Studies Of A Pre-bunched Backward Wave Oscillator

Among various high-power microwave sources, multiwave Cerenkov Generators (MWCGs) developed by Bugaev et al., High Current E1ec:tronics Institute, Russia, have attained top data of output. Radiation powers 3 GW at wave length 9.7 mm and efficiency 20 % were reported. Here a large mean diameter D of the slow wave structure (SWS) is the key point. Although the physical processes involved in the MWCGs are complicated, we here design a large diameter backward wave oscillator (LDBWO) operating at 24 GHz high-power microwaves without decreasing D. The inner radius of the metal surface of the SWS is assumed to vary sinusoidally as R(z)=R0+hcos(2Xz/z ) . The oscillation frequency is raised by cfloosing carefully small values of z o and h. Numerical study is made within the scope of linear pinch point analysis [1 1 for a SWS available in 24 GHz LD-BWO. Especially, we find that there exists a starting energy in addition to starting current in electron beam for initiating microwave oscillation, if the finite length L of SWS is taken into account. The optimized size parameters of SWS are as follows: RO=30 mm, h=1.7 mm, zo=3.4 mm, L=70z -238 mm and annular beam radius R -26.7 mm. !$he beam energy V -65-100 keV an$the beam current Ib up to P-kA are assumed. For Vb=65 keV, I m ( a ) C O , no matter how large Ib would be. No oscillations can be expected. The starting energy is found to be 76.5 keV and it is a decreasing function of L. It must be noted that the starting energy is not the sufficient condition, but the necessary condition for starting oscillation. Additional condition is that the radiation must grow up against the leakage at the both ends. This condition gives the starting current that is increasing function of the leakage. Intensity of output radiation from the designed LD-BWO is expected to be weak, because of small h. However, the output can be drastically enhanced, in the case that the coupling of the radiation with anomalous Doppler electron cyclotron resonance exists. Our exper ment for high-power, high-efficiency oscil ation is presented elsewhere. 5Pll