V. E. Zapevalov

Review of Subterahertz and Terahertz Gyrodevices at IAP RAS and FIR FU

The maximal frequency of radiation higher than 1 THz has been recently obtained in pulse gyrotrons both at IAP (Nizhny Novgorod, Russia) and FIR (Fukui, Japan). CW generation at a 2.2-kW power level is radiated from a 300-GHz gyrotron and used for technological applications. New gyrotrons demonstrate single-mode operation at the second cyclotron harmonic with a frequency of 395 GHz in the CW regime with a power of 100 W and at the third harmonic with frequencies of 371-414 GHz in 10-mus pulses with power of 10-20 kW. Methods of selective excitation of higher cyclotron harmonics, frequency multiplication, and smooth frequency tuning in terahertz gyrotrons are also discussed in the review.

Mirror synthesis for gyrotron quasi-optical mode converters

A new approach for the design of a quasi-optical gyrotron mode converter is proposed. It is based on the synthesis of quasi-optical mirrors using diffraction (Helmholtz-Kirchhoff) integrals. First application of such a method we already described in [1, 2]. Now a more powerful technic is used to provide optimal distributions of the wave beam field. It is shown that by means of specially shaped mirrors parameters of the main converter are enhanced strongly. In this paper we review the search for the optimal scheme of the converter, present and compare results of calculations and measurements of the gyrotron wave beam.

A high harmonic gyrotron with an axis-encircling electron beam and a permanent magnet

A gyrotron with an axis-encircling electron beam is capable of high-frequency operation, because the high-beam efficiency is kept even at high harmonics of the electron cyclotron frequency. We have designed and constructed such a gyrotron with a permanent magnet. The gyrotron has already operated successfully at the third, fourth, and fifth harmonics. The frequencies are 89.3, 112.7, and 138 GHz, respectively, and the corresponding cavity modes are TE/sub 311/, TE/sub 411/, and TE/sub 511/. The permanent magnet system is quite novel and consists of many magnet elements made of NbFeB and additional coils for controlling the field intensities in the cavity and electron gun regions. The magnetic field in the cavity region can be varied from 0.97 to 1.18 T. At the magnetic field intensities, the output powers at the third and the fourth harmonics are 1.7 and 0.5 kW, respectively. The gyrotron is pulsed, the pulse length is 1 ms and the repetition frequency is 1 Hz. The beam energy is 40 kV and the beam current is 1.2-1.3 A. Beam efficiencies and emission patterns have also been measured. In this paper, the experimental results of the gyrotron are described and compared with computer simulations.

Development of 170 GHz/1 MW Russian gyrotron for ITER

The report presents the results of development of the 170 GHz/1 MW gyrotrons in Russia. The main problems of powerful gyrotrons at present (formation of a high quality helical electron beam, selection of a high efficiency cavity mode, decreasing of inner converter losses, reliable collector and output window) are discussed. The design versions of the main parts of gyrotrons including the short-pulse prototype, first and second versions of industrial models and test set-up are presented. Advanced 170 GHz ITER gyrotrons were also investigated including a single-stage depressed collector versions (CPD) as well as an improved window (CVD diamond window from FZK). The possibility to make high efficient 170 GHz/1 MW gyrotron for ITER is proved by calculations and experiments.

Experimental investigation of a 110 GHz/1 MW gyrotron with the one-step depressed collector

The results of experimental investigation of high power gyrotron with depressed collector are given. Total efficiency of a gyrotron with one-step energy recovery of 0.65 at the level of output power about 1 MW was achieved experimentally.

The problems of increase in power, efficiency and frequency of gyrotrons for plasma investigations

Abstract In experiments aimed at the development of gyrotrons for electron-cyclotron (EC) plasma heating, output power of 2 MW at a wavelength of 3 mm was attained. The possibility of increase in frequency of millimetre-wave gyrotron operation by transition to higher EC harmonics at a fixed magnetic field by the use of high-selectivity two-resonator cavities is discussed. Outputs of over 100 kW were generated at the second and third harmonics of EC frequency. The results of calculation of pulsed magnetic systems, capable of providing the operation of near millimetre-wave long-pulsed gyrotrons, are given. The principles of increase in efficiency of gyrotrons by means of a depressed two-stage collector of electrons are discussed.

Advanced Numerical and Experimental Investigation for Gyrotrons Helical Electron Beams

The paper presents a modern approach to studies of characteristics of intense helical electron beams (HEBs) for gyrotrons. The essence of the method consists in finding functions of electron distribution with respect to oscillatory velocities in the beam both by numerical simulation and experimentally. Experimental and calculated data for beams with different topologies were compared in different frequency ranges. Evolution of the electron oscillatory velocities distribution function as the beam current grows has been traced. Influence of the reflected electrons on beam parameters has been simulated numerically.

Experimental tests of a 263 GHz gyrotron for spectroscopic applications and diagnostics of various media

A 263 GHz continuous-wave (CW) gyrotron was developed at the IAP RAS for future applications as a microwave power source in Dynamic Nuclear Polarization / Nuclear magnetic resonance (DNP/NMR) spectrometers. A new experimental facility with a computerized control was built to test this and subsequent gyrotrons. We obtained the maximum CW power up to 1 kW in the 15 kV/0.4 A operation regime. The power about 10 W, which is sufficient for many spectroscopic applications, was realized in the low current 14 kV/0.02 A regime. The possibility of frequency tuning by variation of the coolant temperature about 4 MHz/1 °C was demonstrated. The spectral width of the gyrotron radiation was about 10(-6).

Experimental studies of gyrotron electron beam systems

In order to provide electron beams of powerful gyrotrons, magnetron-injection guns operating in the regime of the temperature limited current are used. The electron beam quality and gyrotron performances are defined both by the cathode emission processes and the processes occurring in the electron beam during its formation and transportation. The results of measurements of the energy spectrum and velocity spread of the gyrotron electron beam in different regimes are given. Experimental data on the parameter of efficient emission inhomogeneity for different regimes are presented, as well as the dependencies of electron beam parameters on efficient inhomogeneity of the cathode.

Coupled cavities with mode conversion in gyrotrons

Abstract The basic features of electrodynamic systems of gyrotrons formed by open coupled cavities with mode conversion (CCMC) are considered. Both the linear and the non-linear theories of gyrotrons with CCMC are given. Experiments on CCMC gyrotrons operating at the second and third cyclotron harmonics have been carried out at power levels up to 150kW. The experimental data are in good agreement with the theoretical results and demonstrate the high efficiency of the systems. The method of plane cross-sections which was used for the calculations is described in the Appendix.