Ionization Waves During the Subnanosecond Breakdown Initiated by Runaway Electrons in High-Pressure Nitrogen and Air

The experimental investigations of a subnanosecond breakdown initiated by runaway electrons in air and nitrogen at the pressures within 0.013-0.4 MPa are performed. The temporal development patterns of the discharge plasma glow in different regions of the discharge gap are registered with a fast photodiode, a four-channel ICCD-camera, and an ultrafast streak camera. It is shown that the breakdown occurs in the form of ionization waves propagating from the electrode with a small radius of curvature. It is found that a runaway electron beam behind the anode foil at the nitrogen and air pressure similar to 0.1 MPa is detected with the collector at the point of time corresponding to the maximum gap voltage.