Quasi-optical Mode Converter Research Articles

Design and experimental results of a 28 GHz, 400 kW gyrotron for electron cyclotron resonance heating

A high-power 28 GHz gyrotron has been successfully developed at the Institute of Applied Electronics, China Academy of Engineering Physics. This gyrotron was designed for electron cyclotron resonance heating (ECRH) in the spherical tokamak XL-50. A diode magnetron injection gun was designed to produce the required gyrating electron beam. The gyrotron operates in the TE8,3 mode in a cylindrical open cavity. An internal quasi-optical mode converter was designed to convert the operating mode into a fundamental Gaussian wave beam and separate the spent electron beam from the outgoing microwave power. A tube has been built and successfully tested. The operational frequency of the tube is 28.1 GHz. For beam parameters at an accelerating voltage of 71 kV and beam current of 16 A, the gyrotron has delivered an output power of 400 kW, with a pulse length of 5 s. The output efficiency is about 50% with a single-stage depressed collector. The gyrotron has been installed on the XL-50 and has played an important role in the ECRH experiments.

Imaging and Mode Content Analysis of the Wave Beam From a Short-Pulse High-Power Gyrotron Using the Millimeter-Wave-Induced Gas Breakdown Initiated by a Metal–Dielectric Screen

The wave beam of a sub-terahertz (THz) gyrotron with a pulsed magnetic field was imaged in a single pulse using a microwave-induced gas breakdown initiated by a metal–dielectric (MD) screen. Images of sub-THz beams, which were recorded at the output of a quasi-optical mode converter, were compared with data obtained by numerical simulation of propagation and transformation of the wave beam from the gyrotron. The analysis of the waveguide mode content in the gyrotron was done. Based on experimental and simulation data, it was shown that the distortion of the sub-THz wave beam is caused by the deformation of the Teflon window at the gyrotron output. Based on the results of the study, the gyrotron was modified, and the quality of its wave beam was improved.

Study on high power gyrotron multimode coupling launcher

In this paper, an improved coupled-mode method is used to design a launcher for a gyrotron quasi-optical mode converter at 170 Ghz with an operating mode of TE32,9. The new method calculates 25 coupled modes, which more accurately simulates the field distribution in the launcher. The diffraction field of the launcher with a spiral cut is calculated by using the scalar diffraction integral method with fast Fourier transform. The radiation power of the launcher is 99.94%, the Gaussian mode content of the radiation beam is 98.40%, the diffraction loss of the launcher is 0.0580%, and the radiation efficiency is 98.34%. The method proposed in this paper is fast in calculation and can be applied to the large-scale high-frequency gyrotron launcher.

A Compact Dual-Mirror Design of Quasi-Optical Mode Converter for Gyrotron Application

The quasi-optical mode converter (QOMC) for high-power fusion gyrotron is a typical beam transformation and shaping device in near-field region. In this letter, the authors report their progress in designing a compact QOMC with only two reflecting mirrors. For achieving the high-efficiency conversion comparable to conventional multimirror converters, the authors exercise modification and optimization on the first mirror based on the classic quasi-elliptical design. It is shown that the optimized first mirror can fulfill multifunctions that have to be achieved by a chain of mirrors in conventional designs. Numerical results show the effectiveness of the optimization, which can be revealed by the achieved 95.97% Gaussian content in the output beam with power transmission efficiency of 99.0%.

The Development of 170 GHz, 1 MW Gyrotron for Fusion Application

This paper presents the overall conceptual design of a 170 GHz, 1 MW gyrotron for plasma heating applications in thermonuclear fusion reactors. The operating mode is carefully selected with consideration of mode competition. The TE25,10 mode is determined as the operating mode in the present study. A weakly tapered conventional resonator is used for the study of the RF behavior, and multimode calculations are carried out for power and efficiencies. The optimized structure of the beam tunnel can further attenuate low-frequency oscillation. The design studies of a triode-type magnetic injection gun, quasi-optical mode converter, and single-disc sapphire window are also described. In addition, novel cold test methods which can effectively validate assembles are designed. In 2021, a prototype gyrotron was fabricated to validate the electronic–optic system and RF system. The current results obtained support an output power of 210 kW and efficiency of 15.9% through an initial low-power experiment. On the basis of the optimized design, an industrial prototype gyrotron is under fabrication as the heating source for the thermonuclear experimental reactor.

