Repetitively-Pulsed Relativistic BWO with Enhanced Mechanical Frequency Tunability

Abstract

Gigawatt repetitively-pulsed relativistic microwave generators with pulse-to-pulse frequency tuning are of interest of some practical applications. Earlier, a 5-15% frequency tuning was demonstrated in single-pulse regime, in particular, in the resonant relativistic BWO, in the feedback-supplied resonant relativistic TWT, and in the two-sectional vircator. However, repetitive operation of these sources meets severe technical problems such as gradual variation of slow-wave structure corrugation period (RBWO), cooling of solenoid producing the guide magnetic field and cooling of electron beam collector (RBWO and RTWT), lifetime of anode mesh (vireator). This paper summarizes our earlier results on mechanical frequency tuning in the relativistic RBWO with resonant reflector. It was shown in theory that it is the effect of electrons" energy premodulation in the resonant reflector area that enables the frequency control. Subsequent experiment demonstrated a 12-% frequency tuning in single-pulse regime simply obtained by mechanically varying the length of the electron drift space included between the reflector and the inlet of the slow-wave structure. This regime of generation was realized in low magnetic fields (below the cyclotron resonance regarding the backward wave). This is essential for repetitive operation of the source observing no necessity for magnetic system cooling, decreasing energy consumption for magnetic field pulse production (having the length about 1 sec) and partially softening the collector cooling due to larger area of e-beam dumping. In experiment, 1-sec hitches of microwave pulses at 50 Hz rate were produced in the RBWO with resonant reflector demonstrating a 9-% pulse-to-pulse gradual frequency tuning (at -3 dB level regarding the maximum peak power) in the low magnetic field of 0.36 T. The maximum peak power over the batch was 2.5 plusmn 0.6 GW at 3.6 GHz while the power efficiency was 20 plusmn 4 % and the microwave pulse width (FWHM) was about 20 ns.