High Frequency Core Localized Modes in Neutral Beam Heated Plasmas on TFTR
Authors: R. Nazikian, Z. Chang, E.D. Fredrickson, S.H. Batha, R. Bell, R. Budny, C.E. Bush, C.Z. Cheng, A. Janos, F. Levinton, J. Manickam, E. Mazzucato, H.K. Park, G. Rewoldt, S. Sabbagh, E.J. Synakowski, W. Tang, G. Taylor, L.E. Zakharov
A band of high frequency modes in the range 50-150 kHz with intermediate toroidal mode numbers 4<n<10 are commonly observed in the core of supershot plasmas on TFTR. Two distinct varieties of MHD modes are identified corresponding to a flute-like mode predominantly appearing around the surface and an outward ballooning mode for. The flute-like modes have nearly equal amplitude on the high field and low field side of the magnetic axis and are mostly observed in moderate performance supershot plasmas with, while the ballooning-like modes have enhanced amplitude on the low field side of the magnetic axis and tend to appear in higher performance supershot plasmas with, where is the equivalent L-mode confinement time. The modes propagate in the ion diamagnetic drift direction and are highly localized with radial widths, fluctuation levels, and radial displacements. Unlike the toroidally localized high- activity observed just prior to major and minor disruptions on TFTR [1], these modes are typically much weaker more benign, and may be indicative of kinetic ballooning modes destabilized by resonant circulating neutral beam ions.