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

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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?

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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 of collaborators like ourselves. In contrast to more established programs, the opportunities on W7-X are wide open to us, limited only by our ability to participate. Now, with W7-X commissioning its technical systems in preparation for research operations starting in 2015, it is the right time to expand our collaboration from the current $2.5M/yr. level of effort. The planned $0.5M/yr. increase for international stellarator collaboration by non-Laboratories starting in FY-15 is an important step but falls well short of the funding needed to take full advantage of the opportunities afforded by W7-X. A few U.S. research tasks are already in preparation but new expressions of interest in W7-X by U.S. Laboratory

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and non-Laboratory fusion researchers far exceeds what the present funding can support. Moreover, it leaves no flexibility to collaborate on LHD, which offers mature capabilities and contrasting design features that can contribute to scientific understanding through comparison with W7-X. Preparation for any new research task on facilities like W7-X or LHD takes time, typically 1-2 years, before it starts to be scientifically productive. For these reasons, an immediate increase of the budget to $5M/yr. is needed, growing as W7-X capabilities mature to $10-12M/yr. (an increase of $7-9M/yr.) by 2019, when W7-X will be fully equipped for high-power steady-state operation. The target figure is based on the U.S. being a ~10% partner in the international W7-X research team, taking into account costs for hardware, for both on-site and remote collaboration, as well as for research staff. While W7-X is the strategic focus, a robust international stellarator program will also include the flexibility to support collaborations on LHD that advance overall scientific aims. 3. New investments in domestic facilities to explore 3D divertor physics. Existing U.S. facilities provide a well-suited platform for development of the tools required to address 3-D divertor physics behavior in a coherent fashion. Enhancement of the existing domestic facilities is proposed to support a domestic initiative that works hand-in-hand with international collaboration toward the goal of developing a 3D divertor solution. The linkage between international and domestic research components is a critical aspect of the proposed initiative. This strategy and its scientific underpinnings are explained in detail in the O. Schmitz white paper. Details of the required facility investments are also provided in that paper, while a brief summary follows. Helically Symmetric Experiment (HSX), Univ. of Wisconsin. The HSX has demonstrated for the first time that a quasi-helical magnetic field structure can be designed and realized to reduce neoclassical losses and optimize plasma confinement. A proposed upgrade will support tests of new 3D divertor concepts and impurity transport in an integrated fashion with optimized neoclassical confinement in the ion root confinement regime where the radial electric field is negative. A neutral beam injection heating system will be added and the existing electron cyclotron heating system will be augmented in order to support a range of ion physics issues requiring higher ion temperature and density, along with divertor and impurity control physics studies.. The core device will be modified with replacement of 8 of the 48 non-planar coils and a new vacuum vessel to provide space for new divertor structures and increased plasma-wall separation. Compact Toroidal Hybrid (CTH), Auburn University. The CTH device is unique in the world in its ability to span the operational space from a stellarator to a tokamak/stellarator hybrid modified by significant amounts of internal Ohmic plasma current. A proposed upgrade will support tests of stability limits when going from tokamak to stellarator plasma equilibria in integrated fashion with 3-D plasma edge understanding. Electron cyclotron heating power will be increased to enable studies of island divertor physics at higher edge temperature and power, and neutral beam heating will be added to explore density limit physics, in particular the differences between tokamaks and stellarators with respect to density limits. Power supplies will be upgraded to improve control in tokamak modes. Compact Non-neutral Torus (CNT), Columbia University. A unique feature of CNT, now operated as an electron-cyclotron-heated neutral plasma confinement device, is the very long magnetic field connection length. An anticipated advantage of the stellarator over the tokamak is that a much longer field-line connection length radially spreads the heat flux in

