High-frequency core localized modes in neutral beam heated plasmas on TFTR (original) (raw)

A bst rad A band of high frequency modes in the range 50-150 ldb with intermediate toroidal mode numbers 40140 are commonly observed in the core of supershot plasmas on "FIR. Two distinct varieties of MHD modes are identified conesponding to a flute-like mode predominantly appearing around the q = 1 surface and an outward ballooning mode for 4 > 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 z , < 2zL, 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 z , > 22,, where z , is the equivalent L-mode confinement time. The modes propagate in the ion diamagnetic drift direction and are highly localized with radial widths Ar-5-10 cm , fluctuation levels E / n, f, / T, < 0.01, and radial displacements 5,-0.1 cm. Unlike the toroidally localized high-n activity observed just prior to major and minor disruptions on TFI'R [ 11, these modes are typically much weaker more benign, and may be indicative of kinetic ballooning modes destabilized by resonant circulating neutral beam ions. fluctuation levels fi / n, fe / T, c 1%. Two distinct varieties are identified corresponding to a flute-like mode predominantly appearing around the q = 1 radius and an outward ballooning mode appearing near the peak of the pressure gradient in the region 1 < 1.5. The flute-like modes are predominantly observed in moderate performance supershots plasmas with z , c 2zL, while the ballooning modes are mainly observed in higher performance discharges with z , > 2zL. Although these two modes appear with similar frequencies and mode numbers, they are rarely observed in the same discharge. Unlike the NOTICE Page(s) size did not permit electronic reproduction. Information may be purchased by the general public from the