å, 4.5 Å and eventually 1.5 Å. The 40-Å FEL is estimated to have 10-GW peak power in 160-fs pulses at 120 Hz. The spectral brightness and transverse coherence are estimated to be several orders of magnitude greater than those of either the ALS or the APS. The construction of a hard x-ray FEL would require a huge step in the development of FELs. For comparison the shortest-wavelength FEL operated to date is at 2400 Å at the Novosibirsk storage ring. In order to achieve coherent emission in the x-ray range, extremely low emittance, high-energy electron beams have to be developed. The high-quality SLAC linear accelerator beam, developed for the linear collider, could provide the starting point for this project, but the existing SLAC electron beam would have to be upgraded. A number of other technical challenges have to be solved. These include (1) the development of a reliable, high-intensity electron gun such as a laser-excited
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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities photocathode RF gun, (2) suppression of space-charge effects between the gun and the high-energy portion of the linear accelerator, and (3) wake-field effects in both the longitudinal and transverse dimensions of the electron beam. (The wake fields, generated by currents in the accelerator structures induced by the leading portion of the electron beam, break up the trailing portion of the pulse.) Other concerns include (4) effects from the tails of the electron beam distribution, (5) electron bunch compression, and (6) the development of a cost-effective package of diagnostics and feedback systems to stabilize all the beam parameters. As one goes to shorter and shorter wavelengths, the problems mentioned above become more difficult to solve. Many of these technical problems could be addressed by research being done for the Next Linear Collider (NLC), since the electron-beam parameters needed for the NLC will be more demanding than those necessary for an x-ray FEL. Alternatively, the development of an x-ray FEL might be a precursor to solving the technical problems of the NLC. The construction and operating costs of an x-ray FEL will be much higher than those of an infrared FEL. For example, the incremental construction cost, based on the assumption that the last third of the 2-mile-long SLAC linear accelerator would be dedicated to an x-ray FEL, is estimated by SLAC to be $30M to $50M. A new, dedicated accelerator built for an x-ray FEL would cost hundreds of millions of dollars. One third of the running cost of SLAC is $13.8M for a 9-month period. This cost includes electricity, maintenance of the accelerator, salaries of the operating and support staff, and 40% indirect costs. It does not include the cost of the scientific programs, salaries of staff scientists and accelerator engineers, general plant costs, and accelerator and facility upgrades. SCIENTIFIC OPPORTUNITIES It is hard to anticipate all of the scientific opportunities made possible by the additional orders-of-magnitude increase in coherence over that of the APS. Increases in peak power and decreases in pulse width of several orders of magnitude are anticipated. An x-ray FEL, especially one operating in the hard x-ray portion of the spectrum, could be an important tool for probing distances at the atomic scale. Currently, the overwhelming bulk of x-ray work is done in the 1-to 1.5-Å range. Experiments that use the transverse coherence of x-ray sources are just beginning. In the current culture among x-ray scientists, coherence properties are ignored because, before the advent of the APS, sources with adequate coherence were not available. Similarly, the scientific community has not yet identified dramatic uses of the very short pulses of the proposed FEL source. Although synchrotron x-ray radiation comes in pulses on the order of 150 ps, the vast majority of the work at these machines is not time resolved. Flash x-ray microscopy and measurements of transient lattice distortions or melting are experiments that one might do with short-pulse x-ray sources, but many of these experiments can also be done at synchrotron sources.
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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities Because of the manner in which x-rays interact with matter, both the scientific opportunities and technical feasibility of the experiments improve as one goes to harder x-rays. For example, soft x-ray optics such as multilayer coatings do not compete with Bragg reflection from crystal lattices. High absorption coefficients in the soft x-ray region also lead to more energy deposition per unit volume, so that there is a greater potential for heating or damage to both optics and samples. In a workshop held at SLAC in October 1992 to explore the scientific opportunities of a 40-Å FEL, a general conclusion was that more exciting science would be possible if one could get to a few angstroms or less. The conclusions of this workshop prompted a second workshop held in February 1994 to discuss the possibilities of a hard x-ray FEL operating in the 4.5-Å and 1.5-Å range. Time-Correlation Spectroscopy Lasers in the infrared and visible wavelength regimes have allowed the study of collective modes, dynamical critical fluctuations near phase transitions, the evolution of conformations and structures, and a host of other phenomena at a distance scale of 100 nm or greater. The variety of methods such as frequency and time-domain spectroscopies and light-scattering techniques that have been developed in the optical domain may be applicable in the x-ray region because of the large increase in the spatial and frequency coherence of planned and proposed x-ray sources such as the APS or an x-ray FEL. For example, a coherently illuminated sample gives rise to a random (speckle) interference pattern, and the time dependence of such a pattern at a particular point in the far field tells about the motion of the scattering sites. Time-resolved x-ray speckle interferometry may allow one to use this technique to probe distances below the 100-nm scale. The motion of defects, the diffusion of particles in liquids, critical fluctuations, liquid crystals, and charge-density waves are examples of phenomena that could be examined with increased spatial and temporal resolution. Millisecond time-correlation experiments are now planned on the APS. These first experiments will use model systems in which the speckle effects are expected to be large. In order to study interesting physical systems with shorter correlation lengths and faster time scales, much higher spatially coherent fluxes will be needed. Ultimately, the proposed intensity of the SLAC FEL could allow a single-shot speckle interferogram to be recorded. If fast beam switching and pulse-delay methods could be developed, the observation of atomic-scale dynamics might be possible. Other X-ray Spectroscopies In addition to time-correlation studies, other forms of x-ray scattering would be made possible with the increased brightness of an x-ray FEL. For example, the magnetic scattering of x-rays at an absorption edge would allow the study of the
