A. A. Bogdashov

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.

Optimization of the frequency step tunable 105-170 GHz 1 MW gyrotron prototype

The report presents results of investigation of the possibility to optimize numerically and experimentally the helical electron beam formation system, mode converter system and processes of mode interaction in the electron beam-cavity system for the prototype 105-170 GHz 1 MW step tunable gyrotron with different operating modes. The influence of ohmic load on the cavity, mode competition, electron beam potential depression and possibility of beam energy recovery is also investigated.

Ka-band resonant ring for testing components for a high-gradient linear accelerator

A new millimeter-wave resonant ring using a traveling TE/sub 01/ wave in an oversized cylindrical waveguide has been developed for high-power tests of accelerator components. Novel low-loss miter bends with flat mirrors that utilize mode mixing to minimize losses were used in the resonant ring. Low-power measurements show a maximum effective power gain factor that exceeds 35:1 at the operating frequency of 34.272 GHz. Total quality factor is approximately 21400, and the reflection coefficient from the input to the ring resonator is less than 1%.

Development of the step tunable 140/110 GHz 1 MW gyrotron for fusion

Electron-cyclotron heating and current control in plasma set-ups require gyrotrons with a level of CW power up to 1 MW at frequencies 80-170 GHz and efficiency not less than 50%. The output window was the key factor that limited the achievable pulse duration in gyrotrons. The CVD diamond single disk window is capable of full 1 MW CW operation. Recent results generated interest in the use of frequency tunable gyrotrons for ECRH plasma experiments. In this paper we discuss the main problems of the step tunable gyrotron and present the design of a step tunable 110/140 GHz 1 MW gyrotron. The 80 kV, 40 A beam is produced by a magnetron injection gun (MIG) with an average perpendicular-to-parallel velocity ratio of 1.2 and a transversal velocity spread below 25%. The microwave circuit has a TE/sub 22.81/-TE/sub 19.61/ cavity, which operates at 140 GHz and 111.5 GHz respectively. Electron beam parameters were optimized for efficient generation of the modes. These modes were chosen from the condition of the gyrotron CVD window transparency at the operation frequencies. The highly efficient quasi-optical mode converter was elaborated for operation with two different cavity modes.

New results in development of MW output power gyrotrons for fusion systems

Gyrotrons are widely used in electron-cyclotron-wave systems of fusion installations. Modern and future installations require microwave sources with power at least 1 MW. CVD diamond windows are implemented into the gyrotrons with frequencies 140 GHz and 170 GHz. The goal of these near-time tests is to demonstrate 0.8-1 MW power in microwave pulse of 10 seconds at least at two frequencies 105 GHz and 140 GHz, corresponding to pass bands of the single diamond disc.

Comparison of Wavebeam Phase Front Retrieval Methods Based on Iterative Algorithm and Irradiance Moments

Two methods for phase front reconstruction of quasi-optical wavebeams are discussed. Both of them allow phase reconstruction using only wavebeam intensity measurements in a few cross sections. The first method applies the iterative convergence procedure for phase synthesis. The second one is based on averaged description of wavebeams, using the irradiance moments. By means of these two methods the field structures was reconstructed in some numerical and practical experiments in mm-range. Since the methods are based on general principles they may be used not only for microwaves, but also for optics and acoustics. It was shown that depending on measurement quality, wavebeam amplitude geometry and number of measured planes, one or another method is more preferable. The optimization of phase retrieval technique by combining iteration algorithm with irradiance moment method is proposed.

Metal reflectivity investigation at 120-140 GHz

The reflectivity (reflection losses) investigation of pure (>99.95%) highly reflective metals namely silver, copper, and real metals and alloys at 120-140 GHz were made. Reflectivity versus roughness is presented.

Design and test of new millimeter wave notch filter for plasma diagnostics

A new design of millimeter wave notch filter was suggested and tested. Fundamental waveguide notch filter consists of a smooth TE301 resonator with a coupling cut-off hole and a frequency tuning plunger. Six resonator notch filter was manufactured and measured. A numerical control machining was used to mill two identical half of the filter. In the experiment the only one resonance was observed in the 53.6 - 78.3 GHz frequency band. Power transmission of -90 dB was measured at 68 GHz, the notch band at the level -60 dB is 600 MHz. Frequency tuning of 0.5 GHz was also demonstrated.

Oversized

A new millimeter-wave barrier window in an oversized metallic circular waveguide has been developed for high-power transmission. The novel design utilizes the traveling-wave regime in an oversized profiled guide and synthesized TE0n mode mixing to minimize field intensity at the alumina ceramics surface. Proper choice of ceramics thickness provides necessary broadening of the operation frequency band. Low-power measurements show a reflection level below -25 dB at the operating frequency band of 34.272 plusmn 0.06 GHz

Ka

Notch filters based on resonant reflection of the incident guided wave of a single-mode rectangular guide designed for frequencies of 28 and 70 GHz are numerically and experimentally studied. As shown by experiments, the 28 GHz two-sectioned resonant notch filter provides the attenuation of -60 dB at the central frequency f=28 GHz, more than -30 dB attenuation within the frequency band /spl delta/f=60 MHz and attenuation-free band up to 36 GHz. For the 70 GHz notch filter, the measured maximum attenuation is -60 dB at the operating frequency. The notch bandwidth at the level of -55 dB is 100 MHz. These characteristics allow use of the notch filters for protection of HF receivers of high sensitivity, for spectral measurements and other related purposes.