Abstract Details
Abstracts
Author: Dov J Rhodes
Requested Type: Poster
Submitted: 2016-02-15 11:52:48
Co-authors: A.J. Cole, D.P. Brennan, J.M. Finn, R. Fitzpatrick, M. Halfmoon, M.E. Mauel, G.A. Navratil
Contact Info:
Columbia University
500 W. 120th St., Mudd Buildin
New York, NY 10027
USA
Abstract Text:
Tokamak operation relies on a proper understanding of the stability limits. The Brennan-Finn study of resistive-MHD modes [1] optimizes feedback control in different plasma response regimes. The regimes are divided by resistive or ideal plasma (rp or ip) with resistive or ideal wall (rw or iw), satisfying the beta-limit ordering rp-rw < rp-iw < ip-rw < ip-iw. Optimizing complex gains on both normal and tangential sensors leads to two main conclusions: (a) Imaginary gain on the normal sensors (Gi) is equivalent to rotation, and thus an effective tool for stabilization up to the rp-iw limit. (b) Above the rp-iw limit rotation is destabilizing, and the plasma is stabilized near an optimal Gi that effectively cancels the plasma rotation (with respect to the wall). A new phase-matching feedback experiment of HBT-EP yields similar results [3]. The present work explores conditions for an alternate beta-limit ordering rp-rw < ip-rw < rp-iw < ip-iw. Reversing the middle two limits expands the domain of effective rotational stabilization above the ip-rw beta-limit. One way to achieve this condition is to increase the plasma-wall coupling by reducing the plasma-wall distance. Alternately, the ip-rw limit may be lowered by toroidal mode-coupling. This destabilizing effect is studied in a first-order toroidal extension of the Brennan-Finn reduced-MHD model [1] and compared with a fully toroidal multi-mode sharp-boundary model based on Fitzpatrick [2]. Initial studies find the alternate beta-limit ordering to exist in a finite domain of the safety factor q. Given the Brennan-Finn model predictions, this finding suggests that toroidal coupling may permit rotation-like feedback stabilization above the ip-rw beta-limit, and possibly even rotation-canceling feedback stabilization up to the ip-iw limit. [1] D.P. Brennan and J.M. Finn, Phys. Plasmas 21, (2014). [2] R. Fitzpatrick, Phys. Plasmas 17, (2010). [3] Q. Peng et. al., Plasma Phys. Control. Fusion 58, (2016).
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