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Burning Plasma: Bringing a Star to Earth 2 Scientific and Technological Value of and Interest in a Burning Plasma INTRODUCTION Fusion energy holds the promise of providing a significant part of the long-term, environmentally acceptable energy supply. At the center of all schemes to make fusion energy is a plasma—an ionized gas that, like the center of the Sun, is heated by fusion reactions. The plasma is said to be burning when alpha particles from the fusion reactions provide the dominant heating of the plasma. All fusion reactors require a burning plasma. The key challenge is to confine the hot and dense plasma while it burns. Two experiments in the 1990s—the Tokamak Fusion Test Reactor (TFTR) in Princeton and the Joint European Torus (JET) in the United Kingdom—obtained significant power from deuterium-tritium (D-T) fusion reactions. However, these early experiments were not large enough or powerful enough to achieve the plasma-confinement conditions for producing a fully burning plasma in which more power is released by the fusion reactions than is used to heat the plasma. In such a burning plasma, the heating of the plasma from fusion reactions is sufficiently high to strongly influence the equilibrium and stability properties of the plasma itself. These earlier D-T experiments in TFTR and JET produced fusion power output levels that were only a fraction of the total input power. The plasma heating induced by this fusion power was measurable, but well below the levels necessary to significantly influence the plasma behavior and thus enter the burning plasma regime.
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Burning Plasma: Bringing a Star to Earth No experiment has yet entered the burning plasma regime, and the physics in this self-heated regime remains largely unexplored. Table 2.1 presents a comparison of some critical parameters expected for a burning plasma experiment in the International Thermonuclear Experimental Reactor (ITER) device with the values achieved to date in D-T experiments. A burning plasma experiment would address for the first time all of the scientific and technological questions that all 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. Scientific advances in the 1990s significantly improved designs for a burning plasma experiment. Tokamaks are the most advanced magnetic-confinement configuration. They alone have established a scientific basis that can be projected to burning conditions with reasonable confidence. Thus, a burning plasma experiment will take place of necessity as a tokamak. TABLE 2.1 Comparison of Design Characteristics of the International Thermonuclear Experimental Reactor (ITER) with Achieved Conditions in Deuterium-Tritium (D-T) Experiments to Date Parameter ITERa Pulsed ITERa Steady State TFTRb (D-T) JETc (D-T) Radius (m) 6.2 6.4 2.5 3.0 Plasma volume (m3) 831 770 38 153 Normalized pressure (percent) 2.8 2.8 1.1 2.6 Normalized confinement (H98y,2) 1.0 1.6 1.3 1.6 Pressure-driven current fraction (percent) 10 48 26 10 Magnetic field strength (T) 5.3 5.2 5.6 3.5 Fusion power (GW) 0.5 0.36 0.011 0.016 Q (fusion power/power supplied) 10 6 0.27 0.64 NOTE: The normalized pressure is the ratio of the average plasma pressure to the vacuum magnetic pressure at the horizontal midpoint of the plasma. aFrom “ITER Technical Basis,” available online at http://www.iter.org/ITERPublic/ITER/PDD4.pdf. Accessed June 1, 2003. bFrom “TFTR Machine Parameters,” available online at http://w3.pppl.gov/tftr/info/tftrparams.html. Accessed July 1, 2003. cFrom “Report on JET Activities,” available online at http://www.jet.efda.org/pages/rep-of-activ.html. Accessed June 1, 2003.
