Abstract Details

icc2006_ellis_final.pdf2006-03-08 10:14:46Richard Ellis

Progress and plans on the Maryland Centrifugal Experiment

Author: Richard F Ellis
Submitted: 2005-12-20 15:36:05

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

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

Abstract Text:
The Maryland Centrifugal Experiment (MCX) has achieved high density (ne > 1020 m-3 ) fully ionized plasmas rotating supersonically with azimuthal velocities in the range of 100 - 250 km/sec with ion temperatures typically 30 eV. Sonic Mach numbers are in the range of 1 to 3 and Alfvèn Mach numbers are somewhat less than unity. Plasmas remain stationary for milliseconds, much longer than MHD instability timescales. MCX exhibits two modes of operation: an ordinary mode (O-mode) and a high rotation mode (HR-mode). MCX has implemented extensive new diagnostics including a multi-chord ion Doppler spectrometer, arrays of magnetic probes, an Ha emission detector array; a two color interferometer is nearing completion. A higher voltage (20 kV) discharge capacitor bank has been implemented to study velocity limits.
Results on radial velocity profiles are reported in an accompanying paper at this meeting and they show velocity shear sufficient to stabilize MHD instabilities for both operating modes.
The H-alpha detectors employ high-speed photodiodes; light is collected from various positions along the axis viewing in the radial direction. The emission in O mode substantially exceeds that in HR-mode and HR-mode emission is much more peaked axially at plasma midplane. These data support the conclusion that HR-mode plasmas are somewhat detached from the end insulators. Multi-chord measurements will be presented employing a single moveable detector to obtain radial neutral density profiles and a multi diode array is being designed.
The magnetic fluctuation diagnostics have been extensively upgraded to include sixteen probe Bz arrays near midplane as well as symmetric axial points close to the mirror throats; four x,y,z triplet probes are also arrayed at varying axial locations. All probes are located at the plasma edge and have frequency response near 500kHz. Data show that magnetic activity varies substantially between the operating modes (O and HR) with deltaB/B of 0.3 to 3 % ; HR mode shows higher activity and higher frequencies. The turbulent frequency spectra are characterized by a small number of broad peaks with widths comparable to the center frequency. At late times coherent modes are observed. Data will be presented on the azimuthal and axial correlation of the fluctuations.
The critical ionization velocity (CIV) phenomenon which may be operating in the vicinity of the end insulators due to a plasma-neutral gas interaction has been studied. It is found that the O-mode speeds are close to the CIV limit while HR-mode speeds are above the limit by greater than a factor of two. The ability to reach the supercritical state would be aided by a marked plasma detachment from the insulators and the presence of supercritical speeds supports the possibility of enhanced centrifugal confinement.
The major upgrade plans include increasing the midplane magnetic field to 1 T and installing extensive surface conditioning.

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