High-power subnanosecond electron beams and volume discharges at a pulsed breakdown in dense gases

New understanding of mechanism of the runaway electrons beam generation in gases is presented. It is shown that the Townsend mechanism of the avalanche electron multiplication is valid even for the strong electric fields when the electron ionization friction on gas may be neglected. The non-local criterion for runaway electrons has been proposed. This criterion leads to the universal two-valued voltage U_cr dependence on pd for a certain gas. This escape curve divides the plane (U_cr,pd) onto the area of efficient electron multiplication and the area where the electrons escape the gas-filled gap without multiplication. On the basis of this escape curves the analogous of Paschen´s curves including a new additional upper branch have been constructed for He, Ne, Xe, N₂, SF₆. The electron beams of subnanosecond pulse duration and amplitude of hundreds of amperes have been created at atmospheric pressure in various gases. The beams of runaway electrons are shown to form at the instant when the discharge plasma approaches to the anode. In this case the electron beam pulse is being formed according the non-local criterion of runaway electrons. A volume nanosecond discharge with high specific excitation power in the absence of gap preionization by an additional external source has been realized. The role of the discharge gap preionization by fast electrons emitted from the cathode non-uniformities and plasma formations on the cathode, as well as electron avalanches propagation from cathode to anode in a dense gas was considered.