Doppler backscattering measurements of radio frequency plasma waves at DIII-D

A novel Doppler backscattering system (DBS) has been developed to investigate radio frequency (RF) waves in DIII-D plasmas. This simultaneously measures density fluctuations from turbulence (f < 10 MHz) and RF waves within a selectable range (20-550 MHz). DBS has been used to probe ion cyclotron waves (f ~ 20 MHz), Alfvenic instabilities (e.g., CAEs), externally launched waves (476 MHz) from DIII-D's helicon antenna, and naturally occurring lower hybrid waves (~500 MHz). The mm-wave launch frequency (O or X-mode) can be remotely stepped over 60-90 GHz, with adjustable dwell time. It can also be steered horizontally and vertically, allowing probe location and wavenumber scan during a single discharge. DBS provides sub-ms temporal resolution and wavenumber coverage k_θ ~ 1-25 cm^-1. Recent data found broadband fluctuations near the helicon frequency (476 MHz) during high-power helicon experiments. Preliminary interpretation suggests this is due to the mm-wave backscattering from plasma turbulence modulated by the helicon wave. In long term, DBS aims to validate predictive modeling (GENRAY or AORSA) of helicon current drive in DIII-D plasmas, assessing wave propagation, amplitude, absorption, and current drive location.