|sarfftalk.pdf||2006-02-21 09:01:40||John Sarff|
Oscillating field current drive in the RFP
Author: John S Sarff
Submitted: 2005-12-20 17:44:53
Co-authors: A.P. Blair, K.J. McCollam, and the MST Team
University of Wisconsin-Madison
1150 University Ave
Madison, WI 53706
Current sustainment is particularly challenging in the RFP and other toroidal confinement configurations with dominant poloidal field. The plasma current is large, but the pressure-driven component is small, even at a high beta of 20%. Oscillating Field Current Drive (OFCD) is an inductive current drive method that operates in steady state. Sinusoidal (more generally zero-time-average) toroidal and poloidal loop voltages are used to maintain a dc current through induction and magnetic relaxation. The OFCD concept was invented considering conservation of magnetic helicity, the underpinning of Taylor's relaxation theory. We present results from OFCD experiments in MST. Key observations include a 10% increase in the plasma current (with the background steady induction maintained fixed), and that the relative phase of the applied ac loop voltages for maximum current is not 90 degrees as predicted by the simple helicity balance . A key issue for OFCD is its impact on MHD tearing turbulence and magnetic fluctuation induced transport. With 10% current drive, the magnetic fluctuations are not observed to increase, in agreement with 3D nonlinear resistive MHD computation. In fact, m=0 fluctuations are measured smaller than for steady induction alone at the maximum current drive oscillator phasing. Because OFCD is inductive, it is expected to have high current drive efficiency, and this is observed in MST. OFCD was the basis for current sustainment in the TITAN reactor system study. Even if fusion-grade confinement cannot be obtained with OFCD, it might be an efficient means to create the plasma current. Used with self-similar-decay programming (a generalization of pulsed poloidal current drive with established good confinement) , a pulsed-burn RFP reactor scenario that is fully inductive is conceivable. The plasma current would not need to be terminated and restarted, possibly reducing stresses associated with conventional pulsed systems. A brief discussion of this hybrid scheme will be presented.
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