Sub-Alfvénic velocity limits in magnetohydrodynamic rotating plasmas

Magnetized plasmas in shaped fields rely on large, supersonic rotation to effect centrifugal confinement of plasma along magnetic field lines. The results of experiments on the Maryland Centrifugal Experiment (MCX) [R. F. Ellis et al., Phys. Plasmas 12, 055704 (2005)] to document velocity limits are reported. Previous results have shown a limit at the Alfven speed, consistent with equilibrium limits from ideal magnetohydrodynamic theory. Another speed limit, previously reported as possibly related to a critical ionization phenomenon and depending only on the ion species and the shape of the confining magnetic field, is investigated here for a broad range of the applied parameters. We show that this speed limit manifests at sub-Alfvenic levels and that, as externally applied torques on the plasma are increased, the extra momentum input shows up as enhanced plasma density or lower momentum confinement time, accompanied by an increase in the neutral radiation level. Several key parameters are scanned, including the mirror ratio, the length between insulators, and the species mass. We show that this velocity limit is consistent with the species-dependent critical ionization velocity postulated by Alfven.