The committee reaffirms the importance of burning plasma research to the development of fusion energy, as well as to plasma science and other science and engineering disciplines.
IMPORTANCE TO THE DEVELOPMENT OF FUSION ENERGY: CONTROLLING A BURNING PLASMA
As explained in the 2004 report of the Burning Plasma Assessment Committee of the National Research Council (NRC), “A burning plasma experiment would address for the first time all of the scientific and technological questions that all magnetic fusion schemes must face. Such an experiment is the crucial element missing from the world fusion energy science program and a required step in the development of practical fusion energy.”1 The integrated challenges of understanding the dynamics of a burning plasma and of applying the high-technology know-how to heat, sustain, and control a burning plasma within the International Thermonuclear Experimental Reactor (ITER) has helped to focus research, improve understanding and predictive capability, and address key concerns such as transients, increasing confidence in the success of ITER as a burning plasma experiment.
Experiments within the United States have led to significant progress in all important areas identified in the 2004 NRC report. These are as described below.
A burning plasma experiment will represent the first time that a confined fusion plasma is dominated by fusion-born alpha particles. Energetic alpha particles from fusion reactions are predicted to drive plasma instabilities, which could, if not mitigated, substantially reduce fusion power produced and potentially damage the reactor inner wall. Consequently, it is vital to understand how energetic alpha particles affect plasma dynamics. In 2004, such instabilities had been observed and their behavior in different circumstances had begun to be characterized. Now, the onset of energetic particle instabilities is understood, and promising techniques to control these instabilities are being investigated. Predictive models are being developed and compared to advanced fluctuation and fast ion diagnostic measurements. Despite considerable progress understanding fusion-born alpha physics, detailed identification of nonlinear mechanisms is just beginning. Beyond validation of theoretical models, important research areas also include methods to control energetic particles instability for helpful purposes such as favorably modifying the current profile or to govern the nonlinear dynamics to control fusion burn.2,3
A burning plasma experiment advances understanding of plasma transport properties from the core to the boundary. A burning plasma can be divided into an inner high-temperature core where fusion
1 National Research Council (NRC), Burning Plasma: Bringing a Star to Earth, The National Academies Press, Washington, D.C., 2004.
2 Chen and Zonca, Physics of Alfvén waves and energetic particles in burning plasmas, Rev Mod Phys 88:015008, 2016.
3 Gorelenkov, Pinches, Toi, Energetic particle physics in fusion research in preparation for burning plasma experiments, Nuc Fusion 54:125001, 2014.
reactions occur, a surrounding insulating layer called the pedestal, and a boundary layer where escaping plasma flows to a protective divertor. Since 2004, major advances in theory and computation have resulted in detailed understanding of turbulent transport in the plasma core and the key physics processes regulating the pedestal structure in high-confinement mode (called “H-mode”) plasmas. Recently developed computational techniques to couple core and edge physics have been extensively tested against experiments, resulting in significantly improved capability to predict fusion performance.4 For illustration, these computational tools predicted new high-performance regimes that were subsequently observed in experiments of the U.S. fusion research program.5 Although there has been considerable progress made in predicting plasma transport, the validity of these predictions must be tested in future burning plasma experiments. Some of the highest performance discharges studied in DIII-D experiments decrease performance when produced with lower injected torque, as expected in ITER.6 Furthermore, additional research is needed to understand confinement scaling towards desirable fusion reactor conditions characterized by high plasma beta, steady state, and compatible divertors.
