Numerical analysis of an injection-locked gyrotron backward-wave oscillator with tapered sections.

Injection-locked operation of a gyrotron backward-wave oscillator (gyro-BWO) is investigated by means of our time-dependent self-consistent code. Numerical results for a 100 kW, Ka-band, TE11-mode gyro-BWO developed at NTHU, Hsinchu, Taiwan, are compared to the experimental ones. The results are in good agreement showing, in both cases, the asymmetric form of the locking bandwidth curve. Comparison of these results obtained for the case of injection of the external signal into the upstream port, as has been done in the experiments, to the results for injection into the downstream port is made. The results of the comparison demonstrate that the asymmetry of the locking bandwidth curves must be attributed to the influence of the injection signal on the electron bunching that takes place in the input tapered section. Moreover, in some cases, this influence results also in a significant increase of the locking bandwidth (up to a factor of 5). An increase of the output power of the locked gyro-BWO over the output power of the free running one is also investigated and compared with the experimental data.