Conversion of Cherenkov Radiation to Transition Radiation by Electron Bunch Post-Acceleration for Extremely Efficient Beam–Wave Interaction

A super klystron-like relativistic backward wave oscillator (RBWO) is presented. In the device, the distributed Cherenkov radiation generated in the slow wave structure (SWS) is absorbed by the electron bunch in the last corrugation and re-accelerated, creating high-energy electrons that overcome the space-charge repulsion force to further compress the bunch length and maximize the fundamental harmonic current, contributing to the extremely efficient microwave generation in the extraction cavity through concentrated transition radiation. A novel elliptical extraction cavity is used to improve the power capacity. A soft magnet is inserted to precisely control the beam path, which provides a higher beam kinetic energy to be extracted and a stronger electric field that interacts with the tighter bunch. In the experiment, as the diode voltage was 585 kV and diode current was 7.5 kA, a microwave pulse with a power of 2.9 ± 0.3 GW measured in the radiation field, a frequency of 8.23 GHz, and a pulse duration of 22 ns was obtained. The corresponding efficiency is up to 66 ± 8%, which is an experimental record for GW-class relativistic vacuum electronic devices.