Far-Infrared Laser Interferometry Measurements on the STP-3(M) Reversed-Field Pinch

Far-infrared laser interferometry at 432 µm was carried out on the STP-3(M) reversed-field pinch. Measurements along two vertical chords showed a change from a parabolic-like to a flat-like electron density profile after field reversal. A density profile inversion and a correlated toroidal magnetic flux perturbation were also observed during the transition from the current rising to the current decay phase. Measurements of electron density fluctuations indicated relative fluctuation levels of ~10% for both chords during the current rising phase and ~5% and ~15% during the current decay phase for the central and outer chords, respectively. Spectral analysis showed a ~30 kHz mode consistent with poloidal mode number m=0 magnetic fluctuations, and a ~90 kHz mode localized to the outer region of the plasma, which was strongly excited during the current decay phase and may be connected to particle and energy transport in STP-3(M).