Quasi-optical Mode Converter Research Articles

RF Beam Parameter Measurements of Quasi-Optical Mode Converters in the mW Range

A measurement system for determining the RF properties of quasi-optical mode converters for gyrotrons operating in the D- and G-bands is built. RF beam parameters are measured. The measurement results are compared with simulations.

Stady on a high efficient Denisov-type launcher for high-power gyrotron oscillators

In this paper, a high-efficiency Denisov-type launcher, a hardcore of quasi-optical mode converters in high-power gyrotron oscillators, is investigated. Based on integral expression of Bessel functions and geometric optics, the propagation of EM wave in circular waveguides, the syntheses of amplitude and phase of nine modes are analyzed in detail. A generalized method for designing the launcher with a dimpled waveguide wall operating in different modes is demonstrated. This method shows how to select a set of appropriate coupled modes to synthesize the Gaussian-like distribution on the inner wall of the launcher. A program is developed and its accuracy is confirmed by a high-power gyrotron at 140 GHz, in TE28,8 mode theoretically. Numerical calculations show a good agreement with reported results.

准光模式变换器研究与设计

准光模式变换器研究与设计

Comparison of Different Methods for Calculating Gyrotron Quasi-Optical Mode Converters

This paper presents the use of combination of three methods for calculation and synthesis of high-efficiency microwave mode converters, such as radiators of gyrotrons. The analytical method yields immediate estimates of mode converter dimensions, the Scalar Integral Equation (SIE) allows one to synthesize efficiently the optimal profile of the mode converter, and the most accurate Electric Field Integral Equation (EFIE) is used to check all transmission characteristics of the converter including calculations of reflection and cross-polarization. The combination of these three methods is an optimal for the mode converter design. Just so the launcher was designed for a quasi-optical mode converter used in the 60 GHz gyrotron in the TE7,3 operating mode. The simulation results agree well with the measured data. The paper also presents for the first time an accurate derivation of the SIE method.

A Power-Optimizing Integrated Design of a Dual-Frequency Gyrotron Quasi-Optical Mode Converter

The conventional design method for multifrequency gyrotron quasi-optical mode converters often results in excessive stray radiation within the gyrotron tube. A new design technique is introduced that integrates the designs of the launcher and mirror system of the converter and optimizes the mirror system for minimal diffractive losses. The new technique is applied to a redesign of a dual-frequency 110- and 124.5-GHz mode converter. It is shown that, by reoptimizing the converter with the new method, its diffractive losses can be reduced by 28.7% and 17.8%, respectively, at the two frequencies, whereas all of the other conversion properties of the output beams remained constant or improved.

Study of a Quasi-Optical Mode Converter for W-band Gyrotron Oscillator

A quasi-optical mode converter for a W-band TE02 mode gyrotron is designed, fabricated, and tested. The system consists of a step-cut waveguide launcher (cut-in-half circular waveguide) and a doubly curved reflector (an elliptical reflector and a parabolic reflector). Based on the geometric optics principle and the vector diffraction theory, the mode converter is optimized and simulated by using numerical calculations. The simulation results show that the good Gaussian mode is converted from the circular waveguide TE02 mode, and the conversion efficiency is 77.9%. Experimental measurements are performed on a quasi-continuous wave gyrotron operating in the TE02 mode at W-band and show a good agreement with theoretical predictions.

Innovation on high-power long-pulse gyrotrons

Progress in the worldwide development of high-power gyrotrons for magnetic confinement fusion plasma applications is described. After technology breakthroughs in research on gyrotron components in the 1990s, significant progress has been achieved in the last decade, in particular, in the field of long-pulse and continuous wave (CW) gyrotrons for a wide range of frequencies. At present, the development of 1 MW-class CW gyrotrons has been very successful; these are applicable for self-ignition experiments on fusion plasmas and their confinement in the tokamak ITER, for long-pulse confinement experiments in the stellarator Wendelstein 7-X (W7-X) and for EC H&CD in the future tokamak JT-60SA. For this progress in the field of high-power long-pulse gyrotrons, innovations such as the realization of high-efficiency stable oscillation in very high order cavity modes, the use of single-stage depressed collectors for energy recovery, highly efficient internal quasi-optical mode converters and synthetic diamond windows have essentially contributed. The total tube efficiencies are around 50% and the purity of the fundamental Gaussian output mode is 97% and higher. In addition, activities for advanced gyrotrons, e.g. a 2 MW gyrotron using a coaxial cavity, multi-frequency 1 MW gyrotrons and power modulation technology, have made progress.