A burning plasma experiment enables critical tests to control plasma transients. Due to the large stored energy of a burning plasma, transient events, which cause rapid energy loss from the plasma, present a significant risk to material lifetimes. Transients include disruptions (i.e., when plasma current and confinement are lost) and edge localized modes (ELMs) (i.e., the outer edge plasma is lost). Since 2004, the United States has made substantial progress understanding transients and demonstrating methods either to avoid or to mitigate transients. Notably, ELMs can be avoided via U.S.-discovered operation regimes, such as the Quiescent H-Mode (Q-H-mode),7 Enhanced Pedestal H-mode (EP-H-mode), or I-mode regime,8 or actively controlled by applying resonant magnetic perturbations9 (a technique pioneered in the United States), pellet injection, and position control.10 The United States has also led the world in the development of techniques for understanding, as well as predicting, avoiding and/or controlling disruptions of the plasma current—the latter by massive gas injection,11 shattered pellets, and shell pellets.12 These techniques are critical for ITER and other burning plasma devices based on the tokamak; however, additional research is needed to understand the science of both ELM suppression and disruption avoidance at the higher temperatures, magnetic energies, and potentially longer current quench times expected in a burning plasma experiment.13
A burning plasma experiment advances divertor science necessary for a fusion power source. Unless controlled, the power escaping from a burning plasma will lead to inner wall damage. Control of escaping heat and particles is made by carefully shaping the magnetic field so that plasma flows along the
4 Kinsey et al., ITER predictions using the GYRO verified and experimentally validated trapped gyro-Landau fluid transport model, Nuc Fusion 51:083001, 2011.
5 Solomon et al., Exploration of the Super H-mode regime on DIII-D and potential advantages for burning plasma devices, Phys Plasmas 23:056105, 2016.
6 Buttery et al., DIII-D research to address key challenges for ITER and fusion energy, Nuc Fusion 55:104017, 2015.
7 Snyder et al., Stability and dynamics of the edge pedestal in the low collisionality regime: Physics mechanisms for steady-state ELM-free operation, Nuc Fusion 47:961-968, 2007.
8 Whyte et al., I-mode: an H-mode energy confinement regime with L-mode particle transport in Alcator C-Mod, Nuc Fusion 50:105005, 2010.
9 Evans et al., RMP ELM suppression in DIII-D plasmas with ITER similar shapes and collisionalities, Nuc Fusion 48:024002, 2008.
10 Loarte et al., Progress on the application of ELM control schemes to ITER scenarios from the non-active phase to DT operation, Nuc Fusion 54:033007, 2014.
11 Hollmann et al., Measurements of injected impurity assimilation during massive gas injection experiments in DIII-D, Nuc Fusion 48:115007, 2008.
12 Commaux et al., Demonstration of rapid shutdown using large shattered deuterium pellet injection in DIII-D, Nuc Fusion 50:112001, 2010.
13 Lehnen et al., Impact and mitigation of disruptions with the ITER-like wall in JET, Nuc Fusion 53:093007, 2013.
plasma boundary14 and into a divertor, where the plasma heat and particle flux can be nearly extinguished by interaction with recycling neutrals.15 Additionally, because carbon-based first-wall materials must have low tritium retention,16 important plasma-material processes such as erosion, tritium co-deposition, dust generation, and neutron-irradiation damage require evaluation in a burning plasma experiment.17 The U.S. research program has significantly advanced understanding of burning plasma boundary physics, including improved understanding of the narrow “scrape-off layer” connecting the confined plasma to the divertor. The U.S. research program has also developed and successfully tested several innovative divertor concepts.18,19 Further developments for a divertor with long lifetime remains a major fusion research challenge.
A burning plasma experiment tests integrated scenarios that simultaneously test the requirements for stability, confinement, fuel purity, and compatibility with plasma-facing components needed for a fusion energy source. Since 2004, plasma operation and control scenarios have been developed and tested in preparation for ITER experiments.20 Additionally, high-fidelity integrated models,21 which take full benefit from advances in high-performance computing, are now routinely used to interpret experimental measurements and make progress in predicting the results of burning plasma experiments.22 The U.S. research program has led the world in the development of quiescent plasma scenarios not subject to damaging transient events23 and the so-called “advanced inductive scenario,”24 which can achieve the same plasma performance at reduced plasma current and so minimize the risk of disruption damage. A burning plasma experiment can also test other advanced scenarios, like the so-called “super H-mode,” which represents an attractive area of innovation aimed to reduce the size of a fusion device with improved confinement. Further research using a burning plasma experiment is needed to develop understanding for integrated scenarios that address the challenges of steady-state operation, robust stability at low plasma rotation and high plasma pressure,25 and compatible divertor concept.26,27
The importance of U.S. advances in these key areas has been broadly recognized. For example, the European Physical Society named plasma physicists working in the United States during 6 of the 18
14 Goldston, Heuristic drift-based model of the power scrape-off width in low-gas-puff H-mode tokamaks, Nuc Fusion 52:013009, 2012.
