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The Maryland Centrifugal Experiment and Velocity Shear Stabilization of Ideal MHD Interchanges
| Author: | Hassam A. B. |
| Coauthor: | A. Case, R. Ellis, R. Elton, J. Ghosh, H. Griem,, Y. Huang, R. Lunsford, R. McLaren, S. Messer, S. |
| Institution : | University of Maryland |
| Abstract text: | The mission of the MCX is to assess velocity shear stabilization of MHD interchange modes and the efficacy of centrifugal confinement. For no rotation, the solenoid + magnetic mirror geometry of MCX is expected to be interchange unstable on ideal MHD time scales. Currently, MCX routinely produces supersonic rotating plasmas. Spectroscopy measurements show Doppler shifts above and below the midplane consistent with average ExB azimuthal rotation. Peak speeds of more than 100km/s are measured, with Doppler broadening ion temperatures of 30-50eV, corresponding to ion-thermal Mach numbers of 1-2 (see also Teodorescu et al on newly discovered \"high-rotation mode\"). The voltage trace across the plasma is relatively steady for as long as the external voltage is applied - this could be as long as 10ms. By comparison, the growth time of ideal MHD interchange modes ranges from 2-20 microsecs. Certainly, no \"major disruptions\" of the discharge are seen. For the MCX geometry, there are no kink modes; interchange modes are not expected to result in termination, rather in strongly degraded confinement. Momentum confinement times have been measured on MCX, from stored energy or by allowing the plasma to spin down freely. These times range from 200-600 microsecs. The steadiness of the plasma voltage over 1000\'s of MHD instability times as well as the large ratio of confinement to MHD times is suggestive of stabilization of flutes but does not rule out low level convection. Since the central neutral density, estimated at less than 1%, is not directly known, residual convection is not easily assessed from confinement times alone. The underlying MHD theory and results of MHD numerical simulations for MCX geometry will be presented and considered with the experimental data. MCX experimental plans to address velocity shear stabilization and residual convection questions will be discussed. |
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