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Burning Plasma: Bringing a Star to Earth Other magnetic configurations—for example, advanced tokamaks, reversed-field pinches, spherical tori, and stellarators—have potential advantages, and all have made significant progress in the past decade. The discovery that confinement can be enhanced by suppressing turbulence and then finding regimes compatible with steady-state operation has enhanced the reactor potential of these configurations. It is too early to predict which configuration has the best potential for becoming a commercial fusion reactor. A tokamak-based burning plasma experiment should produce scientific understanding and technological developments of general use for a wide range of configurations. If it is developed and understood in sufficient detail to provide predictive capability, the scientific knowledge of burning plasmas derived from a tokamak experiment such as ITER will be transferable to other magnetic configurations. The tokamak configuration is closely related to most other leading contenders for fusion energy development, so a wide range of phenomena may be extended from the tokamak to other configurations through theory and computation in the future. These phenomena include alpha-particle confinement and transport, the interaction of alpha particles with instabilities, fusion burn control, interactions of turbulence and magnetohydrodynamic (MHD) phenomena with alpha particles, and so on. The degree to which theory and computation will allow extrapolation to other configurations will evolve with time, but it is already clear that the tools and understanding derived from research in large-tokamak experiments have influenced and in most cases accelerated the development of other members of the family of toroidal configurations. It is reasonable to assume that this influence will continue to extend the knowledge of burning plasma behavior to other attractive confinement configurations in the future. The U.S. fusion program structure is formally defined by its mission to “advance plasma science, fusion science, and fusion technology—the knowledge base needed for an economically and environmentally attractive fusion energy source.”1 The program has defined three goals to achieve in pursuit of this mission: Advance plasma science in pursuit of national science and technology goals; Develop fusion science, technology, and plasma-confinement innovations as the central theme of the domestic program; and Pursue fusion energy science and technology as a partner in the international effort.2 1 U.S. Department of Energy, Strategic Plan for the Restructured U.S. Fusion Energy Sciences Program, DOE/ER-0684, Washington, D.C., August 1996, p. 3. 2 U.S. Department of Energy, Strategic Plan for the Restructured U.S. Fusion Energy Sciences Program, DOE/ER-0684, Washington, D.C., August 1996, p. 3.
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Burning Plasma: Bringing a Star to Earth While the study of burning plasmas will contribute to achieving the first two goals of the fusion science program, it is especially relevant to fulfilling the third goal listed above. Adding a burning plasma experiment to the U.S. fusion program must be considered in the context of these mission goals. To do this effectively, it is necessary to explore the critical motivations for the proposed burning plasma experiment. This chapter addresses that question by analyzing the importance of a burning plasma experiment for fusion energy science and the development of fusion energy, as well as its importance for basic plasma science, for other areas of science, and for fusion technology. Special attention is given to identifying science and technology issues that have particular relevance to the development of fusion energy. In each case, addressing the issue to a degree sufficient for developing the knowledge base for fusion energy requires that it eventually be studied in a burning plasma. For those issues that depend on the presence of a large alpha-particle population of fusion origin, a burning plasma is required. It is only in the burning plasma experiment that the full range of complex interactions between the plasma and its self-generated heat source can be confronted. For this reason, the test of plasma behavior under self-heated conditions is a critical next step for understanding fusion-producing plasmas and projecting to fusion energy production. As important, a burning plasma experiment provides the first opportunity to test many relevant fusion technologies at a reactor scale. SCIENTIFIC IMPORTANCE OF A BURNING PLASMA FOR FUSION ENERGY SCIENCE AND THE DEVELOPMENT OF FUSION ENERGY At each point in the development of fusion science and the implementation of new fusion facilities, new scientific regimes have been explored and important insights have been gained. The approach to the burning plasma regime by TFTR and JET provides a critical example; in these experiments, fusion plasmas were created transiently and with insufficient self-heating to burn, but significant new physics was still uncovered. It is expected, therefore, that a burning plasma experiment at the near-reactor scale will present new scientific opportunities that must be explored and understood. In particular, it can be expected that a burning plasma experiment, owing to its unique plasma parameters and its ability to study these issues in the burning state, will make critical contributions to understanding the following: Plasma behavior when self-sustained by fusion (burning), Fusion-plasma turbulence and turbulent transport, Stability limits to plasma pressure, Control of a sustained burning plasma, and Power and particle exhaust.
