Initial Results from DIII-D High Field Side Lower Hybrid Current Drive Experiment

The first high field side lower hybrid current drive (HFS LHCD) experiments coupled modest power (<200 kW) into a range of plasmas. The HFS is expected to allow improved wave coupling, accessibility, and propagation. HFS LHCD is also potentially an efficient off-axis current drive tool, r/a~0.6-0.8, for advanced tokamak DIII-D discharges. Initial results show the reflected power fraction, an important coupling metric, can be maintained <5% for discharges with inner gap <2 cm and <15% for gaps as large as 10 cm. Non-thermal electron cyclotron emission (ECE) has been observed with injected power as low as 50 kW. The non-thermal ECE arises from the relativistically downshifted cyclotron frequency of the non-thermal electrons driven by LH waves. From the sawtooth modulation of the signals, the nonthermal ECE indicates that the LH absorption is inside the sawtooth radius for lower temperature discharges and within the sawtooth mixing radius for higher temperature discharges or higher density. The modest shift of the deposition location with temperature and density is encouraging and suggests the expected wave upshift to bridge the spectral gap is consistent with experiment. The latest analysis, results and prospects for higher power experiments will be presented.