Design and preliminary test of a 105/140 GHz dual-frequency MW-level gyrotron

A dual-frequency (105/140 GHz) MW-level continuous-wave gyrotron was developed for fusion application at Institute of Applied Electronics, China Academy of Engineering Physics. This gyrotron employs a cylindrical cavity working in the TE18,7 mode at 105 GHz and the TE24,9 mode at 140 GHz. A triode magnetron injection gun and a built-in quasi-optical mode converter were designed to operate at these two frequencies. For the proof-test phase, the gyrotron was equipped with a single-disk boron nitride window to achieve radio frequency output with a power of ∼500 kW for a short-pulse duration. In the preliminary short-pulse proof-test in the first quarter of 2021, the dual-frequency gyrotron achieved output powers of 300 kW at 105 GHz and 540 kW at 140 GHz, respectively, under 5 Hz 1 ms continuous pulse-burst operations. Power upgrade and pulse-width extension were hampered by the limitation of the high-voltage power supply and output window. This gyrotron design was preliminarily validated.

Compact quasi-optical mode converter based on anisotropic metasurfaces

In this paper, a novel compact quasi-optical mode converter based on anisotropic metasurfaces for high-order mode terahertz electronic devices is presented. To demonstrate the design model, a Ka-band metasurface quasi-optical mode converter that converts cylindrical waveguide TE01 mode to circularly polarized Gaussian beam is designed and fabricated. Both electromagnetic simulation and experiment results show that the Gaussian beam can be observed from 35 to 38 GHz, corresponding to over 8.5% of the bandwidth. The maximum scalar Gaussian mode content of 97.85% is observed in the experiment, and the output radiation from the metasurface quasi-optical mode converter is approximate circular polarization. This work unveils the potential of compact quasi-optical mode converter based on metasurfaces.

Development of a 28-GHz/50-kW/30-s Gyrotron System for Fusion Application

A complete 28-GHz/50-kW gyrotron system was developed by the Institute of Applied Electronics, China Academy of Engineering Physics, Mianyang, China, as a plasma heating source for the EXL50 fusion device, a compact spherical tokamak constructed by the ENN Group in 2019. The gyrotron employs a triode magnetron injection gun, a TE02-mode cylindrical cavity, a built-in quasi-optical mode converter, a single-stage depressed collector, and a single-disk boron nitride window. The gyrotron system features a match optical unit, a superconducting magnet, a calorimetric dummy load, a high-voltage power supply system, and other auxiliary components, was delivered to the EXL50 site. In the acceptance test, five successive shots of each power over 50 kW were generated for 30 s at 28 GHz. An average power of 51 kW, a maximum power of 55 kW, and 46% overall efficiency were obtained. The gyrotron system was assembled in the EXL50 and applied to a plasma discharge experiment. The gyrotron is the first practical domestic gyrotron produced for fusion applications in China. The design and test results of this instrument are presented in this article.

Design and Experiment of a Quasi-Optical Mode Converter

Quasi-optical mode converter is an important component to realize the high-efficiency output for a high-power gyrotron oscillator. In this paper, design and experiments of a quasi-optical mode converter, consisting of a Denisov-type launcher and three quasi-optical mirrors, are carried out for the development of 140 GHz/TE28,8 mode gyrotron oscillator. Based on scalar diffraction method, the field distribution at the radiation aperture of the Denisov-type launcher is optimized to make the vector correlation of the aperture field with the ideal Gaussian field reach 96.2%. Based on the geometric optics method and the Gaussian beam matching method, the focusing mirror and the beam shaping mirrors were designed. A 3D full-wave analysis software Surf3D was used to obtain the field distribution on each mirror surface and the output window, which shows that the output beam with Gaussian mode content of 96.67% is obtained at the output window. The total power conversion efficiency of the quasi-optical mode converter is 93.98%. The design and experiments of the quasi-optical mode converter are performed to use the output signal of the self-developed TE28,8 mode generator as its input one under the conditions of the simulated demonstration for its conversion characteristics and the strict control for machining precision as well as the process of the converter assemble and testing. The tested results indicate that the design is in good agreement with the experiment, which is helpful for engineering design and experimental demonstration of the quasi-optical mode converter development.

