Toroidal alfvén eigenmodes driven with ICRF accelerated protons in JT-60U negative shear discharges

Abstract

On JT-60U, for the first time in negative shear plasmas, toroidicity induced Alfvén eigenmodes (TAEs) with frequencies in the range of 90 to 110 kHz and toroidal mode numbers n = 5-8 were observed after a broadening of the density profile due to sequential partial collapses during ion cyclotron range of frequency (ICRF) heating. TAEs are more stable in negative shear plasmas than plasmas with a monotonically increasing q profile. The TAE stability was analysed with the NOVA-K code. It was found that the stability is highly sensitive to the gap alignment just inside q_min and that the theoretical results are consistent with the experimental results. In a weak central magnetic shear discharge, modes with very rapid frequency chirping were observed during ICRF heating. The frequencies of these modes rapidly increased from ~30 kHz up to ~110 kHz in a period of ~150 ms. The large frequency chirping is not well understood by the present theory.