(EX/P6-11) Current Profile Modification Influence on MHD and Non-Solenoidal Plasma Startup in the Pegasus Toroidal Experiment

A.C. Sontag1), D. Battaglia1), R. Bayliss1), M. Bongard1), N. Eidietis2), R. Fonck1), G. Garstka3), C. Hegna1), E. Hinson1), A. Redd1), C. Sovenic1), E. Unterberg4)
1) University of Wisconsin - Madison, Madison, WI, United States of America
2) General Atomics, San Diego, CA, USA
3) TomoTherapy Inc., Madison, WI, USA
4) Oak Ridge National Laboratory, Oak Ridge, TN, USA

Abstract.  The Pegasus Toroidal Experiment operates at A < 1.3 and has achieved record high normalized plasma current for a tokamak, IN > 12. Operation at high IN allows access to high βt and requires j(r) modification to avoid deleterious MHD. Very broad, stable current profiles are obtained when washer-stack current sources (plasma guns) are used to initiate non-inductive discharges via DC helicity injection. This startup technique is scalable and requires no modification of the vacuum vessel. Equilibrium reconstructions of gun discharges show high edge current (li = 0.2) and elevated q ( qmin > 6), which allow access to the high IN regime. Plasma gun discharges relax into a tokamak-like configuration with toroidally-averaged closed flux surfaces, large n = 1 activity and toroidal current amplification up to 3 times the vacuum windup. Maximum Ip is determined by helicity balance and up to 50 kA of toroidal current has been generated with this technique. Nonlinear 3D simulation with NIMROD shows that gun injection excites a line-tied kink that produces amplification of poloidal flux beyond the vacuum wind-up. Experimental evidence of flux amplification includes: reversal of the edge poloidal magnetic flux; increase of the toroidal plasma current over that of the vacuum geometric windup; plasma position subject to radial force balance; and persistence of the plasma current after gun shut-off. Coupling gun discharges to other current drive is straightforward. Gun-only plasmas which reach a maximum plasma current of 20 kA have been coupled to Ohmic drive applied at the time of the plasma gun turn-off and ramped up to 80 kA with 1 - 2 V of loop voltage totaling < 10 mVs of Ohmic flux. Early TSC results qualitatively agree with experimental observations of gun plasma dynamics. Very low TF and high edge currents in Pegasus allow study of peeling-ballooning modes, even in the absence of transition to H-mode. Ideal MHD stability calculations indicate that Pegasus is susceptible to peeling mode instability at low beta. The visible onset of edge filaments believed to be caused by peeling modes coincides with magnetic oscillations in the 20 - 100 kHz range with low to intermediate toroidal mode numbers. The amplitude of these oscillations falls off quickly with increasing radius. Ip ramp-down decreases the edge j || and coincides with the suppression of these modes.

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