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

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Abstract
A band of high‐frequency modes in the range 50–150 kHz with intermediate toroidal mode numbers 4<nq=1 surface and an outward ballooning mode for q≳1. 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 τEL, 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 τE≳2τL, where τL is the equivalent L‐mode confinement time. Both modes appear to propagate in the ion diamagnetic drift direction and are highly localized with radial widths Δr∼5–10 cm, fluctuation levels ñ/n, T̃e/Ter∼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.