5

Strategic Guidance for a National Program for Burning Plasma Science and Technology

This chapter summarizes previous chapters in the context of the committee’s strategic guidance for a national program for burning plasma science and technology covering the next several decades. The committee’s two main recommendations are elaborated. This chapter summarizes program elements, an approximate research timeline, a response to a decision to withdraw from the International Thermonuclear Experimental Reactor (ITER), and budget implications of the committee’s guidance.

This plan has two parts.

• First, the United States should seek full benefit from continued participation in the ITER project as the nation’s burning plasma experiment.
• Second, the United States should start a national program of interconnected science and technology research extending beyond what will be accomplished with ITER and leading to construction of a compact fusion pilot plant at the lowest possible capital cost.

Together, these two research efforts establish the scientific and technical feasibility of fusion power and advance the fusion program from its current feasibility stage along a cost-effective path to the production of electricity from fusion. The strategic guidance strengthens U.S. capabilities in burning plasma science, materials science, fusion nuclear science, and engineering science, continues engagement in the international effort, and promotes innovation and the involvement of industry.

ITER: EXTENDING THE FRONTIERS OF BURNING PLASMA SCIENCE

At the foundation of the U.S. strategy is the production, study, and exploitation of a burning plasma. This regime is characterized by plasmas that are dominantly self-heated by the particles generated by the fusion reactions. Methods to control plasma stability, plasma interactions with first wall materials, plasma confinement, and fusion power output will be tested. Theoretical predictions of energetic particles produced by fusion reactions and methods to sustain a burning plasma will be explored and validated. Equally important are gains in fusion engineering science and industrial capability that result from construction of the world’s first burning plasma research facility at the scale of power plant.

The only existing project to create a burning plasma at the scale of a power plant is ITER, which is a major component of the U.S. fusion energy program. As an ITER partner, the United States receives full benefit from the technology that will establish the feasibility of fusion while providing only a fraction of the financial resources.

Active participation in the ITER project is necessary for the United States to derive full benefit from the ITER project. The committee is not aware of any credible alternative that might achieve burning plasma conditions sooner. Effective participation in ITER requires that the United States continue to invest in domestic tokamak facilities and research to address operational issues for ITER and prepare the physics and fusion technology needed to enable next step devices. U.S. researchers are providing solutions and technical support for successful preparation and operation of the ITER facility. In some areas the United States is uniquely able to provide essential information. Even at this stage of ITER preparation, U.S. fusion research experiments provide knowledge needed to refine burning plasma operating scenarios, to finalize the designs for magnets to control transients, to develop the systems used to mitigate the adverse impact of plasma current disruptions, and to predict the response of the divertor during the expected periods of high plasma heat-flux. Later, during ITER operations, opportunities will exist for the United States to take leadership in upgrading capabilities and the development and testing of ITER operating scenarios that may expand ITER’s scientific impact on the physics and predicative design tools used to lower the cost of fusion development.

Ongoing research targeted to resolve issues related to plasma control and transient reductions should continue. Recent research has been important in developing the ITER plans, which are guided by new techniques to suppress edge localized modes (ELMs) using external control magnets, injection of small pellets of frozen hydrogen, and operation in regimes that are naturally stable to ELMs. A hierarchical approach is needed for mitigating adverse effects of plasma current disruptions, including design of scenarios that are naturally disruption resistant, predictive algorithms for assessing the plasma stability, control tools for stabilizing large-scale

magnetohydrodynamic (MHD), recovery modes for dealing with off-normal faults and reliable mitigation tools when a disruption cannot be avoided. The two largest research facilities within the United States, National Spherical Torus Experiment Upgrade (NSTX-U) and DIII-D, are well poised to address these issues in the near term. Examples of additional research tools include additional plasma heating and plasma current drive systems, expanded magnet systems for precision plasma control, and improved methods to measure plasma phenomena including those diagnostics capable of fully exploiting the physics of plasma current disruptions.

While reaching the frontier of burning plasma science will establish the feasibility of magnetic fusion energy, the full benefit from participation in ITER will result from opportunities to develop and test burning plasma science in order to maximize the potential of a compact fusion pilot plant. Recent advances in theory and simulation validated by results from U.S. experiments have already suggested ways to optimize ITER performance. A strong focus on high performance steady-state scenarios in ITER will advance understanding of the strongly coupled system, where the heating source (primarily fusion produced alpha particles) and current drive (primarily pressure-driven bootstrap current) are both strongly coupled to confinement and energetic particle physics. Advances in understanding toroidal magnetic confinement, plasma control, and integrated solutions to whole-plasma optimization point to improvements beyond the ITER baseline and show how careful design and simulation can be used to lower the cost and accelerate fusion energy development.

The U.S. research participation with ITER should develop and test burning plasma scenarios, validate predictive simulation models, and gain engineering experience that will contribute to reliable operation of a compact fusion pilot plant.

