Investigation of Plasma Detachment From a Magnetic Nozzle in the Plume of the VX-200 Magnetoplasma Thruster

Understanding the physics involved in plasma detachment from magnetic nozzles is well theorized, but lacking in large scale experimental support. We have undertaken an experiment using the 150-m
³
variable specific impulse magnetoplasma rocket test facility and VX-200 thruster seeking evidence that detachment occurs and an understanding of the physical processes involved. It was found that the plasma jet in this experiment does indeed detach from the applied magnetic nozzle (peak field sim~2 T) in a two part process. The first part involves the ions beginning to deviate from the nozzle field 0.8-m downstream of the nozzle throat. This separation location is consistent with a loss of adiabaticity where the ratio of the ion Larmor radius to the magnetic field scale length (r_Li|∇ B|B ) becomes of order unity and conservation of the magnetic moment breaks down. Downstream of this separation region, the dynamics of the unmagnetized ions and magnetized electrons, along with the ion momentum, affect the plume trajectory. The second part of the process involves the formation of plasma turbulence in the form of high-frequency electric fields. The ion and electron responses to these electric fields depend upon ion momentum, magnetic field line curvature, magnetic field strength, angle between the particle trajectories, and the effective momentum transfer time. In stronger magnetic field regions of the nozzle, the detached ion trajectories are affected such that the unmagnetized ions begin to flare radially outward. Further downstream as the magnetic field weakens, for higher ion momentum and along the edge of the plume, the fluctuating electric field enables anomalous cross-field electron transport to become more dominant. This cross-field transport occurs until the electric fields dissipate 2-m downstream of the nozzle throat and the ion trajectories become ballistic. This transition to ballistic flow correlates well with the sub-to-super Alfvénic flow transition (β
_k
). There was no significant change observed to the applied magnetic field.