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Burning Plasma: Bringing a Star to Earth Subsections below address each of these areas in greater detail; each closes with an italicized finding relating the science opportunities to fusion energy science and the development of fusion energy. Behavior of Self-Sustaining Burning Plasmas The expected new phenomena in burning plasma are due to fusion-generated fast alpha particles, which will be the dominant heat source for the plasma if the alpha particles are well confined. The fusion rate increases approximately as the square of the plasma pressure, in the expected temperature range. This nonlinear heating will combine with the turbulent confinement of the plasma to modify the plasma equilibrium and behavior. Under some conditions the alpha particles can collectively generate fluctuations—for example, energetic particle modes and Alfvénic modes—affecting the confinement of the alpha particles themselves or, possibly, the rest of the plasma. The fluctuations could, therefore, allow alpha particles to escape without heating the plasma. The alpha particles stabilize some MHD modes and induce new unstable modes. Thus the nonlinear behavior is exceedingly complex. While these fluctuations have been studied experimentally using externally generated energetic ions, the space and energy distribution of these ions and their anisotropy are significantly different from those of fusion-generated alpha particles, modifying the fluctuations and their impact on the fast ion confinement. In the D-T experiments on TFTR and JET, these instabilities were observed at low amplitude with alpha particles in specially designed experiments (see Figure 2.1). However, the larger size of a burning plasma experiment is predicted to significantly change the spectrum of unstable Alfvénic fluctuations when they occur, generating turbulence and possibly increasing alpha-particle losses. Understanding these complex interactions between large populations of fusion-produced alpha particles and the plasma equilibrium and stability is a critical integrating step in developing the knowledge base for fusion energy. Developing and validating such an understanding require access to a sufficiently large fusion-producing plasma environment. Plasma regimes with these parameters are not accessible in present experiments. Developing and experimentally validating a theory of these Alfvénic fluctuations under conditions of possibly turbulent spectra present a complex and scientifically challenging problem. It will be advantageous to do so in a flexible experiment in which the stability boundary can be challenged in a controlled manner. Linear stability analyses of these instabilities for ITER conditions indicate that they will be marginally stable under normal operating conditions, and hence they should not prevent access to the expected burning plasma regime. However, operating at higher electron temperatures in advanced operation regimes may allow a
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Burning Plasma: Bringing a Star to Earth FIGURE 2.1 Frequency evolution of Alfvénic modes driven by fusion alpha particles in the Tokamak Fusion Test Reactor deuterium-tritium experiments as the central plasma current evolves. Blue contours: reflectometer measurements of mode amplitude inside the plasma. Red contours: magnetic measurements of mode amplitude at the plasma edge. Solid squares connected by lines: predicted frequency evolution from the CASTOR and NOVA-K computer modeling codes using experimental conditions, showing evolution from the cylindrical cascade Alfvén mode to the toroidal Alfvén eigenmode (TAE). Courtesy of R. Nazikian, Princeton Plasma Physics Laboratory. challenge to the stability boundary and allow excitation of these modes. ITER will thus provide a unique opportunity to study these modes in a controlled manner and to provide critical tests of emerging theory. The behavior of an energy-producing fusing plasma will be dominated by the complex nonlinear interactions between plasma heating, stability, and confinement in a plasma heated by the fusion reactions and can only be studied in an integrated manner for the first time in a burning plasma experiment. Fusion-Plasma Turbulence and Turbulent Transport A burning plasma experiment will greatly improve the extrapolation of our knowledge of plasma turbulence and turbulent transport from present experi-
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Burning Plasma: Bringing a Star to Earth ments to the effective size of an energy-producing reactor. The effective plasma size (physical size divided by ion magnetic-gyroradius) must be increased by a factor of 3 to 4 in order to achieve burning conditions; this can be accomplished by increasing either the actual plasma size or the magnetic field strength. It is predicted that an increase in effective plasma size at high plasma temperature will modify many phenomena already studied in existing experiments, such as the saturation of turbulence-generated transport and the onset of macroscopic (tearing) instabilities. Additionally, transport studies in the regime where electron and ion temperatures are comparable (owing to electron heating and equilibration) become possible. These phenomena can determine the plasma pressure that can be confined and thus the level of fusion power produced. Extending the knowledge of plasma confinement and turbulent transport to relative plasma sizes several times larger than those presently available, into the range required for an energy-producing plasma, is necessary for developing a predictive capability of fusion-plasma performance. Stability Limits to Plasma Pressure Since the fusion power produced by a burning plasma increases quadratically with the plasma pressure, maximizing the pressure is crucial for achieving a fusion-heated and fusion-sustained plasma. In tokamaks, the maximum pressure is limited by plasma instabilities. The designs for the proposed burning plasma experiment build on the understanding of these instabilities developed from existing tokamak experiments, such as methods to increase the pressure limits using plasma shaping, control of plasma profiles, and external feedback systems. A burning plasma experiment will test this understanding at larger effective plasma size and in the presence of a substantial alpha-particle population. This study will be especially interesting, because strong self-heating by well-confined alpha particles will control the pressure profile evolution, possibly reducing the effectiveness of existing external control tools. The behavior of the pressure stability limit with strong self-heating may thus lead to the development of new strategies for plasma profile control. Such strategies will be important for validating the basis for the further development of fusion energy. Understanding the interactions between large-scale plasma instabilities and a large, fast alpha-particle population in the presence of strong self-heating is critical for devising effective control strategies and optimizing fusion power production.
