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approvedseltzman_ebw_heating_epr2017.pdf2017-08-09 01:57:43Andrew Seltzman

Observation of Electron Bernstein Wave Heating in the RFP

Author: Andrew H Seltzman
Requested Type: Pre-Selected Invited
Submitted: 2017-06-02 20:20:55

Co-authors: J.K. Anderson, J.A. Goetz, C.B. Forest

Contact Info:
University of Wisconsin-Madison
1150 University of Wisconsin
Madison, Wisconsin   53706

Abstract Text:
The first observation of RF heating in a reversed field pinch (RFP) using the electron Bernstein wave (EBW) has been demonstrated on Madison Symmetric Torus. Challenging heating environment including an overdense plasma with no high field side, porthole field error, mode conversion in the antenna near field, and a stochastic magnetic field required development of novel techniques to measure heated electrons. Efficient mode conversion of an outboard-launched X mode at 5.5 GHz leads to Doppler-shifted resonant absorption (w = n*w_ce – k||*v||) for a broad range (n=1-7) of harmonics. A spatial distribution of solid targets (limiter and target probe) with diametrically opposed x-ray detectors measures the dynamics of EBW-heated electrons. EBW heating produces a clear supra-thermal electron tail in MST. Radial deposition of the EBW is measured from HXR flux from a radially scanned insertable probe. Deposition location was controllable with |B|. In the thick-shelled MST RFP, the radial accessibility of EBW is limited to r/a > 0.8 (~10cm) by magnetic field error induced by the porthole necessary for the antenna. Experimental measurements show EBW propagation inward through a stochastic magnetic field implying EBW feasibility in spherical and advanced tokamak heating scenarios; accessibility in a thin-shelled RFP with actively controlled saddle coils is likely to be r/a> 0.5 in agreement with ray tracing studies. EBW-heated test electrons are used as a direct probe of edge (r/a > 0.9) radial transport, showing a modest transition from ‘standard’ to reduced-tearing RFP operation. Electron loss is too fast for collisional effects and implies a large non-collisional radial diffusivity. EBW heating has been demonstrated in reduced magnetic stochasticity plasmas with Beta=15-20%. Work supported by USDOE.