The Quasi-steady Plasma-neutral Gas Balance in Magnetic Bottles

Neutral gas which penetrates into a hot plasma of characteristic dimension L b, average density n̄, and temperature T, consists mainly of a slow and fast component of densities nns, nnf and temperatures T ns≪T, T nf ≃ T, respectively. These components have the penetration lengths L ns = 1/σcsn̄ and L nf = 1σcfn̄ defining the critical densities ncs = 1/σcs L b and n cf = 1σcf L b where 1/σcf ≃ 100/σcs ≃ 5×1018m-2 for hydrogen in the range 106 < T < 107 K. Thus, hot plasmas can be divided into the classes of permeable dilute, permeable non-dilute, and impermeable systems defined by n̄ ≲ n cs≪ncf,ncs≪n̄ ≲ ncf and ncs≪ncf≪n̄ respectively: (i) These classes have different equilibrium and stability properties, depending on the relative concentrations, spatial distributions, and transport processes of the neutral gas. (ii) The density of the neutral gas immersed in a quasi-steady permeable plasma becomes a measure of the plasma particle loss rate. (iii) The ratio between the neutral gas "blanket" density and the average plasma density increases when transitions are made from the permeable dilute to the permeable non-dilute state, and further to the impermeable state. (iv) The spatial distributions and relaxation times of the plasma. The pressure gradient, which acts as a driving force of instabilities, reaches a maximum value when the transition is made from a permeable to an impermeable state.