BEYOND ITER: SETTING THE NATION’S FUSION ENERGY GOAL

As discussed in the committee’s Interim Report and the technical analysis in the previous chapters, now is the right time for the United States to develop a national strategic plan for fusion energy that benefits from its investment in burning plasma research and takes leadership in the development of fusion electricity for the nation’s future energy needs. The adoption of such a plan provides a grounding for strategic funding decisions and priorities within the national program and helps foster innovation toward commercially viable fusion reactor designs. Additionally, a national fusion energy goal guides national research and innovation programs, helps to engage all participants in the fusion endeavor in the United States, from universities, to national laboratories, to industrial partners, and sets the national priorities of our partners, enabling them to develop key areas of unique expertise.

Many reasons justify the readiness to develop a national strategic plan for fusion energy. Through the work of the United States and other nations, the ITER

project is “back on track,” and confidence has increased that burning plasma science will succeed in its mission. In contrast to the strategic planning proposed more than a decade ago, today the international community is stronger. The world effort has achieved rapid scientific and technical progress, and significant efforts are now under way by our international partners to move fusion beyond ITER toward commercial fusion energy. If the United States seeks to maintain a leading role in the pursuit for abundant fusion power, the development of a national strategic plan for fusion energy that spans several decades is necessary.

Most importantly, the reason behind our committee’s recommendation is scientific progress, technical readiness, and the opportunities presented by recent advances in fusion-relevant technologies. Understanding of toroidal magnetic confinement is now highly advanced: Performance beyond the ITER baseline can now be predicted and new science shows how advanced knowledge and simulation can be used to speed fusion development. Additionally, new technologies (largely developed outside fusion) show great promise to reduce the size and cost of fusion power systems and reduce the cost of fusion research and further progress. Today, with many of the complex physical processes of magnetically confined plasma becoming better understood and the first phase of ITER construction more than half complete, the most critical fusion science research needs are interdisciplinary, combining burning plasma science with materials science, fusion nuclear science, and engineering science. These interconnected science challenges require a comprehensive strategic plan that looks beyond the demonstration of feasibility expected with the ITER device and addresses all of the challenges for developing fusion power. With the initial operation of ITER scheduled to begin within a decade and with the expectation that controlled fusion will be demonstrated in ITER, now is the right time for the United States to develop plans to benefit from its investment in burning plasma research and take leadership in the development of fusion electricity for the nation’s future energy needs

TOWARD FUSION ELECTRICITY: THE COMPACT FUSION PILOT PLANT

The committee’s two main recommendations address the nation’s strategic interest in realizing economical fusion energy in the long term. First, as described in Chapter 3, the United States should remain an ITER partner as the most cost-effective way to gain experience with a burning plasma at the scale of a power plant. Second, as described in Chapter 4, the United States should start a national program of accompanying research and technology leading to the construction of a compact pilot plant which produces electricity from fusion at the lowest possible capital cost. Implementation of both recommendations requires essential research using facilities and programs within the United States and continued active partnership with the international effort.

The committee considered several strategic pathways for the nation’s long-term demonstration of fusion electricity. The objective of each path was fusion electricity, but the size, priority, and number of research facilities were different. For example, a fusion nuclear science facility (FNSF) does not need to make electricity, but instead focuses on the materials science and fusion nuclear science to develop materials that can survive the harsh environment surrounding a fusion system and it enables scientists to understand and control the operation of a burning plasma for many days. However, an FNSF would need to be followed by a second, larger facility to demonstrate fusion electricity. Other nations are pursuing a large, next-step fusion device, called a demonstration fusion power plant (DEMO).^1,2,3 A fusion DEMO would be capable of producing electricity, operating with a closed fuel-cycle, and be the single step between ITER and a commercial reactor. This definition of DEMO calls for larger devices with the mission to produce significant net electricity, establishing routine electricity production and maintenance in order to convince utility companies (and other associated investors) that all aspects of the power source are credible, reliable, safe, and ultimately profitable. Previous committees of the National Academies of Sciences, Engineering, and Medicine and the Department of Energy’s (DOE’s) Fusion Energy Sciences Advisory Committee (FESAC) have recommended that the United States develop a strategic plan leading to either an FNSF or to a DEMO. By contrast, this committee’s recommendation recognizes the advantages for a reduced cost pathway to fusion energy demonstration and the scientific and technical opportunities that make this pathway possible.

Based on scientific progress and the expectation of innovations in technologies that will decrease the size of the magnetic fusion reactor and, as a consequence, also reduce both the capital-cost and construction time, the committee’s strategic guidance for the United States is to target a pilot plant producing power similar to the power levels expected in ITER but in a device much smaller in size and cost and employing design improvements that allow net electricity production. The recommended strategy is faster and less costly than a two-step approach that requires both the construction of an FSNF and a follow-on DEMO device to produce electricity. The recommended strategy is also less costly than those proposed by other nations, which involve a single large DEMO device. This is because the goal of the compact fusion pilot plant is to incorporate science and technology innovations into a single facility, with a staged research plan, that will produce electricity from fusion at the lowest possible capital cost.

