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

The spectra of an ultrashort avalanche electron beam generated by a nanosecond discharge in atmospheric-pressure air were investigated. The temporal characteristics of the beam current pulses, gap voltage, and discharge current in a gas diode were measured with a time resolution of ～0.1 ns. A simple technique was developed for recovering electron spectra from the curves of beam attenuation by aluminum foils. The effect of the cathode design, electrode gap length, and generator parameters on the electron spectra were studied using seven setups. It is shown that generation of electrons with anomalously high energies requires the use of cathodes with increased curvature radius.