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Burning Plasma: Bringing a Star to Earth Controlling Sustained Burning Plasmas A fusion reactor should operate in steady state, minimizing the recirculating power (maximizing the energy gain). In the steady-state advanced tokamak configurations that are envisioned, most of the plasma current is self-generated by the pressure (the “bootstrap current”). In a burning plasma, the heating of the plasma will also depend on the plasma pressure. Furthermore, the distribution of current within the plasma has a large effect on the confinement properties and the stability limit for the plasma pressure. Thus, heating, pressure, and current are coupled so that these configurations are nonlinear and self-organized. Achieving and controlling such a self-organized plasma configuration in a burning condition will be an exciting challenge. Meeting this challenge will require the development of new diagnostics, theoretical and computational models, and feedback control methods. Developing an understanding of and the ability to control sustained, self-organizing burning plasmas is needed in order to specify engineering requirements for energy-producing plasmas and to develop attractive advanced fusion concepts. Power and Particle Exhaust An energy-producing fusion system must not only generate sufficient fusion power, it must also absorb the generated energy at the walls of the device without deleterious effects and provide for elimination of the helium ash. For example, in ITER the total power transported out of the plasma will be about 100 MW, and the helium ash content must be kept below about 5 percent. The heat flow to the divertor must be reduced using impurity radiation, but these impurities must not be allowed to transport into the core plasma, where they would reduce fusion reactivity and increase radiative losses. In addition, instabilities in the plasma edge—known as edge-localized modes—may transiently increase the heat load on the divertor plates to a significant degree; this effect will need to be accommodated. The ITER experiment will explore this challenging issue at the larger scale and power level of a burning plasma. The effective control of heat flow to the chamber walls of the device for sustained operation and control of plasma composition are critical to future fusion concepts and will be tested under more reactor-relevant conditions in the burning plasma experiment than in experiments to date.
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Burning Plasma: Bringing a Star to Earth Conclusion A burning plasma, whose equilibrium and stability properties can be strongly influenced by the presence of fusion-produced alpha-particle heating, offers an environment for the study of several discrete scientific phenomena that influence or are influenced by the alpha heating power. These include the propagation of the fusion burn itself, plasma turbulence and its associated transport at the larger scale of a fusing plasma, pressure limits, sustainability, and the complex interactions in the plasma–wall interface region. While each issue offers unique scientific challenges, it is the integration of all of these phenomena in a complex, self-organizing system with its own heat source that is the overriding and most compelling aspect of the study and understanding of a burning plasma. Indeed, it is only in the burning plasma experiment that these strongly nonlinear and interacting phenomena can be realized simultaneously. In that context, it is important that the burning plasma experiment have sufficient flexibility to modify the susceptibility to these various nonlinearities so that their respective influences on the aggregate behavior of the burning plasma system can be reasonably isolated and tested. SCIENTIFIC IMPORTANCE OF A BURNING PLASMA FOR BASIC PLASMA PHYSICS Basic plasma physics—the study of fundamental processes in the plasma state of matter—is relevant to a variety of fields, including space plasmas, industrial plasmas, astrophysics, and fusion. A burning plasma experiment entails specific scientific goals of great importance to fusion power. It is thus designed specifically to investigate the burning plasma state and cannot replace experiments that are purpose-built to directly address the broader set of basic plasma issues. However, a burning plasma experiment and the scientific program that leads to and supports it may make useful contributions to the basic understanding of plasmas. This section explores this possibility by considering the following four fundamental plasma processes, which are not yet fully understood, and their role in the burning plasma experiment: Magnetic field line reconnection, Plasma turbulence, Abrupt plasma behavior, and Energetic particles in plasmas.
