Electron loss mechanisms in a miniature microwave discharge water neutralizer

This study analyzes the mechanism of electron loss at the discharge chamber wall of a microwave discharge neutralizer via three-dimensional particle-in-cell simulations with Monte Carlo collisions (PIC–MCCs). The neutralizer employs electron cyclotron resonance discharges with two ring-shaped permanent magnets and 4.2-GHz microwaves, where the plasma is confined by a magnetic mirror. The PIC–MCC simulation results show that the electron extraction efficiency of a water neutralizer can be increased by two times in an optimized magnetic field configuration, which is a higher increased rate than that of a xenon neutralizer. However, the efficiency of 20% is still low (e.g., less than half of the xenon one) because many electrons are lost to the magnet surface. The loss is determined to be due to approximately 5-times higher ratio of electrons inside the loss cone in the water neutralizer than that in the xenon neutralizer. The electron velocity distributions of each neutralizer clearly show that the water neutralizer has a larger fraction of electrons parallel to the magnetic field than the xenon neutralizer. This result is attributed to the large number of electron collisions in the water neutralizer owing to the high neutral gas pressure.