Research On The Characteristics Of Low Current Hollow Cathode Discharging With Krypton

With the rapid development of commercial aerospace,the demand for low cost and low power consumption electric propulsion system is increasingly strong.However,it's difficult because xenon is very expensive until now,which is the main propellant of electric propulsion system.Krypton is cheaper and own similar chemical properties with xenon.It has been applied to low power Hall thrusters,but the efficiency degraded.As the core of electric propulsion system,hollow cathode have great influence on the whole system.Thus,we studied the characteristics of low current hollow cathode discharging with krypton,and then optimized its performances.The performances of hollow cathode discharging with xenon and krypton were compared.When discharging with krypton,it was characterized by higher voltage,higher electron temperature,low pressure and narrow stable working range.Besides,the orifice temperature is lower when current is very low.With numerical methods,we found the higher voltage was composed of insert sheath voltage,the voltage drop through the orifice and the plume voltage drop.Therefore,we could optimize the performances of hollow cathode in these aspects.The oscillation characteristics of hollow cathode discharging with krypton was also studied.The current oscillation waveform has two peaks for krypton and xenon.But,it's more smooth and the center frequency was higher for krypton than that of xenon.As the current raise,the oscillation intensified.With the increase of anode current,the correlation between anode current oscillation and plasma density oscillation increases gradually.It's the same when closing to anode.When anode is working,if the keeper power turn on,the oscillation of anode current and voltage will intensify.It was the result of intensification of low frequency oscillation.The effect of keeper current on anode oscillation is related to anode current magnitude.At low anode current,the lower keeper current,the weaker effect on anode oscillation.But at high anode current,it is opposite.With the increasing of anode current,the degree of keeper effect on anode current raise at first and then fall.When the flow was enough large,the effect of keeper on anode current will be suppressed.The plume potential optimization was also studied.With the background pressure increasing,the anode voltage decreased,but the degree would gradually weaken.When the gap between keeper and anode was larger,the spatial distribution of plasma at the exit of keeper will change.It would trigger the change of plume potential,but that of anode voltage was slight.It was the same with changing the diameter of keeper orifice.And with the increasing of keeper diameter,the effect on plume potential spatial distribution would weaken.In view of the operation steady,the keeper diameter should be matched with working range.At last,the performances of hollow cathode was optimized from inner.As the insert length increasing,the insert sheath voltage decreased,then anode voltage decreased,but the degree will weaken gradually with the length increasing.In view of life and efficiency,the length of insert should be matched with current.The larger of orifice diameter,the lower insert sheath voltage and orificed voltage drop,but it was more difficult for self-sustain.Thus,the power consumption and self-sustain should be consider when designing orifice.When the anode is working,if the heater turn on,the discharge performances would be better,especially at low current.It owning to the decrement of insert sheath voltage.With numerical methods,the thermal insulation was assessment.In order to reduce the heat loss of insert,the emissivity of material and thermal conductivity of cathode tube shou be as small as possible.