Abstract Supersonic Rotation in the Maryland Centrifugal Experiment

The Maryland Centrifugal Experiment (MCX) has been built to study the confinement of supersonically-rotating plasmas and velocity shear stabilization of MHD instabilities. Theory predicts improved stability and confinement when a strong radial electric field is introduced into a magnetic-mirror geometry. The resulting radial currents establish a stable highly sheared plasma rotating at supersonic velocities in the azimuthal direction under the influence of J×B forces. This arrangement leads to increased confinement because the supersonic rotation creates an artificial radial gravity which draws the plasma away from the mirrors, closing the mirror loss cone. The large vφ shear stabilizes the plasma and enforces laminar flow. Based on these concepts, we have designed and constructed a machine to produce supersonically rotating highly-ionized plasmas. It typically does this by introducing a radial voltage of 7 kV in a magnetic-mirror geometry, 2 kG at the midplane and 19 kG at each mirror. MCX has completed its main construction phase and is acquiring data, here analyzed primarily in terms of a circuit model which infers plasma characteristics from the radial voltage across the plasma and the total radial current. The theory and simulations supporting the MCX centrifugal confinement scheme are presented here with the data and analysis from its first nine months of operation, including a description of basic plasma characteristics and evidence for both stability and confinement. Theory, simulation, and initial experimental data all indicate that this centrifugal confinement scheme provides good stability and confinement at the temperatures and densities under study, as well as at the larger temperatures, fields, and dimensions expected for a fusion reactor. In particular, spectroscopic and circuit-model data indicate rotational velocities in MCX of up to 100 km/s, ion temperatures of approximately 30 eV, and ion densities upwards of 1020m−3. These parameters give rotational Mach numbers between 1 and 2 and imply ∂rvφ ∼ 106s−1. Measurements of the loss times found via our circuit model indicate the neutral density is typically a few times 1017m−3. Calculations based on a zero-dimensional MHD model indicate that the plasma is collisional and highly ionized. In this paper, we outline the direct and indirect evidence for supersonic flow, high (1020m−3) ion density, scarce neutrals (∼ 1 neutral per 1000 ions), and a plasma state which is at least quasi-stable. Some notes are given on improvements to the models and how these affect the calculations. We also describe planned improvements to the MCX machine and its diagnostics. SUPERSONIC ROTATION IN THE MARYLAND CENTRIFUGAL EXPERIMENT