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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities structure and dynamics of magnetically ordered systems. Inelastic x-ray scattering, the equivalent of Raman scattering in the visible region, would become feasible with a significant increase in x-ray flux. High-energy phonons and magnons, which are difficult to probe with neutron scattering, could be resonantly enhanced by exploiting the laser's tunability. The behavior of quasicrystalline and fluid-phase short-range order, and of quenched disorder in glassy and related materials, could be revealed. While some of these structural studies could be done with third-generation synchrotron sources, an x-ray laser would be required to probe the dynamical behavior of these systems. Materials in which only minute samples are available would also be amenable to study with a brighter x-ray source. Microscopy and Holography For many years, high-resolution elemental mapping—by imaging above and below K-edges of atomic transitions—and three-dimensional pictures have been actively pursued. A number of experiments have shown that these goals are difficult to achieve, and problems encountered with x-ray imaging must be considered in the context of the advances in scanning microscopies such as the tunneling, force, and near-field optical microscopes. X-ray microscopy done in either an imaging or scanning mode has achieved a transverse resolution on the order of 300 Å. Since the numerical aperture of x-ray optics is ≤ 0.1 for the best Fresnel zone plates, the depth resolution is at least 20 times worse. X-ray shadow microscopy in which a sample is placed on top of a high-resolution photoresist such as polymethyl methacrylate has achieved around 100-Å resolution, limited primarily by the damage range of the x-rays as they penetrate into the resist. Realistically, soft x-ray microscopy in the water window could achieve 200-Å transverse resolution in the near future, but achieving dramatically higher spatial resolution will require a breakthrough. Given the lack of high-numerical-aperture x-ray optics, three-dimensional imaging can be done either by tomography or holography. Tomography uses a set of two-dimensional projections to reconstruct a three-dimensional image. The quality of the image is heavily dependent on reconstruction algorithms. Holography uses the interference between light scattered from a sample and light from a reference beam. A true three-dimensional image can be achieved only if holograms are taken from a number of views. A highly coherent x-ray source is a necessary condition for three-dimensional high-resolution imaging. However, it is not a sufficient condition. Atomic resolution demands a high-resolution recording medium and reading capability. The best holographic images, recorded in a high-resolution resist and read out with a scanning tunneling microscope, have achieved a resolution of 560 Å. The resolution in this work was determined by the signal-to-noise ratio of the recording medium, which limited the number of high-frequency oscillations that could be detected.
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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities In two- and three-dimensional imaging, radiation damage is a major concern. The intensities required to achieve an imaging resolution of 200 Å would kill live cells. Furthermore, the local heating of the object could alter its physical structure. Ideally, one would want to take a timed sequence of images in order to follow the microscopic details of biological processes in living cells. A potential solution to this problem would be to take a single-pulse flash picture. The high coherence and single-pulse energies of the proposed x-ray FEL may allow the simultaneous recording of a number of holographic images, and a 200-fs exposure time should be fast enough that the atoms vaporized in the imaging process will not have had time to move enough to blur the object at the 200-Å resolution. FINDINGS Third-generation synchrotron sources, while not as bright or as coherent as an x-ray free electron laser, will provide far more intense and coherent radiation than is currently available. While many scientific studies using x-rays produced by an FEL appear to be possible, the feasibility of these studies should first be explored using third-generation synchrotron sources that are just beginning to operate. This exploration, in conjunction with further research and development of x-ray FEL technology, will provide the basis for an informed decision regarding possible future construction of an x-ray FEL facility. Even if cost were not a factor, construction of an x-ray free electron laser would require significant research and development, particularly at the shorter wavelengths. The costs and risks of building a free electron laser facility would be much higher in the x-ray region than in any other region, and the uncertainties in both the cost and the technology are also much higher. RECOMMENDATIONS Scientific opportunities and the use of coherence in the x-ray spectral region should be explored initially by the use of existing and planned synchrotron sources. The research and development necessary for the possible construction of an x-ray free electron laser should be supported. The goals of this research and development should be improving the technology and lowering the cost. Research and development on other advanced coherent x-ray sources should continue to be supported. One of the goals of this research and development should be the production of devices of appropriate size and cost to be useful for scientific research on a departmental or individual-investigator scale. Construction of an x-ray free electron laser user facility should not be undertaken at the present time.
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