Energy distribution of runaway electrons generated by a nanosecond disharge in atmospheric-pressure air

Summary form only given. The spectra of supershort avalanches electron beams (SAEB) generated during nanosecond discharges in atmospheric- pressure air have been investigated. A novel technique has been developed for recovering electron spectra from attenuation curves. The data on the discharge formation dynamics in an inhomogeneous electric field have been obtained, and the dependences of the electron beam amplitude on various parameters have been determined. The temporal characteristics of the electron beam current, gap voltage, and discharge current in a gas diode have been measured with a time resolution of ~0.1 ns. Electron spectra for various generators and different types of cathodes and electrode gaps have been recovered. It is shown that the breakdown of a gap with a small-curvature-radius cathode by a voltage pulse with an amplitude of >100 kV and a rise time of ~0.5 ns or less is accompanied by the generation of runaway electrons with energies from a few keV to several hundred keV. In the electron spectrum, three groups of electrons with different energies can be distinguished. It is shown that the energy spectrum of the beam electrons depends on the voltage rise time, voltage amplitude, and cathode design. It is found that, in some cases, the main group of electrons consists of two subgroups separated by a dip. The peak of one of these subgroups corresponds to the maximum gap voltage, whereas the electron energy in the lower-energy subgroup depends on the voltage rise time, voltage magnitude during SAEB generation, and electrode gap length. The maximum amplitudes of the SAEB current behind the foil at a fixed gap voltage were obtained using cathodes in the form of a thin-wall (50-100 mum) stainless-steel tube. Under these conditions, the number of electrons with energies higher than the energy gained by electrons in vacuum at a maximum gap voltage is relatively small. These electrons were detected only at a voltage rise time of ~0.5 ns and shorter. - t is shown that the fraction of electrons with anomalous energies in the beam increases with cathode curvature radius. We suppose that the generation of electrons with anomalous energies is caused by their polarization self-acceleration. In this case, the excess negative charge is accumulated at the front of the ionization wave, which propagates toward the anode with a high velocity (~10 cm/ns and faster). This leads to additional acceleration of the beam electrons ahead the front of the ionization wave.