Gregory G. Denisov

Gyro-BWO experiments using a helical interaction waveguide

A helically corrugated waveguide was used for a gyrotron backward-wave oscillator (gyro-BWO) experiment. A thermionic cathode was used to produce an electron beam of 90-215 keV in energy, 2-3 A in current, and pitch alpha of up to 1.6. The oscillator achieved high-efficiency frequency-tunable operation. At a fixed beam voltage of 185 kV and a current of 2 A, the output frequency was tuned by adjusting the magnetic field in the interaction cavity. A maximum power of 62 kW and a 3-dB frequency-tuning band of 8.0-9.5 GHz (17% relative tuning range) with a maximum electronic efficiency of 16.5% were measured. In addition, the interaction frequency could be tuned by varying the electron beam energy. At a fixed cavity magnetic field of 0.195 T, the output frequency and power from the gyro-BWO were measured as a function of tuning electron beam energy while the beam current was maintained at 2.5 A. A 3-dB relative frequency tuning range of 8% was measured when the electron beam voltage was changed from 215 to 110 kV.

Theory and simulations of a gyrotron backward wave oscillator using a helical interaction waveguide

A gyrotron backward wave oscillator (gyro-BWO) with a helically corrugated interaction waveguide demonstrated its potential as a powerful microwave source with high efficiency and a wide frequency tuning range. This letter presents the theory describing the dispersion properties of such a waveguide and the linear beam-wave interaction. Numerical simulation results using the PIC code MAGIC were found to be in excellent agreement with the output measured from a gyro-BWO experiment.

3D wavebeam field reconstruction from intensity measurements in a few cross sections

Abstract An iterative method of 3D amplitude-phase field structure reconstruction using only measured amplitude distributions of the field in a few cross sections of a wavebeam is proposed. This method is based on synthesis of phase fronts providing proper amplitude conversions due to diffraction between chosen cross sections. As a procedure for phase synthesis we chose that published in: J. Radiotechnics and Electronics 12 (1967) 244. The reconstruction method includes a new original algorithm of the fast diffractional Huygens-Kirchhoff integral calculation (FDI). By means of this method, the field structures have been reconstructed in some practical experiments in the mm-range. Since the method is based on general principles, it can be useful for solving a wide class of applied tasks. The software elaborated for the amplitude-phase field structure investigation is intended for a conventional IBM PC.

Ka-Band Gyrotron Traveling-Wave Tubes With the Highest Continuous-Wave and Average Power

The results of experimental investigation of two Ka-band gyrotron traveling-wave tube (gyro-TWT) amplifiers with helically corrugated waveguides are presented. The first tube produces pulsed output power of 130-160 kW within the frequency range of 33.1-35.5 GHz and is capable of operating with a 10% duty factor. Reliability of its major components in the high average power operation regime (about 10 kW) was proven in a continuous-wave (CW) experiment. The second gyro-TWT amplifier delivered CW power of up to 7.7 kW with -3-dB bandwidth of 2.6 GHz and -1-dB bandwidth of 2.1 GHz. Effective implementation of single-stage depressed collectors (to the best of our knowledge, for the first time for gyro-TWTs) enabled the electron efficiencies as high as 36% for the pulsed tube and 33% for the CW tube to be achieved at operation at the second cyclotron harmonic.

Microwave pulse compression using a helically corrugated waveguide

Cylindrical waveguide with a helical corrugation on the inner surface has proven an effective dispersive medium for the compression of smoothly frequency modulated microwave pulses. This paper presents the results of experiments where ~5.6kW, X-band (8.0GHz to 12.5GHz), microwave pulses of 80ns duration and 5% frequency modulation were compressed into 1.5ns pulses with 25 times higher peak power

Selective excitation of high-order modes in circular waveguides

The excitation of very high-order modes in circular waveguides has been performed in a cavity with a connected up-taper with a geometry similar to those used in gyrotrons. A Gaussian beam was coupled to the cavity which was made translucent by an array of holes. With the help of a special optics, the amplitude as well as the phase distribution of the beam was matched to the mode to be excited in the resonant cavity. By simple rotation of one mirror to adjust the phase distribution together with the change of frequency to match the resonance condition, a large number of modes could be produced with one experimental set-up. Field measurements in the output waveguide show a high mode purity of the radiation and confirm the calculations. The method can be used for cold tests of electrodynamic systems operating with these modes, e.g. quasi-optical converters for gyrotrons.

Principles of Synthesis of Multimode Waveguide Units

We propose principles which represent a systematic approach to the synthesis of multimode waveguides converting a specific input field into a different output field. Using these principles, fast and efficient methods of synthesis with specified properties can be constructed for any field analysis method. Several new methods of synthesis for most important applications are offered, and synthesized waveguide components are presented to demonstrate their efficiency.

Microwave System for Feeding and Extracting Power To and From a Gyrotron Traveling-Wave Tube Through One Window

A method, which enables feeding and extraction of microwave radiation to and from gyrotron traveling wave tubes (gyro-TWT) using a single highly oversized barrier window is proposed. The essential conditions for effective implementation of this method are as follows: 1) the operating mode of the amplifier should be a circularly polarized one and 2) the “cold” losses of its interaction circuit should be sufficiently low. A detailed explanation of the method and discussion of the major microwave components are presented.

Progress and First Results With the New Multifrequency ECRH System for ASDEX Upgrade

A multifrequency electron cyclotron resonance heating (ECRH) system is currently under construction at the ASDEX Upgrade tokamak experiment. The system employs depressed collector gyrotrons, step tunable in the range of 105-140 GHz, with a maximum output power of 1 MW and a pulse length of 10 s. One two-frequency GYCOM gyrotron has been in routine operation at ASDEX Upgrade since 2006. A further extension of the system with three more gyrotrons is underway. An in situ calibration scheme for the broadband torus window has been developed. The system is equipped with fast steerable mirrors for real-time MHD control. The gyrotron and the mirrors are fully integrated into the discharge control system. The ECRH system turned out to be essential for the operation of H-modes after covering the plasma facing components of ASDEX Upgrade with tungsten. Deposition of ECRH inside rhotor < 0.2 is necessary to prevent accumulation of W in plasmas with high pedestal temperatures. With respect to the limited loop voltage available in ITER, the use of ECRH for neutral-gas preionization to facilitate plasma breakdown and its application during the current ramp-up to increase the conductivity in order to save transformer flux have been demonstrated successfully for 105 GHz, 3.2 T (O1-mode) and 140 GHz, 2.2 T (X2-mode), corresponding to 170 GHz at ITER with the full and half values of its foreseen toroidal field of 5.3 T.

Cyclotron autoresonance maser with high Doppler frequency up-conversion

An LIA-unit with explosive emission injector was used as a basis for CARM with high Doppler frequency up-conversion when the wave frequency is 7 to 9 times the cyclotron frequency of electrons. Using a high-selectivity Bragg resonator as an electrodynamic system of CARM we investigated two regimes having essentially different properties: the dispersion characteristics of the electron beam and the wave either intersected or were tangential to one another. In the first case, the radiation power amounted to 50 MW at the wavelength of 4.4 mm with efficiency 8%. The efficiency significantly smaller than the design value was evidently caused by a high level of parasitic superluminiscence of the beam. In the second regime of operation at 6 mm, the radiation power was 30 MW with a low level of parasitic superluminiscence and efficiency 10% which was close to the calculated value.