Mechanism Analysis Of Magnetoplasmadynamic Thrusters Based On Particle Simulation Method

A Magnetoplasmadynamic thruster(MPDT)has some advantages including large thrust density,high specific impulse,high power and simple structure.It has broad developing prospects and may be applied into space missions such as orbit transfer of large spacecraft,deep space exploration and interstellar travel.It is also one of the suitable electrical thrusters utilizing controllable fusion technology in the future.However,due to a variety of complex physical processes,the internal operating mechanism on MPDT is extremely complex.So far,due to poor understanding and comprehension to the operating mechanisms on MPDT,it is not clear for some intractable problems and phenomena are how to come into being,and not to mention how to be solved.Therefore,it is of great significance to carry out theoretical research thoroughly on MPDT.The particle method can objectively reflect the motion of plasma,which is suitable for the study on some unclear physical mechanisms.In order to improve the comprehensive performance of MPDT,the operating mechanisms of MPDT were studied thoroughly based on the particle simulation models,especially in solving such some problems as unclear acceleration mechanism,plasma instability,low efficiency,and serious erosion of electrode.Aiming at the problem of large amount of calculation in the plasma particle simulation technology,a high-efficiency calculation method named as scaling law for the PIC/MCC model was proposed.The scaling relationships between the physical quantities before and after the model reduction were derived by ensuring the characteristics of the charged particles and the similarity of the discharge,and were verified by the trajectories of the charged particles and the discharge plasma.The results showed that this scaling law can improve the computational efficiency in terms of reducing the total number of simulated particles and shortening the simulation time to equilibrium of discharge.The utilizing of this method can shorten the simulation time from dozens of days to several hours under the same calculation conditions,which greatly improves the calculation efficiency.The operation mechanisms of AF-MPDT were deeply investigated.First of all,to reveal the motion characteristics of plasma charged particles,the PIC/MCC electrostatic simulation model for the discharge processes of AF-MPDT was set up.The E×B drift characteristics of the charged particles in the cross section electromagnetic field were studied based on this model.The results showed that the electrons in the thruster are magnetized and the ions are not,and the kinetic energy of the plasma near the cathode area is greater than that near the anode area.Then,to reveal mechanisms of the plasma acceleration and energy conversion,the acceleration effects of electric field and magnetic field on plasma were clarified respectively.The three-dimensional particle motion model of thruster was established,and the particle trajectory of the single particle in AF-MPDT was obtained by the particle simulation method for the first time.The expression of plasma magnetic pressure was obtained by solving the MHD equations,and the magnetic nozzle acceleration mechanism was verified by analyzing the energy conversion of single particle in the electromagnetic field.The results showed that the electric field acceleration mechanisms include electron multiplication,ambipolar fields acceleration and electric double layer acceleration,and the magnetic field acceleration mechanisms can be divided into an axial applied magnetic field acceleration inside the discharge chamber and a slowly varying magnetic field magnetic nozzle acceleration at the exit of the thruster.The kinetic energy comes entirely from the work of electric field.And the non-axial kinetic energy can convert to axial kinetic energy in the magnetic nozzle,thereby enhancing the electromagnetic acceleration effect of AF-MPDT.Finally,for the problem of poor working stability of the thruster,the physical principle on the "onset" phenomenon of the thruster was clarified.By solving the plasma wave equation,the dispersion relationship of the plasma electrostatic wave was obtained.PIC/MCC model was used to investigate the cause,development process and type of plasma instability in AF-MPDT for the first time.The results showed that the electrons gather and interact with the plasma electrostatic wave,which in turn causes the wave to grow,and eventually produces thruster plasma instability.Plasma instability will aggravate the electrode erosion,reduce the ion rotation kinetic energy,and decrease the conversion efficiency of electrical energy to kinetic energy.It is confirmed that the plasma instability of AF-MPDT is the electron cyclotron drift instability.The investigation on operating mechanisms of SF-MPDT was carried out,and the PIC/MCC electromagnetic simulation model of SF-MPDT was established.Firstly,the characteristics of electromagnetic field and plasma distribution in the discharge chamber were studied.And the influence of the discharge voltage,the mass flow rate and the induced magnetic field on the plasma density,temperature and velocity and other characteristic parameters were investigated.The results showed that the simulated induced magnetic field distribution was basically consistent with the experimental results in literatures,and the electrode sheath characteristics were obtained for the first time.As the discharge voltage increases,the plasma density inside the discharge chamber will increase firstly and then decrease.The anode lip can accelerate the plasma.Secondly,for the problem of low efficiency in SF-MPDT,the components of anode power deposition and the physical mechanism of anode fall were studied.And the influence of induced magnetic field and mass flow rate on anode fall was analyzed.The results showed that the anode fall depends on the balance of current density between the anode surface and the plasma.By increasing the anode current contact area,the cathode capability of electron thermal emission,and the plasma density on the anode surface can reduce the anode fall,thereby improving the thruster efficiency.Finally,for the serious problem of thruster electrode erosion,the cathode erosion phenomenon and its influencing factors were studied.The results showed that the greater the discharge voltage and the higher the gas mass flow rate,the more severe the cathode erosion.By selecting gaseous with lower ionization threshold,lowering discharge voltage and lowering plasma density near the cathode,the cathode erosion can be relieved,thereby prolonging the service life of the thruster.