Implementation of Heterodyne Detection of RF waves using Phase Contrast Imaging on DIII-D

An upgrade to the Phase Contrast Imaging (PCI) diagnostic [Davis et al, RSI 89 (2018) 10B106] on DIII-D to provide measurements of RF waves up to 500 MHz has been fully implemented and installed. The laser power is modulated at RF by an electro-optic modulator (EOM), a Pockels cell consisting of a CdTe crystal with two biasing plates. The modulated laser and the perturbation due to plasma waves are optically mixed on the PCI detector, providing heterodyne detection of the RF wave. For detection in the ICRF range, the EOM is biased using a high-voltage fixed-frequency source (80% modulation fraction) [Marinoni et al, RSI 93 (2022) 083502] or a scannable moderate power source (8% modulation). For detection of the helicon wave, the EOM is biased at 475 MHz, with a slug tuner providing matching to optimize power transfer from the amplifier to the EOM; a modulation fraction of 18% has been achieved. The initial target of studies with the new system is to provide absolutely-calibrated measurements of the density component of Ion Cyclotron Emission instabilities, with potential future application to RF heating waves and wave-driven instabilities.