This compact fusion pilot plant would be a pre-commercial research facility with a burning plasma at its core and surrounded by a blanket to capture fusion heat and neutrons. In addition to the production of fusion electricity, the pilot plant would ultimately be capable of uninterrupted operation for weeks and produce tritium, the fusion fuel, from lithium-containing blankets that surround the plasma. The compact pilot plant would be a staged facility and constructed at

lower capital cost than previously considered for fusion demonstration devices. The first stage will establish the capability of uninterrupted operation at high-power density in a compact device. The second stage would operate at high fusion power and demonstrate the safe production and handling of the tritium fuel required for sustained power. As a pilot plant, its purpose will be learning, but the knowledge obtained would be sufficient for the next-step to be commercial fusion power systems.

By starting a national research program toward a compact pilot plant, critical science and technology research can be ready in time to use the knowledge learned from ITER operation to demonstrate electricity production by mid-century.

2020-2035: REMOVING THE BARRIERS TO LOW-COST FUSION DEVELOPMENT

The cost of fusion electricity is driven principally by its capital cost and by how many hours the plant can run each year. Key to the achievement of low-cost fusion development are four research challenges: (1) the fusion power density should increase beyond that obtainable in ITER, (2) uninterrupted steady-state operation needs to be demonstrated while learning how to handle reliably the high levels of heat escaping from the plasma, (3) innovations should be encouraged and developed to significantly reduce the size of the fusion power system, improve component lifetime, simplify maintenance, and lower construction cost, and (4) blanket technologies should be developed and tested to efficiently and safely breed tritium and extract high quality heat.

Resolving these barriers to low-cost fusion development will require the design and operation of new facilities and continued engagement in the international effort. Facility decisions should be guided by cost-effective opportunities to achieve critical program goals and by unique opportunities for world-leading contributions. As the research portfolio evolves in time, existing research facilities are phased out as new ones are implemented. One of greatest needs is the control of a continuous high-pressure compact plasma, which will likely require a new intermediate-scale research facility in the United States to establish its feasibility. Another significant challenge is the qualification of the materials and components that surround the plasma and are exposed to fusion irradiation. One of the greatest opportunities is for the United States to take a leading role in the engineering sciences for high-critical-temperature superconductors for fusion purposes.

This national research effort aimed at low-cost fusion development not only addresses U.S. strategic interest in realizing economical fusion energy in the long term but also contributes to several national science and technology goals in high-heat flux material science, advanced nuclear science, and high-field superconducting magnets.

Based on the input received by the committee and the committee’s study, the national program should contain several program elements.

Extending the Frontier of Burning Plasma Science

The U.S. fusion program has been a key contributor to the physics basis for ITER design and operation. Participation in ITER provides the most timely and cost-effective opportunity for the United States to benefit from the science and technology of the burning plasma regime. In the 2020-2040 period, the U.S. program, through its domestic facilities and in collaboration with international partners, will continue playing a key role in burning plasma studies and efforts to fully exploit the capabilities of the ITER facility. Alongside ITER participation, a key program element is the development of scenarios that deliver both higher levels of fusion power gain (i.e., Q > 10) and the achievement of high-power gain at lower plasma current in ITER. The high priority placed on well-diagnosed plasmas in the United States, coupled with continued U.S. leadership in developing physics-based models, will provide the United States with unique capabilities not only to improve the understanding of reactor-grade plasmas but also to use this understanding for improving the performance of fusion systems beyond ITER. Continued involvement in the International Tokamak Physics Activity and future international activities coordinating burning plasma research should remain a U.S. priority.

Sustaining High Fusion Power Density with High Plasma Confinement

The United States has long been a world leader in the development of high performance, steady-state-capable scenarios. The planned capabilities of DIII-D and NSTX-U will remain world-leading in developing the feasibility of high performance, fully noninductive scenarios. This involves research establishing scenarios combining high bootstrap current fraction, compatible divertor configuration, and higher-efficiency current drive technology.

Recent advances in the U.S program, motivated by theoretical studies, point to new research elements that not only offer the United States distinct leadership opportunities but also potentially lead to more cost-attractive fusion systems, a central theme of this strategic plan. The U.S. program in 2020-2035 should focus on these potential breakthroughs with a program to quantify the benefits of such ideas. Early in this period, planned upgrades to DIII-D and NSTX-U should provide sufficient capability to test the basic aspects of the underlying ideas and establish the feasibility of steady-state tokamak operation with good power-handling capability. However, either substantial upgrades to these facilities and/or a new facility are needed to extend these results to burning-plasma-relevant conditions where the self-consistency of transport, stability, current drive, and compatibility with

a reactor relevant boundary solution can be assessed. Note that detailed models predict that the highest performance (and hence highest power density) will be achieved with optimal shaping of the plasma (including aspect ratio, triangularity, and elongation). DIII-D and NSTX-U are well positioned to provide key information on the choice of the required parameters. The committee cannot now determine whether the research needs outlined here require an entirely new facility or an upgrade of an existing facility. The details of the next-step magnetic fusion research facility should be developed through a coordinated community process that includes consideration of multiple mission elements. However, the objective of this facility should be the establishment of the science and technology needed for uninterrupted, high-power-density plasma confinement at a compact size. The resulting upgrades or new facility should be designed, fabricated, and operated by a national team.

