Power deposition measurements during high-power helicon experiments in DIII-D L-mode and H-mode plasmas

Helicon current drive, also called fast wave current drive in the lower hybrid range of frequencies, has long been regarded as a promising current drive tool for reactor grade plasmas. Experiments at DIII-D are underway with a 30-module traveling wave antenna of the comb-line type operating at 476 MHz with n_|| = 3, optimized for DIII-D high-beta target plasmas. To date, up to 0.7 MW has been applied to the input of the vacuum transmission line feeding the antenna in both L-mode and H-mode plasmas. Recent experiments have focused on power deposition measurements looking for a locked-in electron temperature response to modulated helicon power at various modulation frequencies. The core electron temperature was varied using core-absorbed ECH to alter the helicon first-pass absorption. Comparison shots were acquired with modulated ECH, both with strongly absorbed 2^nd harmonic X-mode and with weakly-absorbed 2^nd harmonic O-mode. Estimates of the absorbed power in the plasma are obtained from electron temperature fluctuation data cross-correlated with the modulated power. These estimates are compared to predictions from the GENRAY raytracing code.

[1] B. Van Compernolle, et al, Nucl. Fusion 61, 116034 (2021)