Effects of negative ions on discharge characteristics of water plasma source for a miniature microwave discharge ion thruster

Abstract

This work analyzes the discharge characteristics of water plasma for a miniature microwave discharge ion thruster via three-dimensional particle-in-cell simulations with Monte Carlo collisions. It incorporates the negative ions (H À , O À , and OH À ) into the simulation code, aside from the three major positive ion species (H 2 O þ , OH þ , and H þ ), and investigates their effects on the discharge characteristics. On the one hand, the simulation results indicate that H 2 O þ is the dominant species and the negative ions have little effect on the positive ion density and transport. On the other hand, this study confirms the axial oscillation of the OH À negative ion density, in addition to azimuthal rotation, causing plasma instability with a periodic low-frequency anomalous diffusion. The azimuthal rotation is similar to the phenomena observed in other E Â B devices, where the difference in spatial distributions of magnetized electrons and unmagnetized ions causes the local potential hump and the resultant instability. Moreover, the axial oscillation, not observed in the previous study considering only positive ions, is due to the generation of OH À by the charge transfer collision reaction between H 2 O and H À , and this oscillation frequency is double that of the azimuthal rotation. Both the azimuthal rotation and the axial oscillation result in the periodic double peaks of the potential time evolution. These fluctuations have an influence on electron transport across the magnetic field.Published under license by AIP Publishing. https://doi.

Electron extraction enhancement via the magnetic field in a miniature microwave discharge neutralizer

“…In a further investigation of the time evolution of the electron density, we observed characteristic plasma structures in the different magnetic fields, i.e., rotating spokes similar to the phenomena observed in HiPIMS 27 and the striped pattern fluctuations observed in our previous studies. 8,22,23 In MF-1 and MF-3, both rotating spokes and striped pattern fluctuations were seen, while only striped patterns were observed in MF-2; these differences can be explained by the Bohm diffusion coefficient,…”

“…The 3D PIC-MCC simulations were based on our previous papers. 8,[22][23][24] In the simulations, we employed the following assumptions: (i) only singly charged xenon ions and electrons are treated as particles;…”

Electron loss mechanisms in a miniature microwave discharge water neutralizer

“…A previous study on the water plasma source revealed that the number density of the total negative ions was at least one order of magnitude smaller than that of electrons under the condition shown in Table I. 18 This result indicates that the extracted negative ion currents have little effect on the neutralization performance of the water neutralizer. Hence, we took into account only electrons for the performance evaluation.…”

“…based on our previous papers. [15][16][17][18][19] First, we set the initial conditions; then, we solved Maxwell's equations using the FDTD algorithm for the electromagnetic fields of the microwaves to obtain a steady-state solution without plasma. Second, we conducted electrostatic PIC-MCC simulations using the time-varying electric field of microwaves calculated by the FDTD algorithm, the electrostatic electric field of plasma, and the magnetic fields produced by permanent magnets.…”

“…However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0002336 field configurations, which are referred to as MF-1 and MF-2; MF-1 is the conventional magnetic field used in previous papers [16][17][18][19] , and MF-2 is the optimized magnetic This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.…”