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Burning Plasma: Bringing a Star to Earth C Proposed Burning Plasma Experiments As detailed in the DOE Fusion Energy Sciences Advisory Committee report A Burning Plasma Program Strategy to Advance Fusion Energy and discussed in other reports,1 three burning plasma experiments have been proposed—the International Thermonuclear Experimental Reactor (ITER), the Fusion Ignition Research Experiment (FIRE), and the Italian IGNITOR experiment. These three experiments range from a reactor-scale device using superconducting magnets, to compact, high-field copper-magnet devices. While each of the three devices is capable of addressing fusion physics and technology issues to some extent, they vary markedly in their missions, schedules, and budgets. INTERNATIONAL THERMONUCLEAR EXPERIMENTAL REACTOR ITER is an international facility that is designed to demonstrate the scientific feasibility of fusion as an energy source. It will also develop and test key features of 1 Fusion Energy Sciences Advisory Committee (FESAC) Panel Report, A Burning Plasma Program Strategy to Advance Fusion Energy, September 2002, available online at http://www.ofes.fusion.doe.gov/More_HTML/FESAC/Austinfinal.pdf; Proceedings of the 1999 Snowmass Fusion Summer Study, available online at http://www.ap.columbia.edu/SMproceedings/; Snowmass 2002 Fusion Summer Study, Executive Summary, available online at http://web.gat.com/snowmass/exec-summary.pdf.
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Burning Plasma: Bringing a Star to Earth TABLE C.1 Parameters for Burning Plasmas in the International Thermonuclear Experimental Reactor (ITER) Quantity Value Major radius 6.2 m Minor radius 2.0 m Magnetic field 5.3 T Plasma current 15 MA Fusion power 500 MW Q (fusion power/power in) ≥10 Burn time ≥400 s Wall loading 0.57 MW/m2 Plasma volume 837 m3 Heating/current drive power 73 MW SOURCE: Information obtained from the ITER Web site, http://www.iter.org/. Accessed September 1, 2003. the technology that will be required for a fusion power plant. A cutaway figure of the device is shown in Figure 1.1 in Chapter 1 of this report, and the ITER operating parameters are summarized in Table C.1. ITER is a $5 billion device that utilizes reactor-relevant fusion technologies, including superconducting magnets and techniques for control of the plasma profiles, to create self-heated plasmas. The ITER project has benefited greatly from the expertise and scrutiny of fusion-plasma researchers throughout the world. The present design is the result of a decade of effort. This work included one major redesign that reduced the anticipated cost by a factor of 2 by reducing the size and eliminating some of the capability to test fusion power components and technologies. The engineering design of ITER is well developed, and prototypes for many of the systems have been built. ITER has been designed to accommodate a range of heating and current drive technologies and to have the most complete set of plasma diagnostics of the three proposed burning plasma experiments. It will facilitate studies of plasmas for pulse lengths much longer than the plasma current redistribution time, which will enable studies of steady-state operation. The long pulse capability, the range and flexibility of heating and current drive technologies, and the extensive diagnostic set provide the capability to explore and evaluate advanced, steady-state operating regimes. The present ITER design would demonstrate the integrated operation of some of the important technologies for fusion power. It also has the capability to test some of the key nuclear components necessary for a fusion power plant, such as tritium breeding blanket modules required to close the deuterium-tritium (D-T) fuel cycle.
