Overview of Results in the MST Reversed-Field Pinch

1. Overview of Results in theMST Reversed-Field Pinch D. J. Den Hartog University of Wisconsin – Madison Center for Magnetic Self-Organization 20 October 2006

2. Four major challenges for the RFP on the path to fusion: • Confinement • Beta • Current sustainment • Resistive wall instabilities

3. MST has recently advanced the RFP concept in three of these areas: • Confinement • good confinement of both thermal and large-orbit ions • Beta • total beta (tot) nearly doubles to 26% with pellet injection • Current sustainment • current drive of 10% of Ip by oscillating field current drive • Resistive wall instabilities • outstanding advances by RFX (Italy) and T2 (Sweden)

4. Contributors to this work: J-W. Ahn 1), A. F. Almagri 1), J. K. Anderson 1), A. D. Beklemishev 2), A. P. Blair 1), F. Bonomo 3), M. T. Borchardt 1), D. L. Brower 4), D. R. Burke 1), M. Cengher 1), B. E. Chapman 1), S. Choi 1), D. J. Clayton 1), W. A. Cox 1), S. K. Combs 5), D. Craig 1), H. D. Cummings 1), V. I. Davydenko 2), D. R. Demers 6), B. H. Deng 4), W. X. Ding 4), F. Ebrahimi 1), D. A. Ennis 1), G. Fiksel 1), C. Foust 5), C. B. Forest 1), P. Franz 3), L. Frassinetti 3), S. Gangadhara 1), J. A. Goetz 1), R. W. Harvey 1), D. J. Holly 1), B. F. Hudson 1), A. A. Ivanov 2), M. C. Kaufman 1), A. V. Kuritsyn 1), A. A. Lizunov 2), T. W. Lovell 1), R. M. Magee 1), L. Marrelli 3), P. Martin 3), K. J. McCollam 1), M. C. Miller 1), V. V. Mirnov 1), P. D. Nonn 1), R. O’Connell 1), S. P. Oliva 1), P. Piovesan 3), S. C. Prager 1), I. Predebon 3), J. A. Reusch 1), J. S. Sarff 1), V. A. Svidzinski 1), T. D. Tharp 1), M. A. Thomas 1), Yu. A. Tsidulko 2), M. D. Wyman 1), T. Yates 4) 1) The University of Wisconsin–Madison, Madison, Wisconsin, and the Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas 2) Budker Institute of Nuclear Physics, Novosibirsk, Russia 3) Consorzio RFX, Associazione EURATOM-ENEA sulla Fusione, Padova, Italy 4) The University of California at Los Angeles, Los Angeles, California 5) The Oak Ridge National Laboratory, Oak Ridge, Tennessee 6) Rensselaer Polytechnic Institute, Troy, New York This work is supported by the U. S. Department of Energy and the National Science Foundation.

5. Outline • Brief introduction to the Reversed-Field Pinch and MST • RFP advancement • thermal and fast ion confinement • high ne and  from pellet injection • oscillating field current drive • Summary

6. The RFP: toroidal confinement at low |B|. • Toroidal field BT is ~10X smaller in RFP than same-current tokamak • BT ≈ BP with high total  (typically ≥10%) • low BT at edge  high engineering beta (reduced coil stress)

7. MST is an ohmic RFP operated at moderate current. R = 1.5 m, a = 0.5 m Ip ≤ 600 kA, |B| ≤ 0.6 T ne≤ 4  1019 m-3; Te, Ti ≤ 2 keV

8. Current profile control enables MST to reduce stochastic transport and achieve tokamak-like confinement. • Standard RFP • substantial MHD tearing mode activity • stochastic transport, E~ 1 ms • Improved-confinement RFP • apply inductive current profile control  reduce MHD activity • reduced stochasticity, x10 confinement improvement, E~ 10 ms • limitation: inductive technique is transient • Previous work concentrated on electron confinement in improved confinement RFP • New results: • confirm that ions are also well-confined • high  and improved confinement can be achieved simultaneously

9. Confinement:thermal ions

10. Ion heating (“self-heating”) occurs in ~0.1 ms throughout the plasma during a global reconnection event. • RFP reconnection event has some similarities to tokamak sawtooth • Ti normally drops rapidly following a reconnection event Ti versus time Ti versus radius r/a = 0.37 after (+0.15 ms) Ti (keV) Ti (keV) before (-0.3 ms)

11. Ti is sustained ≥ 1 keV during improved confinement period. • Ion energy is retained by initiating improved confinement following a reconnection event reconnection events 20 improved confinement   15 ~ B (gauss) 10 5 0 3.0 2.0 Ti (keV) 1.0 0 10 15 20 25 time (ms)

12. Ti is sustained at a high level throughout the plasma during the improved confinement period. • Both impurity and majority Ti are ≥ 1 keV during improved confinement • Impurity C+6 measured with CHERS (shown above) • Majority D measured with Rutherford scattering Ti (eV) E,i ≈ 10 ms r/a

13. Electron temperature Te is also greater than 1 keV in these improved-confinement discharges. • Improved confinement applies to both electrons and ions • Both Te and Ti now simultaneously ≥1 keV with few MW ohmic power Te (eV) r/a

14. Confinement:large-orbit (“fast”) ions

15. Fast ions are well-confined and slow down classically. • Short pulse, 1.3 ms, of 25 keV D neutrals injected into MST deuterium plasma • fusion D-D neutrons from fast ion/plasma ion collisions are recorded • Confinement time of injected fast ions is > 20 ms • encouraging for neutral beam heating and  confinement in the RFP

16. Fast ions are well-confined even in the stochastic field of a standard RFP. • Full orbit simulation shows that fast ion drift orbits do not follow the magnetic field lines • Rotational transforms of fast ions and magnetic field lines are different fast ion trajectory Magnetic field line puncture plot for standard RFP

17. Beta

18. Pellet injection triples density in improved-confinement plasmas, while maintaining fluctuations at a reduced level. pellet ablation improved confinement • Gas puffing to high density does not maintain low fluctuations

19. Total beta (tot) nearly doubles to 26% with pellet injection. • Toroidal beta (toroidal) during pellet injection is ≈ 80% • Beta limit does not appear to have been reached

20. Current sustainment

21. OFCD uses phased oscillations of the surface poloidal and toroidal loop voltages to drive a net emf at the plasma edge. • OFCD has efficiency of ohmic current drive (~0.1 A/W) • 3D MHD simulations show OFCD can sustain full RFP plasma current • can be steady-state since <Vt>=0,<Vp>=0 • key issue will be compatibility with good confinement • OFCD “oscillating field current drive”

22. OFCD as implemented on MST adds ~10% to Ip. • OFCD subtracts an equivalent amount of current when phased for anti-drive • Results limited by pulse length and source voltage Iplasma (kA)

23. Summary • Recent results from MST have demonstrated • good ion confinement • sustained Ti ≥ 1 keV, E,i ≈ 10 ms • fast ion confinement time > 20 ms • high  operation with pellet injection • tot = 26% • a plasma current sustainment technique • current drive of 10% of Ip by OFCD • In context with previous results, the RFP has now demonstrated • good confinement of thermal and fast electrons, • good confinement of thermal and fast ions, • simultaneous with high  operation.