A. V. Chirkov

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.

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.

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).

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.

Terahertz gyrotrons: State of the art and prospects

The state of the art for terahertz gyrotrons that are needed for various scientific research and practical applications is presented. Powers of 5 kW and 200 kW are obtained at frequencies of 1 and 0.7 THz using pulsed gyrotrons with pulse durations of tens of microseconds. A power of 100 W is demonstrated for cw gyrotrons at frequencies ranging from 0.2 to 0.5 THz.

Simulation and experimental study of a remote steering system for ECRH/ECCD antenna beams

Abstract The present design for the ITER electron cyclotron wave launcher is based on individual circular corrugated waveguides running up to the vacuum vessel. At each waveguide end near to the plasma, a pair of movable mitre bends provides steering of the beam between 0° and 40°. An alternative to this concept could be corrugated square or rectangular waveguides. These waveguides show imaging characteristics, which can be used for remote scanning of the beam, thus avoiding movable parts near to the plasma. To obtain a safe data base for the realisation of this concept, theoretical and experimental studies were carried out. The calculations show that a scanning range of more than ±10° is possible with negligible loss into side lobes. Furthermore, concepts to improve the scanning range can be derived from the calculations. Measurements of amplitude and phase distribution in the output plane of the waveguide and in the far-field show very good agreement with theory for a beam polarisation perpendicular to the scanning direction. For beams polarised parallel to the scanning direction, where the propagation in the waveguide is determined mainly by the grooved walls, a reduced beam quality is measured, which can be attributed to imperfect machining of the grooves. The results show also, that the imaging characteristics are in principle maintained after the introduction of a pair of mitre bends with the bending plane perpendicular to the scanning plane. Finally, the application to ITER is discussed and concepts to improve the scanning range are given.

Mode content analysis from intensity measurements in a few cross sections of oversized waveguides

A method of mode content analysis in oversized waveguides basing on intensity measurements in a few cross sections is suggested. The method is based on iterative synthesis of phase fronts of the radiation measured in a few cross sections of an oversized waveguide. The phase reconstruction makes it possible to calculate field distribution as well as the mode content at any place of the waveguide. The method was tested in experiments with circular and rectangular waveguides with impedance corrugation of the walls [8].

Multi-Frequency Gyrotron with BN Brewster Window

A multi-frequency gyrotron equipped with a circular Brewster window made on the base of high-temperature-brazed ceramic disc was tested. Efficient gyrotron operation at 11 modes (frequencies) in the range of 105 GHz -143 GHz was demonstrated. Gyrotron output power (0.8 MW/0.1 sec) was limited by the used power supply. The gyrotron scheme and the test results are used in the design of long-pulse gyrotron with a CVD diamond window.

Perspective gyrotron with mode converter for co- and counter-rotation operating modes

A gyrotron oscillator operating efficiently at modes of both rotations was developed and tested. The gyrotron operation can be switched between two modes: co- and counter rotating ones with respect to electron rotation in a resonance magnetic field. A synthesized mode converter provides output of both waves in the form of two different paraxial wave beams corresponding to direction of the mode rotation. Measured gyrotron power (up to 2 MW), interaction efficiency (34%), and diffraction losses in the mode converter (≈2%) agree well with the design values. The proposed gyrotron scheme alloys principal enhancement in the device parameters—possibility of electronic switching of output wave beam direction and possibility to arrange an effective scheme to provide frequency/phase locking of a gyrotron-oscillator.

Methods of Wavebeam Phase Front Reconstruction Using Intensity Measurements

Two methods for phase front reconstruction of paraxial linear polarized wavebeams are presented. Both of them are based on wavebeam field intensity measurements. The first method requires a reference wavebeam with known characteristics. The second one allows phase reconstruction using only wavebeam intensity measurements in a few cross sections. The methods described are successfully applied in the millimeter range for wavebeam diagnostics in mode converters, transmission lines and antenna elements. Since the methods are based on general principles they may be used not only for microwaves investigation.