Observation of Beam Driven Modes during Neutral Beam Heating on the National Spherical Torus Experiment
Authors: E.D. Fredrickson, N. Gorelenkov, C.Z. Cheng, R. Bell, D. Darrow, D. Johnson, S. Kaye, B. LeBlanc, J. Menard, S. Kubota, and W. Peebles
Date of PPPL Report: October 2001
Presented at: the 28th European Physical Society (EPS) Conference on Controlled Fusion and Plasma Physics, Madeira, Portugal, June 18-22, 2001.
With the first injection of neutral beams on the National Spherical Torus Experiment (NSTX), a broad and complicated spectrum of coherent modes was seen between approximately 0.4 MHz and 2.5 MHz (where fci for deuterium is approximately 2.2 MHz). The modes have been observed with high bandwidth magnetic pick-up coils and with a reflectometer. The parametric scaling of the mode frequency with density and magnetic field is consistent with Alfvénic modes (linear in B, inversely with the square root of density). These modes have been identified as magnetosonic waves or compressional Alfvén eigenmodes (CAE) excited by a cyclotron resonance with the neutral-beam ions. Modes have also been observed in the frequency range 50-150 kHz with toroidal mode numbers n = 1-5. These lower frequency modes are thought to be related to the TAE [Toroidal Alfvén Eigenmode] seen commonly in tokamaks and driven by energetic fast ion populations resulting from ICRF [ion cyclotron range of frequency] and NBI [neutral-beam injection] heating. There is no clear indication of enhanced fast ion losses associated with the modes.