Ion Collection by Probing Objects in Flowing Magnetized Plasmas.

摘要

A new one-dimensional collisionless kinetic model is developed for the flow of ions to probing structures in drifting plasmas. The cross-field flow into the presheath is modelled by accounting consistently for particle exchange between the collection flux tube and the outer plasma. Numerical solutions of the self-consistent plasma/sheath equations are obtained with arbitrary external ion temperature and parallel plasma flow velocity. Results are presented of the spatial dependence of the ion distribution function as well as its moments (density, particle flux, temperature, and power flux). The ion current to the probe is obtained and the ratio of the upstream to downstream currents is found to be well represented by the form R = exp(Ku(sub d)), where K = 1.66 for T(sub i) = T(sub e) and u(sub d) is the drift velocity in units of (T(sub e)/m(sub i))(sup 1/2). The results agree well with comparable recent fluid calculations but show substantial deviations from other models which ignore particle exchange out of the presheath. No evidence is found of the formation of shocks in the downstream wake, contrary to the implications of some fluid theories. We have also extended the previous kinetic model by generalizing cross-field transport and adding ionization to the source of the Boltzmann equation along the presheath. Ion sheath current density and ratio(R) of upstream to downstream current are obtained as a function of plasma drift velocity, equivalent viscosity, ion temperature, and ionization rate. Constants(K) in the form R = exp(Ku(sub d)) are obtained in terms of viscosity, ion temperature, and ionization rate. The effect of an electrical bias applied to the object on the presheath characteristics is discussed.