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The radiation-frequency locking by an external signal is experimentally studied for a megawatt gyrotron. An external signal from a magnetron is applied to the operation space of the gyrotron at a frequency of 35 GHz via a synthesized two-mirror quasioptical converter developed at the Institute of Applied Physics of the Russian Academy of Sciences. The radiation spectra are measured for both the frequency locking regime and the frequency beats. The locking region is constructed on the plane of two parameters, namely, the external-signal power and frequency. The experimental and theoretical results are compared to show a good agreement.

We report a design of a new quasi-optical input coupler developed for the 35-GHz two-cavity gyroklystron. The coupler is a section of longitudinally corrugated waveguide placed between two cutoff irises. It operates with an eigenmode being a superposition of the rotating on azimuth TE7,4 and TE13,2 modes in the input cavity. The corrugated cavity is designed with holes in the walls to ensure necessary mode selection, because in a broad frequency band the operating eigenmode only does not have fields at the holes. In experiments, the gyroklystron equipped with this coupler has shown a peak output power of 4.2 MW with a nearly 1- $\mu \text{s}$ pulse duration, a power gain of 24 dB, and an efficiency of about 15%.

The frequency capture of radiation by an external signal was experimentally studied for the first time for a megawatt-power gyrotron. An external signal from the magnetron at a frequency of 35 GHz entered the working space of the gyrotron through a synthesized two-mirror quasi-optical converter that allowed transforming the input signal into a working mode. This converter has been recently developed and implemented in the Institute of Applied Physics, Russian Academy of Sciences. The radiation spectra were experimentally obtained in modes of frequency capture and frequency beats. The dependence of the frequency capture bandwidth on the power of the external signal was found. Comparison of experimental results with the data of numerical modelling of gyrotron equations with an external signal showed their good agreement.

Previously, relativistic gyrotrons with thermionic magnetron-injection electron guns have been demonstrated successfully to produce radiation in the cm and long-mm wavebands with the pulse power exceeding 10 MW. This brief presents a new step toward generating higher-frequency microwaves with a relativistic gyrotron operated in the 3-mm wavelength band. The gyrotron employed with a 250 kV, 100 A, $1~\mu \text{s}$ helical electron beam. The operating wave was the rotating TE12,5,1 mode of a circular waveguide cavity. The mode was transformed into the Gaussian wave beam by a built-in mode converter. In the optimized single-mode regime, the gyrotron delivered 5.6 MW of the peak power in pulses with durations of about 0.5 $\mu \text{s}$ .

The relativistic gyrotron with thermionic magnetron-injection gun was developed and experimentally tested at 3-mm wavelength band. The gyrotron yielded stable generation of the rotating TE12,5,1 mode of a circular waveguide cavity with 250 kV/100A helical electron beam. The mode was transformed into the Gaussian wave beam by a built-in mode converter. Output power of the gyrotron was over 5 MW in pulses with durations of about 0.5 μs.

A magnetic analyzer of a helical subrelativistic electron beam formed by a three-electrode magnetron-injector gun in the field of a cryomagnet is described. Electrons are separated in transverse velocities by a magnetic field adiabatically increasing along the beam direction. The analyzer was used to measure the electron spread in velocities and the pitch factor of a microsecond helical electron beam with an electron energy of 300 keV and a current of 100 A.

We present the results of studying the start-up scenario for a high-power pulsed gyrotron operated at the TE53 mode for two variants of the current–voltage characteristic of a three-electrode magnetron-injection electron gun. In the standard variant of feeding the voltage to the first anode from the resistive divider, sequential generation of the TE73 and TE63 modes, and, finally, the operating TE53 mode was observed at the edge of the pulse. The current–voltage characteristic for which only the operating mode is excited is obtained by decreasing the growth rate of the voltage applied to the first anode.