Powerful sources of the millimeter wavelength range are of considerable interest for a number of applications, such as plasma heating in controlled thermonuclear fusion devices, power supply systems for charged particles linear accelerators with an ultra-high acceleration rate and new radar schemes. The most promising versions of multi-megawatt sources operating in the band seem to be relativistic gyrotrons and gyroklystrons.A new version of the pulsed relativistic gyrotron operating at the 3-mm wavelength range was developed and tested in experiment. Earlier, significant results were achieved in the realization of centimeter and long-wave millimeter range relativistic gyro-devices in the IAP RAS [1,2]. These achievements was based on the detailed study [3], which showed the possibility of obtaining high operation efficiency in gyrotrons at any initial electron energy, including the relativistic one. However, the decrease in the working wavelength, while maintaining the energy parameters of the beam and radiation, entails increasing of the cross-section of the electrodynamic system and, accordingly, the transition to higher-order operating modes. Such a transition makes more complex solving of the key problems of the gyrotron development, first of all mode selection end electron beam formation. Also, an important part of the work was the conjugation of a new short-wave device with elements of the highvoltage electron accelerator already in existence.Scheme of the experiment. The gyrotron is based on the "Saturn-F" electron accelerator [4], which can generate electron beams with a particle energy of up to 500 keV and a current of up to 200 A in pulses with the duration and repetition rate being equal to 1 μs and 10 pps, respectively. Overall view and the schematic diagram and of the relativistic gyrotron is presented in Fig. 1 and Fig. 2, correspondingly. A helical electron beam is formed by a three-electrode magnetron-injection gun. Theoretical analysis and optimization of the gun was undertaken with the ANGEL code [5]. Calculations showed it feasible to obtain an electron beam at 250 kV of the total voltage and 80-100 A of the beam current with the pitch-factor being equal to 1.3 and an acceptable spread of transverse velocities (20-25 %).The rotating TE 12,5 mode was chosen as the operating one. Single-mode numerical simulations predicted 7-8 MW of the output power with an efficiency of about 35%.Earlier relativistic gyrotrons were equipped with axial radiation outputs. That approach results, obviously, in very complicated output field structures, corresponding to the operation mode of the gyrotron, and it also has several other drawbacks. The higher-order mode scatters because of the irregularities of the output waveguide, and it results in an increase in the radiation losses.The new gyrotron was designed with a built-in mode converter, which provides transformation of the operating mode into the Gaussian wave beam and, simultaneously, separation between the regions of propagation of the RF wave and transport...
