Localized measurement of short wavelength plasma fluctuations with the DIII-D phase contrast imaging diagnostic

A novel rotating mask system has been designed and implemented on the DIII-D phase contrast imaging (PCI) diagnostic to produce the first spatially localized PCI measurements of a tokamak plasma. The localization technique makes use of the variation in the magnetic field component perpendicular to the viewing chord as a function of chord height. This new capability provides measurements in the range of 2<k<30 cm-1, 10 kHz<f<10 MHz, and 0.7<r/a<1. This technique provides a spatial resolution of 10 cm at k=15 cm-1 and can realistically provide measurements at a rate of 10 profiles/s. Calibration measurements show accurate characterization of the system transfer function making feasible a time dependent analysis that results in improved localization. Initial measurements show turbulence to peak near the plasma edge. This upgrade is part of a broader program to operate the DIII-D PCI at wave numbers up to 40 cm-1 to probe electron scale turbulence in the plasma core.