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the divertor region. Heating and diagnostic upgrades are planned at CNT to explore long connection length regimes for heat flux spreading in stellarators. Limiters and island divertors will be easily replaceable and movable in the large vessel, and heat fluxes will be imaged by infrared cameras. Hybrid Illinois plasma Device for Research and Applications (HIDRA), Univ. of Illinois. The HIDRA device is based on the WEGA stellarator, newly acquired from the Max Planck Institute for Plasma Physics. It will be developed as a toroidal plasma-facing component test facility aimed at liquid metals and enabling technology development. A focused program on plasma materials issues in 3D geometries will cover approaches to create workable divertors for stellarators as well as to test high-powered liquid metal solutions more generally. Detailed budget estimates and time profiles for each of these programs are provided in the O. Schmitz paper. Overall, an increase of $6-7M/yr. over the current funding level for these facilities is needed to support implementation of these plans on a reasonable time-phased schedule. 4. Construction and operation of QUASAR. QUASAR (formerly NCSX) will provide an integrated physics of test of a high-beta quasi-axisymmetric stellarator configuration. The facility will include the medium-scale (major radius 1.4 m, magnetic field 2 T) QUASAR stellarator device and an array of heating, diagnostic, divertor, and control systems sufficient to study a wide range of physics issues. Perspectives on the science case are provided in the white papers by J. Freidberg and by the Max Planck Institute for Plasma Physics scientific leadership. In addition, multiple reviews of NCSX/QUASAR by FESAC and other community studies provide consistently strong support for the project on its scientific merits. The QUASAR device would be constructed using the coils (both non-planar and planar) and the vacuum vessel that were fabricated by the NCSX project prior to its termination. A preliminary estimate of $130M to complete construction of the stellarator and equip it for research was reported to the panel at its June meeting. Operating costs of $60M/yr. were reported, based on its similarity in scale and scope to NSTX-U. However, as subsequently explained in the QUASAR white paper from PPPL, there are realistic possibilities to obtain international contributions to construction, potentially reducing U.S. construction costs by up to $40M. Moreover, if QUASAR is sited at PPPL while NSTX-U continues to operate, both operating up to 15 weeks per year, the additional cost is only $30M/yr. due to the more efficient use of the PPPL infrastructure and staff. 5. Development of design options and mission need for a next-step U.S. stellarator. As noted at the outset, stellarator optimization defines the U.S. initiative, so in a sense this element integrates the results from the other parts of the program. The first task is to re-establish a national stellarator optimization effort aimed at continuing to improve stellarators. Some of the needed improvements have already been mentioned. The next machine must be optimized for divertor performance and impurity control as well as for core plasma equilibrium, stability, and confinement. The potential for improved alpha particle confinement and reduced turbulent transport must be fully explored. Coil systems need to be simplified in terms of both their geometry and tolerances in order to improve the engineering attractiveness of the concept. The requirements for such a program are relatively well known from the experience of designing NCSX. A team of both physicists and engineers is needed to develop the tools, explore the design space, and incorporate advances from ongoing theory

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and experimental research. The scope of this effort requires funding at the level of about $5M/yr. and the lead time to develop and qualify the configuration for a new experiment is several years. Configuration choices are necessarily linked to technology and mission choices. While QUASAR will provide the first integrated physics test of QA physics, the next facility will extend the understanding of quasi-symmetric stellarators to high temperature, high pressure, sustained plasmas with excellent energetic particle confinement at approximately the JET scale. This facility would address scientific issues requiring high performance, for example the variation of confinement properties with system size and pulse length, and integration of 3D divertor and plasma-facing component designs with a steady-state core plasma at near- burning performance. Either superconducting or water-cooled normal conductor technology could be considered. Deuterium-tritium operation, component testing taking advantage of the stellarator’s steady-state capability, and reactor maintenance prototyping are within the range of mission possibilities. Priorities The proposed initiative has been designed as a coherent program whose multiple elements are focused on a single scientific mission, namely to extend all aspects of stellarator optimization over the next decade. The initiative is a bold one, whose scope and breadth are justified by the urgency of developing a steady-state solution for MFE and the absence, at this time, of a clear winner. The plan calls for an increase in funding for stellarator research of $24-$28M/yr., not counting QUASAR. The increase is almost $60M/yr. when QUASAR is included, assuming both QUASAR and NSTX-U continue to operate at PPPL, and almost $90M otherwise. These may seem like large numbers, but they are not unreasonable when compared to other leadership class tokamak and stellarator programs in the world. If, in the worst case, fusion program budgets remain flat at the FY-15 request level of $266M, a $90M/yr. stellarator program involves a ~33% shift in program priorities over 10 years, a challenge to be sure, but appropriate for a budget-constrained program that nonetheless aspires to maintain a world leadership role. If the program sees some funding growth, and can take advantage of joint operation of QUASAR and NSTX-U, then the relative perturbation becomes less and the expansion of the domestic stellarator program is more manageable. The priorities are best understood by realizing that the various elements can be implemented in a time-phased manner, and that they must be viewed not only in competition with each other, but also with other parts of the fusion program. We explain it as follows: 1. Increased 3D theory and computation. The requested $3-4M/yr. increase should be phased in over at most 2 to 3 years, starting immediately. The immediacy is based on the need for an effective partnership with the experimental efforts now beginning on W7-X, the importance of 3D issues for tokamaks, and the need for theory leadership in developing solutions to stellarator problems. 2. Expansion of the U.S. partnership on W7-X. With W7-X about to go into operation, the budget should be increased by $2.5M/yr. (including a $0.5M/yr. increase for non- Laboratories already planned by DOE) without delay. The W7-X program is moving forward independently of U.S. plans, and we must increase our participation now to take full advantage of the rich scientific opportunity, increase the involvement of both non-Laboratory and Laboratory scientists, and benefit scientifically from our hardware investments. Further

