Review of ST Panel Activity - US DoE Fusion Research Needs Workshop

1. Office of Science Supported by Review of ST Panel Activity - US DoE Fusion Research Needs Workshop S.A. Sabbagh1, N. Gorelenkov2, C.C. Hegna3, M. Kotschenreuther4, D. Majeski2, J.E. Menard2, Y.-K. M. Peng5, A.C. Sontag5, V. Soukhanovskii6, D. Stutman7 1Department of Applied Physics and Applied Mathematics, Columbia University 2Princeton Plasma Physics Laboratory 3University of Wisconsin, Madison 4University of Texas, Austin 5Oak Ridge National Laboratory 5Lawrence Livermore National Laboratory 5Johns Hopkins University International Spherical Torus Workshop October 22 - 24, 2009 UW Madison, Madison, WI V1.2

2. Review of ReNeW ST Panel Activity - Outline • ReNeW Purpose and Product • ReNeW Structure in Brief • ReNeW ST Panel and approach • ReNeW Report • ST Research Theme Chapter • ST Research “Thrust” • Comments re: guiding ST research forward

3. DOE ReNeW process to aid in defining the “ITER-era” (20 yr) research program • ReNeW: Research Needs Workshop • To inform the Office of Fusion Energy Sciences (OFES) in preparing a strategic plan for research in each major area of the Fusion Energy Sciences Program • To allow U.S. fusion community to explain research goals, methods to achieve them • Including communication to new administration • Six month process (January – June 2009) • Document (419 pages) http://burningplasma.org/renew.html • Part 1: Defines scientific research issues and requirements needed to fill “gaps” in present understanding • Guidance from Greenwald report, FESAC TAP and EPA reports • Divided into 5 “themes” comprising magnetic fusion research • Part 2: Defines 18 “research thrusts” to carry out this research • Basis for detailed program plan to be constructed by OFES

4. ReNeW organized into 5 fusion research themes Spherical Torus sub-theme Structure/gaps FESAC Toroidal Alternates Panel Report Structure/gaps Energy Policy Act task group report Structure/gaps Priorities, Gaps, and Opportunities Panel Report (“Greenwald Report”) Reports available at: http://burningplasma.org/renew.html

5. The ReNeW Spherical Torus Panel had broad expertise and institutional base • In addition, more than 30 advisors agreed to help • Special thanks to Brian Lloyd, Ray Fonck, Dick Majeski (as Thrust 16 coordinator) • Special thanks to Theme 5 ExCom leaders J. Sarff, M. Zarnstorff, S. Barish ReNeW ST Panel membership

6. ReNeW tasks and ST panel approach to them had many positive elements • Communication • Numerous conference calls, individual calls, small meetings, two large ReNeW group meetings over a 6 month process • Community outreach • ST panel was prolific in extending drafts to the community; posting to ReNeW web page • Open process • All were invited to participate • Identification of cross-cutting research • ST topic perhaps best situated

7. ReNeW theme chapter addresses ST research goals • ReNeW ST panel largely adheres to TAP ITER-Era Goal • Some important clarifications of emphasis, and alterations • “properly informing the design of a demonstration fusion power plant requires that research extend past the needs of an ST fusion nuclear science component testing facility” • “Important that research examine a high level of plasma control flexibility and performance beyond baseline ST-CTF design needs to minimize performance risk for ST-CTF” • ST Panel further suggests that during the ITER-era: “research aggressively pursue improvements to the ST concept that advance an ST-based DEMO” Goal: “Establish the ST knowledge base to be ready to construct a low aspect-ratio fusion component testing facility that provides high heat flux, neutron flux, and duty factor needed to inform the design of a demonstration fusion power plant.”

8. ReNeW theme chapter refocuses TAP research needs • Physics of twelve TAP report critical research issues retained • Issues re-grouped and slightly redefined • For more logical arrangement of topics; smaller number of areas (12  6) • To better emphasize required research in all areas; avoid a perception that one can choose one or two areas out of 12 • To align with the proposed actions in Research Thrust 16 • Six topical research areas (focus on ST specific components) • Plasma initiation and ramp-up (difficulties due to compact ST geometry) • Plasma-material interface (uniquely high ST heat loads) • Electron energy, ion scale transport (low A differences; EM turbulence) • Stability and steady-state control (unique low li, high bN regime) • Technological development (unique ST challenges, eg. magnets) • Integration at high b(self-consistent, low A physics: ST-CTF, Hybrid, DEMO)

9. Sub-division of ST panel research requirements • Further detail of research needs comprises the majority of the ST section • See full document • This talk: research needs will be addressed in the Thrust 16 actions