High-Efficiency Quasi-Optical Mode Converter for the Coaxial Cavity Gyrotron

A 2-MW, continuous wave, TE34,19 mode, 170-GHz coaxial cavity gyrotron for the International Thermonuclear Experimental Reactor (ITER) is under development within the European GYrotron Consortium (EGYC). This paper presents the improved design of the quasi-optical mode converter for this gyrotron. The simulation results show that with the improved quasi-optical mode converter, the fundamental Gaussian mode content of the rf beam can be enhanced to 99.1% at the output window plane and the stray radiation in the quasi-optical mode converter could be decreased to 1.2%.

Design of a quasi-optical mode converter for 94 GHz gyrotron

Based on geometry optics theory and vector diffraction theory, mode converter is studied to convert the gyrotron and other high-power microwave oscillator output mode into quasi-optical Gaussian beam, using Vlasov launcher and two quasi-optical reflector achieved quasi-Gaussian mode TEM00 transverse output.The working mechanism of Vlasov launcher in analyzed, the radiation field is calculated by using vector diffraction theory, and the surface radiation field is also calculated by using the surface current distribution method. A built-in quasi-optical mode converter is designed to convert the 94GHz, TE62 model millimeter wave into quasi-optical Gaussian beam through programming.

High Efficiency Mode Converter for Low-Frequency Gyrotron

A high efficiency quasi-optical (QO) mode converter for high-power, low-frequency gyrotron have been designed and tested. For low-frequency gyrotrons, the scales of the mode converter are comparatively small on the wavelength scale, thus causing significant diffraction losses. Over-1 MW power gyrotron with TE8,3 cavity at 28 GHz have been developed, which has a high efficiency mode converter designed by the use of numerical methods for launcher optimization. This calculation is sufficiently optimized to maximize the fractional Gaussian content of the far field. The total transmission efficiency from the mode converter to output window is 94.7%. For the experimental result of first tube, the output power of more than 1 MW has been obtained with about 40% efficiency and output burn pattern agrees fairly with the calculated profiles, which imply the design appropriateness. Besides, the frequency dependence for diffraction loss is discussed, and these results give the guiding design principle of the mode converter for high-power, low-frequency and long-pulse gyrotrons.

Continuous-Wave Operation of a Frequency-Tunable 460-GHz Second-Harmonic Gyrotron for Enhanced Nuclear Magnetic Resonance

The design, operation, and characterization of a continuous-wave (CW) tunable second-harmonic 460-GHz gyrotron are reported. The gyrotron is intended to be used as a submillimeter-wave source for 700-MHz nuclear magnetic resonance experiments with sensitivity enhanced by dynamic nuclear polarization. The gyrotron operates in the whispering-gallery mode TE(11,2) and has generated 16 W of output power with a 13-kV 100-mA electron beam. The start oscillation current measured over a range of magnetic field values is in good agreement with theoretical start currents obtained from linear theory for successive high-order axial modes TE(11,2,q). The minimum start current is 27 mA. Power and frequency tuning measurements as a function of the electron cyclotron frequency have also been carried out. A smooth frequency tuning range of 1 GHz was obtained for the operating second-harmonic mode either by magnetic field tuning or beam voltage tuning. Long-term CW operation was evaluated during an uninterrupted period of 48 h, where the gyrotron output power and frequency were kept stable to within ±0.7% and ±6 ppm, respectively, by a computerized control system. Proper operation of an internal quasi-optical mode converter implemented to transform the operating whispering-gallery mode to a Gaussian-like beam was also verified. Based on the images of the gyrotron output beam taken with a pyroelectric camera, the Gaussian-like mode content of the output beam was computed to be 92% with an ellipticity of 12%.

