MHD-induced Energetic Ion Loss during H-mode Discharges in the National Spherical Torus Experiment (NSTX)

Abstract

MHD-induced energetic ion loss in neutral beam heated H-mode discharges in NSTX is discussed. A rich variety of energetic ion behavior resulting from magnetohydrodynamic (MHD) activity is observed in NSTX using a horizontally scanning neutral particle analyzer whose sightline views across the three co-injected neutral beams. Reconnection events, sawteeth and bounce fishbones are observed to cause prompt loss (or redistribution) of energetic ions (E ~ 10 -100 keV) on a time scale of ≤ 1 ms and, consequently, a precipitous drop in the neutron yield. On the other hand, onset of low-n MHD modes leads to a much slower decay of the energetic ions and neutron yield. MHD-induced ion loss during H-mode operation in NSTX exhibits yet another phenomenon. After H-mode onset, the NPA spectrum usually exhibits a significant loss of energetic ions mainly for E > E b /2 where E b is the beam injection energy, although this loss occasionally extends to lower energy. The magnitude of the energetic ion loss diminishes with increasing tangency radius of the NPA sightline, increasing toroidal field and increasing NB injection energy. Modeling suggests that MHD-induced ion loss is enhanced during H-mode operation due to an evolution of the q and beam deposition profiles that feeds both passing and trapped ions into the region of the plasma affected by the low-n MHD activity. Analysis of the particle interaction with a model magnetic perturbation supports the energy selectivity of the observed MHD-induced loss. Transport analysis using a fast ion diffusion model to emulate MHD-induced energetic ion loss shows significant modifications of the heating produced by the neutral beam which changes the inferred power balance, thermal External Distribution

Modeling of Low Frequency MHD Induced Beam Ion Transport In NSTX

“…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 as reported by Medley [1]. In NSTX experiments of interest typical plasma parameters were major radius R 0 = 0.85 − 0.9m, minor radius a = 0.67m, plasma current I p = 0.3 − 1.5M A, and toroidal field B T = 0.3 − 0.6T .…”

“…In NSTX experiments of interest typical plasma parameters were major radius R 0 = 0.85 − 0.9m, minor radius a = 0.67m, plasma current I p = 0.3 − 1.5M A, and toroidal field B T = 0.3 − 0.6T . In NSTX Neutral Particle Analyzer (NPA) [1] was measuring energy spectrum signal of confined beam ions initially injected with maximum injection energies in the range E b0 = 80 − 100keV . Note that in NSTX Neutral Beam Injection (NBI) is characterized by multiple injection energies, E b0 , E b0 /2, and E b0 /3.…”

“…ORBIT is a guiding center code, which follows beam ion drift orbits. For realistic simulations we employ numerical equilibrium and model MHD activity based on the experimental observations [1]. In the analyzed discharge #108730 MHD activity was measured as being dominated by one m = 4 poloidal harmonic with the toroidal mode number n = 2.…”