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Experimental evidence of MHD plasma centrifugal confinement in a shaped open magnetic field configuration

Author: Catalin Teodorescu
Requested Type: Consider for Invited
Submitted: 2009-12-02 21:38:57

Co-authors: W.C. Young, R.F. Ellis, A.B. Hassam, C.A. Romero-Talamas

Contact Info:
University of Maryland
1202B Energy Research Building
College Park, MD   20742
USA

Abstract Text:
The three central goals of the Maryland Centrifugal Experiment are to demonstrate supersonic ExB rotation, stability to flute modes, and confinement of plasma on open field lines from centrifugal forces of rotation. The first two goals have been met previously, with spectroscopic confirmation of supersonic sheared rotation and routine demonstration of disruption-free discharges staying relatively quiescent for 1000’s of flute growth times and 10’s of momentum confinement times. In this work, we demonstrate for the first time parallel density confinement by direct interferometric measurements of electron density. The MCX is an axisymmetric, strongly magnetized plasma, created in a shaped open-field magnetic mirror configuration. A central core biased at high voltage sets up azimuthal supersonic rotation. Simultaneous interferometric measurements of plasma density are made at the magnetic minimum (midplane) and 85 cm axially off-midplane. Complete time histories of density at these two locations are obtained and compared to deduce the efficacy of axial confinement. Other key parameters are also directly measured at midplane (rotation velocity profiles, Doppler-broadening ion temperature, and diamagnetic flux) and off-midplane (diamagnetic flux). The observed scaling of the density ratio of midplane to off-midplane is obtained as a function of the shape of the magnetic field (mirror ratio) and of the sonic Mach number, and the data are compared with the ideal MHD Grad-Shafranov equation extended to include azimuthal supersonic rotation. The sonic Mach number and the mirror ratio are key parameters in the theory. The data are shown to be in good agreement with the predictions of the ideal MHD equilibrium theory. Ratios between the density at midplane and off-midplane of up to 15 times are obtained for mirror ratios of 8 and sonic Mach number around 2. This ratio decreases with decreasing mirror ratio. In particular, the density ratio becomes of order unity for when the field shape is such that the two interferometers are on an “isobar”, and the density ratio flips for low mirror ratios corresponding to when the magnetic minimum moves to off-midplane.

Characterization: C,E3

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