Stellarators as fusion-fission reactor candidates

Transcription

1 Stellarators as fusion-fission reactor candidates Jeffrey H. Harris Donald A. Spong Fusion Energy Division Oak Ridge National Laboratory Managed by UT-Battelle for the Department of Energy Third Fusion-Fission Hybrids Workshop College Park, Maryland 9-11 March 2009

2 Topics Characteristics of stellarators What we (really) know about stellarator performance Key issues for stellarator development for fusion Use a stellarator as driver in a fusion-fission hybrid? 2

3 Distinguishing characteristics of stellarators Flux surfaces & rotational transform ( ) from external windings Non-axisymmetric magnetic configuration Great design flexibility Avoid (or minimize need) for plasma current Steady-state without disruptions 3 Potential for real fusion ignition (P ext ~ 0) Torsatron/ Heliotron Broad and deep experience base 1970-present: 30 stellarator devices successfully built & operated in 7 countries R: 0.12 m 3.9 m B: 0.05 T 3 T (including superconducting) P: MW 18 MW Harder to build than tokamaks... but easier to operate 3-D construction with challenging coil shapes Accuracy in construction: ~1 part in (resonant components) Improved construction strategies are developing: metrology, etc. Advanced Stellarator

4 Stellarators from large to small have been succesfully constructed 4 LHD, Japan; 1997-present R = 3.9 m, a = 0.6 m, B = 4 T superconducting coils 1500 tonnes UST-1, Spain; 2006-present R = 0.12 m, a = 0.02 m B = 0.1 T; auto batteries V. M. Queral; self funded Capital cost ~ $5000

5 Stellarators are achieving outstanding results Quiescent high beta plasmas 5 Limited by heating power & confinement LHD = 5.2% transiently; 4.8% sustained W7AS > 3.2% for 120 E E similar to ELMy H-mode ELMs occur in narrow parameter ranges Improved confinement with orbit optimization W7AS, HSX High density operation Limited only by heating power and magnetic field Up to 3 equivalent Greenwald density (W7AS) LHD n e (0) ~ m -3 at B=2.7T with pellets 3-D divertor controls recycling, excludes impurities Steady-state operation LHD ~0.7 MW pulse lengths ~1 hr (utility limit)

6 Useful stellarator performance predictor tools developed Energy confinement: International Stellarator Scaling 2004 [1] ISS E / fren a R P ne B 2/3 [s, m, m, MW, m -3 ] f ren = configuration factor ~ 1; correlates with degree of orbit optimization Maximum density limited by radiative collapse (power/volume) [2-4]: n c 0.48 P 14.6 B V p 0.54 Large stellarators easily attain n > m -3 ; LHD has reached n(0) > m -3 ; Reduces slowing down time & instability drive; Reduces wall damage from escaping s Maximum normalized pressure [5-6]: 5% [10 19 m -3, MW, m -3 ] Easily stated, but immensely important. 6 References [...] on the last slide.

7 Configuration optimization is major goal of stellarator program Goal: make stellarator fusion reactor smaller. (Would also help hybrid). Reduce effective helical ripple to improve orbits of thermal & particles US: quasi-symmetry (helical, axi- (toroidal), or poloidal) Other goals/trade-offs include tailoring of bootstrap current, flows, etc. New configurations developed numerically: W7X, HSX, NCSX, QPS... HSX (U. Wisconsin) already showing improved confinement in exp ts Helically Symmetric EXperiment 7 quasi-helical symmetry

8 Ref. baseline parameters: NCSX-like (QA): 3 periods R = 7.75 m R / a ~ 4.5 a = 1.72 m n = 4.0 x m 3 T = 6.6 kev B axis = 5.7 T = 6.4% H(ISS04) = 1.1 I plasma = 3.5 MA (bootstrap) 25% of rotational transform P(fusion) = GW P(electric) = 1 GW Fully ignited (P ext = 0) 8 US compact stellarator research program is developing basis for attractive reactor concepts, e.g. ARIES-CS Aries- Blanket COE(92) -I RS CS LiPb/FS AT LiPb/SiC 47.5 alpha loss 5% divertor heat load ~ 5-18 MW/m 2 -CS LiPb/SiC (core radiation fraction ~75% as in ARIES tokamaks) 48.

9 Need to integrate stellarator performance & alpha physics into fusion-fission hybrid scenario fraction slowing-down time power 9

10 Stellarator optimization path will take time to follow Modular advanced stellarators have a large number of complex parts that must be assembled to high accuracy. Complex projects do not play well with risk-averse funding agencies. Start of W7X (R = 5.5 m) delayed to 2014; NCSX and QPS canceled. Older designs simpler, if not optimal for pure fusion. Torsatron/ Heliotron Advanced Stellarator Could a less-optimized stellarator drive a steady-state hybrid while we sort out the more complex designs for fusion use? 10

11 If there is a real rush... we have a lot of experience with LHD-like configurations LHD (NIFS) ATF (ORNL) CHS (NIFS) 11 Heliotron-E (Kyoto Univ.)

12 Conclusions Characteristics of stellarators Gains may outweigh complexity to yield an elegant fusion reactor. What we (really) know about stellarator performance Good database from which to extrapolate, and tools to do it with. Key issues for stellarator development for fusion How well does configuration optimization work in practice? Must make construction more predictable. Divertor and wall. Use a stellarator as driver in a fusion-fission hybrid? Demonstrated stable operation at high density is key. Reculer pour mieux sauter: Use less optimized configuration, not most advanced? 12

13 Stellarator Performance Estimator Toolkit [1] H. Yamada, J. H. Harris, A. Dinklage et al, Nucl. Fusion 45 (2005) [2] L Giannone, J Baldzuhn, R Burhenn, et al, Plasma Phys. Control. Fusion 42 (2000) 603. [3] M. Greenwald, Plasma Phys. Control. Fusion 44 (2002) R27. [4] K. Itoh, S-I. Itoh, L. Giannone, J. Phys. Soc. Japan 70 (2001) [5] S Sakakibara, K Y Watanabe, Y Suzuki, et al, Plasma Phys. Control. Fusion 50 (2008) [6] A Weller, K Y Watanabe, S Sakakibara, et al International Stellarator/Heliotron Database Activities on High-Beta Confinement and Operational Boundaries, submitted to Nucl. Fusion (2009). 13