Abstract
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.
Highlights
Electric propulsion is used in a wide variety of high-ΔV space applications, such as satellite constellations, all-electric geosynchronous satellites, very-low-Earth-orbit satellites, and deep space exploration.1,2 To maintain the electric potential of a spacecraft during the operation of electric thrusters, such as gridded ion thrusters and Hall thrusters, neutralizer cathodes are required.Hollow cathodes are commonly used as neutralizer cathodes.3 For Hall thrusters, a hollow cathode serves as a neutralizer and effectively contributes to the production and maintenance of the discharge
The cathode has the same anode currents as those of a flight model in the diode mode
We developed a microwave discharge cathode with a small optical window whose performance is similar to that of the flight model for Hayabusa2
Summary
Electric propulsion is used in a wide variety of high-ΔV space applications, such as satellite constellations, all-electric geosynchronous satellites, very-low-Earth-orbit satellites, and deep space exploration. To maintain the electric potential of a spacecraft during the operation of electric thrusters, such as gridded ion thrusters and Hall thrusters, neutralizer cathodes are required.Hollow cathodes are commonly used as neutralizer cathodes. For Hall thrusters, a hollow cathode serves as a neutralizer and effectively contributes to the production and maintenance of the discharge. A hollow cathode can be operated under a wider current range (from hundreds of milliamperes to hundreds of amperes) compared to that for a microwave discharge cathode.. At more than a few amperes, a hollow cathode can operate in the selfheating mode, which maintains the plasma without heater power, improving thrust efficiently.. We conduct a parametric study in which the microwave power, xenon flow rate, anode current, and microwave antenna condition are varied. The excited ion densities Xe II (3P2)6p[3]5/2 inside and outside the microwave discharge cathode are evaluated based on previous probe measurements in the plume region. The relationship between the measured IVDFs and anode current oscillation is examined This is the first paper to investigate plasma parameters inside and outside a microwave discharge cathode.
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