Results from the High-Power Helicon Current Drive System at DIII-D

Heating and current drive by helicon waves, i.e., fast waves in the lower hybrid frequency range, is predicted to be efficient in reactor plasmas. Experiments at DIII-D use a 'comb-line' traveling wave antenna at 476 MHz. Recent upgrades have enabled coupled power levels of > 0.5 MW for pulse lengths of > 2 s. The system was shown to be load-resilient in ELMing H-mode plasmas with no indication of impurities associated with injection. Recent experiments have studied power deposition by seeking a core electron temperature response phase-locked to modulated helicon power. Coherent T_e modulations were observed on core ECE channels (rho < 0.4) at a range of modulation frequencies. Comparison shots were acquired with 2^nd harmonic X-mode and weakly-absorbed 2^nd harmonic O-mode ECH. High-frequency Ion Cyclotron Emission magnetic probes measured nonlinear sidebands near 476 MHz, correlated with spectral peaks at edge ion cyclotron harmonics. In parallel, new diagnostics are coming online, including Doppler back scattering (DBS), a helium beam edge density diagnostic, Doppler-free saturation spectroscopy (DFSS), and a high-frequency phase contrast imaging (PCI).