Investigation On Plasma Mechanisms During The Operation Of A Pulsed Plasma Thruster

Pulsed Plasma Thruster（PPT）could be applied to space tasks such as attitude control,drag compensation and constellation phase control of micro satellite due to its advantages of flexible power,simple structure,high reliability,high specific impulse under low power and so on.With the increasing attentions,developing PPT with high performance becomes an urgent problem to be solved.The plasma generation and acceleration processes directly influence the performance of the thrusters,so it is necessary to deeply investigate the principles of plasma generation and variation in the discharge channel during the operation of PPT,and the mechanisms of the influence of the thruster design parameters on the operation such as the configurations,in order to provide suggestions for the optimization and promotion of the thrusters from the view of mechanism.The detailed contents are as follows.The magnetic field model considering the configuration features of the electrodes is built on the basis of the PPT operation characteristic and the electrode configurations.The variations of the magnetic field along with time and the field distribution under different electrodes configurations conditions are analyzed.The investigation of the magnetic field indicates that the bending part of the anode could increase the magnetic flux density around the propellant surface,which promote the electromagnetic acceleration of the plasma.The variation of the gap between electrodes and the flared angle of the electrodes would influence the magntic field and further have effect on the electromagnetic acceleration,which demonstrates that the optimization of the electrodes configuration is a crucial approach to promte electromagnetic acceleration and improve thruster performance.A two dimensional Particle in cell（PIC）model is established basing on Monte Carlo Collision（MCC）,and the plasma dynamics of the PPT operation is numerically simulated.The simulation reveals the motion and diffusion mechanisms of plasma.The simulation results demonstrate that the electron density decreases from the cathode to the anode area,while the distribution feature of electrons from the upstream to the downstream varies with time.Electrons firstly diffuse in the direction from the cathode to the anode and then move to the downstream,which is related to the varying electromagnetic field.The electric field provides electrons with the initial kinetic energy,and the Lorentz force drives the electrons which are equiped with considerable kinetic energy to move downstream.When the discharge current reaches the peak value,the electron density is on the level of～10²³ m^-3.After the current peak moment,the electrons in the intermediate and downstream area are magnetized.Such phenomenon is related to the strong magnetic and weak electric field condition,in which electrons conduct cyclotron motion,which prolong the residence time of electrons between the electrodes and enlarge the collision frequency of electrons and atoms.Considering the variation of plasma characteristics in PPT,a diagnostic method based on optical emissive spectroscopy to determine the relative ion density is proposed.According to the results of the diagnostic research,the evolution processes of plasma characteristics are concluded and the mechanisms of the plasma generation and development processes in the discharge channel are explained.The experimental results reveal the relative variation of the electrons,ions and atoms.The electrons are firstly generated and enriched near the propellant surface,constituting the carrier of the arc at the early stage of the discharge.The electron number density reaches the level of 10²³m^-3.The evolution of electrons,ions and atoms near the propellant surface indicates that the physical and chemical processes of avalanche,decomposition and ionization in PPT discharge happens sequentially,and that the desynchrony of these processes would be one of the reasons which are responsible for the low efficiency of PPTs.These are intrinsic processes in the principle of PPTs,which are difficult to change,so the approaches to promote ionization performance of PPT should be considered from the view of mechanisms.Two groups of ions with different average velocities of20.2km/s and 12.1km/s are detected.When the second group of ions are generated,the discharge current and voltage descend to a low level and could not provide a highly acceleration contition for the ions,leading to the low velocity of this group of ions.On the basis of mechanism analysis and aiming at improving the performance of thrusters,further studies are carried out from the aspects of promoting electromagnetic acceleration and ionization performance.The discharge and ablation process on the propellant surface,and the ionization and the acceleration process of the plasma mainly take place in the discharge chamber of a PPT,so the configuration of the discharge chamber deeply influence the working process and the performance of the thruster.Discharge characteristics analysis and electromagnetic impulse bit evaluation on thrusters with different discharge chamber configuration parameters are conducted to reveal the mechanisms of the effects of the configurations on the thruster operation and performance.The results show that the increase of the aspect ratio of electrodes leads to a higher value of the inductance gradient,while the peak value of the discharge current decreases with the increase of the gap between the electrodes near the propellant surface.Considering the discharge current concentrates on the area near the propellant surface,the current peak is mainly affected by the gap between electrodes in the upstream of the discharge chamber.So the structure of electrodes with increased flared angle only in the downstream of the discharge chamber,which increases the aspect ratio,is applied to the thruster.Such separation could settle the paradox of increasing the current peak value and increasing the inductance gradient,and then the electromagnetic impulse bit of PPT is enhanced.From the view of promoting ionization,the stage-by-stage discharge characteristics of the two-stage discharge PPT is investigated by theoretical analysis combining with experimental research.The effects of the electric parameters on the discharge process are systematically studied,and the discharge phenomenon in the secondary discharge area is studied with the aid of optical diagnostic methods.The charging phenomenon in the secondary discharge is detected in the discharge characteristics tests.Electric field simulation results show that the electric potential difference between the primary and the secondary anodes leads to the diffusion of plasma between the anodes,resulting in the charging phenomenon of the second stage capacitor.The filter graphs and optical spectroscopic diagnostic results indicate that singly charged carbon ions are generated in the secondary discharge,and that the plasma generated in the primary discharge could be further ionized when it moves to the secondary discharge area under the mechanism of the stage-by-stage discharge in the operation condition in this paper.With the same total energy,when the ratio of the primary and the secondary discharge energy is 1.77,the specific impulse improves by52%relatively compairing with the traditional working mode,and the efficiency improves from 5.6%to 8.4%.