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A hollow cathode is an important component of the discharge circuit in Hall thruster. Different axial positions of the hollow cathode will cause the electrons emitted by the cathode to enter the discharge channel differently, thereby affecting the energy acquisition of the electrons and influencing the ignition of Hall thruster. Thus, a high-speed camera with a charge-coupled device was used to examine the plume features over time when the cathode was located at different axial positions. To better understand the changes in the plasma parameters in the discharge channel and plume during ignition, a two-dimensional axisymmetric fully kinetic particle-in-cell/Monte Carlo collision model was used to calculate the ion density and potential distribution at different times during the ignition process of Hall thruster. The results indicate that the ion density generated in the initial stage of ignition is larger at the exit of the discharge channel when the cathode axial position is close to the exit of the discharge channel. The electron avalanche ionization occurs earlier, and the peak value of the ignition pulse current is larger. These differences may occur because the energies of the electrons (emitted by the cathode) entering the discharge channel from different cathode axial positions differ. • The influence of cathode axial position change on the HET ignition is studied. • The CCD images of HET ignition under two different cathode positions are given. • A 2D-3V fully kinetic PIC-MCC model was used to calculate the plasma parameters. • The ion density and potential at different times during HET ignition are given. • The change of cathode position related to different energy acquisition of electrons. [ABSTRACT FROM AUTHOR]

Abstract A high speed charged coupled device (CCD) camera was used to examine the plume features over time when the cathode was located inside and outside the magnetic separatrix. Due to the different positions of the cathode, there are obvious differences in the electron impact excitation process, the characteristics of the plasma bridge, and the transition process from the end of ignition to the steady-state discharge process. The main reason for these differences may be due to the different conduction paths followed by emitted electrons into the acceleration channel. The CCD images method can be used as a technique to characterize the ignition process of Hall thruster. Highlights • Images of HET ignition with cathode inside and outside magnetic separatrix is studied. • The CCD images of Hall thruster ignition under two different structures are given. • The ignition of Hall thruster affected by position of cathode and magnetic separatrix. • The change of cathode position related to different conduction paths of electrons. [ABSTRACT FROM AUTHOR]

Changing the operating parameters of a cathode affects the ignition process of a Hall thruster mainly by changing the micro-plasma parameters emitted by the cathode at the moment of ignition. However, the micro-plasma parameter that dominates this effect is not clear. Therefore, the ignition voltage thresholds with different cathode operating parameters were first measured. Then, to better understand how these operating parameters affect the ignition voltage threshold of a Hall thruster, the two-dimensional distribution characteristics of the electron density, electron temperature, and plasma potential in the cathode plume region were measured using a Langmuir probe. The results showed that increasing both the keeper current and mass flow rate of the cathode causes an increase in the electron density emitted by the cathode and a decrease in the electron temperature. It was also found that increasing the cathode mass flow rate has a more pronounced effect than increasing the keeper current. Furthermore, when both the mass flow rate and keeper current of the hollow cathode are increased, the ignition voltage threshold of the Hall thruster decreases significantly. This is mainly because of a significant increase in the electron density resulting from the increases in the mass flow rate and keeper current. • The effect of cathode operating parameters on the ignition process of HET is studied. • The ignition voltage thresholds of HET under different operating parameters are given. • A Langmuir probe was used to measure the plasma parameters of cathode plume. • The plasma parameters at different operating parameters of cathode plume are given. • The change of cathode operating parameters related to different plasma parameters. [ABSTRACT FROM AUTHOR]

Hall thrusters must be frequently ignited and extinguished during operation in orbit. The thruster is typically extinguished by terminating the anode voltage. However, the equilibrium state of the plasma in the discharge channel is lost when the external electric field disappears. This consequently changes the state of the plasma, causing rotating spokes to appear. However, spoke rotation ceases after nine rotations. Therefore, in this study, a high-speed camera was used to study the instantaneous spoke rotations of a Hall thruster during the extinguishing process. Additionally, the characteristics of the azimuthal rotating spoke during this process were studied. It was found that, because the anode voltage does not instantaneously decrease to 0 V during the extinguishing process, the spoke mode changes from multi-mode to single-mode. Furthermore, as the voltage dropped from 250 to 0 V, a single-mode spoke appeared, and the spoke speed linearly decreased. • Spokes in Hall thruster channel found during extinguishing process. • The channel discharge image during extinguishing process of HET is studied. • The comparison between the voltage and the speed of the spokes is given. • Spoke speed is linearly attenuated during extinguishing process. [ABSTRACT FROM AUTHOR]