15 Krasheninnikov et al., Edge and divertor plasma: Detachment, stability, and plasma-wall interactions, Nuc Fusion 57:102010, 2017.
16 Skinner et al., Recent advances on hydrogen retention in ITER’s plasma-facing materials: Beryllium, carbon, and tungsten, Fusion Sci and Tech 54:891, 2008.
17 Roth et al. Recent analysis of key plasma wall interactions issues for ITER, J Nuc Materials 390-91:1-9, 2009.
18 Kugel et al., Evaporated lithium surface coatings in NSTX, J. Nuc Materials 390-91:1000-1004, 2009.
19 Umansky et al., Attainment of a stable, fully detached plasma state in innovative divertor configurations, Phys Plasmas 24:056112, 2017.
20 Solomon et al., DIII-D research advancing the scientific basis for burning plasmas and fusion energy, Nuc Fusion 57:102018, 2017.
21 McClenaghan et al., Transport modeling of the DIII-D high beta(p) scenario and extrapolations to ITER steady-state operation, Nuc Fusion 57:116019, 2017.
22 Sips et al., Progress in preparing scenarios for operation of the International Thermonuclear Experimental Reactor, Phys Plasmas 22:021804, 2015.
23 Hubbard et al., Physics and performance of the I-mode regime over an expanded operating space on Alcator C-Mod, Nuc Fusion 57:126039, 2017.
24 Luce et al., Development of advanced inductive scenarios for ITER, Nuc Fusion 54:013015, 2014.
25 Evans et al., ELM suppression in helium plasmas with 3D magnetic fields, Nuc Fusion 57:086016, 2017.
26 Wenninger et al., Advances in the physics basis for the European DEMO design, Nuc Fusion 55:063003, 2015.
27 Ongena et al., Magnetic-confinement fusion, Nat Phys 34:398, 2016.
years since awarding the prestigious Hans Alfvén Prize.28 Also, 8 of the 11 Nuclear Fusion Awards were presented to U.S. scientists working on scenarios, transport, stability, transient control, boundary, and pedestal physics.29
IMPORTANCE TO THE DEVELOPMENT OF FUSION ENERGY: FUSION TECHNOLOGY
While burning plasma science has progressed since the 2004 NAS burning plasma assessment, significant advancements in fusion technology are needed for a burning plasma reactor. Below are brief descriptions of a selected number of important science and technology contributions from fusion technology research and their impacts on fusion energy development.
Fusion blanket design, tritium breeding, fuel processing. A fusion breeding blanket—that is, a nuclear system that creates tritium via interaction of the fusion-produced 14-MeV neutrons with lithium—is a key fusion nuclear technology needed for the development of fusion energy. Fusion reactors must operate with more tritium produced and recovered than is burned. The vast majority of the fuel injected in a fusion chamber will not be burned in a single pass. Unburned deuterium-tritium fuel will be continuously transported to the plasma edge, where it must be exhausted, stripped of impurities, and then reinjected into the plasma. A burning plasma experiment provides the opportunity to test and evaluate the performance of prototypical blanket modules and demonstrate technologies for tritium extraction from blankets and for fuel processing systems that can be operated efficiently at large scale.30,31,32
Fusion safety, remote handling, and waste management. A burning plasma experiment offers the opportunity to begin development of the technologies needed for a fusion reactor, including important safety-related technologies. Many components and systems needed for fusion’s safety objectives are unique, such as source diagnostics and cleaning technologies, state-of-the-art safety analyses tools, technologies for the remote handling of large activated components, technologies for the control of routine tritium releases, and innovative approaches for the control of tritiated and mixed waste streams.33 A burning plasma experiment will be an integrated demonstration of the safety, reliability, and effectiveness of these technologies.34
Fusion materials science. The behavior and integrity of materials in a fusion system are of great importance to the long-term viability of fusion energy.35 The high flux of energetic neutrons to the vessel and structural materials poses a serious materials problem that will require substantial testing, some of
28 Alfvén Prize winners: Marshall N. Rosenbluth (2002), Liu Chen (2008), Allen Boozer (2010), Patrick Diamond and Akira Hasegawa (2011), Miklos Porkolab (2013), and Nathaniel Fisch (2015). (See http://plasma.ciemat.es/eps/awards/alfven-prize/).
