Investigation of the possibility of generation of runaway electrons in subnanosecond gas discharges of high and ultrahigh pressure in the vicinity of microprotrusions on the cathode surface

In the pressure range of 1–40 atm, experimental and theoretical studies of the processes of initiation and development dynamics of the initial stage of the self-sustained subnanosecond discharge in nitrogen, developing in a uniform electric field with the participation of runaway electrons, were carried out. Data on the maximum achievable values of the electric field strength in the discharge gap at the pre-breakdown stage of the discharge development and photographs of the microrelief of the surface of a stainless steel cathode formed during its training by subnanosecond high-voltage pulses were obtained. These data served as the basis for numerical 3D modeling of the development of an electron avalanche initiated by a field emission electron in a small region of enhanced electric field near a microinhomogeneity on the cathode. The possibility of transition of electrons in these avalanches to the runaway regime was studied. Cone-shaped microprotrusions, metal drops, and boundaries between pores and microcraters were considered as microinhomogeneities. It has been shown that the initial energy obtained by an electron near the microinhomogeneity can significantly facilitate its transfer into the runaway regime. This effect is especially noticeable at gas pressures higher 10 atm. As a result, at the stage of a self-sustained subnanosecond discharge formation, the runaway mode of an electron can be realized at the average reduced electric field strengths in the discharge gap, which are significantly lower than required by the runaway criterion.