Power Exhaust Solutions for High-Power Density Fusion Systems

A key enabling element of any U.S. strategic plan should be the development of power exhaust solutions and materials that can handle the very high heat flux intrinsic to these systems. In the United States, DIII-D and NSTX-U have world-leading diagnostic sets and the ability to vary divertor conditions over a wide range. Additionally, ITER will offer a significant opportunity to assess boundary solutions at heat flux levels much higher than in presently available devices and therefore should be an integral part of this plan.

However, a key aspect of projecting these solutions to devices beyond ITER is the ability to confidently predict the divertor and first wall configurations compatible with very high plasma heat and particle fluxes while preventing material contamination of the burning plasma core. Because projecting boundary solutions for future devices is quite uncertain, a key aspect of any plan going forward is to reduce the predictive uncertainties through a science-driven, model validation approach to elucidating key features of the boundary solution and identifying phenomena that are not captured properly by measurement or by simulation. Additionally, several innovations to improve the heat flux handling capability of the divertor, called “Snowflake,” “Super-X,” and “Small-Angle Slot” divertors and including innovative material choices (e.g., liquid metal walls) should be assessed. While each of these can be developed and tested in existing facilities, confidence in such solutions for future devices may require substantially increased capability in order to explore heat fluxes and their dissipation at the high levels and densities expected in fusion power systems. For this reason, an additional requirement of the next-step magnetic fusion research facility discussed above should be tests of advanced divertor schemes with robust power and particle handling capabilities applicable to the compact fusion pilot plant.

Large-Bore High-Temperature Superconducting Coils

Recent advances in the technology of high-temperature superconductors (HTS) have the potential of reducing the cost of fusion energy and providing unique benefits for increasing the performance limits of fusion systems including very high current density, operation at much higher magnetic field, and the potential for improved maintenance, for example through jointed, demountable magnets.

In order for the U.S. program to be in a position to utilize HTS magnet technology on the timeline articulated in this strategic plan, a development program should begin as soon as possible. Early stages of this effort should focus on assessing relevant means to produce coils from the HTS conductor and possible performance degradation as the bore size is increased. The United States should consider hosting a toroidal magnet test facility similar to the Large Coil Test Facility (LCTF) that was hosted by Oak Ridge National Laboratory and facilitated by an implementing agreement of the International Energy Agency’s Fusion Power Co-ordinating Committee. The LCTF was a successful partnership of magnet scientists from the United States, Switzerland, Europe, and Japan and between national laboratories and industry. Just as the LCTF established the engineering science for low-temperature superconductors, like those used in ITER, this new facility would invite proposals from industries around the world and establish the engineering basis for higher-field high-temperature superconductors for fusion magnets.

Materials that Deliver High Performance and Long Lifetime

Since the economics of fusion power depend critically on the amount of time available for power production, the lifetimes of materials are a critical factor in reactor design and operation. Rapid material degradation due to plasma erosion or neutron bombardment could severely limit the benefit that could be gained from a high-power core/boundary solution. In this regard, the presence of 14 MeV neutrons and their deleterious effects are very specific to the fusion environment and therefore data quantifying such effects is sparse.

To resolve key materials challenges, two branches of research are envisioned. First, the effect of long-term exposure of plasma-facing materials to divertor-like conditions needs to be understood, leading to the development and qualification of materials that meet the stringent demands of a fusion system. Second, these plasma-facing materials as well as structural materials that can maintain under high neutron fluence their critical properties, transfer properties, tensile strength, and fracture toughness would need to be developed, tested, and qualified. The development of these materials requires a better scientific understanding of the processes that modify material properties. This understanding requires continued improvement in theory, numerical simulations, and experimental capabilities.

During the 2020-2035 period, the U.S. program should develop plans for new facilities to address these materials research needs. First, the United States would bring online a nonnuclear material test facility that has the capability to expose materials to relevant heat fluxes over the range of plasma conditions expected in future fusion systems. Such a facility (similar to the proposed Material Plasma Exposure Experiment facility) would provide a unique testbed for new materials including composites and liquid metals. Second, and later in this period, the United States would develop a facility and research plan leading to construction of a fusion neutron source for tests of modest-scale-sample materials and components in a 14-MeV neutron environment. This research should consider the benefit, cost, and timing of nuclear materials testing using a spallation source. A new fusion neutron source would be a world-leading scientific instrument, enabling U.S. and international researchers to explore effects not possible at any other facility and would complement the efforts of other nations that are focused on exposure of smaller material samples (for example, a beam-driven neutron source like the International Fusion Materials Irradiation Facility). Delivering the necessary capabilities in a cost-attractive manner should be a key factor in the choice of concept.