10. ST available means for research - general timeline • Address research needs in existing international devices when possible • Upgrades to existing devices to fill “near-term” research • ST physics at long pulse/high field: Open question • does this step require a new device? • what capabilities would such a device need to have? • Subject of extensive discussion at June meeting • Guidance / decision to keep timeline general and qualitative

11. ST research requirements summarized in one page • Gives some quantitative guidance on ST research next steps

12. ReNeW Thrust 16 suggests ST research actions to address research needs • Thrust 16: “Develop the spherical torus to advance fusion nuclear science” • Elements of thrust aim to: • Leverage extensive knowledge-base of higher-A tokamak • Extend understanding to unique ST high-b + low n operation regime • Advance ST as reduced size and cost configuration for fusion

13. Proposed actions for Thrust 16: Develop the ST to advance fusion nuclear science • Exploit and understand magnetic turbulence, electromagnetic waves, and energetic particles for megampere plasma current formation and ramp-up. • Develop innovative magnetic geometries and first-wall solutions such as liquid metals to accommodate multi-megawatt per square meter heat loads. • Utilize upgraded facilities to increase plasma temperature and magnetic field to test the understanding of ST confinement and stability at fusion-relevant parameters. • Implement and understand active and passive control techniques to enable long-pulse disruption-free operation in plasmas with very broad current profiles. • Employ energetic particle beams, plasma waves, particle control, and core fueling techniques to maintain the current and control the plasma profiles. • Develop normally conducting radiation-tolerant magnets for low-A applications. • Extend the ST to near-burning plasma conditions in a new or further upgraded device.

14. (16.1) Develop plasma startup and ramp-up with no, or very low transformer flux • Develop reliable plasma initiation and growth schemes • Establish science basis for helicity injection • Current limiting mechanisms, resulting plasma characteristics • Implement EBW/ECH/ECCD coupling to startup plasma • Demonstrate ramp-up to full current • Effective coupling to RF/NBI • Develop predictive modeling capability • Validate transport-based CD modeling against ST database • Effects of fast-particle transport on NBCD • Assess feasibility of central induction • Mineral insulated conductor • Retractable solenoid technology • Modeling & engineering assessment of iron-core • Demonstrate integrated test at ST-CTF relevant level of performance • Similar transport regime, magnetic field, Ip, etc. Non-solenoidal startup with HI and PF induction

15. (16.2.1) Demonstrate and understand a viable plasma-material interface at high heat flux and low density Compact geometry drives need for high power handling PFCs • Develop divertor and wall power reductionand handling solutions • Innovative divertors, flux expansion, stochastic edge • Divertor targets: liquid metals, moving “pebbles” as PFCs • Develop edge transport and turbulence models for predictive capability • Reduce divertor heat flux from 20-60 MW/m2 to <10 MW/m2 • Transient loads to < 0.5 MJ/m2 • Nuclear environment, long pulse • Develop particle control for continuous, low density H-mode operation • Cryo-pumping and/or liquid metals for impurity, helium, density control • Achieve steady-state ne/nG ~ 0.2-0.3 • Demonstrate integration with high-performance pedestal and core high-b plasma in upgraded and scaled ST configurations  ST presents the opportunity to push to higher neutron wall loading  Common to other compact alternates (spheromak, FRC, RFP)

16. 150 +20% With Lithium 100 Without Lithium +44% Welectron (kJ) We (kJ) Deuterium BT = 0.45 T Ip = 0.9 MA PNBI = 4.0 ± 0.2 MW 50 Average  std. dev. 0 0 100 200 300 Total stored energy (WMHD, kJ) WMHD<EFIT> (kJ) (16.2.2) Deploy liquid metal, especially lithium, PFCs in the ST • Implement full liquid lithium wall in an ST, with NB core fueling • Does performance continuously improve in the ST as recycling is reduced? • Implement a full liquid lithium divertor in an ST, with core fueling • Is a low recycling divertor sufficient? • Characterize PMI, transport in a low recycling ST Integration of PMI, confinement in the ST Electron confinement improves with reduced recycling - NSTX • Validate models for the core, edge, and PMI, for arbitrary global recycling • Determine optimum global recycling coefficient for the ST • Construct an “optimized”, reduced recycling liquid metal-walled DD ST  ST provides rapid, cost-effective test of flowing liquid metal PFCs discussed in Thrust 11