29 The Nuclear Fusion Award has been given annually since 2006. U.S. award recipients are Tim Luce (2006 General Atomics), Todd Evans (2008 General Atomics), Steve Sabbagh (2009 Columbia University), John Rice (2010 MIT), Pat Diamond (2012 University of California, San Diego), Dennis Whyte (2013 MIT), Phil Snyder (2014 General Atomics), and Rob Goldston (2015 Princeton University). See http://www-pub.iaea.org/books/iaeabooks/Nuclear_Fusion/NF/NFAward.
30 Sawan and Abdou, Physics and technology conditions for attaining tritium self-sufficiency for the DT fuel cycle, Fusion Eng and Design 81:1131-1144, 2006.
31 Giancarli et al., Overview of the ITER TBM Program, Fusion Eng and Design 87:395, 2012.
32 National Research Council, Burning Plasma: Bringing a Star to Earth, The National Academies Press, Washington, D.C., 2004.
33 Girard et al., TER, safety and licensing, Fusion Eng Des 82:506, 2007.
34 Bornschein et al., Tritium management and safety issues in ITER and DEMO breeding blankets, Fusion Eng Des 88:466, 2013.
35 Zinkle and Snead, Designing radiation resistance in materials for fusion energy, Annu. Rev. Mater. Res. 44:241, 2014.
which may be done on a burning plasma experiment.36 The high energy neutrons from the D-T fusion reaction generate between 50- to 100-times-higher He/dpa in materials such as ferritic steels than does fission reactor irradiation. Burning plasma experiments will also aid in the development of high-heat-flux components and will serve as testbeds in which to evaluate the performance of the components in a reactor-like fusion environment. The heat loads on components in a burning plasma experiment will be comparable to those expected in a reactor and will require the application of state-of-the-art high-heat-flux technology using materials that satisfy requirements of tritium retention, safety, structural integrity, lifetime, and plasma compatibility.37,38,39
Plasma heating and current drive systems for fusion. Plasma heating by electromagnetic waves and neutral particle beams are needed to heat the plasma to a burning state,40 sustain plasma current,41 modify temperature and current profiles, and control plasma instabilities.42 Ion cyclotron heating is one primary method for heating the bulk plasma, while lower hybrid current drive is perhaps the most efficient radio-frequency method to drive a steady-state toroidal current. Sources are available for both applications.43 Electron cyclotron resonance heating can also be used for bulk electron heating, profile control, pre-ionization/startup, and current drive in burning plasmas, but here further source development is still needed. Fusion reactor research continues to push the frontiers of high power mm-wave and radio-frequency technology.44
High-field magnet technology for fusion. Strong magnetic fields are critical to the success of magnetic fusion as a source of energy. Achieving higher magnetic field strength extends the allowable plasma properties to higher plasma density, higher plasma current, and higher plasma pressure while retaining the same dimensionless scaling parameters found at lower magnetic field strength. This extended range of plasma parameters from high-field magnets allows more compact tokamak devices that may provide a lower cost path to future fusion reactors. ITER’s superconducting magnet system will be the largest ever made and is designed to operate with the highest practical magnetic field strength for large toroidal field coils made of Niobium-Tin superconductors and consistent with the strength of steel.45 New developments of rare-earth barium-copper-oxide high-temperature superconductors may lead to larger magnetic field strength and potentially improve the prospects for magnetic fusion energy.46,47 However, the costs and performance of these advanced superconductors will not be fully understood
36 NRC, Burning Plasma: Bringing a Star to Earth, The National Academies Press, Washington, D.C., 2004.
37 Raffray et al., High heat flux components-Readiness to proceed from near term fusion systems to power plants, Fusion Eng and Design 85:93-108, 2010.
