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 (H2
, 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, H2
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.
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