Electron heating mode in magnetic field diffusion region of microwave discharge ion thruster

Abstract

In magnetic field design principle of microwave discharge ion thruster, it is generally agreed that enlarging the magnetic mirror region can confuse more electrons to acquire better energy utilization rate, while reducing the magnetic field diffusion region can prevent electrons lose at wall to decrease the discharge loss. However, a newly report of integrated simulation proposes a hypothesis that electrons can also be heating at magnetic field diffusion region if considering the Child-Langmuir sheath as a constraint condition for electrons. Therefore, the paper designed a magnetic field structure for the magnet array microwave discharge ion thruster to verify the hypothesis, in which the magnetic field diffusion region is located near the screen grid. Then, an integrated simulation was conducted for study the initial discharge and ion beam extraction stages of the thruster. The simulation results show that at magnetic field diffusion region, the electron temperature is 4-8 eV without grid system voltages, while the electron temperature is 4-12 eV with grid system voltages. And the plasma density of the latter has one order of magnitude higher than that of the former. It means that electrons are obviously heated at magnetic field diffusion region under being constrained between the Child-Langmuir sheath, the plasma sheath at antenna surface, and magnetic mirror. This electron heating mode produces more high energy electrons outside the magnetic mirror region to generate plasma in front of the grid system, which can significantly increase the plasma density and ion beam current density. The result shows that under the conditions of 0.3 sccm Xenon gas flow, 1 W input microwave power, 300 V screen grid voltage and -50 V acceleration grid voltage, the ion beam current and its density is 0.47 mA and 0.60 mA/cm<sup>3</sup> for magnet array microwave discharge ion thruster, while the ion beam current and its density is 1.2 mA and 0.38 mA/cm<sup>3</sup> for 2-cm microwave discharge ion thruster. The ion beam current density increases by 57.9%. Through the integrated simulation, a new electron heating mode at magnetic field diffusion region has been proved theoretically, which will provide the theoretical basis for the magnetic field structure optimization of microwave discharge ion thruster.