Theoretical And Experimental Investigation On The Property Of Propellant Ablation And Propulsion Performance Of Laser Ablation Magnetoplasmadynamic Thruster

A novel Laser ablation magnetoplasmadynamic thruster（LA-MPDT）is proposed by applying laser technology to the electromagnetic acceleration thruster.It can provide an efficient and advanced propulsion system for the future deep space exploration,manned lunar landing and Mars development in China.In this dissertation,the design and simulation of thruster power supply,theoretical prediction of propulsion performance,modeling and simulation of propellant ablation and plume movement,ablated experiment of modified propellant,experimental measurement of propulsion performance and analysis of influencing parameters are systematically investigated by theoretical analysis,numerical simulation and experimental method.Aiming at the operating characteristics of LA-MPDT,the scheme on multi-stage ignition and discharge power supply of LA-MPDT is proposed.The simulation models of ignition circuit and main discharge circuit are established respectively,and the ignition and main discharge process of the thruster are simulated numerically.A novel electromechanical model is established to predict thrust performance on the thruster.The influences of discharge current,anode radius,mass flow rate of propellant and applied magnetic field intensity on thrust performance are simulated and analyzed.The simulating results show that if increasing discharge current,anode radius and applied magnetic field intensity while reducing the mass flow rate of propellant,these will be beneficial to improving thrust performance of the thruster.Aiming at physical characteristics of aluminum ablation induced by short pulse laser,the specific ablation model is established,and taking into account temperature dependent material properties,phase transition,dielectric transition and phase explosion.Meanwhile,a two-dimensional plume dynamic model is established by considering plasma ionization,plume absorption and shielding.By coupling calculation on propellant ablation and plume movement,the whole process of propellant ablation and plume movement is described.After that,the propellant ablation characteristics,plume movement and ionization characteristics are numerically analysed by utilizing the above models.The gained results show that with the increase of laser energy density,a large number of materials removal from the surface of propellant into plume due to phase explosion,hence the temperature of propellant surface maintains at about 5400K.With the increase of laser energy density,the ionization rate of ablation plume increases,and the absorption and shielding of the ablation plume on the laser energy are also enhanced,but most of the laser energy is still deposited in the propellant.The peak velocity,peak temperature and ionization region of the ablation plume are always presented at the front of the plume.The collision between the ablation plume and the inner surface of the cathode tube significantly improves the temperature,velocity and ionization rate of the ablation plume near the inner surface of the tube.With the increase ambient gas pressure,the temperature,velocity and ionization rate of the ablation plume decrease.In view of the physical characteristics of PTFE ablation induced by short pulse laser,an ablation model of PTFE is established by taking into account temperature dependent material properties,phase transition and non-fourier effect.Meanwhile,a plume dynamic model and thermochemical model are also established by considering the chemical reactions among plume components,plasma ionization,plume absorption and shielding.By coupling computation of the propellant ablation,plume movement and chemical reactions,the whole process of propellant ablation and plume movement is described.Then the propellant ablation characteristics,plume movement and thermochemical characteristics are numerically analysed by utilizing the above models.It is found that the response time of temperature variation increases with increasing of thermal relaxation time due to non-fourier thermal effect.The ablation mass flow rate of propellant firstly increases and thereafter decreases with increasing of absorption coefficient,and the ablation mass flow rate reaches at the highest value when the absorption coefficient is 1.0×10⁵cm^-1.The lower the ambient gas pressure,the more intense the chemical reactions between the plume components,and the higher the ionization rate of plume.Based on theoretical analysis of ideal propellant characteristics suitable for LA-MPDT,a scheme and method of PTFE-based modified propellant filling with metal or metal oxide are put forward,and a variety of modified propellants with different components and mass fractions are manufactured,then the laser ablation characteristics of the modified propellants are studied experimentally.It is found that the thermal conductivity of the modified propellant is significantly improved by filling Al in PTFE,but the phase transition temperature is almost unchanged.The ablation mass of the modified propellant increased linearly with Al content and single pulse laser energy,and the ablation mass per unit laser energy of 40%Al60%PTFE propellant reaches2.6×10-4g/J.With increasing of Al ingredient,the internal structure of the modified propellant become to be more compact,the laser ablation reaction becomes to be more intense,ablation materials are more converted into plume,and the condensate around the ablation spot decreases,but the black carbon deposition increases.With increasing of Al ingredient,the ionization rate and plasma density in ablation plume of the modified propellant increase.With increasing of laser energy,the velocity,ionization rate and plasma density of the ablation plume also increase.In order to obtain specific propulsion performance of LA-MPDT,an impulse measurement system based on inverted cantilever beam and an electromagnetic calibration device based on impulse ampere force are established.Then the experimental study on the influence parameters of LA-MPDT propulsion performance is carried out.It can be inferred from the variation of voltage and current during the discharge process,the plasma arc in the thruster changes from generation to enhancement to extinguishment.Based on the same laser parameters（1000W,1ms）,with increasing of discharge energy,the propulsion performance of the thruster is significantly improved.The specific impulse reaches at 4800s and the thrust efficiency reaches at 9.1%with the discharge energy of 78J.Based on the same discharge energy（50J）,with increasing of laser energy,the specific impulse decreases gradually,and the thrust efficiency increases first and then decreases,the maximum thrust efficiency reaches at 18.5%with the laser energy of 2J.The mass loss of the propellant is mainly caused by laser ablation,while the thrust mainly comes from the electromagnetic acceleration of plasma.