A Monte Carlo method was used to investigate the early stages of ionization of neutral helium, as produced by large-amplitude magnetic pumping in a stellarator. The competing processes considered were ionization, excitation, elastic scattering and electron diffusion in and out of the pumping region. The e-folding time τe for the electron density was computed for various values of the modulation M (ratio of pumping field to static confining field), mirror ratio R (ratio of the static field in the tube to its value in the pumping region) and neutral density n0. Other pertinent parameters were the pumping frequency ν, length of pumping region l, the remaining machine length L. These were for the most part confined to values concordant with those of the B-2 stellarator (ν = 200 kHz, l = 50 cm, L = 500 cm).It was found for values of M > 0.7, that τe varied roughly as (1—M)−l. The dependence on R and n0 was slight. In addition it was found that when the average number of inelastic collisions is greater (less) than one during a pumping cycle, the electron temperature is less (greater) than the ionization energy. A phenomenological theory based on the results of these calculations agrees with the observed modulation required for breakdown in B-2.
Non-resonance plasma heating by radiation in the millimetre range in toroidal devices