Experimental observation and integrated modeling of electron heating by helicon waves in DIII-D

Helicon current drive is an attractive solution for driving off-axis current for advanced scenarios in reactor conditions. Dedicated DIII-D experiments have been conducted to assess power absorption and current drive for a MW-level helicon system. By modulating the injected helicon power in time, cross spectral analysis techniques were used to detect coherent electron temperature fluctuations at the modulation frequency on multiple core-localized ECE channels, indicating core electron heating. In a series of progressively hotter discharges, the measured fluctuations were larger in the plasmas with stronger first pass absorption predicted by the GENRAY ray tracing code. Time-dependent, integrated modeling was performed with TRANSP coupled to GENRAY and the MMM turbulence code in order to account for nontrivial transport effects when deriving the total absorbed helicon power from the measured temperature fluctuations. Hardware upgrades have enabled power injection from either end of the antenna, facilitating the quantification of the driven current profile. Results from recent helicon experiments performed in plasmas with 100% predicted first pass absorption and strong predicted current drive will be discussed.