A model of collisionless ion-beam plasma

Abstract

Summary form only given, as follows. On the base of the experimental results a self-consistent theoretical model have been developed, which describes the behavior of low energy (ϵ<1 keV) broad (D=50 mm) intense (I/sub b/=1÷50 mA) ion beam passing through the neutral gas at low pressure within conducting walls. The collisionless approach is used which means absence of collisional relaxation of the beam. The theory is used to derive the plasma potential and electron temperature within the beam. The energy and particle balance of this system are explored as a function of the physical parameters of the primary beam and of the gas pressure. The approach adopted here is qualitatively described below. Electrons, produced by ionization, are trapped within the beam by it's own space charge, resulting in a decrease of the potential well depth. Slow ions, produced by ionization and resonant charge exchange, escape at walls without collisions. Low energy trapped electrons have large lifetime and collisions are especially important for electron gas parameters formation. Collisions with heavy atoms and ions promote isotropization of the electron distribution function. Electron-electron collisions lead to development of a Maxwellian distribution, characterised by electron temperature T/sub c/. Diffusion in velocity space allows the electrons to escape through high energy tail of distribution function. Special attention is paid to electron loss on the dielectric surfaces which inevitably are present in a vacuum chamber. The electron particle balance is governed by the rate of electron production by ionization and the rate of escape, which is determined by the depth of the potential well, the shape of electron energy distribution, and area and location of the dielectric surfaces. The electron temperature and plasma potential are calculated from self-consistent energy and particle balance equations. As a result of solving of energy balance equation analytical expressions for T/sub c/ has been obtained in some particular cases. Numerical solution in general case of the equations was performed on personal computer. Electron energy distribution function had been calculated oh the base of Fokker-Planck equation (spherical symmetry was assumed). This equation was solved numerically. In some particular cases the distribution have been expressed analytically The results of calculations agree well with experimental data.