Theoretical And Experimental Study On Operating Process And Modified Propellant Of A Laser-electric Hybrid Thruster

With the combination of laser ablation and electromagnetic acceleration,the laser–electric hybrid thruster is now perceived as an advanced propulsion system to support the precise maneuvering of microsatellites.When the hybrid acceleration thruster operates,the laser beam decomposes and ionizes the solid target into quasi-neutral plasma.An electric current is generated between the electrodes to induce a high-intensity magnetic field.The impulse bit is generated from the interactions between the plasma flow and the magnetic field.Compared with other thrusters,this hybrid acceleration thruster is expected to achieve high-propulsion performance and smaller volume,thus retaining an increased payload for microsatellites.To address the further applications of the hybrid thruster,this dissertation conducts three methods including theoretical analysis,numerical simulation and experimental investigation to explore the multi-physical characteristics of the thruster and design an advanced composite propellant for the hybrid thruster.Firstly,a new laser-electric hybrid thruster composed of coaxial anode,hollow cathode,annular insulating sleeve and propellant supply frame is designed and tested.The composition of the total thrust in the new hybrid thruster is experimentally tested and theoretically analyzed,the proportion of laser ablation thrust,electromagnetic thrust and gas aerodynamic thrust is quantified,and the effects of different working parameters on the propulsion performance of the thruster are explored.The results show that:(1)When the thruster is operated with the laser enegy of 1 J and initial discharge voltage of 300 V,its impulse bit reaches 400 μN·s and specific impulse exceeds 760 s,being superior to the similar thrusters at home and abroad;(2)The thrust of the thruster is mainly generated from the electromagnetic acceleration,while the laser ablation process is mainly used to produce the plasma plume required for electromagnetic acceleration;(3)Increasing the laser energy could improve the propulsion performance but too high laser energy will reduce the specific impulse and propulsion efficiency due to the surge of ablation mass;(4)When the discharge width is slightly larger than the laser width,the propulsion performances are expected achieve the best results.Secondly,a numerical model which can reflect the multi-physical field characteristics of the hybrid thruster and predict the macro performance is established.Based on the numerical model of laser-electric hybrid thruster,the effects of initial discharge voltage,capacitance,loop resistance and inductance on propulsion performance are studied.The results show that:(1)increasing the initial voltage of the capacitor can improve the performance;(2)The specific impulse increases with the increase of capacitance,while the propulsion efficiency decreases with the increase of capacitance;(3)The loop resistance and inductance will deteriorate the performance.Thirdly,a broad comparison of composite material samples,which are made of polymeric matrix filled with different dopants according to the same preparation steps,as propellants on a laser-electric hybrid thruster with a common design base is conducted.The impulse bit,specific impulse,efficiency and discharge success rate of the propellant samples are measured and analyzed under the same operation.The experimental results illustrated that:(1)The alloy dopants can improve the impulse bit,and impulse coupling coefficient,but the specific impulse and propulsion efficiency are low due to the surge of ablation mass;(2)Oxide dopants can reduce the mass loss caused by the thermal effect in the laser ablation process,so the oxide doped propellant samples have the high specific impulse and propulsion efficiency,but their discharge success rate is low;(3)The doping of salt can improve the discharge success rate,and their specific impulse and propulsion efficiency are greater;(4)Carbon dopants can enhance the laser energy absorption rate,and improve the ionization degree of the ablation plume,so their impulse bit,impulse coupling coefficient,specific impulse,propulsion efficiency and discharge success rate are high;(5)The low doping ratio of cobalt chromium alloy doped propellant samples has better performance under the conditions of short discharge pulse width and low laser energy,while the high doping ratio of cobalt chromium alloy doped propellant samples has better performance under the conditions of long discharge width and high laser energy;(6)The performance of titanium dioxide doped propellant sample is expected to be improved with the low doping ratio,its maximal specific impulse reaches 9000 s and efficiency exceeds 22%;(7)The optimum doping ratio of graphene doped propellant samples is 5%.Fourthly,a numerical model of laser ablation of polymer is established considering the decomposition reaction of polymer materials,ablation surface retreat and non-Fourier heat conduction effect.Based on the numerical model,the effects of different laser energy fluences and thermal relaxation times on the ablation process are investigated.The results show that:(1)The variation trends of ablation surface temperature and ablation depth of polymer materials are generally similar under different laser energy fluences.The higher the laser energy density,the greater the ablation surface temperature and ablation depth,and the earlier the ablation surface temperature reaches the peak;(2)The thermal relaxation time will lead to the ablation surface temperature and ablation depth lower than the traditional Fourier heat conduction process,but it is more in line with the experimental results.Fifthly,the multi-physical field process of the interaction between laser and composite propellant is analyzed and divided into three stages and three regions,and a high fidelity dynamic numerical model considering laser energy thermal deposition,phase transition,moving boundary and chemical dynamics is established.Based on this numerical model,the relationship between the ablation characteristics of composite propellant and the laser energy fluence is explored.It is found that:(1)The variation trend of ablation surface temperature under different laser energy fluences is generally similar,which first stands still for a while,then continues to rise to the peak,and finally remains stable;(2)The higher the laser energy fluence,the greater the peak value of ablation surface temperature;(3)The temperature distribution of solid layer under different laser energy fluences is equal and the formation time of bubble layer is almost the same;(4)When the bubble layer is formed,the higher the laser energy fluence is,the higher the temperature of the bubble layer is.Lastly,the effects of different dopants on the characteristics of ablation plume are analyzed by using high-speed photography technology and image processing method.The characteristic parameters such as ablation plume length,expansion velocity,plume area and relative light intensity distribution of different propellant samples are compared,and the effects of doping ratio on plume characteristics are explored.It is found that:(1)The ablation plume of metal doped composite propellant samples contains uniformly distributed bright jets and large plume clusters at the tail,and their plumes have the large plume length,expansion velocity and plume area;(2)The ablation plume of composite propellant doped with oxide keeps a certain distance from the ablation surface,which has poor continuity and uneven light intensity distribution;(3)The ablation plume area of carbon doped composite propellant samples does not increase continuously with the ablation time,but remains dynamically stable after increasing to the peak value.This dissertation conducts three methods including theoretical analysis,numerical simulation and experimental investigation to explore the multi-physical characteristics of the thruster and design an advanced composite propellant for the hybrid thruster.The experimental prototype,numerical model and composite propellant are obtained,which could contribute to the performance optimization and engineering application of laserelectric hybrid thruster.