Development of a High Frequency Heterodyne Phase Contrast Imaging System to Detect Helicon Waves on DIII-D

The Phase Contrast Imaging (PCI) diagnostic on the DIII-D tokamak is being upgraded with a novel optical heterodyne scheme to measure the density perturbation induced by the recently commissioned helicon antenna [1]. The PCI, being an absolutely calibrated interferometer that images in real space line-integrated electron density fluctuations, is uniquely able to measure both the internal structure and the absolute amplitude of such a wave, thereby extending the purely temporal measurements that are otherwise typically available. In order for the imaging method to be applicable to the helicon wave at 476 MHz, the intensity of the PCI laser beam must be modulated in such a way that the low-frequency sideband falls within the PCI 2 MHz detector bandwidth. A transverse Pockels cell made of a water cooled CdTe birefringent crystal is driven by a cascade of two RF amplifiers and a tuned cavity providing 2 kV pk-pk voltage at 475 MHz. Calibration data on the spectral purity of the sinusoidal modulation as well as on its depth will be presented.

[1] B. Van Compernolle et al, Nucl. Fusion 61, 116034 (2021)