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Burning Plasma: Bringing a Star to Earth ITER provides excellent opportunities to address key physics issues. Of the three proposed burning plasma experiments, the relevant dimensionless physics parameters of ITER are closest to those expected for a fusion power plant. The operating regime of ITER facilitates the study of alpha-particle-driven instabilities at temperatures relevant to a power plant. The flexible plasma control capability and long pulse duration will permit the exploration of self-driven current regimes, permitting studies relevant to steady-state operation. Two phases of operation are planned for ITER. In the first phase, physics issues related to controlled burn will be evaluated. Assuming successful long pulse (up to 3,000 s), high-fusion-power operation, the second phase of the experiment will concentrate on the nuclear testing of materials components, although not at the flux and fluence levels required for a power plant. All of the burning plasma experiments under consideration are based on the D-T reaction, chosen because of its large cross section and relatively low reaction temperature. There is sufficient tritium available for these experiments. However, tritium does not occur naturally, and so it must be bred in the fusion reactor itself to make fusion power a reality. This can be accomplished using the fusion-produced neutrons in a lithium-containing “blanket,” which surrounds the burning plasma. The second phase of ITER is planned to have the capability to address this important technology issue by testing prototype breeding blankets using the neutrons from an actual burning plasma. Two challenges for ITER require further physics and technology research and development. One challenge involves the expected significant erosion of the divertor owing to repetitive oscillations of the plasma edge (edge-localized modes, or ELMs). The other issue is that the projected tritium retention in redeposited carbon has the potential to increase the machine downtime because of the need to remove the trapped tritium. These topics have been identified as high priorities for ongoing research. A more complete predictive understanding of the characteristics of the plasma edge in high-confinement regimes would reduce the uncertainty and increase confidence in the performance projections for ITER (as well as any other burning plasma experiment). Developing this understanding should also be a key element of the ongoing R&D program. FUSION IGNITION RESEARCH EXPERIMENT FIRE is a U.S. design study in the advanced preconceptual phase. Preliminary estimates indicate a cost of approximately $1.3 billion for this device, not including diagnostics. FIRE is intended as a major next step in magnetic fusion research. The mission of FIRE is to attain, explore, and optimize magnetically confined, fusion-dominated plasmas in order to provide the physics knowledge base for the
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Burning Plasma: Bringing a Star to Earth design of a fusion reactor. The FIRE option involves somewhat smaller extrapolations in physics and technology than those required for ITER and defers the integration of the fusion physics and technology to later experiments. The design is based on cryogenically cooled copper magnets, with a relatively high magnetic field and modest size as compared, for example, with ITER. FIRE employs strong plasma shaping and internal feedback control coils, both of which improve the capability to operate at high “beta” (i.e., plasma pressure normalized by the confining magnetic field) and at a relatively large fraction of internally generated (i.e., bootstrap) current. FIRE can operate at pulse lengths up to a couple of current redistribution times. The FIRE design facilitates the achievement of self-consistent, near-steady-state operation with large self-driven currents. However, in the present design, the plasma heating and current drive needed to achieve and control these discharges are limited. A key element of an ongoing R&D program for FIRE will be the development of electrical insulators for the magnets that are less susceptible to neutron damage. While the number of full-power D-T pulses will be sufficient for the investigation of burning plasma physics, if current materials are used, the useful life of the device will be limited by neutron damage. As in the case of ITER, divertor deterioration from plasma edge oscillations (ELMs) is an important issue that will benefit from further R&D. While FIRE is a technically sound design as presently proposed, it is a U.S.-centered project and hence does not benefit from the cost sharing and additional expertise that can be gained by international cooperation. FIRE would cost the United States as much as its participation in ITER but would pursue a more limited scientific mission and offer less in the development of new fusion technology. The FIRE design should thus be viewed as a contingency to be revisited, among several concepts, if the ITER project does not proceed. The attractiveness of the tokamak as a practical energy source would be increased significantly if it could be operated in steady-state and high-performance regimes. Thus, the ability of a burning plasma experiment to explore such advanced tokamak (AT) operating regimes is highly desirable. Important factors include the flexibility to effect strong plasma shaping, plasma profile control, active magnetohydrodynamic (MHD) control, long pulses, and detailed profile measurements. Both FIRE and ITER have significant AT capabilities and plans to study aspects of these regimes. If successful in ITER, for example, these operating modes would be used for the second phase of ITER operation, in which long pulses and high neutron fluence are required. FIRE can explore AT regimes with strong plasma shaping and active MHD control, which are both advantageous features in producing high self-driven currents and high performance. ITER can explore high
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Burning Plasma: Bringing a Star to Earth self-driven current regimes with a flexible array of heating, current drive, and rotational drive systems, with good profile measurements. THE IGNITOR EXPERIMENT IGNITOR, an Italian project, is a compact, cryogenically cooled, copper-magnet device capable of operation at high magnetic field. It is designed to achieve ignition in D-T plasmas and to study alpha-particle confinement and the heating and control of ignited plasmas. While potentially cost-effective in achieving the burning plasma regime, the resulting plasma conditions and flexibility of the device are more limited in the reactor-relevant physics that can be addressed. The IGNITOR design also raises a number of concerns, including less well established performance projections, questions about whether the required peak pressure profiles can be realized, and issues surrounding the structural integrity of the vessel.2 2 R. Bangerter, G. Navratil, and N. Sauthoff, 2002 Fusion Summer Study Report, 2003, pp. 5, 66-67, 69. Available online at http://www.pppl.gov/snowmass_2002/snowmass02_report.pdf. Accessed September 1, 2003.
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