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Oscillating Field Current Drive Studies in MST
| Author: | McCollam K.J. |
| Coauthor: | A.P. Blair, F. Ebrahimi, P.D. Nonn, J.S. Sarff, D. Craig, D.J. Den Hartog, B.H. Deng, W.X. Ding, G. |
| Institution : | University of Wisconsin |
| Abstract text: | Oscillating field current drive (OFCD) is a candidate for steady-state RFP equilibrium sustainment using inductive electric fields. In OFCD the application of AC poloidal and toroidal loop voltages, typically with 1/4-cycle relative phase, injects DC magnetic helicity to sustain a DC plasma current. OFCD sustainment is studied theoretically and partial sustainment is being tested in the Madison Symmetric Torus (MST). Nonlinear 3D resistive MHD calculations [1] predict that OFCD can fully sustain an RFP equilibrium. A significant modulation of the equilibrium field is required that decreases with decreasing plasma resistance. Core-resonant modes are not seen to grow much larger than for steady induction. However, large amplitude modes resonant in the edge, outside the toroidal field reversal surface, appear during the portion of the cycle when the modulated edge toroidal field is most negative. Low-power oscillators are used for the MST partial sustainment experiments. The observed change in current between drive and anti-drive phases is consistent with helicity balance, but a net increase in current has not been reproducibly found. The AC magnetic field has been detected penetrating to the core under some conditions, but its amplitude is small, consistent with expectations. Some observations suggest that the 550 Hz oscillation used to date is near the upper frequency limit for plasma relaxation in MST, so the oscillators have been modified to run at 275 Hz for upcoming experiments. Under some conditions, anomalous ion heating synchronous with the applied voltage is observed, perhaps associated with synchronous modulations in the tearing mode amplitudes.
[1] F. Ebrahimi et al., Phys. Plasmas 10, 999 (2003).
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