Charge and Mass Considerations for Plasma Velocity Measurements in Rotating Plasmas

Abstract

Velocity of hydrogen plasmas rotating due to imposed E × B fields at the Maryland Centrifugal Experiment (MCX) [R. F. Ellis, et al., Phys. Plasmas 12, 055704 (2005)], where E is the electric field in the radial direction and B the magnetic field in the axial direction of a cylindrical configuration, has traditionally been measured using Doppler shifts of atomic spectra from impurity elements such as carbon. Ideally, the gyrocenter of trace particles rotates at the bulk plasma velocity, regardless of the charged state or trace particle mass. However, for sufficiently large applied |E/B| (or equivalently, a sufficiently large ratio of bulk plasma rotation frequency and particle gyrofrequency), charged particles may have gyroradii that depart significantly from quasi-circular orbits drifting about the B field axis. This effect is investigated numerically with a single particle code that includes scattering, as well as experimentally at MCX. Numerical findings are compared to experimentally measured Doppler shifts of singly inonized helium and oxygen, and doubly ionized carbon atoms.

Isorotation and differential rotation in a magnetic mirror with imposed E×B rotation

“…Particle mass-to-charge ratio, q/m, was recently realized to be an issue in rotating plasmas. 18 The relatively high E fields required to sustain discharges in MCX would yield particle trajectories that significantly differ from ideally circular trajectories, depending on q/m. He II was chosen because q/m is the most similar to that of hydrogen, while still emitting visible radiation that can be captured by our spectrometer.…”