Blanket Systems that Breed Tritium and Extract High Quality Heat

A very important component of any fusion energy system will be the ability to efficiently breed and extract tritium and convert the fusion energy to high quality heat for electricity production. In particular, achieving sufficient tritium breeding and extraction efficiency is absolutely critical to the success of fusion given the very limited availability of tritium. As noted above, the worldwide fusion program has developed plans for significant technical development of blanket systems through the ITER Test Blanket Module (TBM) program and other dedicated research worldwide. The United States should invest sufficiently in blanket research and development (R&D) to ensure that we are capable of leveraging this international investment while addressing specific issues associated with a compact fusion reactor. A decision whether or not to become a supporting participant in the TBM program would be part of a national evaluation of research options considering the timeline for the final design, licensing, and installation of TBM modules. Examples of research issues to be addressed in the national program include heat generation and removal at very high-power density and potential new blanket solutions that are predicted to achieve very high thermal efficiency and provide a path to tritium self-sufficiency.

In addition to the research elements above, the committee views other considerations as important guidance for the national strategic plan for fusion energy.

Effective Leadership and Participation in the ITER Research Program

ITER will provide the first laboratory for studying the behavior of a burning plasma, and as such will offer an unprecedented opportunity to U.S. fusion scientists to move into what we all acknowledge as the next frontier in our field. The U.S. fusion research community should be prepared to fully embrace that opportunity. Maintaining technical leadership is essential for that readiness, but it is just as important to optimize the organization of the ITER research program to facilitate that participation. This will eventually be decided in negotiation between the Domestic Agencies and the ITER Organization Central Team, but a U.S. Burning Plasma Organization Task Group has already prepared “Recommendations for ITER Experimental Operation, U.S. Team Formation and Participation”⁴ to help guide the process.

Maintaining Readiness to Move to Next Steps After ITER

The United States has not constructed a large facility since the 1990s, when Alcator C-Mod and NSTX came into operation. Several of our international partners in ITER are in the midst of a long period without a major domestic tokamak facility, and much of their expertise developed in the past has been lost through attrition. If the United States wishes to maintain scientific and technical leadership in this field, the nation needs to maintain the skills to design, construct, and operate a world-class fusion research facility. This potential for leadership should engage the participation of universities, national laboratories, and industry in the realization of commercial fusion power for the nation.

FUSION SCIENCE PREDICTIVE MODELING AND EXASCALE COMPUTING

Fundamental to this strategic plan is the continued role of theory, simulation, and computation in motivating innovative approaches to improving the prospects of fusion energy. This encompasses research in burning plasma physics, materials science, fusion nuclear science and engineering sciences. These efforts benefit tremendously from the availability of exascale computing capabilities to tackle complex problems and high-capacity computing for scoping studies, machine learning, and data analysis. Utilizing these tools, validation of important physics models should be a strong emphasis of the R&D program supporting this plan.

The creation of the DOE Exascale Computing Project⁵ in fiscal year 2017 has provided the impetus for the realization of a high-fidelity whole device model of fusion plasma applicable to a high-performance advanced tokamak regime, integrating the effects of turbulent transport, plasma collisions, and neutral particles,

energetic particles, plasma-material interactions, as well as heating and current drive. At the present time, the Exascale Computing Project is one of the largest projects in the DOE Office of Science.⁶ Whole device modeling holds the promise of being a powerful predictive tool for current and future fusion devices that will access hitherto unrealized plasma regimes, and has the potential to produce scientific discoveries of new and emergent behavior of burning plasmas that have been so far studied piecemeal. The project will be developed in a computational ecosystem that brings together plasma physicists with applied mathematicians, computer scientists, and other application scientists using a diverse range of software technologies and several co-design centers. The continued growth of exascale computing, which is supported by the DOE Advanced Scientific Computing Research program, is a valuable opportunity for U.S. fusion scientists and represents at the present time the largest project in the Office of Science.

PROMOTING DISCOVERY IN FUSION ENERGY SCIENCE AND ENGINEERING

To reduce risk and encourage discovery, the long-term research strategy should develop promising innovations in burning plasma science and fusion engineering science that can accelerate fusion development or improve and reduce the cost of fusion as a source of electricity. New insights and discoveries are expected to occur in all of the interconnected research in burning plasma science, materials science, fusion nuclear science, and engineering science. But the committee feels that research on less developed and therefore more speculative topics should continue to be a feature of the U.S. program. This might include some research previously provided by the Advanced Research Projects Agency-Energy (ARPA-E) ALPHA program⁷ that supports the national program leading toward a compact fusion pilot plant. An ongoing program promoting discovery in fusion energy science would be similar to the existing DOE Office of Fusion Energy Sciences (FES) program in Discovery Plasma Science, except any program in Discovery Fusion Science would need to be open to the possibility of intermediate scale research facilities involving multiple institutions. While some research would be supported by competitive peer review, the selection of intermediate scale facilities should follow a conceptual design effort and be guided by the evaluation of a national team of experts.

