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IT11 · Overview of fast ion driven MHD in NSTX E.D. Fredrickson, D. Darrow, S. Medley, J. Menard, H. Park, L. Roquemore, Princeton Plasma Physics Laboratory, Princeton, NJ Abstract: A wide variety of fast ion driven instabilities are excited during neutral beam injection (NBI) in the National Spherical Torus Experiment (NSTX) by the large ratio of fast ion velocity to Alfvén velocity, Vfast/VAlfvén, together with the relatively high fast ion beta. The modes can be divided into three categories; chirping Energetic Particle Modes (EPM) in the frequency range 0 to 120 kHz, the Toroidal Alfvén Eigenmodes (TAE) with a frequency range of 50 kHz to 200 kHz and the Compressional and Global Alfvén Eigenmodes (CAE and GAE, respectively) between 300 kHz and the ion cyclotron frequency. Calculated fast ion distributions find a bump-on-tail in the perpendicular energy direction that satisfies the Doppler-shifted ion cyclotron resonance believed to excite the CAE. The same resonance is believed to drive the GAE, and is found to support behavior similar to “hole-clump pairs” under special circumstances. The internal amplitude, and to a lesser extent the spatial structure, of the TAE, CAE, GAE and EPMs has been measured with Heterodyne reflectometry in L-mode plasmas. Soft x-ray cameras provide more detailed information on the structure of the lower frequency EPMs. The TAE bursts have internal amplitudes of up to ń/n 1% and toroidal mode numbers 2 ≤n ≤6. The EPMs are core localized, kink-like modes similar to the fishbones in conventional aspect ratio tokamaks. Unlike the fishbones, the EPMs can be present with q(0) > 1 and can have a toroidal mode number n > 1. The range of the frequency chirp can be quite large and the resonance can be through the fishbone-like precessional drift resonance, or through a bounce resonance. Fast ion driven modes are of particular interest because of their potential to cause substantial fast ion losses. In all regimes of NSTX NBI heated operation we see transient neutron rate drops, correlated with bursts of TAE or fishbone-like EPMs. The CAE and GAE may also affect fast ion confinement, but there is little direct evidence for direct fast ion losses caused by these instabilities. The fast ion loss events are predominantly correlated with the EPMs, although losses are also seen with bursts of multiple, large amplitude TAE. The latter is of particular significance, because the transport of fast ions from the expected resonance overlap in phase space will reveal a "sea of Alfvén modes" kind of physics similar to that expected in ITER. *This work supported by U.S. DoE Contract DE-AC02-76CH03073. |