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

Abstract

Neutral gas which penetrates into a hot plasma of characteristic dimension L_b, average density , and temperature T, consists mainly of a slow and fast component of densities n_ns, n_nf and temperatures T_nsT, T_nf T, respectively. These components have the penetration lengths L_ns = 1/σ_cs and L_nf = 1σ_cf defining the critical densities n_cs = 1/σ_csL_b and n_cf = 1σ_cfL_b where 1/σ_cf 100/σ_cs 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 n_csn_cf,n_cs n_cf and n_csn_cf 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.