Study On Discharge Range Limit Mechanism And Extension Method Of Hollow Cathode

With the rapid development of commercial spaceflight,electric thrusters have become the mainstream with their advantages of high specific impulse and high efficiency.In recent years,10k W Hall thruster has become a new development direction for deep space detection.The multi-mode operation of the hall thruster requires the hollow cathode–the core components to work in a wide range.At present,the cathode has such problems as narrow discharge range,high working temperature and complex oscillation.In this paper,the limits of self-sustaining temperature and oscillation characteristics on the discharge range of hollow cathode are discussed,and the range performance has been extended by changing emitter materials and changing key dimensions.The limitation of temperature characteristic of hollow cathode to its current range under different working conditions is discussed.The experimental results show that the anode voltage increases when the current changes from the design condition to a high current,which is not conducive to the performance improvement of the thrusting device.Meanwhile,the temperature in the cathode hole area increases rapidly,leading to the risk of melting keeper.The anode voltage also increases when the current is low,but the heat gained by the emitter is not enough to maintain the self-sustaining discharge.Simulation has analyzed the influence of current as energy input on internal plasma parameters and the reason of temperature limiting current extension is expounded from the perspective of thermal deposition change of plasma on the emitter.The limiting factors of the oscillation characteristic of hollow cathode under different working conditions are discussed.The electric signal peak-to-peak value is used to characterize the oscillation intensity.The experiment shows that the oscillation of cathode weakens with the increase of flow rate and current.According to the frequency characteristic of the oscillation will be divided into the loop type and ionization type oscillations both modes,using high-speed cameras to capture specific frequency plasma parameters of the cycle evolution law,combined with single Langmuir probe and RPA measured electron energy distribution function and the ion energy distribution function to analyze oscillation influence on plasma parameters.The oscillation frequency of the loop type oscillation is similar to that of the ionization type oscillation,and the inverse phase relationship between voltage and current exists in both of them.The cause of this oscillation is explained by the plasma throttling instability.The traditional polycrystalline LaB₆ is replaced by a monocrystal LaB₆ with lower work-function and higher thermal conductivity to expand the working scope.Experiments show that single crystal LaB₆ can expand the range to reduce the voltage,but the oscillation is more violent when deviating from the design condition.The effects of the two materials on plasma parameters were simulated,and it was found that single crystal LaB₆ could increase the thermal emission capacity to reduce the voltage and reduce the corrosion of tungsten orifice.Compared with the thermal conductivity coefficient,the cathode discharge performance can be improved more when the escaping work function is reduced,so it should be taken as the first factor in the selection of emitter materialThe working range of the cathode was expanded by changing the keeper hole and orifice of the cathode,changing the position of insulating ceramics and adding shielding structure.Expanding the orifice can continuously reduce the voltage,but it is difficult to maintain under low current condition.Simulation results show that it affects the voltage by changing the thermal deposition.There is an optimal aperture in the keeper hole to ensure the maximum working range of the cathode,which affects the performance mainly by affecting the electronic transport capacity in the plume area.The thermal insulation ceramics must have a gap with the cathode tube to improve the ambient radiation temperature in order to optimize discharge.Contact with the cathode tube will increase the heat loss and reduce the discharge range.Shielding structure can be added to prevent plasma leakage at high current and high flow rate at the expense of partial discharge performance of cathode and reduce corrosion effect to extend the life of cathode.