Demonstration of high-gradient electron acceleration driven by subnanosecond pulses of Ka-band superradiance

We report the high-gradient acceleration of electrons driven by a subnanosecond pulse of Ka-band Cherenkov superradiance (SR). The experiments are carried out in a combined ``generator-accelerator'' scheme, powered by two electron beams from a coaxial explosive-emission graphite cathode. The outer tubular beam (≈300 keV; ≈2.3 kA) propagates along the periodic slow-wave structure (SWS) and generates a backward moving gigawatt-level SR pulse, which pumps a low-Q ``pill-box'' resonator located at the SWS input. The inner paraxial test beam (≈250 keV; ≈150 A) passes through a hole in the resonator wall and is accelerated in extreme SR fields exceeding 500 MV/m. The energy of accelerated electrons is estimated by measuring the test beam current after it passes through aluminum filters (foils) that absorb low-energy electrons. It is shown that the test beam contains fractions with a maximum energy of 1.25 MeV, which, taking into account the pill-box length of 4 mm, corresponds to the extremely high averaged accelerating gradient of 250 MV/m.