The 2018 DOE report of the FESAC Transformative Enabling Capabilities for Efficient Advance Toward Fusion Energy⁸ listed several revolutionary ideas that would “dramatically increase the rate of progress toward a fusion power plant.” These breakthroughs include substantial increase in fusion performance, simplification of fusion enabling technologies, reduction in fusion system cost or time to delivery, or improvements in reliability and safety. Example transformative enabling capabilities (TEC) include: (1) advanced algorithms, like machine learning

and integrated data analysis, to improve the methods to control a burning plasma and to facilitate predictive understanding from advances in exascale computing, (2) high critical temperature superconductors, (3) new material designs, advanced fabrication methods, and additive manufacturing offering the potential for local control of material structure, and (4) novel technologies for tritium fuel cycle control. Each of these TECs presents a tremendous opportunity to accelerate fusion science and technology toward power production. Dedicated investment in these TEC areas for fusion systems is needed to capitalize on the rapid advances being made for a variety of non-fusion applications so that their transformative potential for fusion energy is fully realized.

In addition to the Transformative Enabling Capabilities report, the committee was presented with two additional examples for promising discovery in fusion energy science. These are described below.

“Hidden Symmetry” versus “Axisymmetry”

Magnetic fusion energy requires a toroidal confinement region with strong magnetic field to contain the high-pressure plasma. As described in Chapter 2, the tokamak configuration is most successful and best understood configuration for magnetic fusion energy. ITER is also a tokamak as are the two major U.S. research facilities, DIII-D and NSTX-U. While tokamaks use relatively small non-symmetric magnetic fields, the other configuration, called the stellarator, is strongly non-symmetric by design. The two largest stellarators are the Japanese Large Helical Device and the German Wendelstein 7-X (W7-X) stellarator. Both of these experiments are built with superconducting magnets, and both are conducting successful ongoing investigations of the potential for uninterrupted magnetic confinement for fusion with a more complex, non-symmetric set of magnets. U.S. scientists are active participants in the German W7-X experiment.

The stellarator has benefited from advances in theoretical understanding leading to improved particle confinement at high plasma temperature. This improvement is achieved through careful optimization of the shape of the magnetic field. The present day focus of the U.S. stellarator program creates a “hidden symmetry” of magnetic field, called the quasi-symmetric approach. A stellarator with a “hidden symmetry” has favorable single particle orbits and pressure driven currents that are similar to axisymmetric tokamak configurations. Validating the effectiveness of these optimization approaches defines a major element of the existing experimental stellarator program. Presently, the world’s only stellarator with hidden symmetry is the Helically Symmetric Experiment (HSX) located at the University of Wisconsin. While the HSX program has definitively demonstrated neoclassical confinement of thermal electrons, the small size of the HSX device has prevented the investigation of thermal and energetic ion confinement and plasma flows in quasi-symmetric

magnetic systems. Because the behaviors of a plasma confined with “hidden symmetry” informs the predictive capability of all magnetic configurations, including the axisymmetric tokamak, a unique opportunity for discovery exists through exploration of a larger stellarator experiment where “hidden symmetry” can be evaluated at a device size sufficient to fully investigate confinement effectiveness and understand how plasma flow physics can positively impact turbulent transport, MHD physics, and impurity confinement.

Beam-Driven Plasma Neutron Source versus Fission-Based Neutron Source

A key facility needed to prepare for initial operation of a compact fusion pilot plant is a prototypic 14 MeV fusion neutron source capable of testing fusion blanket concepts, as well as to obtain the neutron-induced degradation data required for the initial stage of pilot plant licensing. The U.S. fusion nuclear engineering community has long advocated a dedicated facility with a reliable steady-state plasma source for this purpose. Concerns about the costs of fusion neutron sources need to be addressed in arriving at the most robust fusion neutron source for cost-effective testing. Various approaches for blanket and materials testing are characterized by the useful irradiation volume and the neutron energy spectrum. Research facility options include fast fission reactor test facilities, accelerator-based ion sources, or somewhat non-prototypic neutron spectra involving spallation neutron sources. A beam-driven plasma source may provide a lower cost approach while also providing a prototypic neutron spectrum. A beam-driven plasma source consists of a relatively small magnetic confinement device that is operated continuously by injecting energetic neutral beams of deuterium and tritium atoms that generate a fusion neutron spectrum for testing. A low-cost linear mirror device, called the gas dynamic trap, as well as beam-driven tokamak or alternate plasma-based, accelerator-based, or innovative fission reactor facilities, should be evaluated to determine which may provide an innovative solution to fusion’s nuclear testing needs.