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growth can occur over the next 4-5 years, paced by need and the maturation of W7-X capabilities. 3. New investments in domestic facilities to explore 3D divertor physics. The upgrades to the four facilities can be implemented in a time-phased manner, but the process needs to start without delay. All should be implemented within 5-6 years and must be followed up with healthy research support in order to scientifically exploit the investments. 4. Construction and operation of QUASAR. The U.S. can have a scientifically exciting and important stellarator program without QUASAR. But if the U.S. is to have a recognizably world-leading program and spearhead the development of quasi-axisymmetric stellarators, QUASAR must be built. Any new facility or facility upgrade on this scale involves a major political decision and requires a strong commitment to a particular scientific direction. Under flat or nearly-flat budgets, it may mean large shifts in priorities as well. QUASAR will likely compete for consideration with other major facility proposals that have been presented to the panel, such as ADX and DIII-D upgrade. A decision to go forward with any of these may take some time, but in the case of QUASAR the immediate need is to begin exploring the possibility of reducing U.S. construction costs through international partnerships. This requires funding of about $5M to U.S. partners PPPL and MIT (at least) to explore partnerships and update plans and costs for critical items. 5. Development of design options and mission need for a next-step U.S. stellarator. Re- establishment of a robust U.S. stellarator optimization activity is the first task, but can build up slowly in the first few years, giving preference to moving forward with theory and experimental research elements, which provide the knowledge base for optimization. Exploration of next-step mission options can begin any time, but realistically it makes sense to settle the question of QUASAR first.

3. Beyond the Ten-Year Horizon The vision of a Fusion Nuclear Science Facility (FNSF) as a long-term direction for U.S. fusion research sparks excitement among fusion researchers from all disciplines- plasma physicists, materials scientists, technologists, and engineers. Timely construction of an FNSF could keep the U.S. at the forefront of fusion research for decades to come. To fulfill its central mission of testing in-vessel components in a real fusion environment, an FNSF must reliably provide a high-performance steady-state plasma to generate sustained neutron wall loads of >1 MW/m2. An essential pre-condition is the demonstration of a sound basis for stable, steady-state plasma confinement for periods ranging from hours to weeks. For this reason, U.S. stellarator research has a crucial role to play in the preparation for an FNSF. With its equilibrium provided by 3D coils instead of plasma current and its lack of disruptions, the stellarator starts with clear advantages as a candidate magnetic configuration for an FNSF. With no need for current drive, it has the potential for high fusion gain and avoids the uncertainties and development costs associated with current drive physics and technology. Higher density limits and simpler controls compared to tokamaks are additional advantages. The lack of any requirement for current drive has major practical benefits for an FNSF. It minimizes wall penetrations and the attendant loss of tritium breeding capacity; it simplifies control requirements and eliminates entire sub-systems; and it reduces recirculating power requirement allowing higher gain and the potential for net electricity generation even in a compact plant.

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Stellarators have been studied for a very long time, with many experimental facilities and extensive theoretical study. Achievements include a multi-machine confinement data base, empirical confinement scaling similar to those used for tokamak projections, and many theory- based analysis codes. Still, it can be rightly said that stellarators have been much less thoroughly studied than tokamaks. The depth and breadth of stellarator physics understanding is less, the tools available for design and analysis of future stellarators are more limited, and in terms of performance the peak triple-product (nTτ) values achieved in stellarators are well below those of tokamaks. With no stellarator counterpart to ITER on the horizon, the stellarator clearly lags the tokamak in its maturity or technical readiness. However, the persistent problem of disruptions and the lack of a clear steady-state solution mean that the prospects for a tokamak-based FNSF are far from certain. Alternatives must be pursued in parallel to mitigate the risk to the FNSF mission, and the stellarator is the obvious choice. Implementation of the proposed U.S. stellarator initiative will only improve the prospects for developing a suitable magnetic configuration for an FNSF. A successful U.S. effort to develop quasi-symmetric stellarators could open a path to an attractive FNSF and arguably the optimum DEMO. The path to FNSF readiness is likely to be paced not by the magnetic configuration but by solving problems common to all magnetic configurations, particularly in materials and technology areas. In other words, stellarators can be developed to a level of readiness for an FNSF without delaying the critical path. As fusion researchers we of course welcome any solution that finally succeeds, be it a tokamak or a stellarator, but to ensure success it is essential to move ahead with a vigorous U.S. stellarator program starting now.

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