The parameters of X-ray radiation and runaway electron beams (RAEBs) generated at long-pulse discharges in atmospheric-pressure air were investigated. In the experiments, high-voltage pulses with the rise times of 500 and 50 ns were applied to an interelectrode gap. The gap geometry provided non-uniform distribution of the electric field strength. It was founded that at the voltage pulse rise time of 500 ns and the maximum breakdown voltage Um for 1 cm-length gap, a duration [full width at half maximum (FWHM)] of a RAEB current pulse shrinks to 0.1 ns. A decrease in the breakdown voltage under conditions of a diffuse discharge leads to an increase in the FWHM duration of the electron beam current pulse up to several nanoseconds. It was shown that when the rise time of the voltage pulse is of 500 ns and the diffuse discharge occurs in the gap, the FWHM duration of the X-ray radiation pulse can reach ≈100 ns. It was established that at a pulse-periodic diffuse discharge fed by high-voltage pulses with the rise time of 50 ns, an energy of X-ray quanta and their number increase with increasing breakdown voltage. Wherein the parameter Um/pd is saved.

Repetitive nanosecond-pulse discharge with a highly inhomogeneous electric field was investigated in air at atmospheric pressure. Three repetitive nanosecond generators were used, and the rise times of the voltage pulses were 15, 1, and 0.2 ns, respectively. Under different experimental conditions, X-rays and runaway electron beams were directly measured using various setups. The variables affecting X-rays and runaway electrons, including gap distance, pulse repetition frequency, anode geometry, and material, were investigated. It was shown that it was significantly easier to record the X-rays than the runaway electrons in the repetitive nanosecond-pulse discharge. It was confirmed that a volume diffuse discharge was attributed to the generation of runaway electrons and the corresponding X-rays.