High-frequency core localized modes in neutral beam heated plasmas on TFTR

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 [R- Hawryluk, Plasma Phys. Controlled Fusion 33, 1509 (1991)] Two distinct varieties of magnetohydrodynamic (MHD) modes are identified, corresponding to a flute-like mode predominantly appearing around the q = 1 surface and an outward ballooning mode for q>1. The flute-like modes have nearly equal amplitude on the high-field and low-held side of the magnetic axis, and are mostly observed in moderate performance supershot plasmas with τ_E<2τ_L, while the ballooning-like modes have enhanced amplitude on the low-field side of the magnetic axis and lend to appear in higher performance superhot plasmas with τ_E>2τ_L, where τ_L is the equivalent L-mode confinement time. Both modes appear to propagate in the ion diamanagnetic drift direction and are highly localized with radial widths Δr∼5-10 cm, fluctuation levels n̈/n, T̃_e/T_e<0.01. and radial displacements ξ_r.∼0.1 cm. Unlike the toroidally localized high-n activity observed just prior to major and minor disruptions on TFTR [E. D. Fredrickson et al., Proceedings of the 15th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville, Spain (International Atomic Energy Agency, Vienna, 1995), No. IAEA-CN-60/A-2-II-5], these modes are typically more benign and may be indicative of MHD activity excited by resonant circulating beam ions.