Design and Construction of an Electron Trap for Studying Cross-Field Mobility in Hall Thrusters

Abstract

Excessive cross-field electron mobility in Hall thrusters has a negative effect on thruster efficiency and has been shown experimentally to be much larger than predicted by classical collisional transport theory. An electron trapping apparatus was constructed at Michigan Tech's Isp Lab in order to study electron dynamics in the defining electric and magnetic fields of a Hall-effect thruster. This apparatus was designed to stably trap a non-neutral electron plasma in a confining volume in order to study these dynamics in a highly controlled environment. Electrons are confined using only electric and magnetic fields in the absence of ions and dielectric walls, which are present in a typical Hall thruster. Mobility studies on a low-density, non-neutral plasma provide several advantages over a typical Hall thuster's quasi-neutral plasma, including a well-defined electric field and the ability to take internal electrostatic probe measurements in the "acceleration" region. Cross-field electron mobility was investigated in response to magnetic and electric field strengths and background neutral density. Experimental design considerations including loading mechanisms, trapping potential, magnetic field design, calibration, and diagnostic techniques are presented along with preliminary experimental results. In this investigation, measured cross-field mobility is much larger than classical theory predicts and appears to be consistent with Bohm-like mobility rather than classical mobility. = permittivity of free space e = elementary charge E = electric field vector E z = axial electric field F µ = magnetic mirror force F E|| = parallel electric field force Nomenclature= current density at the anode J ez = cross-field electron current density J p = current density at the probe m e = electron mass µ, µ 0 = magnetic moment of gyrating particle µ ez = cross-field electron mobility n e , n " = electron number density n 0 = neutral particle density

Energy Efficiency in Low Voltage Hall Thrusters

“…There is considerable research regarding relationship between current efficiency and magnetic field curvature and strength. 8 The magnetic field controls the Hall current and subsequently much of the electron mobility characteristics contributing to large discharge current/low current efficiency. The acceleration efficiency's dependence on the magnet field strength, however, is largely unknown.…”