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Burning Plasma: Bringing a Star to Earth Magnetic Field Line Reconnection Magnetic field line reconnection is the process by which magnetic topology changes sometimes suddenly. Reconnection is often accompanied by the generation of fast particles and flows, the sudden release of energy in heat and waves, and nonlocal changes in plasma resistivity and turbulence. Reconnection is believed to be a key process in solar flares, magnetospheres, and astrophysical processes. Several basic reconnection experiments have been performed in the past, representing first steps in forming a greater understanding of the phenomenon, but much is still unknown. It is expected that tearing modes, disruptions, and sawtooth oscillations will be seen in a burning plasma experiment; they may limit the accessible plasma pressure. These phenomena all involve at least some reconnection of field lines. Careful diagnosis of these phenomena will contribute to our understanding of reconnection. The codes developed to model reconnection in the burning plasma experiment will be immediately useful in simulating reconnection in space and astrophysics. Plasma Turbulence Plasma turbulence is now under intense investigation both numerically and in laboratory experiments (see Figure 2.2). Turbulence in fusion devices dominates the transport of heat, and the minimizing of turbulence is a major goal of fusion science. Plasma turbulence also controls the behavior of accretion disks around black holes and the dynamics of the solar corona. The discovery that shear flows suppress turbulence in tokamaks is a fundamental advance in understanding, as well as a practical method for increasing the performance of the burning plasma experiment. Since this suppression is key to the desired high-confinement mode (H-mode)—as well as to discharges with internal transport barriers—it is being investigated extensively. Turbulent transport of heat by electrons is less well understood than transport by ions. This issue will also be addressed extensively by a burning plasma experiment, and it is hoped that the experiment will lead to a better fundamental understanding of the interaction between turbulence on different scales. Gyrokinetic simulation, which was developed to simulate the turbulence and predict the performance of fusion plasmas, has found a wide range of application to basic plasma physics. The demands of simulating turbulence in the burning plasma experiment will undoubtedly lead to improved computational algorithms that will find subsequent use in other areas of plasma science.
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Burning Plasma: Bringing a Star to Earth Abrupt Plasma Behavior Many plasmas exhibit abrupt changes in behavior. Examples include solar flares, disruptions in tokamaks, flux ropes, coronal mass ejections, and magnetic substorms (see Figure 2.3). Very little is understood about these processes. Disruptions at the pressure limit in a burning plasma experiment are extremely problematic, as they can cause damage to the walls of the device. Although the physics of disruptions and edge-localized modes (ELMs) is not fully understood, their phenomenology is. Thus, avoidance of their most serious consequences is expected. One can expect some such events, however, and the data from these events will help unravel the mysteries of abrupt plasma behavior. Energetic Particles in Plasmas Burning plasmas by definition have a significant population of fast alpha particles. Many naturally occurring plasmas also have an energetic component—cosmic rays in the Galaxy and ring current protons in the magnetosphere, for example. Energetic particles (such as those from an avalanche of runaway electrons in a fusion plasma) can drive instabilities, including the toroidal Alfvén eigenmodes (TAE) observed in tokamaks and discussed in the subsection above entitled “Magnetic Field Line Reconnection.” Clearly, the burning plasma experiment will contribute greatly to our understanding of such plasmas. Conclusion In summary, it is clear that the burning plasma experiment will contribute to many areas of basic plasma science. In essence, a burning plasma program’s benefits to basic plasma physics will be threefold. First, critical phenomena in the burning plasma involve fundamental plasma processes. These phenomena will be studied in the burning plasma experiment and the supporting parts of the base program, as discussed above. Second, the burning plasma scientific program will develop tools—for example, computer codes for analysis—that will be of use to basic plasma science. Third, it is highly likely that new issues will arise from the studies on the burning plasma experiment. They will motivate new theoretical activity and focused investigations on nonburning experiments to develop and confirm a detailed understanding of the basic processes. However, the extreme conditions in a burning plasma experiment and other large fusion experiments make any detailed measurement a challenge. Notwithstanding the promise of a burning plasma experiment in increasing our understanding of plasma science,
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Burning Plasma: Bringing a Star to Earth
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Burning Plasma: Bringing a Star to Earth FIGURE 2.2 Edge plasma turbulence is composed of the superposition of many waves—a combination of both drift and shear Alfvén waves; the waves have correlation lengths on the order of the skin depth perpendicular to the magnetic field and much longer correlations along the direction of the magnetic field. Under controlled studies in the laboratory, one can produce single waves of either type to understand their evolution and propagation; by understanding the components of edge turbulence in detail, significant insight into the full turbulent process has been gained. In this figure, results from the Large Area Plasma Device (LAPD) experiment at the University of California at Los Angeles are shown. (a) The measured magnetic field of a shear Alfvén wave launched from a fluctuating current on the order of the skin depth in radius. The antenna is