Numerical Analysis of a Miniature Microwave-discharge Ion Thruster Using Water as the Propellant

Abstract

We have conducted three-dimensional particle-in-cell simulations with Monte Carlo collisions method (PIC/MCC) to investigate the discharge characteristics of water for a microwave-discharge ion thruster, which can replace a high-pressure gas storage system. In the calculation model, three ion species (H 2 O + , OH + , and H +) are taken into account. The PIC/ MCC results indicate that the electron density, the potential, and the electron temperature are the highest near the ECR region, and in order to obtain a similar electron density, high absorbed power is required compared to the results using xenon as the propellant. For the ion composition ratio of the water discharges, H 2 O + and OH + dominate over H + and occupy more than 97%. However, about 10% of the ion current density is derived from H + because of its lightweight and the effect of hydrogen ions would not be negligible for ion beam extraction.

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

“…This result is consistent with the results of previous work, wherein only the positive ions were considered. 13 The presence of negative ions is perceived to have little impact on both the density and the velocity distribution of the positive ion species as indicated by the total negative ion density occupying approximately only 10% of the electron density. In other words, without negative ion inputs, the PIC-MCC simulation gives unmagnetized positive ion profiles with low calculation cost, which would be useful for the calculation of ion beam profiles.…”

“…14 A fully kinetic PIC code was developed for investigating the discharge characteristics of water plasma discharges having negative ions; the same code was employed in the previous paper, with the exception of the collision model. 13,15 A brief description of the model is provided in Secs. II A-II C. Figure 1 schematizes the calculation model and contour plots of the magnetic field strength, for a 20 Â 20 Â 4 mm 3 plasma source with a ring-shaped antenna and permanent magnets.…”

“…With the magnetized electrons, a high-density region appeared along the magnetic field lines passing through the ECR area and its distribution was confined by magnetic mirror fields, leading to the low density near the z-axis, consistent with the results of previous work. 13 Conversely, the unmagnetized negative ions formed a high-density region that was confined in the plasma potential with a relatively high-density distribution even near the zaxis. The existence of the negative ions mitigated the concentration of negative charge near the ring-shaped antenna, where the ECR occurs, thus resulting in a little more uniform distribution of the current density than that without negative ions shown in Fig.…”

“…The difficulty in conducting plasma diagnostics in experiments and identifying the dominant ion species in a small plasma source is understood in this work; hence, water plasma discharge characteristics are analyzed in threedimensional (3D) particle simulations. 13 Three dominant positive ion species (H 2 O þ , OH þ , and H þ ) were considered for the authors' previous simulations with particle-incell Monte Carlo collisions (PIC-MCCs). However, the presence of negative ions in water plasma discharges accounted for predictions of their impact on the plasma potential structure and charged particle transport across the magnetic field.…”

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

“…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;…”