Study and design of a quasi-optical mode converter for W-band whispering-gallery mode gyrotron

A quasi-optical mode converter for a whispering-gallery mode gyrotron，consisting of a Vlasov helically-cut launcher and two curved-mirror reflectors, is studied and designed. Firstly, the operation mechanism of the Vlasov type quasi-optical mode converter is investigated using geometric optics theory. And then, applying the aperture field integration and the surface current source integration of the vector diffraction theory, a numerical simulation code is programmed. Finally, the code is used to study a quasi-optical converter system of a W-band whispering-gallery mode gyrotron, and the operation mechanism of the mode conversion in the converter is revealed in detail. Numerical results indicate that the TE12,2 whispering-gallery mode inside the W-band gyrotron is converted into a highly Gaussian-like beam at the output window.

Synthesis of quasioptical waveguide mode converters on the basis of the Fox–Li method

We consider a generalization of the Fox-Li method to the problems of analysis and synthesis of weakly irregular quasioptical waveguides. It is shown that the field representation in the form of integral expansions in terms of convergent and divergent cylindrical waves makes it possible to efficiently solve such problems with allowance for the effects of nonparaxiality of wave beams and the vector character of electromagnetic fields. An example of synthesizing a converter of the output radiation of a gyroklystron with an operating frequency of 30 GHz at the TE 5,3 mode to the Gaussian wave beam is considered.

Highly Efficient Quasi-Optical Mode Converter for a Multifrequency High-Power Gyrotron

A highly efficient quasi-optical mode converter with a bandwidth of 38 GHz has been designed and tested. The mode converter combines low-diffraction losses and a Gaussian mode content up to 97% for a set of nine modes in the range of 105 to 143 GHz for a 1-MW CW gyrotron. This was achieved using a dimpled-wall waveguide antenna (launcher), one quasi-elliptical mirror, and two toroidal mirrors. The optimization of the launcher was done using coupled-mode theory. The simulation results show a well-focused Gaussian beam for all nine operating modes. The curvature radii of the toroidal mirrors were determined by Gaussian mode transformation (ABCD-law) and subsequently optimized for a multimode operation. The simulations of the quasi-optical mode converter are based on the electric field integral equation and, thus, are 3-D. Experimental low-power measurements show close agreement with predictions.

Two-dimensional phase unwrapping to help characterize an electromagnetic beam for quasi-optical mode converter design

An improved two-dimensional phase unwrapping procedure is discussed that uses a weighted least-squares algorithm, a congruence operation, and a filter to unwrap the phase distribution of an electromagnetic beam. These improvements make possible several advances for mirror designs used in gyrotron quasi-optical mode converters. The improved phase unwrapping procedure is demonstrated by applying it to a measured beam and a simulated beam that are used to design mirrors. The unwrapping procedure produces a smooth unwrapped phase that does not change the characteristics of the beam. The smooth unwrapped phase distribution is also used to find an estimate for the wavenumber vector distribution that is needed to design the mirrors.

Mirror Design for Use in Gyrotron Quasi-Optical Mode Converters

An iterative planar- and cylindrical-based mirror design procedure is discussed for use in a gyrotron quasi-optical mode converter. The planar-based mirrors were designed using the iterative Katsenelenbaum-Semenov phase corrector procedure with an input beam obtained from a planar phase reconstruction. Many improvements were made to the design procedure including the use of an advanced phase unwrapping technique. The more robust design procedure resulted in smooth mirror surfaces. It was found that the fraction of power coupled between the target beam and the output of the planar-based mirrors measured in a cold test setup was 0.982. A similar design process was used for a cylindrical-based mirror pair. This design was performed using a simulated output of the launcher. It was found that the fraction of power coupled between the target beam and the theoretical output of the system was 0.990. The fraction of power coupled between the target beam and the phase-reconstructed beam at the output of the second mirror was 0.961. By using a cylindrical phase reconstruction, it was found that the reason for the slightly degraded performance of the cylindrical-based mirrors tested in the low power test setup was due to inaccuracies in modeling the field radiated from the launcher. These inaccuracies are presumed to come from impurities in the mode generator. The techniques discussed in this article can be used to improve the efficiency of quasi-optical mode converters and thus improve the operation of gyrotrons.