a small mesh disk inserted on a radial shaft (in the upper right). The figure shows the magnetic field of the wave in two green and blue isosurfaces and as a vector field. Each “cigar-shaped” pattern is a half wavelength long; the magnetic field of the wave is zero on axis. The data are superimposed on a model of the interior of the LAPD device. The wave data have been scaled by a factor of 2 in the radial direction for easier viewing. The inset shows the wave magnetic field at one instant of time on a plane perpendicular to the background magnetic field. The disk antenna is superimposed in the figure but is 1.54 m from this plane. Note that the vectors switch direction along a radial cut. This is because the wave has a finite perpendicular component of the wave vector, which, in turn, gives it a parallel electric field. (b) In three snapshots of time (Dt), the measured magnetic field of a drift Alfvén wave that was caused by a channel of field-aligned hot electrons with a radius on the order of the skin depth. The fluctuations have the shear mode polarization and rotate in the electron diamagnetic drift direction. In this case the magnetic field fluctuations are associated with a density fluctuation, which has the shape of a rotating spiral. (c) Measurement of magnetic turbulence on the edge of the LAPD plasma. The fluctuations below 2 kHz are correlated with density fluctuations and are believed to be drift Alfvén modes; the remainder of the spectrum is that of shear Alfvén waves. SOURCES: Courtesy of W. Gekelman, University of California at Los Angeles. (a) W. Gekelman, S. Vincena, D. Leneman, and J. Maggs, “Laboratory Experiments on Shear Alfvén Waves and Their Relationship to Space Plasmas,” J. Geophys. Res., 7225-7236 (1997); (b) A. Burke, J. Maggs, and G. Morales, “Experimental Study of Fluctuations Excited by a Narrow Temperature Filament in a Magnetized Plasma,” Phys. Plasmas, 1397-1407 (2000); (c) data courtesy of T. Carter, University of California at Los Angeles.
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Burning Plasma: Bringing a Star to Earth FIGURE 2.3 Kink instability of the central column produced in spheromak formation experiment at the California Institute of Technology. This kink, believed to be intrinsic to spheromak self-organization, occurs when the jetlike central column becomes sufficiently long to satisfy the Kruskal-Shafranov instability threshold. It can also be considered as a good simulation of the kink instability of an astrophysical jet. Courtesy of S.C. Hsu and P.M. Bellan, California Institute of Technology. systematic studies of any basic process are best done on the smallest or simplest laboratory devices that can access the appropriate regime. GENERAL SCIENTIFIC IMPORTANCE OF A BURNING PLASMA Progress in plasma physics, and fusion-plasma physics in particular, can lead to progress in other subfields of physical science. A burning plasma experiment will likely lead to progress in new regimes. There will undoubtedly be unexpected discoveries as well; only a few examples of such connections are mentioned here. Astrophysics and space science are replete with evidence that heat, magnetic flux, and angular momentum are transported much more quickly than is predicted by straightforward physics.3 Enhanced transport leads to dramatic energy 3 National Research Council, Frontiers in High Energy Density Physics: The X-Games of Contemporary Science, Washington, D.C.: The National Academies Press, 2003.
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Burning Plasma: Bringing a Star to Earth release events such as solar flares, geomagnetic substorms, and x-ray emissions from the vicinity of black holes. On the basis of data, theory, and advanced numerical methods, laboratory plasma physics has already led to substantial insights into these processes. Burning plasmas will generate the highly energetic ions and large temperature gradients that characterize many astrophysical systems and will provide the opportunity to study enhanced transport under these more realistic conditions.4 Another example of general scientific interest in burning plasma physics is self-organization, which occurs in many astrophysical, space, and geophysical settings. Self-organization is characterized by phenomena on small spatial scales acting in concert to produce phenomena on large scales. One example is large-scale planetary and solar flows driven by small-scale turbulence; another is large-scale magnetic fields driven from small-scale motions. The large-scale rotational flows observed in laboratory plasmas share many common features with these self-organized flows in nature, as do the large-scale, self-sustained magnetic fields observed in some laboratory plasmas. A burning plasma experiment would offer an opportunity to observe self-organization in a new setting, with much stronger drivers and correspondingly weaker external constraints than in experiments to date. Another set of applications involves shared diagnostic techniques rather than shared phenomena. Innovative techniques for image reconstruction can be used in many fields, including medical imaging and surface science. Probes in burning plasma must operate in a hostile environment similar to conditions in space and industrial settings.5 Spectroscopy of heavy and highly charged ions in a burning plasma faces issues similar to those in astrophysical observations and often uses similar instrumentation. A burning plasma experiment can offer substantive and important contributions to other fields of science connected to plasma physics, primarily through experimental access to the fundamental and/or extreme conditions offered by such a state. TECHNOLOGICAL IMPORTANCE FOR FUSION ENERGY SCIENCE AND THE DEVELOPMENT OF FUSION ENERGY The previous sections have considered the scientific importance of a burning plasma experiment. The most compelling scientific importance is, for obvious 4 National Research Council, The Sun to the Earth–and Beyond, Washington, D.C.: The National Academies Press, 2003. 5 National Research Council, Plasma Science: From Fundamental Research to Technological Applications, Washington, D.C.: National Academy Press, 1995.
