Millimeter-wave reflectometry for electron density profile and fluctuation measurements on NSTX

A millimeter-wave reflectometry system for electron density profile and fluctuation measurements is being developed and installed on the National Spherical Torus Experiment. The initial frequency coverage will be in the bands 12–18, 20–32, and 33–50 GHz, provided by frequency-tunable solid-state sources. These frequencies correspond to O-mode cutoff densities ranging from 1.8×1012 to 3.1×1013 cm−3, which will span both the plasma core (ρ=r/a<0.8) and edge (ρ>0.8) regions. Operated as a broadband swept-frequency (frequency-modulated continuous-wave) reflectometer, the diagnostic is expected to provide routine (shot-to-shot) time- (⩽50 μs) and spatially resolved (∼1 cm) density profiles. The previous hardware can be easily reconfigured as a fixed-frequency reflectometer for density fluctuation measurements. The combination of measurements would be valuable for studying phenomena such as possible L- to H-mode transitions and edge-localized modes.