Abstract Details

Presentation:submitted:by:
austin.pdf2006-03-09 16:21:07catalin teodorescu

Measurements of radial profiles of rotational velocity on MCX and assessment of velocity shear stabilization

Author: catalin teodorescu
Submitted: 2005-12-20 14:28:28

Co-authors: R. Elton, J. Ghosh, A. Case, R. Clary, R. Ellis, A. Hassam, R. Lunsford

Contact Info:
University of Maryland
1202L Energy Research Facility
College Park, MD   20742
USA

Abstract Text:
The mission of MCX is to test the concept of centrifugal confinement and investigate velocity shear stabilization of the interchange instability of simple mirrors. Measurements of the radial profiles of rotational velocity have been made employing multi-chord high resolution spectroscopy of impurity ion spectral lines and these profiles show velocity shear sufficient to satisfy simple models of shear stabilization. MCX has a mirror geometry with an inner coaxial core biased with respect to the vacuum vessel producing a radial electric field which drives azimuthal rotation; plasma fills the radial region between the core and the wall. Spectroscopic measurements were made of Doppler shifted spectral lines for five chords viewing at different radial impact parameters across the plasma column. Abel-like inversion techniques are then employed to obtain the plasma species averaged rotation velocity at five radial locations. Emission lines were measured for C+, C2+, and C3+ ions and neutral H (Hα and Hβ lines). The system employs an ICCD camera suitable for 2-D scans and either a 1-m Czerny Turner design stigmatic spectrometer with a reciprocal dispersion in the first order of 0.692 nm/mm or a 2-m spectrometer with a dispersion of 0.182 nm/mm. For these data MCX was operated with mirror ratio of 5-9 and midplane B of 0.2-0.3T; the plasmas were high density (n>3x1020 m-3) and highly ionized. A small number of discharges have been analyzed in detail for C+, C2+, and C3+ spectral lines. They show peak rotational velocities of ~ 100 km/s and relatively smooth parabolic type profiles with small rotation at the inner and outer plasma radii. The emissivity profiles peak near the plasma edges and are often very high near the inner core of the vacuum vessel. Within substantial error bars the different carbon species are seen to rotate with approximately the same angular velocities, consistent with E×B being the dominant drift speed as evaluated for the MCX parameters. C+ emission in many other discharges is also analyzed. The velocity shear in the flanks of the profiles is compared to that theoretically necessary for stabilization of the interchange mode. All the impurity line data show that the shear stabilization criterion is exceeded. This is consistent with the relatively quiescent I-V traces of MCX with confinement times that exceed MHD instability growth times by a large factor. Magnetic fluctuations have been measured with magnetic probes in an azimuthal array as well as probes at different axial locations and these data will also be presented to assess MHD stability. Results from Hα and Hβ spectral lines also show hollow emissivity profiles and the angular rotation profiles will be presented. Comparative spectroscopy at different axial positions on MCX is underway; rotation data for heavy impurities is relevant for testing the efficacy of centrifugal confinement, since heavy impurities are expected to be very well confined.

Characterization: A5

Comments:

The University of Texas at Austin

Innovative Confinement Concepts Workshop
February 13-16, 2006
Austin, Texas

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