Modeling of Low Frequency MHD Induced Beam Ion Transport In NSTX
Authors: N.N. Gorelenkov and S.S. Medley
Date of PPPL Report: July 2004
Presented at: the Thirty-First EPS Conference on Plasma Physics, 28 June - 2 July 2004, London, United Kingdom.
Beam ion transport in the presence of low frequency MHD activity in National Spherical Tokamak Experiment (NSTX) plasma is modeled numerically and analyzed theoretically in order to understand basic underlying physical mechanisms responsible for the observed fast ion redistribution and losses. Numerical modeling of the beam ions flux into the NPA in NSTX shows that after the onset of low frequency MHD activity high energy part of beam ion distribution, Eb > 40keV , is redistributed radially due to stochastic diffusion. Such diffusion is caused by high order harmonics of the transit frequency resonance overlap in the phase space. Large drift orbit radial width induces such high order resonances. Characteristic confinement time is deduced from the measured NPA energy spectrum and is typically ∼ 4msec. Considered MHD activity may induce losses on the order of 10% at the internal magnetic field perturbation δB/B = O; (10-3), which is comparable to the prompt orbit losses.