First Experimental Observation of Helicon Current Drive in the DIII-D Tokamak

Non-inductive rf current drive by externally-launched helicon waves (whistlers, fast waves in the lower hybrid range) has been measured in a tokamak for the first time. Ray-tracing models have long predicted such a result would be possible with a technology by which waves of the correct properties could be launched. Wave power deposition on electrons was studied using square-wave amplitude modulation of the 0.476 GHz helicon power, and central deposition has been observed under conditions in which the ray-tracing model predicted central first-pass absorption of ~40% in L-mode discharges. This level of absorption is achieved by raising the electron pressure in the core with neutral beam and electron cyclotron preheating. In ELMing H-mode plasmas, where ray-tracing predicts nearly complete (>90%) first-pass absorption in the core, core heating was observed even at relatively low levels of power per particle compared with the lower density L-mode cases. No impurity influx was observed to be correlated with injection of the helicon power in any of the plasmas studied; this property facilitated clear observation of electron heating. With unmodulated pulses to maximize the time-averaged coupled power, the helicon power raised the central electron temperature by nearly 1 keV compared to discharges without the helicon waves, at coupled power levels of ~0.3 - 0.4 MW. Current drive near the center of a reproducible quiet, sawtooth-free L-mode plasma was measured with co-current helicon waves using the Motional Stark Effect diagnostic. Shots with co-current helicon power at different times in the discharge, without helicon power, and with incremental ECH at comparable power levels and deposition locations were repeated to improve the statistics of the measurements. The time in the discharge at which sawteeth begin is reproducibly affected by the helicon power consistent with core current drive from the helicon.