Effects in a high power BWO induced by the exhausted electron beam

Summary form only given, as follows. As a result of the interaction with the operating wave in a relativistic BWO, electrons obtain a great velocity spread for practically any efficiency of beam-to-microwave energy conversion. Calculations show that the distribution of electrons is almost symmetric with respect to the average electron energy. The main portion of the electrons is found in two fractions: one of them has energy almost twice that of the average value, whereas the electrons of the second fraction have very low energy that lead to a decrease in the value of the space-charge-limiting current. When the electron collector is placed far from the interaction space, local virtual cathodes appear in the output channel, even when the electron beam propagates close to the wall. Part of the reflected electrons reach the cathode and result in a decrease of the beam current and the subsequent disappearance of virtual cathodes. As a result, modulation of the output radiation appears, the depth of modulation depending on the collector conditions. In experiments performed at the Institute of Applied Physics using a Sinus accelerator, a 0.5 MV, 17 ns X-band relativistic BWO radiated microwave pulses of 10 ns duration. The placement of a massive copper ring directly at the end of the electrodynamic system resulted in the microwave pulse duration equaling the voltage pulse duration. In the case without the massive ring, pulse shortening could be attributed to a decrease in electron emission owing to electrons reflected from the collector; the electron beam current would fall below the start oscillation current, thereby preventing microwave generation. Therefore, conditions at the collector of a relativistic BWO can interrupt the regime of stationary microwave generation.