A heterodyne Phase Contrast Imaging diagnostic for detection of Ion Cyclotron Emission and Helicon waves

The Phase Contrast Imaging (PCI) diagnostic on DIII-D has been upgraded with a novel optical heterodyne scheme to detect Ion Cyclotron Emission (ICE) and Helicon waves. The PCI, an absolutely calibrated interferometer that creates an image of line-integrated electron density fluctuations, is uniquely able to measure the internal structure of such waves thus extending the purely temporal measurements that are otherwise typically used. Arrays of cryogenically cooled detectors used by worldwide PCI systems operate within a 2 MHz bandwidth which, although suitable for broadband turbulence, preclude the study of faster phenomena. The laser beam power is modulated in such a way that the frequency of the wave of interest is within the PCI detector bandwidth, thus making the imaging method applicable at higher frequencies. A transverse Pockels cell made of a water cooled CdTe birefringent crystal, driven by a matched oscillator providing 2 kV pk-pk voltage, is used for the beam modulation. Bench-top tests using a fixed 10 MHz oscillator are in agreement with the expected response. Calibration data will be presented using both a variable frequency oscillator to detect ICE at various values of the confining magnetic field (18-32 MHz), and a fixed frequency oscillator locked to the 476 MHz high power Helicon antenna.