In this paper,we adopt the modified Morozov secondary electron emission model to investigate the influence of the characteristic of a space-charge-saturated sheath near the insulated wall of the Hall thruster on the near-wall conductivity,by the method of two-dimensional (2D) particle simulation (2D+3V).The results show that due to the sharp increase of collision frequency between the electrons and the wall under the space-charge-saturated sheath,the near-wall transport current under this sheath is remarkably higher than that under a classical sheath,and equals the near-wall transport current under a spatially oscillating sheath in order of magnitude.However,the transport currents under a space-charge-saturated sheath and a spatially oscillating sheath are different in mechanism,causing different current density distributions under the above two sheaths,and a great influence of channel width on the near-wall transport current under a space-charge-saturated sheath.
A preliminary investigation is conducted to study the characteristics of sheath damping near a dielectric wall with secondary electron emission (SEE). Making use of the linear analysis of the sheath stability, we have found two major contributions to the sheath damping, one similar to the Landau damping in uniform plasmas and another determined by local electric field and electron density of the steady-state sheath. It indicates that in a classical sheath regime the damping in the sheath region monotonically increases towards the wall and decreases with the enhancement of SEE effect. In order to verify the theoretical analysis, sheath oscillation processes induced by an initial disturbance are simulated with a time-dependent one-dimensional (1D) sheath model. Numerical results obtained are consistent with the theoretical analysis qualitatively.
In order to investigate the effects of secondary electrons,which are emitted from the wall,on the performance of a thruster,a one-dimensional fluid model of the plasma sheath in double walls is applied to study the characteristics of a magnetized sheath.The effects of secondary electron emission (SEE) coefficients and trapping coefficients,as well as magnetic field,on the structure of the plasma sheath are investigated.The results show that sheath potential and wall potential rise with the increment of SEE coefficient and trapping coefficient which results in a reduced sheath thickness.In addition,magnetic field strength will influence the sheath potential distributions.
A semi-analytical method is introduced to study kink instability in cylindricalplasma with line-tied boundary conditions.The method is based on an expansion for magnetohydrodynamics(MHD)equations in one-dimensional(1D)radial eigenvalue problems by usingFourier transforms.The MHD equations then become an ordinary differential equation.Thismethod is applicable to both ideal and non-ideal MHD problem.The effect of plasma pressure(P_0)on kink instability is studied in a cylindrical geometry.Complex discrete spectra are presented.Two-dimensional(2D)eigenfunctions with the line-tied boundary conditions are obtained.The growth rate and radial eigenfunctions are different in the two cases of P_0=0 and P_0≠0,which indicate that the effect of plasma pressure can not be ignored if it is large enough.Thismethod allows us to understand the role of individual radial eigenfunctions,and is also computationallyefficient compared to direct solutions of the MHD equations by the finite differencemethod.
A one-dimensional slab model of the plasma sheath in the stationary plasma thruster(SPT)chamber is developed in this study.It is considered that secondary electrons emitted fromceramic walls are partially trapped by the bulk plasma in the SPT chamber;some secondaryelectrons drift across the sheath where they are generated and the bulk and move towards theopposite sheath.Thus both the secondary electron emission(SEE)from one sheath and thepartially trapped secondary electrons from the opposite sheath contribute to this sheath.Theresults indicate that both the SEE coefficient and trapping coefficient have a significant impactnot only on the distributions of both electrons and ions of the SPT sheath but also on the energyflux loss to the SPT wall.When the trapping coefficient increases,the energy flux of electronsdeposited to the walls will increase whereas that of ions will decrease.Besides,the critical electrontemperature will decrease greatly with the increase of the trapping coefficient.
