Study On Lifetime Optimization Of An Electric Propulsion Hollow Cathode In Wide Operating Condition Range

The electric propulsion system with wide thrust range has become a hotspot of the research on space electric propulsion technology in recent years.The demands for hollow cathodes that provide electrons for ion thrusters and Hall thrusters have been further increased.In this paper,the research is conducted on the problem of hollow cathode that occures with variable working condition.The main contents are as follows:First of all,the effect of varying the discharge current in a wide range on the discharge characteristics and erosion characteristics of the hollow cathode materials are studied.The results show that when the discharge current changes in a wide range,the discharge voltage of the hollow cathode increases.The temperature of the emitter increases as the discharge current increases.When the discharge current is lower than the rated current,the emitter temperature drops faster.The energy and quantity of xenon ions in the plume region of hollow cathode increase with the increase of discharge current.In addition,the high-speed camera imaging technology is used to study the evolution of hollow cathode plasma during the ignition process.The method of increasing the inner gas pressure of the hollow cathode is adopted to reduce the material erosion rate of the hollow cathode during the ignition process.The discharge characteristics of the hollow cathode basically remain unchanged in the 10,000 times cyclic ignition test,and only slight erosion occurres on the exit of the orifice plate and the the emitter.The effect of discharge current on the hollow cathode ematerial erosion rate is studied by combing the plasma emission spectroscopy and the impactradiation model.The results show that the erosion rate of emitter material accelerates under high current.Secondly,the discharge characteristics and erosion characteristics of hollow cathodes within a wide gas flow rate range are studied.The results show that the discharge characteristics of the hollow cathode are optimized with increased gas flow rates.While the gas flow rate exceeds a certain degree,the optimization effect slows down.Aiming at the difficulty in the measurement of trace-level erosion products of hollow cathode,a time-of-flight mass spectrometer detection platform is built.The mass spectrometry results show that when the zenon gas flow rate increases from 4sccm to 8 sccm,the erosion rates of the emitter,keeper plate and orifice plate drop by1-2 orders of magnitude.It is shown that high-temperature evaporation is the main factor in the wear of emitter material,the erosion rate of the orifice plate depends on both the high-temperature evaporation and the high-energy ion bombardment.Thirdly,the mechanism of local overheating of hollow cathode and material erosion acceleration within wide operating condition range has been studied.A numerical model is established to obtain the internal parameters of the hollow cathode which are difficult to measured directly.It is found that local overheating under high discharge current is the direct limiting factor to the discharge current range of the hollow cathode,the plasma spatial distribution changes with the operating conditions is the root cause.The spatial distribution of the plasma determines its heat deposition and the effect of the bombardment intensity to the hollow cathode components.The hollow cathode structure is designed for constant working condition.The contradiction between its structure and the requirement of wide operating condition is that the current density that passes through the orifice region changes synchronously with the discharge current.Based on this,two basic characteristics of a hollow cathode with wide discharge current range are discussed: a more uniform emitter temperature distribution and an adaptive current density through the orifice region.Finally,the hollow cathode with an inner placed orifice plate is proposed and experimentally verified,and its current self-adapting mechanism is studied.The results show that the discharge characteristics of the hollow cathode with an inner placed orifce plate are optimized.The inner placed orifice plate can smooth the temperature gradient of the emitter and reduce the working temperature of the hollow cathode.The emitter outside the orifice plate shares part of the electron emission task,and the proportion of the emitted electron increases with the increase of the discharge current.The erosion simulation results show that the inner placed orifice plate can reduce its erosion rate under steady-state operating.The discharge performance of the new structure hollow cathode is stable during 10,000 cyclic breakdown and ignigion experiment.After the experiment,there is no visible deformation of the emitter material,the erosion rate of the orifice plate also meets to the requirement of 10,000 times ignition.