M. Glyavin

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.

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.

Computer simulation of axis-encircling beams generated by electron gun with permanent magnet system

Results from computer aided design of a novel electron gun generating axis-encircling beams are presented and discussed. Numerical experiments were performed by the new version of the software package GUN-MIG named GUN-MIG/CUSP. It is based on a self-consistent relativistic model and is developed as a problem oriented tool for analysis of electron-optical systems with magnetron injection guns (MIG) and electron guns with field reversal (cusp guns), forming axis-encircling beams. As a result of the simulations an electron-optical design of a novel electron gun with permanent magnet system was accomplished. The gun is expected to form high quality beams with small velocity spread and beam ripple. Parameters of the generated beams are appropriate for a prospective weakly relativistic high harmonic large orbit gyrotron (LOG). The development of such device is in progress now at the Research Center for Development of Far-Infrared Region (FIR Center) at Fukui University.

Development of THz Gyrotrons at IAP RAS and FIR UF and Their Applications in Physical Research and High-Power THz Technologies

In this paper, we present some of the most significant recent results that characterize the state of the art in the development of sub-THz and THz gyrotrons at IAP-RAS and FIR-UF after 15 years of collaboration as well as their applications in various novel and prospective research fields and advanced technologies.

Influence of reflections on mode competition in gyrotrons

A general theory is presented for studying the influence of reflections on mode competition in gyrotrons. Strong effect is found in the illustrative example: the coaxial cavity TE/sub 28,16/ gyrotron operating close to the optimal point in the general parameter spare. In some cases, the fixed-field approximation and the self-consistent approach lead to qualitatively different predictions, which emphasizes the need for self-consistent calculations in the final stage of design of a gyrotron.

Development of a Magnetic Cusp Gun for Terahertz Harmonic Gyrodevices

A magnetic cusp gun (MCG) is being developed to generate an axis-encircling electron beam, which is called the large orbit beam, which is going to drive a 0.396-THz fourth-harmonic gyrotron. Developing an MCG imposes crucial challenges on a simultaneously minimizing guiding center deviation and velocity spread of the electron beam, particularly because an ultrahigh magnetic compression ratio is unavoidable, as is the case for a terahertz (THz) gyrotron. The study of the electron dynamics in the MCG reveals that, close to the emitter, a pair of focusing electrodes are employed to construct a special focusing and accelerating electric field as a way to balance the space-charge influence and guiding center deviation. Investigation indicates that both the electron-beam generalized-angular-momentum spread and the guiding center distribution are the critical factors contributing to beam parameter spread. Intensive optimization generates a high-power MCG with a pitch factor of 1.5, the highest magnetic field of 4 T, minimum transverse velocity spread of 1.1%, and a beam current of 2 A. The key parameters exhibit excellent stability tuning over a wide range of beam current and magnetic field. These merits enable the harmonic gyrotrons or even the frequency-tunable THz gyrotrons to be developed.

Recent test results on broad-band gyro-TWT and gyro-BWO with helically grooved operating wave-guides

The paper presents recent test results on low relativistic (20-80 kV) gyro-TWT and gyro-BWO based on a microwave system in the form of a waveguide with helical surface. In particular, in experiments the Ka-band gyro-TWT showed the following output parameters: pulse power up to 180 kW, gain 25-30 dB, instant frequency band 10%, efficiency at second harmonic operation 27%. The gyro-BWO was designed for CW operation with output power of 5-7 kW at frequencies around 24 GHz. Smooth frequency tuning in the band of 10% was demonstrated.

NUMERICAL ANALYSIS OF WEAKLY RELATIVISTIC LARGE ORBIT GYROTRON WITH PERMANENT MAGNET SYSTEM

High-harmonic gyro-devices with axis-encircling electron beams known as large orbit gyrotrons (LOG) represent an appealing alternative to the conventional gyrotrons. In this paper we investigate the feasibility of such device operating with low current and low energy electron beams formed by a novel electron gun with a permanent magnet system. The results from the numerical experiments indicate the possibility to excite TE41 mode at fourth harmonic of the cyclotron frequency. Simulations predict generation of microwave radiation with frequency 104 GHz and output power near 1 kW.

Experimental investigation of technological gyrotrons

Powerful gyrotrons for technological applications, including material processing, are presented. 30 GHz gyrotrons at the second cyclotron harmonic TE/sub 02/ mode with oil-cooled solenoid were tested. On CW regime with output power 10 kW obtained efficiency 42% and output power 30 kW with efficiency 35%. The highest for the second harmonic gyrotrons efficiency 50% with 13 kW output power level was observed after modification of the tube. Experimental dependencies of output power and gyrotron efficiency before and after modification magnetic field profile are given. Conditions of high efficiency operation taking into account space-charge effects, velocity spread, RF field and static magnetic field profiles are discussed. Results of quality testing of cathodes and dependencies of the gyrotron efficiency on cathode emission inhomogeneity are presented.

Mode Competition in a Two-Mode Gyrotron With Delayed Reflections

Effect of delayed reflection from the remote load on mode competition in a two-mode gyrotron is considered. A system of quasi-linear delay-differential equations for slowly varying amplitudes of the modes is derived. The most typical case of competition of two modes with close frequencies at the center of a generation zone is studied in detail. Transitions between different generation regimes with increasing of reflection of the spurious mode are investigated analytically and numerically at different values of delay.