| As a new thruster which is combined by laser propulsion technology and pulse plasma thruster,a laser-assisted pulsed plasma thruster(LAPPT)has such advantages as high propulsion efficiency,high specific impulse,wide selection range of propellant,precise controllable impulse bit and high thrust adjustability.It is expected to provide accurate thrust for attitude control,orbit maneuver and formation flying of micro-satellites.By using the method of combining experiment and simulation,the modelling and simulation of LAPPT operating process were carried out systematically,and the experimental investigation on the ablation and modification characteristics of propellant was also carried out.Meanwhile,the influencing factors of thruster performance as well as the working mechanism were studied.A LAPPT operating process simulation model with applied magnetic field was established,and reliability of the simulation model was verified by some experimental results with three different energy level thrusters.The model was used to study the influence of different electrical parameters,initial state parameters of the current sheet,the plasma parameters and the applied magnetic field on the performance of the thruster,which provides effective theoretical guidance for design of the thruster.Aiming at the metals and polymers commonly used in propulsion,the characteristics of nanosecond laser ablation were studied by experiment.The results show that:(1)During single-pulse laser ablation,the propellant supply of polymer is much greater than that of metal.(2)The PTFE、HDPE and PMMA ablation surfaces have uneven structures,which is not conducive to the reuse of the same ablation surface,so they are not the preferred objects of the propellant.(3)Black POM is more sensitive to laser energy,and its ablation amount is less and ionization degree is lower when laser energy is low.(4)The expansion distance of plume front at the background pressure0.005 Pa is linear with time,and the expansion speed of the plume front increases with the increase of laser energy.When the background pressure is 5 Pa,a shock wave is formed at the interface between plume front and background gas,and its expansion can be described by S-T theory.(5)The phenomenon of visible plume splitting is observed in plasma plume.The conclusion of plume splitting exists under any background pressure is obtained,and the inevitability of plume splitting is analyzed.At the same time,it was found that the plume splitting degree of polymer was higher than that of metal.(6)The laser ablation surface of black PVC is relatively uniform,and it can maintain high single pulse ablation mass even at lower laser energy.Therefore,black PVC can be used as the preferred propellant for thrusters.To solve the problem of low adhesion between aluminum particles and PTFE particles,the ablation characteristics of Al/PTFE composite propellant doped with different binders and coupling agents were studied.After optimizing the preparation method with excellent effects,the Al/PTFE composite propellant in different ratios was prepared and nanosecond laser ablation experiments were carried out.The results show that:(1)The binder cannot improve the adhesion between the aluminum powder particles and the PTFE particle interface,and polyimide reduces the reaction conditions of the Al/PTFE composite propellant.(2)The coupling agent KH560 can effectively inhibit the proportion of the propellant being ablated by denudation,and the mechanism of the coupling agent KH560 inhibiting ablation is obtained through experiments.That is,the coupling agent and aluminum powder have a chemical bonding effect in the modification process.(3)Doping a certain proportion of Al in PTFE can improve the propellant supply of thruster and the laser ablation uniformity under the same laser energy condition.(4)The study of nanosecond laser ablation Al/PTFE plasma expansion dynamics shows that the plume expansion process of laser ablating Al/PTFE plasma can be divided into three stages and this phenomenon has not been reported in the literature.The Al powder doped in PTFE will block part of the laser transmission into the propellant,thus reducing the laser absorption depth of the propellant.In the case of short pulse laser ablation,the reaction rate between Al and PTFE is optimal when the reductant is slightly higher than the oxidant.When the background pressure is 5 Pa,the overall light intensity of plasma plume is weaker,the size is smaller and the retention time is shorter by comparison with the background pressure of 0.005 Pa.Aiming at physical process of femtosecond laser ablation in different stages,the dual-phase-lag model was combined with energy conservation equation of different stages to establish the corresponding hyperbolic heat conduction equation,while plasma expansion and shielding models were coupled in it to establish a model of femtosecond laser ablation metal.The effects of plasma shielding,delay time ratio,laser energy density and target initial temperature on characteristics of femtosecond laser ablation were analyzed by using the model.The validity of the model was verified by comparing with some experimental results from the literature.The results show that:(1)Plasma shielding and the ratio between two delay times have a great influence on femtosecond laser ablation process.The ablation mechanism of femtosecond laser ablation is dominated by phase explosion.(2)The heat affected zone for femtosecond laser ablation is small,and the heat affected zone is less affected by laser fluence.(3)Increasing initial temperature of target can effectively reduce laser energy density under same propellant supply,thereby reducing requirements of thruster for laser is obtained.Increasing initial temperature of target can effectively reduce the phase explosion threshold energy density.Based on the performance simulation of thruster and the ablative characteristics of propellant,an experimental prototype of the thruster with easy adjustment to energy storage elements,propellant,electrode plate material and structural parameters was designed and fabricated.The effects of ceramic isolator,electrical parameters,electrode shape,anode configuration and propellant on the discharge characteristics,discharge parameters and spatiotemporal evolution of discharge plasma were deeply studied.The results show that:(1)The ceramic isolator can effectively avoid the ablation of propellant by discharge arc,which can inhibit the generation of late-time ablation.(2)The most significant time for the ionization and acceleration on propellant in the discharge channel is the second half cycle of discharge.(3)With the increase of initial voltage,the propulsion performance generated by unit discharge energy increases first and then tends to be stable,and continues to increase afterwards.The existence of attached plasma arc has an important effect on improvement of thruster performance.(4)The configuration of small capacitance and high voltage is beneficial to improvement of propulsion performance.(5)Among rectangle,trapezoid and tongue electrodes,the tongue electrode can obtain the best propulsion performance,which is mainly achieved by increasing the inductance gradient to obtain better propulsion performance than the other two plate shapes.(6)The new type of LAPPT with a segmented anode was designed and its superiortity was proved through experiments.The reason why the segment anode configuration can improve the thruster was grasped.That is,by limiting the speed of the anode arc point moving to the anode end,the current density between the plates is improved,thereby improving the performance of the thruster.(7)The utilization efficiency of PVC is higher than 20%Al80%PTFE.The change of propellant will cause differences in discharge delay time,ionization degree,and ionization products of LAPPT. |
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