The electron runaway mechanism in dense gases and the production of high-power subnanosecond electron beams

New insight is provided into how runaway electrons are generated in gases. It is shown that the Townsend mechanism of electron multiplication works even for strong fields, when the ionization friction of electrons can be neglected. The non-local electron runaway criterion proposed in the work determines the critical voltage – $pd$ relationship as a two-valued function universal for a given gas ($p$ being the gas pressure, and $d$ the electrode spacing). This relationship exhibits an additional upper branch as contrasted to the familiar Paschen's curves and divides the discharge gap into two regions: one where electrons multiply effectively, and the other which they leave without having enough time to multiply. Experiments on the production of electron beams with subnanosecond pulse duration and an amplitude of tens to hundreds of amperes at atmospheric pressure in various gases are addressed, and the creation of a nanosecond volume discharge with the high density of excitation power and without preionization of the gap by a supplementary source is discussed.