Measurements of rotational velocity shear and interchange stabilization in the Maryland centrifugal experiment

The Maryland Centrifugal Experiment (MCX) produces supersonically rotating plasmas in a mirror geometry with a radial electric field produced by a coaxial core biased at high voltage. MCX has achieved high density (n > 1020 m-3) fully ionized plasmas rotating supersonically with velocities of ∼100 km/sec for times exceeding 8 ms under a wide range of conditions. Ion temperatures are 30 eV and confinement times ∼100 microseconds. Sonic mach numbers are 1-2 and Alfven mach numbers somewhat less than 0.5 for standard discharges. MCX has also demonstrated an enhanced mode of operation with sonic mach numbers greater than 3, confinement times of several hundred microseconds and Alfven mach numbers near one. Plasmas remain grossly stable, or steady, for many milliseconds, much longer than MHD instability timescales for MCX, though significant magnetic fluctuations are measured with magnetic probes. Measurements of rotational velocity profiles are made employing a five channel high resolution spectroscopy system, viewing across the plasma. Profiles for impurity emission lines and neutral Hydrogen are measured. Abel-like inversions of the emissions from the five chords clearly show that the profile of angular velocity is parabolic type. The peak and average velocities are fully consistent with the direction and the magnitude of the rotation velocities inferred from the voltage and B field. Rotational velocities for different charge states (C+ and C++) agree within error bars, which should be the case since the E x B drift dominates other drifts. The velocities measured for Hα are lower than those for the charged states, as expected. The inferred emissivity radial profiles all show a hollow structure with the plasma interior substantially depleted. Most importantly the shear in the rotation profile is calculated. In a database consisting of both O and HR modes, the HR mode shear exceeds the theoretically required shear over most of the radial profile and exceeds the threshold by up to factors of 5. For the O mode, the observed shear is of the same order as and somewhat exceeds the theoretical threshold. These observations are essential in establishing the physics of velocity shear stabilization of MHD interchanges in rotating plasmas. (author)