Nonlinear dynamics of fast ions in cylindrical hall thruster plasmas

Abstract

Research and development of the electric propulsion systems has been performed for a variety of space missions by virtue of advantages of high specific impulse and low propellant mass. A Hall thruster is a promising electrostatic propulsion device which basically utilizes the crossed electric and magnetic fields to accelerate the ion beam and to generate the thrust. A cylindrical type Hall thruster (CHT) is the Hall thruster with the reduced central magnetic core which increases the plasma volume-to-surface ratio and is considered as the more promising type for low power operation. A Hall thruster has cylindrical symmetry to discharge azimuthally uniform plasma. The thruster anode is designed to provide propellant gas into the discharge channel, and its azimuthal uniformity is one of most important factors in the anode design. A mathematical method is developed to reconstruct azimuthal distributions of the propellant gas from the optical emission images of the Hall thruster plasma. The reconstructed distributions are analyzed by using an equivalent electrical circuit model (EECM) to obtain the optimized design for the Hall thruster. A mathematical method is developed to reconstruct the ion energy distribution functions (IEDFs) from the measured EB probe spectrum. The CHT plasmas were observed to have the high energy tail in the reconstructed entire IEDF, and the tail formation is found to be mainly due to singly charged ions. It is parametrically investigated to show strong correlation with the oscillating discharge current at a frequency of ion transit-time scale (360 kHz). Analytical and numerical investigations confirm that the IEDF tail formation is basically due to nonlinear ion acceleration under the ion transit-time field oscillation.