Real-time plasma rotation diagnostic for measuring small Doppler shifts

Abstract

A noninvasive velocity measurement has been developed for use in high-temperature plasma confinement devices, having particular use where multi-kilohertz measurement of plasma rotation is needed. The most interesting aspect of the technique is that the Doppler shift is determined from the ratio of the light intensity from two detectors rather than by resolving the emission line with a spectrometer. One detector views the plasma through an interference filter whose passband has a negative slope, and the other channel views the identical volume of plasma through a positive-slope filter. The signal ratio varies as the line is shifted across the passbands, but is not sensitive to changes in plasma emission. For interference filters with linear passbands, that is, constant slopes, the ratio is not sensitive to ion temperature, and the shifted wavelength reduces to a simple function of the signal ratio, the channels’ relative responsivity, and the two filters’ transmission curves. Filters with sharp passband slopes (100%–200% per nm) and ≈0.1% deviation from linearity have been fabricated. For an uncertainty in the signal ratio of ±1%, the expected error in the wavelength shift is ±0.001 nm (Δvφ≈±1 km/s).