Disk-Shaped Bunch of Runaway Electrons Formed in a Magnetized Air Diode

We report the first experimental results on the generation in a magnetically isolated air-filled diode of the flow of sub-relativistic runaway electrons (RAEs) with a duration of approximate to 20 ps and a charge of approximate to 1 nC resembling a slightly curved disk with a transverse size (approximate to 20 mm) noticeably higher than that in the propagation direction. RAEs originate from the boundary of plasma blobs near electric field enhancers protruding at the graphite cathode end face. Accounting for the transformation of the incident voltage front permits profiling the edges of concentric protrusions so that the field on them synchronously reaches the critical value for RAE appearance. It is shown that approximate to 45% of RAEs have energies >125 keV. Emission moment shifts of the bunch radial fractions, their rms spread (approximate to 1.2 ps), and the total bunch current (approximate to 50 A) were determined.