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Burning Plasma: Bringing a Star to Earth reasons, the advancement of fusion energy science. What, however, is the importance of a burning plasma experiment, such as ITER, to the technological advance of fusion energy? This question is explored below with regard to the following issues: Breeding blanket development, Tritium processing, Magnet technology, High-heat-flux component development, and Remote handling technology. Breeding Blanket Development A burning plasma experiment could provide the opportunity to test and evaluate the performance of prototypical blanket modules. The blanket in a reactor is the structure immediately surrounding the plasma. It is typically about 1 m thick and is fabricated in modules designed to be remotely replaceable several times during a fusion plant’s lifetime. The blanket serves the multiple functions of removing most of the energy from the fusion-produced 14-MeV neutrons, providing adequate shielding for the vacuum vessel and magnets and breeding tritium via interaction of the neutrons with lithium. The coolants used for a fusion reactor need to operate at high temperature in order to optimize plant efficiency—both for plants intended to produce only electricity and for plants that could produce both electricity and hydrogen. The principal nondefense source of tritium is the Canadian Deuterium Uranium (CANDU) reactors. While the Canadian supply is expected to be adequate for providing the fuel for the ITER experiment without additional breeding, any fusion reactors beyond ITER must clearly produce and recover more tritium than is burned if fusion energy is to be viable.6 A blanket providing this function is a critical fusion technology; it must be developed on ITER to ensure a tritium fuel supply for future fusion facilities. A burning plasma experiment of the scope of ITER provides the opportunity to evaluate the tritium-breeding ratio and extraction process, the thermome- 6 Any arrangement to use Canadian tritium would of course have to be negotiated between ITER management and the Canadian government. Another possible source of tritium would be the target devices being developed for providing tritium for the U.S. weapons program using commercial reactors. However, this approach is still being developed for what probably are the small amounts needed for defense purposes and would have to be examined when and if that program became successful.
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Burning Plasma: Bringing a Star to Earth chanical performance, and the plasma compatibility of near-full-scale test blanket modules. In particular, the 3,000-s pulse length available in the second stage of ITER operations is well in excess of all relevant plasma time constants and is sufficiently long to ensure that all in-vessel components, including blanket test modules, come to thermal equilibrium. This is adequate for testing the breeding and thermomechanical performance of blanket modules. The behavior and integrity of materials in a fusion system are of great importance to the long-term viability of fusion energy. The high flux of energetic neutrons poses a serious materials problem that will require substantial development and testing. The fluence—that is, the integrated neutron flux—in the burning plasma experiments under consideration will be too low (by as much as two orders of magnitude) to explore the lifetime characteristics of materials and components needed for a reactor. The main structural material specified for ITER construction is 316L(N) stainless steel. This is not considered to be a low-activation material, but the change in its structural properties due to the neutron fluence over ITER’s lifetime is well characterized and small enough that the machine’s structural integrity will not be challenged. It should be noted that neither the evaluation of blanket performance under fusion reactor neutron fluences nor the evaluation of materials lifetime for reactors is part of the ITER mission. Future dedicated facilities may be needed for this purpose. The development of efficient and robust reactor blanket modules is required in order to provide a means of extracting energy from the plasma, to breed the required fuel, and to provide shielding of external subsystems in future reactor concepts. A burning plasma experiment provides the first opportunity to test such blanket concepts. Tritium Processing Most of the fuel injected into a fusion reactor will not be burned in a single pass. Unburned deuterium and tritium will be continuously transported to the plasma edge, where it must be collected; stripped of impurities; separated into deuterium, tritium, and hydrogen isotopes; and then reinjected as fresh D-T fuel. Elements in the fuel-processing system such as the step of separating into isotopes have already been developed on a small scale. The fuel-recovery system designed for ITER would operate online under quasi-steady-state conditions using technology that would be prototypical of that needed for a reactor. The successful demonstration of an integrated steady-state fuel-processing capability in ITER would therefore establish this technology at the reactor scale. A related issue concerns the level of tritium inventory in the plasma chamber
