We present the results of experiments on the generation of periodical trains of subnanosecond $K_a$-band pulses in the electron generator, which is based on passive mode locking. This technique is broadly used in laser physics for the generation of ultrashort pulses (USPs) but has not been previously employed in high-power microwave electronics. The microwave USP generator comprises a helical-waveguide gyrotron traveling wave tube and a saturable absorber in the feedback loop. Saturable absorption is implemented in an auxiliary section using the cyclotron resonance interaction with an initially rectilinear electron beam, for which nonlinear saturation is caused by the effect of the relativistic dependence of the gyrofrequency on the particle’s energy. In agreement with theoretical predictions, periodical trains of 0.4-ns pulses with a peak power of 100 kW and repetition of 2.5 ns are measured. Phase coherence of radiated pulses is demonstrated based on analysis of the autocorrelation function. The experiments performed open a wide range of possibilities for the generation of coherent broadband radiation, which is in high demand for numerous applications.

• Received 6 July 2021
• Revised 4 October 2021
• Accepted 5 November 2021

DOI:https://doi.org/10.1103/PhysRevApplied.16.054045