Scaling of the Ionization Fraction in a Self-Field MPD Thruster
Using spectroscopic measurements of ArI and ArII line intensities in the discharge chamber of a steady-state self-field magnetoplasmadynamic thruster (MPDT) operated over a wide range of operation conditions, it is shown that the axial profiles of the ionization fraction α in the chamber scales with a similarity parameter ξ (the “MPDT scaling number"), which combines the operation parameters in a non-dimensional number, and was found previously to be a fundamental parameter scaling many aspects of MPDT performance. The ionization fraction at a given axial location within a large upstream portion of the chamber was found to exhibit a linear scaling with ξ irrespective of the dimensional operation parameters. The scaling reveals a distinct transition between two modes of operation, ξ ≤ 1 and ξ > 1. This transition, and the flattening of the axial profiles of α at a high value as ξ nears 1, suggest that by raising ξ through 1 the thruster transitions from a mode where the acceleration and ionization power sinks are competing, to a more efficient mode where the ionization sink is largely saturated, thus allowing the acceleration power to increase more vigorously with input power.