Magnetic fluctuation measurements from first high-power helicon experiments on DIII-D

Parametric Decay Instabilities (PDI) have been observed in experiments with slow and fast wave coupling in the lower hybrid range and were predicted to occur in helicon current drive experiments at 476 MHz in DIII-D [1]. Initial experiments with a helicon system at DIII-D have been conducted with a 30-module comb-line traveling wave antenna to couple up to 0.3 MW of RF power at 476 MHz with n_|| = 3 to L- and H-mode plasmas with toroidal fields B_T(0)= 1.85—2.0 T. The Ion Cyclotron Emission (ICE) diagnostic was used to obtain magnetic fluctuation spectral measurements from 1 to 100 MHz. ICE was expanded to acquire high-frequency (‘HICE’) signals up to 500 MHz with pickup loops near the midplane on both the high- and low-field sides, enabling the observation of PDI in helicon experiments. Measurements showed the ‘pump’ peak at 476 MHz as well as sidebands up to -10 dB of the pump amplitude separated by where is the ion cyclotron frequency near the outboard plasma edge. Corresponding ion cyclotron harmonic quasi-modes were also observed at the low frequency end of the spectrum.



[1] M. Porkolab et al, Bull. Am. Phys. Soc. (2021) https://meetings.aps.org/Meeting/DPP21/Session/UO07.7