Ion Rotational Velocity Diagnostic for the Centrifugal Mirror Fusion eXperiment

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. The cylindrical, mostly hydrogen CMFX plasma is set into azimuthal E x B rotation in a magnetic mirror. The resulting rotational velocity shear can suppress magnetohydrodynamic instabilities. Therefore, the measurement of ion rotational velocity profiles is critical to the characterization of the CMFX plasma. 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 neutrals (H I, 656.3 nm). 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 updated design of the IDS system and the inversion method, along with the calibration procedure and initial spectroscopic measurements.