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Burning Plasma: Bringing a Star to Earth of a fusion reactor. Owing to its superior heat and thermal shock characteristics, carbon has been the first-wall material used in most tokamak experiments (including the “large tokamaks” TFTR, JET, and the Japanese JT-60U) during the past decade. In a process known as co-deposition, hydrocarbons form in the interaction of plasma with the wall, leading to a buildup of hydrogen in thick films deposited on components within the plasma chamber. In ITER, this could result in a limit of 10 to 100 shots before the tritium in the chamber reaches the maximum permitted by the in-vessel tritium inventory. Of necessity, a burning plasma experiment must address this problem either by excluding carbon from being the choice of first-wall material or by developing techniques to mitigate the formation and/or retention of the hydrocarbon films. The control and recycling of the tritium fuel, while minimizing the tritium inventory in the plasma chamber, will be required for the routine operation of a burning plasma experiment, similar to requirements for the routine operation of future reactors. Magnet Technology The superconducting magnets required for ITER are of unprecedented size and scale, being comparable to those foreseen to be required for a fusion reactor. Their development will not only continue the advances being made in niobium tin (Nb3Sn)-based magnets but could also stimulate the research and development of magnets using still more advanced conductors and cable design. Higher field, higher current density, and higher-temperature operation can all contribute to improving the economic projections for fusion energy. A result of the production of hundreds of tons of Nb3Sn superconducting strand for ITER could be the development of a worldwide industrial capacity that would lower the cost and improve the performance and quality of this high-field superconductor. The U.S. fusion program has been coordinating Nb3Sn development efforts with the U.S. High Energy Physics program. The development of about 30 metric tons of Nb3Sn strand for the ITER Engineering Design Activity (EDA) model coil programs in the 1990s resulted in an immediate increase in both performance and production capacity. The U.S. High Energy Physics program has since advanced this type of strand performance significantly for its application to very high field accelerator magnets, such as those required for the Very Large Hadron Collider. The extremely large-scale production of Nb3Sn required for ITER would result in significant improvements in worldwide industrial production capacity and in the quality of this superconductor, and the costs would be lowered as a result of high-volume production. This development would directly
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Burning Plasma: Bringing a Star to Earth benefit the High Energy Physics program and would also allow for improved and low-cost advanced superconducting wire for many other high-field magnet applications, such as those used in high-field research magnets—for example, for nuclear magnetic resonance. The ability to construct efficient high-field superconducting magnets will directly impact the economic prospects of a fusion reactor. The construction of such magnets for ITER can help drive this technology for fusion and other applications. High-Heat-Flux Component Development Burning plasma experiments will need to develop high-heat-flux components; in the operating phase they will serve as testbeds in which to evaluate the performance of these components in a reactor-like fusion environment. The heat loads on divertor or limiter targets in burning plasma experiments will be comparable to those expected in a reactor. Handling the heat loads requires 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. However, as in the case of materials testing, the burning plasma devices under consideration will not have the integrated operating time necessary to qualify key internal components for use in a demonstration reactor. Deploying technology to handle the high-heat fluxes in a burning plasma will allow tests of these components at the reactor-heat levels expected in a fusion environment. Remote Handling Technology In a fusion reactor, it is critical that the first wall and high-heat-flux components as well as ancillary components such as radio-frequency heating antennas and diagnostics can be remotely repaired, with tolerable downtime for maintenance. The scientific success of a burning plasma experiment will be critically dependent on the successful use of remote handling tools to minimize lost experimental time owing to component failure. Prototypes of the tools exist; a burning plasma experiment would provide an integrated demonstration of their reliability and effectiveness. The development and use of remote maintenance capabilities are necessary for both a burning plasma experiment and a future reactor. The burning plasma experiment will provide unique tests of these technologies in a fusion environment.
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Burning Plasma: Bringing a Star to Earth Conclusion A burning plasma experiment such as ITER could offer an early opportunity to begin the development of essentially all of the technologies needed for a fusion reactor. These include components and systems unique to fusion’s energy goal; plasma technologies such as heating, current drive, and fueling systems; hardened diagnostics; and superconducting coils of unprecedented size and energy. In addition, by operating safely, reliably, and within the structural code requirements used by the nuclear industry, a burning plasma experiment can demonstrate the favorable safety characteristics of a fusion reactor.
Representative terms from entire chapter: