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youngepr2014.pdf2014-08-27 23:51:59William Young

Electron Heating During m=0 Mode Activity on the MST RFP

Author: William C Young
Requested Type: Consider for Invited
Submitted: 2014-05-30 23:01:22

Co-authors: L. A. Morton, E. Parke, D. J. Den Hartog

Contact Info:
University of Wisconsin-Madison
1150 University Ave
Madison, WI   53706
USA

Abstract Text:
MST plasmas with low density (~ 0.7×10^19 m^-3) and strong toroidal magnetic field reversal routinely exhibit spontaneous transition to enhanced confinement, with a reduction in magnetic fluctuations and a three-fold improvement to energy confinement. These plasmas produce toroidally localized magnetic bursts associated only with edge-resonant m=0 modes, unlike sawtooth crashes that otherwise emerge regularly in MST plasmas entailing strong coupling between the m=0 modes and core-resonant m=1 modes. In response to these bursts, Te in the core was found to rise ~4% over a 100 μs time window. This is accompanied by a 1% reduction in parallel current density in the core, along with a 0.5% reduction in magnetic energy. Previous measurements on MST have shown ion heating during sawtooth crashes and these m=0 bursts, but these are the first measurements to show electron heating in conjunction with a burst of magnetic mode activity and a reduction of stored magnetic energy. The Te increase is followed by a return to the initial temperature, with evidence of an inward-propagating cold pulse, consistent with previous SXR measurements. These measurements were enabled by a Bayesian analysis method developed to model relative changes in measurements ensembled about bursts of mode activity. This method was applied to Thomson scattering measurements made using both MST’s established pulse-burst laser, with bursts of 6 pulses at 25 kHz, and the newer high-repetition-rate laser producing bursts of 15 pulses at 75 kHz (and recently tested at 250 kHz). Application of the same method to magnetic reconstructions by the MSTFIT equilibrium quantified the reduction in magnetic energy and parallel current density.

Characterization: 3.0

Comments:
Place with other MST abstracts where appropriate.

Workshop on Exploratory Topics in Plasma and Fusion Research (EPR) and US-Japan Compact Torus (CT) Workshop
August 5-8, 2014
Madison, Wisconsin

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