Local helicon wave measurements in DIII-D via Doppler Backscattering*

Experiments have been performed in DIII-D to develop high power (1 MW) 476 MHz helicon waves as a current drive tool for future fusion reactor plasmas. A prototype Doppler backscattering system to measure the helicon wave amplitude and spatial distribution for validation of helicon propagation and absorption models is being tested. It launches a fixed frequency beam (60–80 GHz range), simultaneously measuring turbulent and radio-frequency (RF) wave density fluctuations. During ~ 300 – 500 kW helicon injection, broadband fluctuations are observed in a ~ 2 MHz band around 476 MHz in L-mode and H-mode plasmas. Preliminary analysis shows the waves are localized near  ~ 0.5, with  < 6 cm^-1.  In ELMy H-modes, their amplitude is seen to diminish significantly around the time of the ELM crash. The RF frequency range is adjustable, which allowed the system sensitivity to be tested via measurement of ion cyclotron harmonic waves (20–40 MHz) spontaneously excited by neutral heating beams. The new system demonstrates significant improvements in sensitivity over previous efforts to measure these waves [N. A. Crocker, IAEA FEC 2020]. A synthetic diagnostic model is under development for interpretation measurements.