Electron extraction mechanism of magnet array microwave discharge neutralizer

Abstract

Microwave discharge neutralizer is an important part of microwave discharge ion thruster system, which plays a vital role on maintaining potential balance of spacecraft and neutralizing ion beam. Its electron extraction property directly affects the operation condition of ion thruster system. In order to break through the power limit of miniature microwave discharge ion thruster, a magnet array microwave discharge ion thruster system was designed and tested. During the magnetic field structure testing experiments of magnet array microwave discharge neutralizer, an interesting phenomenon was found. When turning around the magnet array, the I-V curves of electron current show a notable difference. Defining forward direction of magnet array can normally extract electrons, then backward direction of that can hardly extract electrons. Because the diameter of discharge chamber is only 10 mm, it is too tiny to use Langmuir probe diagnosis. And thus, an integrative particle-in-cell method was used to simulate the neutralizer operation processes of two different magnetic field structures, in which the real vacuum permittivity was applied for accuracy. The simulation results show great agreement with experiment phenomenon. In initial discharge process, it is found that the magnetic field gradient leads to different plasma distributions; in electron extraction process, it is found that the potential distribution near the orifice determines the electron extraction property of the neutralizer. Through comparing the plasma parameter distributions under different magnetic field structures and operating voltages, an assumption that the ion is an important factor in electron extraction process was proposed. Then, a simulation that ions disappear artificially outside the orifice was conducted, and the simulation results show that electrons cannot be effectively extracted without ions near the orifice. According to the simulation and experiment results, two necessary conditions are summarized for electron extraction of the neutralizer. The first one is magnetic field structure: the magnetic field gradient should point towards the orifice to lead plasma migrate towards the orifice. The second one is potential distribution: there should be enough ions to lift the potential near the orifice for decreasing or breaking the potential well. These two conditions can help understand the electron extraction mechanism of microwave discharge neutralizer and provide theoretical reference for performance optimization of the neutralizer in future.