1D profile measurement of ion velocity distribution with glass-tube-pair type Doppler probe array

Abstract

We developed a glass-tube-pair type Doppler probe array for ion velocity distribution function measurement with high spatial and time resolution. By four mirrors and four optical fibers fixed in two parallel glass tubes, the light from ions in each measurement area (3.2 × 2.5 × 1.0 cm3) between tubes is collected in three different directions, and the fibers are led to a Czerny–Turner spectrometer and finally to an ICCD camera (the exposure time of 2 μs) for Doppler spectra measurements with high time resolution. Since the Doppler spectrum profile in each viewing line represents the distribution of the ion velocity component in the line, we can reconstruct 2D velocity distribution by the measured spectra in three viewing lines with tomography techniques. We tested the accuracy of some reconstruction methods from spectra to velocity distribution in numerical simulation and chose the iterative method as the best one. By use of this new diagnostic, we successfully measured the radial profile of ion velocity distribution during magnetic reconnection of merging spherical tokamak plasmas and revealed that their ions have anisotropic velocity distributions.