38 NRC, Burning Plasma: Bringing a Star to Earth, The National Academies Press, Washington, D.C., 2004.
40 Omori et al., Overview of the ITER EC H&CD system and its capabilities, Fusion Eng and Design 86:951-954, 2011.
41 Cesario et al., Current drive at plasma densities required for thermonuclear reactors, Nature Comm 1:55, 2010.
42 Sauter et al., On the requirements to control neoclassical tearing modes in burning plasmas, Plasma Phys Control Fusion 52:025002, 2010.
43 Hill et al., DIII-D research towards resolving key issues for ITER and steady-state tokamaks, Nuc Fusion 53:104001, 2013.
44 Thumm, M., Recent advances in the worldwide fusion gyrotron development, IEEE Trans Plasma Sci 42:590-599, 2014.
46 Fietz et al., Prospects of high temperature superconductors for fusion magnets and power applications, Fusion Eng Des 88:440, 2013.
47 Takayasu et al., Investigation of HTS twisted stacked-tape cable (TSTC) conductor for high-field, high-current fusion magnets, IEEE Trans Applied Superconductivity 27:1, 2017.
Integrated systems engineering for fusion. Systems engineering combines plasma physics and engineering constraints into a self-consistent integrated design for large-scale fusion facilities. Systems engineering studies have been carried out for various types of tokamak reactors, including the advanced tokamak,50 high-field tokamak,51 spherical tokamak, and stellarator.52 The recent Advanced Reactor Innovation and Evaluation Study—Advanced and Conservative Tokamak tokamak studies53 are a good example covering the four possible options of optimistic versus conservative physics and/or engineering. The value of these studies is to learn the strengths and weaknesses of any given concept and to point out which physics or engineering quantities have high leverage in improving reactor performance and economics.
IMPORTANCE TO PLASMA SCIENCE AND OTHER SCIENCE
The process of creating a fusion-based energy supply on Earth has led to technological and scientific achievements of far-reaching impact that touch every aspect of our lives. Those largely unanticipated advances span a wide variety of fields in science and technology and were the focus of a 2015 Fusion Energy Sciences Advisory Committee report, Applications of Fusion Energy Research: Scientific and Technological Advances Beyond Fusion.54 There are many synergies between research in plasma physics and other fields, including high-energy physics and condensed matter physics, dating back many decades. For instance, the formulation of a mathematical theory of solitons, solitary waves which are seen in everything from plasmas to water waves to Bose-Einstein Condensates, has led to an equally broad range of applications in the fields of optics, fluid mechanics, and biophysics. Another example, the development of a precise criterion for transition to chaos in Hamiltonian systems has offered insights into a range of phenomena including planetary orbits, two-person games, and changes in the weather.55 Burning plasma physics also contributes to understanding important plasma processes like magnetic reconnection,56 kinetic turbulent processes in magnetized plasma,57 nonlinear wave-particle interactions and resonances,58 and multi-scale phenomena that are also common to space and astrophysical plasma.59
48 Green and Strauss, Things to think about when estimating the cost of magnets made with conductors other than Nb-Ti, IEEE Trans on Applied Superconductivity 27:1, 2017.
49 Whyte et al., Smaller and sooner: Exploiting high magnetic fields from new superconductors for a more attractive fusion energy development path, J Fusion Energy 35:41, 2016.
50 Chan et al., Physics basis of a fusion development facility utilizing the Tokamak approach, Fusion Sci and Technology 57:66-93, 2010.
51 Whyte et al., Smaller and sooner: Exploiting high magnetic fields from new superconductors for a more attractive fusion energy development path, J Fusion Energy 35:41, 2016.
52 Menard et al., Prospects for pilot plants based on the tokamak, spherical tokamak and stellarator, Nuc Fusion 51:103014, 2011.
53 C.E. Kessel et al., The ARIES Advanced and Conservative Tokamak Power Plant Study, Fusion Science and Technology 67:1-21, 2015, doi: 10.13182/FST14-794.
