Radially resolved measurements of plasma rotation and flow-velocity shear in the Maryland Centrifugal Experiment

Abstract

In diagnosing the Maryland Centrifugal Experiment (MCX) [R. F. Ellis et al., Phys. of Plasmas 8, 2057 (2001)], earlier spectroscopic measurements of averaged plasma rotation velocities have been upgraded to include radial distributions, using a five-channel fiber-optic collection system. Detailed information from each view is now possible with an 8-times increase in spectral resolution, by using a 2m spectrograph and a 2400lines∕mm grating. Inversion of the integrated chordal radiation into a radial dependence of local emissions is performed by two methods: (a) an iterative simulation beginning with assumed emissions in five axially concentric cylindrical zones followed by summation along the five viewing chords, and (b) inversion of a combination of dual Abel-type matrices. The radial profiles of the absolute velocities derived cover a range from 20to70km∕s for both C+ and C++ impurity ions. Previous apparent differences in velocities between ions from a single chordal observation are now explained by the measured radial dependence of velocities and relative emissions. An important result is the first direct and quantitative measurement on MCX of a radial shear in rotational flow velocity as large as 9×105s−1, 9 times a threshold of 1×105s−1 for magnetohydrodynamic stability. Stark-broadened hydrogen Balmer-series spectral lines provide both a value for electron density of Ne=(8.5±1.5)1014cm−3 and supporting data for radial particle distributions.