Mode-locking Regimes in a Helical Waveguide Gyro-BWO

We present results of experiments on generation of periodic sequences of ultrashort microwave pulses based on a Ka-band helical-waveguide gyro-BWO. Two operating regimes are studied, namely, self-locking of the gyro-BWO due to an external feedback loop and passive mode-locking when a saturable absorber is introduced into this loop. The saturable absorber is based on the cyclotron-resonance interaction of radiation with an initially rectilinear electron beam guided by a magnetic field. In both regimes, periodic sequences of nanosecond-duration microwave pulses are obtained. At the same time, in the passive mode locking regime, an increase in peak power and a decrease in the duration of the generated pulses are demonstrated.