54 U.S. Department of Energy (DOE), Applications of Fusion Energy Sciences Research: Scientific Discoveries and New Technologies Beyond Fusion, Fusion Energy Sciences Advisory Committee, Office of Science, September 2015, https://science.energy.gov/~/media/fes/fesac/pdf/2015/2101507/FINAL_FES_NonFusionAppReport_090215.pdf.
56 Yamada, Kulsrud, and Ji, Magnetic reconnection, Rev. Mod. Phys. 82:603, 2010.
57 Howes, Kinetic Turbulence, pp. 123-152 in Magnetic Fields in Diffuse Media (Lazarian, de Gouveia Dal Pino, and Melioli, eds.), Springer Berlin Heidelberg, Berlin, Heidelberg, 2015.
58 Breizman, Nonlinear consequences of energetic particle instabilities, Fusion Sci and Tech. 59:549-560, 2011.
59 Burch et al., Magnetospheric multiscale overview and science objectives, Space Sci Rev 199:5-21, 2016.
Materials research in support of burning plasma science contributes to better understanding of irradiated materials.60,61 Additionally, fusion facilities can be used to advance fundamental and non-fusion plasma physics.62
In assessing the importance of burning plasma research to other fields of science and technology, the committee notes that the Department of Energy Office of Fusion Energy Sciences distinguishes “burning plasma research” from the “discovery plasma science” component of the its program. It is not, in the committee’s opinion, possible to justify the construction of a burning plasma experiment based on its ability to answer questions of relevance to other fields (for example, astrophysics), yet the broad program that must necessarily be in place to exploit the results from such an experiment will have a profound effect on other fields. For example, the tremendous advances made in computational plasma physics addressing burning plasma issues have had, and will continue to have, important impact on space and astrophysical questions where the intrinsic multi-scale, multi-physics nonlinear interactions can only be addressed by large-scale computations.63 Generally speaking, burning plasma research acts as an important driver for the development of novel concepts and methods at the interface between plasma physics, materials science,64 applied mathematics,65 and computer science,66 with wide visibility and impact.67
The substantial impacts of burning plasma research on science, technology, and engineering were identified in the 2004 Burning Plasma Assessment Committee report,68 and these have continued in several areas: (1) basic plasma science, (2) low-temperature plasmas, (3) space and astrophysical plasmas, (4) high energy density laboratory plasmas and inertial fusion energy, and (5) particle accelerator technology. Another area of technology which has benefitted from fusion research is high frequency high power millimeter wave sources (e.g., gyrotrons69), which have medical and industrial processing applications.70
60 Zinkle and Snead, Designing radiation resistance in materials for fusion energy, Annu. Rev. Mater. Res. 44:241, 2014.
61 Bai et al., Efficient annealing of radiation damage near grain boundaries via interstitial emission, Science 327:1631, 2010.
63 Schekochihin et al., Astrophysical gyrokinetics: Kinetic and fluid turbulent cascades in magnetized weakly collisional plasmas, ApJS 182:310, 2009.
64 Odette, Alinger, and Wirth, Recent developments in irradiation-resistant steels, Ann Rev Mat Res. 38:471-503, 2008.
65 Dongarra, Hittinger (Co-Chairs) et al., Applied Mathematics Research for Exascale Computing, Report of DOE Working Group on Exascale Mathematics, 2014, http://science.energy.gov/~/media/ascr/pdf/research/am/docs/EMWGreport.pdf.
66 Batchelor et al., Simulation of fusion plasmas: Current status and future direction, Plasma Sci and Techn 9:312, 2007.
67 DOE, Scientific Grand Challenges: Fusion Energy Science and the Role of Computing at the Extreme Scale, Report from the DOE Workshop, held March 18-20, 2009, Washington, D.C.
68 NRC, Burning Plasma: Bringing a Star to Earth, The National Academies Press, Washington, D.C., 2004.
69 Rzesnicki et al., 2.2-MW record power of the 170-GHz European Preprototype Coaxial-Cavity Gyrotron for ITER, IEEE Trans Plasma Sci 38, pp. 1141-1149, 2010.
70 Sabchevski et al., A dual-beam irradiation facility for a novel hybrid cancer therapy, J. Infrared Millimeter and THz Waves 34:71, 2013.