Reconstruction of energy spectrum of runaway electrons in nanosecond-pulse discharges in atmospheric air

This paper presents an experimental investigation into the runaway electron spectrum with a gas diode composed of a rough spherical cathode and plane anode under the excitation of a nanosecond-pulse generator in atmospheric air. The runaway electron beams are measured by a collector covered with aluminum foil with a thickness from 0 μm (mesh grid) to 50 μm. The energy spectrum is calculated by an improved Tikhonov regularization called the maximum entropy method. The experimental results show that the transition state of the discharge consisted of multiple streamer channels stretched from the cathode with glow-like plasma uniformly distributed over the anode. The number of runaway electrons measured by the collector is in the order of 10¹⁰ in atmospheric pressure air with a gap spacing of 5 mm and applied voltages of 70–130 kV. The cathode with a rough surface creates a more inhomogeneous electric field and larger emission site for the runaway electrons around the cathode, providing conditions for the coexistence of filamentary streamer and diffuse discharge. The reconstructed spectra show that the energy distribution of the runaway electrons presents a single-peak profile with energies from eU_m/2–2eU_m/3 (U_m is maximal voltage across the gap).