Analysis of a ${\rm TE}_{22,6}$ 118-GHz Quasi-Optical Mode Converter

A quasi-optical mode converter for the European TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22,6</sub> 118-GHz gyrotron has been continuously improved for long pulse operation. Measurements at the last generation show a double peak structure at the output window plane, but simulations have not been able to explain this behavior in the past. The overall output system was analyzed carefully. Besides measurements, the first full 3D electromagnetic analysis of a complete quasi-optical mode converter was performed. This gives the indication of the double-peak structure at the output window plane. An enhancement of the system for a single-peak Gaussian-like output beam will be presented.

A High Efficiency Launcher and Mirror System for Use in a 110 GHZ TE22,6 Mode Gyrotron

A four-mirror internal quasi-optical mode converter designed for a CPI gyrotron is discussed. Many improved methods were used in designing the mode converter. The launcher was designed and analyzed using the TOL and Surf3d codes developed by CCR. Mirrors 1 and 2 were designed using analytic surfaces to shape the general wavefront curvature of the beam radiated from the launcher. Mirrors 3 and 4 were iteratively designed using improved techniques that produced smooth mirror surfaces. Good agreement was found between theory and low-power cold-test measurements. The fraction of power coupled between the target Gaussian beam and the output of mirror 4, inferred from measurements, was 0.990. The stray radiation from the internal quasi-optical mode converter, inferred from measurements, was only 1.9% of the total power radiated from the launcher. This resulted in a high overall system efficiency of 97.0%, which is greater than previous designs.

THE DESIGNS OF HIGH EFFICIENCY LAUNCHER OF QUASI-OPTICAL MODE CONVERTER FOR HIGH POWER GYROTRONS

A high efficiency launcher of quasi-optical (QO) mode converters for high power gyrotrons have been designed and tested. A helical cut launcher radiates the RF power via its straight cut onto the first phase correcting mirror. The launchers have been optimized for the TE31.8 mode at 170 GHz and TE22.6 mode at 110 GHz by numerically optimizing a launcher surface. The helical cut of the launcher has been optimized by taking the taper angle into account. Further more, the amplitude of the surface perturbation have been optimized for improved focusing in order to reduce the diffraction losses at the helical cut. Low power measurement shows a good agreement with the design. High efficiency characteristics of the design have also been calculated on the assumption of frequency downshift due to the thermal expansion of the cavity and stepwise frequency tuning by changing the operating mode. Besides, the possibility of high efficiency launcher for higher mode is discussed, and these results give the prospect to high efficiency long pulse gyrotrons.

Dreidimensionale Feldberechnung für elektrisch große Geometrien und ihre Anwendung für multifrequente Hochleistungsgyrotrons

At the Research Center Karlsruhe (Forschungszentrum Karlsruhe) high-power millimeter wave sources, so called gyrotrons, are developed. Gyrotrons generate radio frequency (RF) in the frequency range of a few gigahertz up to a couple of hundred gigahertz at power levels up to megawatts. The RF-output coupling is done by an internal quasi-optical mode converter. It focuses the millimeter waves to a pencil like beam. This report deals with the simulation of quasi-optical mode converters, which are large compared to the wavelength. For a fast simulation inside the waveguide the method of coupled modes is used and in free space the propagation is done by the Kirchhoff diffraction integral. To avoid approximations and to enhance accuracy a three-dimensional simulation method based on the electric field integral equation (EFIE) was developed. The performance of this method will be shown with an analysis of a quasi-optical mode converter of a 118 GHz TE 22,6 -gyrotron. The second focus of this work is the optimization of a broadband quasi-optical mode converter in the frequency range of 105 GHz to 143 GHz. The optimization was done with the fast simulation methods, which were mentioned above. Beside a high Gaussian mode content of the output beam for every frequency, the mode converter was optimized for a Brewster angle window. A Brewster window has no dependency on the frequency for the transmitted wave beam, but the useable aperture of the window is much smaller than for a mono-frequent window. By optimization the stray radiation could be reduced to two to three percent depending on the cavity mode with a Gaussian mode content of more than 90%. The work is based on a dissertation which was done at the Institut fur Hochleistungs-impuls- und Mikrowellentechnik at the Forschungszentrum Karlsruhe and accepted by the University of Karlsruhe (TH), Germany.