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Stellarator Initiative Overview R3 Solutions for Steady-State High Performance MFE: A U.S. Stellarator Program for the Next Ten Years by Members of the National Stellarator Coordinating Committee* 1. Introduction This white paper summarizes the ten-year stellarator research initiative that is being proposed for consideration by the FESAC Strategic Planning Panel. In addition to the six talks presented at the June and July meetings, several white papers have been submitted to the panel documenting the various elements of the initiative in detail: • “3D theory and computation: A cost-effective means to address ‘long-pulse’ and ‘control’ gaps,” by M. Landreman, et al. • “Development of 3-D divertor solutions for stellarators through coordinated domestic and international research,” by O. Schmitz. • “MIT-PSFC Makes the Case for QUASAR,” by J. Freidberg, et al. • “Status and Prospects of the U.S. Collaboration with the Max-Planck Institute for Plasma Physics on Stellarator Research on the Wendelstein 7-X Device,” by T. Klinger, et al. • “A Perspective on QUASAR,” by T. Klinger, et al., Max Planck Institute for Plasma Physics. • “A Management Strategy for QUASAR,” by Princeton Plasma Physics Laboratory. • “Control of High-Performance Steady-State Plasmas: Status of Gaps and Stellarator Solutions,” by the National Stellarator Coordinating Committee. The authorship of these papers (including members of the NSCC*) includes many early- and mid-career scientists ready to lead the program into the future as well those who have been involved in stellarator research for decades. The last paper in the above list reviewed the current status of gaps to a magnetic fusion energy (MFE) DEMO that were documented in the 2007 Greenwald report. In brief, that paper found that the high-level gap G-2, “Demonstration of integrated, steady-state, high-performance (advanced) burning plasmas, including first wall and divertor interactions,” is still a wide one and it argued that the U.S. fusion program for the next decade must include a strong stellarator effort as a component of its strategy to close that gap. 2. The Proposed Initiative The case for stellarator research is simple: stellarators are inherently steady-state and they eliminate or greatly reduce the problem of disruptions. The importance of these issues to the success of MFE is so great, and the prospects for tokamak solutions sufficiently uncertain, as to justify pursuit of stellarator solutions in parallel with advanced tokamak research. Indeed, Europe and Japan have vigorous stellarator programs, including large facilities, for that reason. Nowhere in the world, however, is there a program to develop the innovative concept of quasi- symmetric stellarators beyond the current exploratory level of research. This class of stellarators lies between the advanced tokamak and the optimized W7-X stellarator, and offers a path to a * J. P. Allain, D. Anderson, A. Boozer, D. Currelli, J. Freidberg, D. Gates, J. Harris, M. Landremann, J. Lore, D. Maurer, H. Neilson, A. Reiman, D. Ruzic, O. Schmitz, D. Spong, J. Talmadge, F. Volpe, H. Weitzner, G. Wurden, M. Zarnstorff – 1 – steady-state fusion power plant with improved neoclassical transport at about half the size (in major radius) of a plant based on the optimized W7-X. Timely understanding of this path, both its benefits and risks, is needed to inform choices regarding the steps toward next-step fusion devices that will be made in the next decade. There is a need and a leadership opportunity, that exists now, for the U.S. to move this science forward over the next ten years. Science Strategy The goal of stellarator research is steady-state, ignited, disruption-free reactor operation with predictable performance and minimum requirements for external control. Since the performance of a stellarator is determined by its 3D magnetic field, the central scientific challenge is to optimize the 3D configuration targeting the important performance attributes: MHD equilibrium and stability, alpha particle confinement, transport reduction, and plasma exhaust through a divertor. The challenge is to develop and validate techniques that allow both optimization and confident extrapolation of designs to reactor-scale devices. The U.S. has unique capabilities to lead in this area, especially as pertains to quasi-symmetric stellarators. The proposed U.S. initiative aims to extend all aspects of stellarator optimization over the next decade. Its several elements will work in an integrated fashion to experimentally test and validate theory, to extend understanding and tools, and to continue to improve designs. The targeted attributes will span multiple domains, extending from the core plasma (equilibrium, stability, and transport), to the divertor (plasma exhaust, impurity