Study on a gyrotron quasi-optical mode converter for terahertz imaging

In this paper, a quasi-optical mode converter for terahertz imaging is presented. This design is based on a 0.22 THz TE03 gyrotron oscillator. It includes a Vlasov launcher, an elliptical reflector and a parabolic reflector, which is used to convert TE03 cylindrical waveguide mode to a Gaussian-like beam. The quasi-optical system is analyzed in terms of geometric optics theory and the vector diffraction theory. Numerical optimization code is programmed by MATLAB software for analyzing the quasi-optical system. Simulation results show that the power transmission efficiency of the operating cavity mode to a fundamental Gaussian distribution is 82.14%, the output Gaussian beam can be propagated over 0.8 m without divergence and the scalar Gaussian mode content is 98.2%. A cold test experiment platform is built up to verify the design based on numerical calculation. In experiment, the scalar Gaussian mode content of 97.0% is measured which shows good agreement with numerical calculation.

Design and analysis of a broadband quasi-optical mode converter with a Denisov launcher

A terahertz quasi-optical mode converter with wide bandwidth was studied and designed.The quasi-optical mode converter used Denisov launcher,which had high efficiency,and worked in TE6,2 mode.It was used to realize mode conversion in gyrotron.As the parameters of Denisov launcher were the main factors affecting the broadband performance of quasi-optical mode converter,the optimal design of the radiator parameters could increase the bandwidth of the mode converter.Using the self-developed quasi-optical simulation program for simulation,the bandwidth of the mode converter can reach 2 GHz(center frequency 94 GHz).

Review of Experimental Verification Methods of Gyrotron Quasi-optical Mode Converters

This survey presents a review of experimental methods relied upon while implementing gyrotron higher mode generation techniques and performing near electromagnetic field measurements in launcher and quasi-optical mode converters. In particular, the paper focuses on low power (cold) testing of gyrotron quasi-optical mode converters outside of the gyrotron, without the presence of high electromagnetic power and electron beams

Design of Quasi-Optical Mode Converter for 170-GHz TE32,9-Mode High-Power Gyrotron

In this paper, a design of quasi-optical mode converter for 170-GHz TE32,9-mode high-power gyrotron is presented in detail. The quasi-optical mode converter consists of a launcher and a mirror system. A MATLAB code for analysis of the electromagnetic fields at the launcher wall and in mirror systems has been developed. The numerically optimized launcher can generate a Gaussian-like mode with a scalar correlation coefficient of 99.6% and a vector correlation coefficient of 99.1% at the launcher aperture. The radiation fields of the launcher are analyzed in the free space. The mirror system consists of four analytic surface mirrors and a phase-correction mirror. After the phase correction, the scalar Gaussian mode content and vector Gaussian mode content on the output window are 99.1% and 97.1%, respectively. The performance of the mirrors is calculated and analyzed. The power transmission efficiency of each mirror is above 99.8%, and the efficiency of the quasi-optical mode converter is 96.77%.

Poynting Vector Method for Reflector Beam Shaping

A reflector beam shaping method is presented, which includes an efficient algorithm. This efficient algorithm based on the Poynting vector tracing method is employed to analyze the procedure of beam propagation and reflection. The Poynting vector ray tracing method is naturally integrated into the reflector reshaping procedure, which brings significant advantage over existing beam shaping methods. The beam shaping process by this algorithm also shows distinct advantage in efficiency over competing integration theories. It has comparable accuracy with typical physical optics. The validity of this reflector shaping method is demonstrated by performing iterative phase correction toward a gyrotron quasi-optical mode converter.

