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Compact Stellarators as Reactors

Compact Stellarators as Reactors

J. F. Lyon, ORNL

NCSX PAC meeting

June 4, 1999

- Earlier Stellarator Reactor Studies
- Comparison with ARIES Reactors
- Extrapolation of QA to 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

• Compact, power density similar to tokamaks

• Without disruptions, feedback, or external current drive

- R = 22 m, B0= 5 T, Bmax= 10 T (NbTi SC coils)
- <> = 5%; based on conservative physics, technology

1993-95 ARIES Stellarator Power Plant Study

•SPPS based on W7-X

like configuration but

aimed at smaller size

•R0 = 13.9 m, <> = 5%

•B0 = 4.9 T, Bmax = 16 T

•Pelectric = 1 GW

modular

coils

blanket

and shield

plasma surface

ARIES SPPS Study Developed

a Feasible Maintenance Approach

Fusion Power Core

- R0, pwall not the most important measures!
- Higher value of Qeng compensates for R0, pwall

- A configuration is chacterized by the ratios A = R0/, Ap = R0/<a>, and Bmax/B0
- The minimum reactor size is set by R0 = A(D + ct/2) where D is the space needed for scrapeoff, first wall, blanket, shield, coil case, and assembly gaps
- B0 is set by (16 T)/(Bmax/B0)

- Vary distance for compact stellarator configurations
- calculate sheet-current solution at distance from plasma that recreates desired plasma boundary
- calculate Bmax/B0 at distance ct/2 radially in from current sheet

- Choose maximum credible distance R0 = A(D + ct/2)
- R03Pfusion/B04, so want high B0 for smaller reactor; however
- B0decreases with increasing (Bmax/B0 increases)
- Coil complexity (kinks) increases with increasing

- Choose minimum ct/2 that satisfies two constraints
- Ampere’s law: B0 = 20Njct2/(2R0); coil aspect ratio = 2 assumed
- B0 = (16 T)/(Bmax/B0); Bmax/B0 increases as ct decreases

- Bmax/B0 is larger for actual modular coils, so use 1.15Bmax/B0
- Need to redo in future for real modular coils

nonplanar

coil contour

poloidal angle

9.67 15.3 m

7.25 11.6 m

5.8 9.3 m

4.83 7.7 m

toroidal angle

Bmax/Bo Scan for Reactor-Scale QA’s

Bmax/Bo calculated at coil inner edge

(on a surface shifted inward by half coil depth)

from Nescoil surface current solution at coil center

P. Valanju

C93

A = 4.1

C82

Operating

Point

- R0/ = 5.8 case, 21 coils, 2:1 coil aspect ratio; Bmax = 16 T
- Based on surface current distribution, not modular coils

j = 3 kA/cm2

based on

1.15Bmax/B0

================

- <> = 5%, H’ = 0.64; not optimized yet for a reactor

- ARIES study is needed to determine realistic plasma-coil spacing and estimated COE

- 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 Qeng)
- 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 GWe 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

- Optimize Bmax/B0 vs for
- QA sheet-current configurations with Ap = 3.4 and 4.1

- Simple 0-D spread sheet reactor optimization
- include variation of Bmax/B0 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 Te(r) and Ti(r) for 1-D anomalous e,i and electric-field-dependent e,i with fixed n(r),(r): Shaing, Mynick
- Self-consistent solution for Te(r),Ti(r), ne(r), ni(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

- 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
- R0/<a> = 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