and recycling control), to the magnets, blankets, and structures (construction, maintenance). Optimized high-beta QA stellarator core designs have been available since the early 2000s. Promising strategies to improve the core by targeting turbulent transport reduction and to improve coil geometries for ease of construction and maintenance have emerged in recent years and need to be developed and integrated into the design process. The next optimization challenge is the divertor, where models of 3D plasmas, plasma-material interactions, high heat-flux materials, and thermal-mechanics must be integrated to optimize the configuration of both the edge magnetic configuration and the plasma-facing structures. Plasma exhaust scenarios compatible with simultaneously high core performance and long material lifetimes must be developed. In all these domains, optimization tools must be developed, exercised, and experimentally tested and the underlying understanding must be validated. These imperatives define the needed initiative. Program Elements and Costs The U.S. stellarator program currently includes scattered theory efforts, two exploratory experiments (at Wisconsin-Madison and Auburn), and international collaboration. Though small, this program does provide a base of activities and experienced personnel from which it could expand rapidly. The elements of an expanded program follow. 1. Increased 3D theory and computation. As stated in the M. Landreman paper, a vibrant 3D theory and computation program is crucial to identifying the most interesting regions of parameter space. Techniques and computational tools required for stellarators are also required to understand and address non‐axisymmetric effects in tokamaks. A well posed set of questions for an expanded 3D theory and computation program is already defined. Fast particle confinement: How much can alpha particle losses be reduced in non- axisymmetric configurations? Can lost alpha footprints be localized to areas of the wall specially designed to handle them? – 2 – Sensitivities. How accurately must the components in tokamaks and stellarators be manufactured and assembled? Can tolerances be eased to reduce costs? Coil geometry. Can superconducting materials, magnetic materials, or saddle coils be incorporated into stellarator designs for improved access, flexibility, and simplified coil shapes? Can stellarator optimization be more targeted to the spatial modes of the magnetic field which can be efficiently produced from a distance and to which the physics performance is most sensitive? 3D equilibrium, islands, and stochasticity. Can modern nonlinear extended‐MHD codes accurately compute 3D equilibria, including islands? Can 3D equilibrium tools help interpret the effect of non-axisymmetric ELM‐control perturbations in tokamaks? Shaping for reduced turbulent transport. To what extent can 3D shaping reduce turbulent transport? Can the availability of nonlinear gyrokinetic codes and advances in analytic understanding of stellarator microinstabilities be combined to optimize stellarator design for reduced turbulent transport? An increase of $3-4M, which would more than double the current level of effort in 3D theory and computation, is called for. 2. Expansion of the U.S. partnership on W7-X. The Wendelstein 7-X (W7-X) program will provide the first large-scale test of an optimized stellarator and will extend the pulse length for high-performance plasma operation to 30 minutes. For the U.S., collaboration in W7-X offers the opportunity to participate in the advancement of stellarator physics at the pulse- length/performance frontier. In particular, we will link our research on W7-X with targeted domestic experiments, theory, and modeling efforts to develop a 3D divertor solution for stellarators over the next decade, as explained in the white paper by O. Schmitz. The U.S. will also participate in W7-X core physics research from the beginning and increasingly so over time. A U.S. partnership with W7-X is already well established, with several U.S. contributions to construction having been made already. The national W7-X team of PPPL, ORNL, and LANL is expanding now to include participation of non-Laboratory groups. A unique aspect of W7-X is that the host, Max Planck Institute for Plasma Physics (IPP), is planning the research program to be an international collaboration from the start. Under a formal partnership agreement between DOE and IPP, the U.S. will have a voice in shaping the W7-X research program with a seat on its International Program Committee. We will have opportunities for full participation in W7-X research and access to all data. IPP has strong academic ties, and they are organizing themselves to support the training of students and young researchers, including those
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