Initiation of Explosive Electron Emission and Runaway of Electrons during Pulsed Breakdown of Dense Gases

A mechanism for the initiation of explosive electron emission at the interface between the cathode and a dense gas is proposed. It is based on the accumulation of positive ions, which appear in the gas ionized by field-emission electrons, near natural microprotrusions. The distance at which ions appear decreases with increasing gas density, which leads to an increase in their Coulomb field on the emitting surface. As a result, the emission current density in a high-pressure gas (tens of atmospheres) increases explosively, leading to the formation of many explosive-emission centers in tens of picoseconds. They initiate the development of plasma channels growing toward the anode. Runaway electrons are generated at the tops of the plasma tips and ionize the gas, providing its subnanosecond breakdown. This scenario of the breakdown development can be implemented at a critically low reduced electric field (i.e., the ratio of its strength to the pressure), when the characteristic time of avalanche multiplication of thermal electrons is longer than the duration of the voltage pulse.