Localized Measurement of Helicon Wave Power Distribution via Doppler Backscattering on DIII-D*

Recent experiments on DIII-D demonstrate the first localized measurements of the complex helicon wave power distribution using a novel millimeter-wave Doppler Backscattering (DBS) system. Positioned 60º toroidally from the helicon antenna, the DBS probes the plasma from a low-field-side ECH launcher above the midplane. By sweeping 60–90 GHz during an H-mode discharge, the system probes radial locations from ρ∼0.86 to ρ∼0.4 between the launcher and the magnetic axis. A high-frequency spectral feature near the helicon frequency (476 MHz) is observed, attributed to scattering from turbulence modulated by helicon-driven E×B velocity. The ratio of power in this feature (P_bb) to background turbulence (P_turb) reveals the helicon power at this scattering location (ρ). Measurements show peak power near ρ∼0.5, decreasing at larger and smaller ρ. 3D GENRAY simulations predict helicon ray trajectories closely approaching the DBS scattering region, showing the closest approach (~10 cm) for the ρ∼0.5 scattering location, consistent with the observed peak. The measured helicon power decreases with increasing ray separation, validating GENRAY predictions. The observed E×B modulation highlights an interesting RF–turbulence interaction, introducing a novel method for measuring RF waves in fusion plasmas.