Profile Modifications Resulting from Early High-Harmonic Fast Wave Heating in NSTX
Authors: J.E. Menard, B.P. LeBlanc, J.R.Wilson, S.A. Sabbagh, D. Stutman, and D.W.Swain
Date of PPPL Report: May 2001
Presented at: the 14th Topical Conference on Applications of Radio Frequency Powers in Plasmas held in Oxnard, CA, May 7-9, 2001. Proceedings to be published by the American Institute of Physics (AIP).
Experiments have been performed in the National Spherical Torus Experiment (NSTX) to inject high harmonic fast wave (HHFW) power early during the plasma current ramp-up in an attempt to reduce the current penetration rate to raise the central safety factor during the flattop phase of the discharge. To date, up to 2 MW of HHFW power has been coupled to deuterium plasmas as early as t = 50 ms using the slowest interstrap phasing of k(parallel) always approximately equal to 14 m-1 (nf = 24). Antenna-plasma gap scans have been performed and find that for small gaps (5-8 cm), electron heating is observed with relatively small density rises and modest reductions in current penetration rate. For somewhat larger gaps (10-12 cm), weak electron heating is observed but with a spontaneous density rise at the plasma edge similar to that observed in NSTX H-modes. In the larger gap configuration, EFIT reconstructions (without MSE) find that resistive flux consumption is reduced as much as 30%, the internal inductance is maintained below 0.6 at 1 MA into the flattop, q(0) is increased significantly, and the MHD stability character of the discharges is strongly modified.