Simulation of Microwave Frequency Combs Generation in a W-band Helical Waveguide Gyro-TWT Locked by a Saturable Absorber

We present results of simulations which demonstrate experimental feasibility of W-band microwave generators with passive mode-locking based on helically-waveguide gyro-TWTs. Similar to lasers, passive mode locking is realized when a gyro-TWT is equipped by a feedback circuit with an installed saturable absorber. Two schemes of saturable absorbers are considered, namely, the cyclotron-resonance absorber with an initially rectilinear electron beam, as well as an absorber with a rotating beam interacting with radiation in the Kompfner dip regime due to proper adjusting the cyclotron resonance detuning. The advantage of the Kompfner absorber is the possibility of interaction at the 2
^nd
cyclotron harmonic with a multiple decrease in the guiding magnetic field. According to simulations for both considered scheme and a W-band gyro-TWT with electron energy of 60 kW and current of 10 A, it is possible to generate periodical trains of sub-nanosecond ultrashort pulses with the peak power of about 400 kW.