Supershort avalanche electron beams in air and other gases at high pressure

In recent years, an interest to the generation of runaway electron beams and X-radiation which are generated in the gas discharge plasma at increased pressures has grown. We were suggested to call an electron beam generated in the gas diode a supershort avalanche electron beam (SAEB) and we will use this term. In this paper, the conditions of generation SAEB from the plasma of nanosecond diffuse discharges in atmospheric pressure air and other gases were investigated. It is shown that under conditions of SAEB generation, the formation of plasma in a gap with a non-uniform electric field may occur during the time not exceeding 100 ps. At that, concentration of the charged particles in the plasma being created is sufficient for displacement of the electric field from a part of the discharge gap occupied with dense plasma. It is shown that at the reduction of the voltage pulse duration across the gap (to ~100 ps at half-height) the inter-electrode gap length which is optimal for the SAEB generation is decreased. It is shown as well that the SAEB current pulse duration is influenced by the difference of the path length of the beam electrons at their motion towards the center and edge of a foil. At the small voltages across the gap (~25 kV), a SAEB pulse FWHM of ~200 ps was obtained. Generation of the runaway electron beam (SAEB) registered with maximum amplitudes behind the gas diode foil is a complicated process which is determined by the following factors. Firstly, it is determined by the field gain near the cathode and appearance of the field emission electrons. Some part of these electrons (fast electrons) passes into the runaway mode realizing a pre- ionization of the near-cathode region and promoting the plasma creation near the cathode. Secondly, it is determined by reaching of a critical field between the front of the polarized plasma of the diffuse discharge and anode. And thirdly, it is determined by the influence on the electron acceleration in the gap of th- ionization wave which propagates from the cathode to anode. Generation of the runaway electrons owing to the electric field gain at the electrodes and through the gap is a usual phenomenon at the pulse discharges in a non-uniform electric field but their registration requires applying of special techniques and equipment. The runaway electrons make essential influence on the time delay of the gap breakdown and on the spatial waveforms of the nanosecond discharges.