Other opportunities for discovery in fusion energy science having high scientific merit and technical readiness should be promoted provided that they significantly advance burning plasma science, materials science, fusion nuclear science, or engineering science.

RESPONDING TO A U.S. DECISION TO WITHDRAW FROM THE ITER PROJECT

Chapter 3 describes the importance of burning plasma research, explains why continued participation as an ITER partner is important to U.S. fusion energy research, and describes how ITER participation will inform the design of a compact fusion pilot plant as a new element of the U.S. magnetic fusion program. The

benefits of continued U.S. participation in ITER are compelling for several reasons: (1) ITER is the only existing experiment with a mission to explore burning plasma physics at the power plant scale, (2) ITER research is currently a major focus of the U.S. fusion research program, (3) the development of national expertise in burning plasma science requires the hands-on participation by experts, (4) ITER construction is more than half complete, and the first plasma experiments are expected to begin in less than 10 years, and (5) as an ITER partner, the United States fully shares in the technology that will establish the feasibility of fusion while providing only a fraction of the costs. However, should the United States decide to end its participation in the ITER project, the need to address key burning plasma physics issues remains. Advancing toward a national fusion energy goal requires progress in all of the interconnected program elements needed for a low-cost demonstration of fusion electricity: burning plasma science, materials science, fusion nuclear science, and engineering science.

The committee’s response to the scenario in which the United States is not an ITER partner is based on the scenario in which the United States continues participation in the ITER project. Irrespective of whether the United States remains an ITER partner, the committee recommends the United States should start a national program of accompanying research and technology leading to the construction of a compact pilot plant at the lowest possible capital cost and the production of electricity from fusion. In this way, the committee’s long-term strategic guidance is generic and applies to both scenarios.

All previous strategic plans reviewed by the committee call for construction and operation of a burning plasma experiment and the demonstration of scientific and technical feasibility prior to construction of a facility capable of electricity production. This committee concurs with this assessment. A burning plasma experiment is a critical next step toward the realization of fusion energy, and the science and technology gained from a burning plasma experiment, like ITER, will answer key questions needed to design a compact pilot plant. The study, control, and manipulation of a burning plasma will give scientists their first opportunity to demonstrate many technical capabilities needed by an energy-producing magnetic fusion device. With access to a burning plasma experiment, scientists will have the means to answer fundamental questions pertaining to energetic alpha particles created by fusion reactions, plasma transport processes in fusion reactor conditions, methods to control of plasma transients, divertor science, and the integrated scenarios that simultaneously test the requirements for stability, confinement, fuel purity, and compatibility with plasma-facing components needed for a fusion energy source. If the United States wishes to take leadership in fusion energy development and pursue a program toward a compact pilot plant, national expertise in burning plasma science needs to be developed through hands-on operational participation and scientific study by U.S. fusion scientists.

For the scenario with the United States remaining an ITER partner, research toward the second goal of compact, attractive fusion power generation will build upon the ITER experience and focus on high power density plasmas, and the integration of core and edge physics in the regime required for a compact fusion pilot plant. However, if the United States were to withdraw from ITER, the United States would need to design and construct a larger and more ambitious research facility with a capability to explore burning plasma science with deuterium-tritium operation. The direct study of high-gain burning plasma physics and access to research opportunities necessary to evaluate long plasma duration and burning plasma control methods are central long-term goals of the U.S. program. As an alternative to ITER, the expanded fusion nuclear program for a high-power density burning plasma facility would be expensive for the United States to undertake without international support, and it would likely delay progress in the field. Such an expanded fusion research program, however, would be critical for directly addressing the physics of a strongly coupled burning plasma, and addressing the key challenges discussed above.

A decision by the United States to withdraw from ITER would make international collaboration far more difficult. Nevertheless, the United States would need to explore other avenues for collaboration and international cost-sharing, and important avenues for collaboration may still remain. In particular, the United States has already been engaged in the design of the China Fusion Energy Test Reactor, and if that collaboration could be maintained, it would provide valuable insight on fusion technology. However, such collaboration, particularly in the event of a U.S. withdrawal from ITER, would require the United States to have its own vibrant fusion program with value to offer to those collaborating with the United States.

Given the resources required, the primary initial focus of a U.S. program without ITER participation would still be a high-power density burning plasma tokamak. However, the mission for this new facility would need to be expanded to include study and control of self-heated burning plasma. Design studies for such an experimental device, building on results from new experiments on DIII-D and NSTX-U, state-of-the art theory and simulation, and possibly technology innovations, would need to begin as quickly as possible in response to a U.S. decision to withdraw from ITER. These design studies would need to include the licensing and facility requirements for safe handling of tritium. Once constructed, this new facility would give U.S. fusion scientists the necessary means to study burning plasma science and technology and to maintain progress toward the long-term development of commercial fusion power.

