Ion Rotational Velocity Measurements for CMFX

A multi-chord ion Doppler spectroscopy (IDS) diagnostic has been developed for the Centrifugal Mirror Fusion Experiment (CMFX) at the University of Maryland, College Park. H-He or D-He plasma is set into azimuthal E x B rotation in a cylindrical magnetic mirror, and the resulting rotational velocity shear can suppress magnetohydrodynamic instabilities. A fiber-optic array consisting of ten tangential viewing chords is set up to take ten simultaneous line-integrated spectroscopic measurements of the intensity, Doppler broadening, and Doppler shift of introduced impurity helium emission (He II, 468.6 nm) at the midplane. The system is easily reconfigured to measure emission from hydrogen (H I, 656.3 nm) or deuterium (D I, 656.1 nm) neutrals, depending on the input gas configuration. The diagnostic can be moved to different positions along the axis of symmetry. Radial profiles of emissivity and rotational velocity are obtained via an Abel-like matrix inversion of the spectra. We present the design of the IDS system, with a new fiber bundle, along with the inversion method, calibration procedure, and spectroscopic measurements.