Steady supersonically rotating plasmas in the Maryland Centrifugal Experiment

Abstract

The Maryland Centrifugal Experiment MCX [R. F. Ellis, A. B. Hassam, and S. Messer, Phys. Plasmas 8, 2057 (2000)] studies supersonic rotation and enhanced confinement produced by the application of an electric field perpendicular to an axial confining mirror magnetic field; radial shear in the rotation is predicted to stabilize magnetohydrodynamic (MHD) interchange modes. The MCX mirror field is 2.6 m in length, maximum mirror field 1.9 T, maximum midplane field 0.33 T; an inner coaxial core is driven by a 10 KV capacitor bank, producing the radial electric field which drives azimuthal rotation. MCX produces high density (n>1020m−3) fully ionized plasmas and has two operating modes. In the O (ordinary) mode the plasma rotates supersonically with azimuthal velocities in the range of 100 km/s for discharge times exceeding 8 ms. Ion temperatures are ∼30eV and momentum confinement times 100–200 μs. Sonic Mach numbers (uφ∕vti) in the range 1–2 and Alfvén Mach numbers (uφ∕vA)∼0.3 have been achieved for O mode discharges which remain steady for many milliseconds, much longer than MHD instability time scales; plasma lifetime is limited by the capacitance of the capacitor bank. MCX also has an enhanced mode of operation [higher rotation (HR) mode] with higher rotation velocities (>200km∕s), sonic Mach numbers greater than 3, Alfvén Mach numbers >∼0.5, and momentum confinement times of several hundred microseconds. HR mode occurs at higher B fields and lower discharge currents but is transient, transitioning to O mode after a few milliseconds. Both O and HR mode show spectroscopic evidence of radial velocity shear sufficient to satisfy the simplest criterion for MHD stability, but both modes also show significant fluctuations on magnetic probes.