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

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

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)

Contributors to this work: This work is supported by the U. S. Department of Energy and the National Science Foundation. 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

Outline Brief introduction to the Reversed-Field Pinch and MST RFP advancement –thermal and fast ion confinement –high n e and  from pellet injection –oscillating field current drive Summary

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

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

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

Confinement: thermal ions

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 T i normally drops rapidly following a reconnection event T i versus timeT i versus radius r/a = 0.37 T i (keV) after (+0.15 ms) before (-0.3 ms)

T i is sustained ≥ 1 keV during improved confinement period. Ion energy is retained by initiating improved confinement following a reconnection event B (gauss) ~ T i (keV) time (ms) reconnection events improved confinement 

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

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

Confinement: large-orbit (“fast”) ions

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

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

Beta

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

Total beta (  tot ) nearly doubles to 26% with pellet injection. No pelletPellet injection (10 13 cm -3 ) %26% Toroidal beta (  toroidal ) during pellet injection is ≈ 80% Beta limit does not appear to have been reached

Current sustainment

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 =0, =0 –key issue will be compatibility with good confinement OFCD “oscillating field current drive”

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 I plasma (kA)

Summary Recent results from MST have demonstrated –good ion confinement sustained T i ≥ 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.