Plasma parameters measured inside and outside a microwave-discharge-based plasma cathode using laser-induced fluorescence spectroscopy

An understanding of the plasma physics inside a microwave discharge cathode is key to extending the lifetime of microwave ion thruster systems. However, probes can only measure the plume region due to their low spatial resolution and electromagnetic disturbance. In this study, we develop a microwave discharge-based cathode with a small optical window in the discharge chamber that provides visual access to the cathode interior. The cathode has the same anode currents as those of a flight model in the diode mode (anode voltage error is within 7%). Laser-induced fluorescence spectroscopy is applied to the cathode. The axial and radial ion velocity distribution functions (IVDFs) in the plume region and the axial IVDFs inside the cathode are measured. The measured functions, which represent the number density of Xe II (3P2)6p[3]5/2, are compared to a previously reported number density of Xe II measured by an electrostatic probe in the plume region. The functions exhibit multimodal characteristics. Theoretical models based on the measured current oscillation support these characteristics.