SUSTAINING THE NATIONAL PROGRAM

The demonstration of fusion electricity is a long-term undertaking, requiring progress at both national and international levels and sustained support. Besides

the scientific and technical challenges to continued progress, fusion energy research needs solid support from the broad scientific community and throughout the political system. The 2009 National Research Council report A Review of the DOE Plan for U.S. Fusion Community Participation in the ITER Program⁹ recommended that steps should be taken to “seek greater U.S. funding stability for the international ITER project to ensure that the United States remains able to influence the developing ITER research program, to capitalize on research at ITER to help achieve U.S. fusion energy goals, to participate in obtaining important scientific results on burning plasmas from ITER, and to be an effective participant in and beneficiary of future international scientific collaborations.”

The first half of the committee’s recommendations is continued participation in the ITER project as the nation’s primary experimental burning plasma component within a balanced long-term strategic plan for fusion. The committee concurs with the conclusion from the Secretary of Energy’s Report to Congress in May 2016, “ITER remains the best candidate today to demonstrate sustained burning plasma, which is a necessary precursor to demonstrating fusion energy power.”¹⁰ The studies carried out with ITER will inform accompanying research and technology programs needed to progress beyond ITER to a commercial fusion reactor.

The second half of the committee’s recommendations recognizes that if the United States is to profit from its share of the ITER investment, the nation’s strategic plan for fusion should combine its ITER experience with the additional research needed to realize fusion. In addition to burning plasma science, the interconnected research in materials science, fusion nuclear science, and engineering science should be expanded. Without this additional research, the United States risks being overtaken by other nations as they advance the science and technology required to deliver a new and important source of energy.

BUDGET IMPLICATIONS

The committee was asked to consider the budget implications of its guidance. Estimates were examined for the cost and schedule for the two main research activities: (1) construction and operation of the ITER burning plasma experiment and (2) a national program of accompanying R&D leading to the construction of a compact fusion pilot plant. The committee also examined the schedule and budget implications of a decision by the United States to withdraw from the ITER project. Because the committee’s long-term strategic guidance covered the next several decades, all cost and schedule estimates are necessarily approximate. Implementation of the committee’s strategic guidance will require significant planning and thought by the fusion research community, involvement with international partners, and oversight by DOE. Additionally, because the committee’s strategic plan involves research and technology development over several decades, the impact of

unanticipated discoveries, breakthroughs, or technical setbacks that would influence the schedule and cost of the strategic plan could not be determined.

With the baseline cost and schedule for U.S. contributions to ITER’s first plasma subproject now formalized, the committee’s recommendations imply a sustained national funding for more than two decades at a level that is about $200 million higher than the presently enacted funding levels. About half of this additional amount is required to meet ITER commitments and the other half is needed to launch the science and technology supporting the research leading to a compact fusion pilot plant. Appendix H summarizes the input used by the committee in its considerations of the budget implications of its recommended strategic plan for U.S. burning plasma research.

Based on information received, including the Updated Long-Term Schedule for ITER, the 2016 DOE Report to Congress,¹¹ and the 2017 DOE Project Execution Plan for U.S. ITER Subproject-1,¹² continued U.S. participation in the ITER project requires additional annual funding near $100 million, representing half of the required incremental funding.

The start of a national program of accompanying science and technology leading a compact pilot plant at the lowest possible capital cost will also require additional funds of $100 million annually, representing the other half of the required incremental funding. This estimate is based on recent reports of the DOE FESAC, including the 2014 Report of the Subcommittee for Priorities Assessment and Budget Scenarios,¹³ the 2013 Report of the Subcommittee on the Prioritization of Proposed Scientific User Facilities for the Office of Science,¹⁴ and the 2013 Report of the Subcommittee on the Priorities of the Magnetic Fusion Energy Science Program.¹⁵ All of these reports recommended additional funding to address the full range of scientific, technical, and engineering challenges for fusion energy. They also recommended an evolution of the research portfolio as existing research facilities are phased out and new ones are implemented.

Programmatic decisions for new facilities and programs mentioned in this report should be based on cost-benefit analysis and technical input from the U.S. fusion research community as recommended in Chapter 6. If the required engineering and science studies begin soon, new world-class research facilities could be available to enable resolution of the critical issues needed to finalize the design and begin construction of a compact fusion pilot plant having the lowest possible capital cost. Such a program with a cost-attractive goal for the demonstration of fusion electricity would also provide important scientific opportunities for U.S. researchers and U.S. industry and deliver technical know-how to the nation’s effort to provide abundant fusion power.

In summary, based on information received and described in Appendix H, including the updated long-term schedule for ITER participation and previous strategic planning efforts of the DOE FESAC, the committee expects the implemen-

tation of its recommendations, including both continued participation in ITER and the start of a national research program for a compact pilot plant, to require an annual funding level about $200 million larger than presently enacted levels, about half required to meet ITER commitments and half needed to launch the science and technology supporting the research leading to a compact fusion pilot plant. This funding would need to be sustained for several decades.

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