17. (16.3) Understand electron and fast/thermal ion confinement in high, lowST plasmas BT scaling of E in NSTX • Does stronger BTT,  scaling of E in present STs lead to improved fusion performance at low-A? • Study transport in upgraded (x2) and new (x4) STs (near term focus: electrons) • Measure high and low-k EM turbulence (n.t. focus: ETG ‘streamers’+ -tearing islands) • Develop predictive understanding (models+control) • Will large population of super-Alfvenic energetic particles affect heating/current drive in a ST reactor? • Study EP transport over extended range of * (n.t. focus: intermediate *) • Study EP interaction with background plasma via AEs (n.t. focus: ion heating/alpha channeling, electron transport) ~BT0.91 Modeling of Alfven eigenmode - induced electron transport

18. (16.4) Understand stability and develop control of low li, high b ST plasmas • Achieve, understand global mode stability near low li current-driven kink limit, over full range of bN, with controlled broad pressure, Vf profiles • Steady-state current profiles; near-burning plasma conditions • Explore/validate key stability science for low A at reduced collisionality (~order of magnitude; near burning plasma levels), at increased field • Extend capability to alter Vf and shear (e.g. expanded NBI, 3-D fields) • Understand stability at intermediate Vf; effects of Vf shear on NTMs • Characterize disruptions at low A and li • Demonstrate/study high bN with minimal disruptivity or transients using multiple controls - compatible with a high neutron fluence environment • Active and passive RWM and ELM control,q0 > 2 for NTM control  Operate continuous high N beyond CTF (N > 5.8), approaching ST-DEMO levels (N > 7), with flexible controls

19. (16.5) Employ beams, plasma waves, particle control, core fueling to maintain the current and control plasma profiles • Reduce collisionality 1 - 2 orders of magnitude at high beta • Toroidal field, current, heating power increase by ~ 2 -4 • Pumping to reduce recycling, density, collisionality  Assess impact on ST transport and stability • 100% sustained non-inductive current-drive, at  50% bootstrap • Non-inductive ramp-up to MA, or multi-MA, level in the ST  Control of core safety factor profile; optimize stability, confinement • Mitigate steady-state, high heat and particle exhaust in the ST • Increase plasma pulse length by 1 -3 orders of magnitude • Control fully-non-inductive ST for many current relaxation times • Develop disruption avoidance, mitigation for integrated ST conditions  Sustained high performance with equilibrated first-wall conditions

20. (16.6) Develop normally-conducting radiation-tolerant magnets • Design, evaluate single-turn centerpost magnet • Evaluate candidate multi-turn designs • Develop matching low impedance power supplies • Construct, test candidate TF magnet + PS • High field (2-3T); ~10 sec. pulse • Design, test radiation-tolerant OH solenoids • Backup for non-inductive approach • Implement a testing program for radiation tolerance • DPA limits, tritium migration, joints, electrical, mechanical degradation Thrust element can provide the core of a high field, moderate pulse-length ST ST-CTF Example Rotor Liquid Metal

21. (16.7) Extend ST performance to near-burning-plasma conditions • Potential need for new (large) device(s) • Device upgrades advance knowledge base for ST fusion nuclear science applications • Factor of 5- 10 reduction in collisionality, increase in pulse duration by doubling of the field, current, and heating and current drive power in upgraded ST facilities • Depending on upgrade results and worldwide tokamak program, a new ST device with increased performance and capabilities may be needed • Demonstrate and understand physics insufficiently addressed at that point • Configuration and control flexibility to reduce risk before moving to goal device • Actions • Reduce collisionality to near-burning-plasma conditions to assess • Current drive for ramp-up / sustainment at high current, core and pedestal transport, MHD stability, PMI solutions. • Operate a high-performance ST for very long pulse lengths with actuators relevant to a high neutron fluence environment to assess: • Sustained plasma current drive and profile control, reliable disruption prediction, avoidance, and mitigation for integrated ST conditions • Compatibility of sustained high-performance with high power and particle exhaust mitigation techniques, with equilibrated divertor and first-wall conditions and low hydrogenic retention. • Test high-field, long pulse magnets under conditions directly relevant to ST applications

22. Some suggestions to guide ST research forward in the next several years • Quantification of ReNeW goals • Advantage: ST milestone of an ST fusion nuclear science facility (CTF) is arguably closer than “standard” tokamak goal of DEMO • ReNeW process didn’t aim at detailed quantification of physics goals • The STCC is suggesting quantification of physics goals as next step • Creation/adoption of design for next-step device(s) • Would have been useful many times during ReNeW • New assessments of ST for fusion burn applications • ARIES-ST study somewhat outdated • Re-assessment of the relative cost of low vs. high aspect ratio device • Increase ST constituency • Entire process should be open, engaging innovative physics / ideas