Studies towards an upgraded 1.5 MW gyrotron for W7-X

Studies towards a 1.5 MW, 140 GHz CW gyrotron, with the capability of MW-class operation also at 175 GHz, are ongoing at Karlsruhe Institute of Technology in view of a possible future upgrade of the ECRH system of the stellarator W7-X. The upgrade of the existing 1.0 MW, 140 GHz European gyrotron for W7-X has been chosen as a development path. Detailed designs of the cavity, the non-linear uptaper, and the quasi-optical launcher for the upgraded gyrotron have been obtained and have been validated numerically. In parallel, a mode generator, intended for low-power tests of the quasi-optical mode converter system of the upgraded gyrotron, has been designed, manufactured, and successfully tested.

IMPLEMENTATION OF TWO METHODS FOR DESIGNING THE PROFILES OF MIRRORS IN QUASI-OPTICAL MODE CONVERTER FOR 170 GHZ TRANSVERSE OUTPUT GYROTRON

IMPLEMENTATION OF TWO METHODS FOR DESIGNING THE PROFILES OF MIRRORS IN QUASI-OPTICAL MODE CONVERTER FOR 170 GHZ TRANSVERSE OUTPUT GYROTRON

A 94 GHz continuous operation second harmonic gyroton with 1.8 T solenoid

The continuous operation of a W-band gyrotron without superconduct magnet was realized. The gyrotron operated at the second harmonic cyclotron frequency. Its operational frequency was 94.08 GHz and the operating mode was TE02. The required 1.8 T magnetic field was generated by a DC solenoid cooled by water. The power consumption of the solenoid was 28 kW and the driving current was 500 A. The inner bore diameter was 66 mm. The gyrating electron beam was generated by a triode magnetic injection gun. A high efficiency internal quasi-optical mode converter was used to separate the wave from the electron beam. In the test, 5 min stable operation was realized. The output power was 12 kW with an electron beam of 45 kV, 1.7 A. The output efficiency was 15.7%.

Design of a Vlasov mode converter of 263 GHz gyrotron oscillator for DNP‐NMR

This paper presents design and simulation of a quasi-optical mode converter of 263 GHz gyrotron oscillator operating in the circular TE03 mode for dynamic nuclear polarisation enhanced nuclear magnetic resonance (DNP-NMR). The converter consists of a Vlasov stepped-cut launcher, and additional wave beam-shaping reflectors, including an elliptical reflector and a parabolic reflector. First, the preliminary parameters of the launcher are designed with the geometric optics theory. Next, both geometric optics and vector diffraction theory are used to simulate the propagation of wave beam in the mode converter system. Finally, operation of the mode converter was simulated, based on the physical model of Vlasov mode converter for 263 GHz TE03 mode gyrotron oscillator. The simulated results show that a good fundamental Gaussian mode is converted at the output window from the TE03 mode, and that the power conversion efficiency and the vector correlation are 90.11 and 85.23%, respectively.

Asymmetrical mirror optimization for a 140 GHz TE22,6 quasi-optical mode converter system**Project supported by the National Natural Science Foundation of China (Grant No. 61671032).

We introduce an asymmetrical mirror design to a 140 GHz TE22,6 quasi-optical (QO) mode converter system to correct the asymmetry of the beam’s field distribution caused by the Denisov launcher. By such optimization, the output beam with better symmetrical distribution is obtained at the system’s output window. Based on the calculated results, the QO mode converter system’s performance is already satisfying without iterative phase correction. Scalar and vector correlation coefficients between the output beam and the fundamental Gaussian beam are respectively 98.4% and 93.0%, while the total power transmission efficiency of the converter system is 94.4%. The assistance of optical ray tracing to the design of such QO mode converters is introduced and discussed as well.

Quasi-Optical Mode Converter for a 0.42 THz TE17,4 Mode Pulsed Gyrotron Oscillator

A quasi-optical mode converter (QOMC) for a 0.42 THz, TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17,4</sub> mode pulsed gyrotron oscillator is designed, fabricated and tested. Based on the coupled-wave theory, geometric optics theory and vector diffraction theory, a computer code is developed to design and optimize the QOMC which converts the high order TE17,4 mode into a fundamental Gaussian distribution. The numerical results show that the scalar fundamental Gaussian mode content of the wave beam at the output window is about 98.1%. The experiment result shows that a well-focused Gaussian-like beam with 95.2% scalar Gaussian mode content has been obtained by using an infrared video camera. The measured results agree well with the theoretical predictions.