Spatial Distribution of Helicon Wave Amplitude Using Turbulence-induced Doppler Backscattering Measurements at DIII-D*

A novel, frequency-stepped Doppler Backscattering System (DBS) has been employed at the DIII-D tokamak to investigate high-frequency broadband fluctuations observed near the frequency (476MHz) of externally launched helicon waves. These fluctuations are hypothesized to result via direct backscatter from plasma turbulence modulated by the externally injected helicon waves. By analyzing and comparing simultaneously measured low-frequency turbulence with the above high-frequency broadband helicon signal, we can infer the electric field amplitude of the helicon wave (E_hel) together with its spatial profile. Preliminary estimations of E_hel, as obtained through DBS, will be presented. The findings reveal specific spatial distribution patterns of E_hel within the plasma, highlighting the system's capability to measure these distributions. These measurements represent a crucial step toward utilizing helicon waves for current drive applications in DIII-D. Understanding the amplitude and spatial distribution of helicon waves will aid in optimizing their use for efficient current drive, a key component for the future of controlled fusion energy.