1. Compact Stellarators as Reactors J. F. Lyon, ORNL NCSX PAC meeting June 4, 1999

2. TOPICS Earlier Stellarator Reactor Studies Comparison with ARIES Reactors Extrapolation of QA to Reactor

3. Compact Stellarators Have the Potential for an Attractive Reactor Steady-state operation without external current drive Disruption immunity at the highest plasma parameters Stability against external kinks and vertical instability without a close conducting wall or active feedback systems Reduced size for higher wall loading Reduced size for higher wall loading

4. Compact, power density similar to tokamaks Without disruptions, feedback, or external current drive

5. HSR Reactor Based on Wendelstein 7-X R = 22 m, B 0 = 5 T, B max = 10 T (NbTi SC coils) = 5%; based on conservative physics, technology

6. SPPS based on W7-X like configuration but aimed at smaller size R 0 = 13.9 m, = 5% B 0 = 4.9 T, B max = 16 T P electric = 1 GW 1993-95 ARIES Stellarator Power Plant Study plasma surface modular coils blanket and shield

7. ARIES SPPS Study Developed a Feasible Maintenance Approach Fusion Power Core

8. Most Important Measure of Reactor Attractiveness is COE R 0, p wall not the most important measures! Higher value of Q eng compensates for R 0, p wall

9. Minimum Reactor Size Is Determined by  A configuration is chacterized by the ratios A  = R 0 / , A p = R 0 /, and B max /B 0 The minimum reactor size is set by R 0 = A  (D + ct/2) where D is the space needed for scrapeoff, first wall, blanket, shield, coil case, and assembly gaps B 0 is set by (16 T)/(B max /B 0 )

10. Extrapolation of Compact Stellarators to a Reactor Extrapolation of Compact Stellarators to a Reactor Vary distance  for compact stellarator configurations – calculate sheet-current solution at distance  from plasma that recreates desired plasma boundary – calculate B max /B 0 at distance ct/2 radially in from current sheet  Choose maximum credible distance   R 0 = A  (D + ct/2) R 0 3  P fusion /B 0 4, so want high B 0 for smaller reactor; however – B 0 decreases with increasing  (B max /B 0 increases) – Coil complexity (kinks) increases with increasing  Choose minimum ct/2 that satisfies two constraints – Ampere’s law: B 0 = 2  0 Njct 2 /(2  R 0 ); coil aspect ratio = 2 assumed – B 0 = (16 T)/(B max /B 0 ); B max /B 0 increases as ct decreases B max /B 0 is larger for actual modular coils, so use 1.15B max /B 0 Need to redo in future for real modular coils

11. Minimum Credible A  Value for C82 is 5.8  Minimum R 0 ­ 9.3 m 9.67  15.3 m7.25  11.6 m 5.8  9.3 m 4.83  7.7 m nonplanar coil contour poloidal angle toroidal angle

12. B max /B o Scan for Reactor-Scale QA’s B max /B o calculated at coil inner edge (on a surface shifted inward by half coil depth) from Nescoil surface current solution at coil center P. Valanju

13. Coil Half-Depth Is Chosen to Minimize R 0 R 0 /  = 5.8 case, 21 coils, 2:1 coil aspect ratio; B max = 16 T Based on surface current distribution, not modular coils j = 3 kA/cm 2 C82 A = 4.1 C93 based on 1.15B max /B 0 OperatingPoint

14. QA Reactors Are Closer to ARIES-RS than SPPS = 5%, H’ = 0.64; not optimized yet for a reactor       ================ 

15. C82-Based Reactors Are Sensitive to Plasma-Coil Spacing ARIES study is needed to determine realistic plasma-coil spacing and estimated COE

16. Compact Stellarators Could Lead to a Better Reactor Compact Stellarators Could Lead to a Better Reactor 14-m SPPS (with lower wall power density) was competitive with 6-m ARIES-IV and 5.5-m ARIES-RS because of its low recycled power (high Q eng ) C82 can retain low recycled power of SPPS, but has smaller size (lower cost) and higher wall power density However, the power produced is more than the 1 GW e assumed in the ARIES studies (  margin) The details of size, field, and wall power density need to be studied further to optimize a reactor by the ARIES group, as was done for SPPS

17. Cost of Electricity Decreases with Plant Size

18. Plans for Reactor Scoping Studies Plans for Reactor Scoping Studies Optimize B max /B 0 vs  for – QA sheet-current configurations with A p = 3.4 and 4.1 Simple 0-D spread sheet reactor optimization – include variation of B max /B 0 vs  and reactor physics Full systems code reactor optimization (OPTOR) (minimize COE: ARIES algorithms, benchmark with ARIES-RS) – simple 0-D transport models – solve for T e (r) and T i (r) for 1-D anomalous  e,i and electric- field-dependent  e,i with fixed n(r),  (r): Shaing, Mynick – Self-consistent solution for T e (r),T i (r), n e (r), n i (r), and  (r) with fixed particle source (pellets or gas) – study sensitivity to transport models and energetic losses ARIES group look at impact of key issues, COE

19. CONCLUSIONS QA Compact Stellarators lead to more attractive reactors, but not smaller reactors Ultimate figure of merit for a toroidal reactor is the cost of electricity, not major radius or wall power density, when comparing different concepts However, major radius and wall power density are important when optimizing a particular concept R 0 / = 3.4 and 4.1 QA configurations lead to smaller reactors closer to ARIES-RS than the earlier competitive SPPS QA configurations so far have not been optimized for a reactor; need to reduce A  further ARIES study will be needed for better optimization