Influence of applied magnetic field in an air-breathing microwave plasma cathode

Abstract

The air-breathing electric propulsion (ABEP) concept refers to a spacecraft in very-low Earth orbit (VLEO) ingesting upper atmospheric air as propellant for an electric thruster. This compensates atmospheric drag and allows the spacecraft to maintain its orbital altitude, removing the need for on-board propellant storage and allowing an extended mission duration which is not limited by propellant exhaustion. There is a need for development of a robust, high current density and long life cathode (or neutralizer) for air-breathing electrostatic thrusters as conventional thermionic hollow cathodes are susceptible to oxygen poisoning. An Air-breathing Microwave Plasma CAThode (AMPCAT) is proposed to overcome this issue through the use of a microwave plasma discharge, producing an extracted current in the order of 1 A with 0.1 mg/s of air. In this paper, the effect of varying magnetic-field strength and topology is investigated by using an electromagnet coil, which reveals a significantly different behaviour for air compared to xenon. The extracted current with xenon increases by 3.9 times from the zero-field value up to a peak around 150 mT magnetic-field strength at the antenna, whereas an applied field does not increase the extracted current with air at nominal conditions. A non-zero magnetic-field with air is however beneficial for current extraction at reduced neutral densities. A distinct increase in extracted current is identified at low bias voltages with air for a field strength of around 50 mT at the internal microwave antenna, consistent across varying field topologies. The effect of a lowered magnetic-field strength in the orifice region is investigated through the use of a secondary coil, resulting in an extracted current increase of 25 % for a relaxation from 6 mT to 1 mT, and demonstrating the beneficial impact of a locally reduced field strength on electron extraction.

Performance and plasma diagnostics of the Air-breathing Microwave Plasma CAThode (AMPCAT) coupled to a cylindrical Hall thruster

“…The operating principles of the AMPCAT discharge are detailed further in previous publications. 13,14 Differently from other microwave cathodes:…”

Inception of positive wire-cylinder corona discharges in air in crossed electric and magnetic fields

“…Studies of plasma properties under the joint action of electric and magnetic fields are of great interest for a broad range of applications, including plasma propulsion, magnetron sputtering, magneto-hydrodynamic flow control and power generation, plasma processing techniques for thin-film deposition and surface modification, among others. The effects of applied magnetic fields on the kinetics and transport properties of electrons [1][2][3] and on the characteristics of discharges of various types ( [4][5][6][7][8][9] and references therein) are actively studied. One of the important effects is the influence of applied magnetic fields on discharge inception and breakdown voltages [10].…”