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 ≈ 20 ps and a charge of ≈1 nC resembling a slightly curved disk with a transverse size (≈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 ≈45% of RAEs have energies >125 keV. Emission moment shifts of the bunch radial fractions, their rms spread (≈1.2 ps), and the total bunch current (≈ 50 A) were determined.