Gaussian Mode Content Research Articles

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.

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.

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 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%.

Report of recent experiments with the European 1 MW, 170 GHz CW and SP prototype gyrotrons for ITER

The European 1 MW, 170 GHz industrial CW prototype gyrotron has been designed within EGYC (European GYrotron Consortium) in collaboration with the industrial partner Thales Electron Devices (TED) and under the coordination of Fusion for Energy (F4E). This is a conventional (hollow) cavity gyrotron that is based on the 1 MW, 170 GHz short-pulse (SP) modular gyrotron, which has been designed and manufactured by KIT in collaboration with TED. The SP prototype has been tested in multiple experimental campaigns since 2015 and the nominal cavity mode TE32,9is exited at 170.1 GHz, producing RF power above 1 MW with 35 % interaction efficiency. The first phase of the experiments with the CW industrial gyrotron was successfully completed at KIT in 2016, verifying most of the ITER specifications. Short pulses (<10ms) deliver RF power higher than 0.9 MW with a total efficiency of 26 % (in non-depressed collector operation). The Gaussian mode content of the RF beam is 97 %. Pulses with duration of 180 s (limited by the high-voltage power supply at KIT) produce power more than 0.8 MW with maximum efficiency 38 % (in depressed collector operation). In this work the achievements with the SP and the CW prototype gyrotrons are summarized.

Theoretical Study on the Operation of the EU/KIT TE34,19-Mode Coaxial-Cavity Gyrotron at 170/204/238 GHz

The 170 GHz 2 MW TE34,19-mode coaxial-cavity modular short-pulse pre-prototype gyrotron at KIT was recently modified in order to verify the multi-megawatt coaxial-cavity technology at longer pulses. In parallel, theoretical investigations on a possibility to operate the 170 GHz TE34,19-mode coaxial-cavity prototype at multiple frequencies up to 238 GHz have been started, with a goal to find a configuration at which the tube could operate in the KIT FULGOR gyrotron test facility using the new 10.5 T SC magnet. This paper indicates which adjustments have to be made and show the feasibility of the multi-frequency operation. Small modifications at the gyrotron cavity will support an RF output power of more than 2 MW at 170/204 GHz. Furthermore, a new gyrotron launcher has been designed capable of producing a Gaussian microwave beam with a Gaussian mode content of more than 96% at these frequencies.

Improved Simulation of Quasi-Optical Launchers for High Power Gyrotrons with Smoothing Algorithm

Numerically optimized launchers for the quasi-optical output coupler of high power gyrotrons have been developed to provide RF beams with high Gaussian mode content. Generally, the profiles of numerically optimized launchers are quite complicated, so the improvement of their tolerance to fabrication errors is very important. In order to reduce the stray radiation generated by launchers with quite complicated wall profile, and also to reduce their sensitivity to fabrication errors, a method for the smoothing of the numerically optimized launcher wall has been developed at KIT, which is based on the spectrum reconstruction approach. As an example, a launcher designed for the KIT TE34,19-mode, 2 MW CW coaxial-cavity gyrotron has been investigated.

Waveguide Embedding of a Double-Metal 1.9-THz Quantum Cascade Laser: Design, Manufacturing, and Results

We present the details of a coupling structure to embed a 1.9-THz double-metal quantum cascade lasers (QCLs) into a 120- $\boldsymbol{\mu }$ m-wide full-height rectangular waveguide. We describe the split-block manufacturing of the waveguide coupler together with a diagonal feed horn and present power and beam shape measurements of two different devices. The two devices differ in coupling factor, which can be chosen in a wide range by the mounting position of the QCL. Our waveguide embedding allows coupling of a large fraction of the laser power into the waveguide and the subsequent horn antenna emitting with a large Gaussian mode content, enabling efficient integration into a diffraction limited optics setup. This is illustrated by a self-mixing experiment and by using the embedded QCL as a local oscillator in a heterodyne receiver.

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.

Optimization and Measurement of a Smoothly Profiled Horn for a W-Band Gyro-TWA

In this paper, a smoothly profiled horn was designed and manufactured to replace a corrugated output horn for a W-band gyrotron traveling-wave amplifier with improved ultrahigh vacuum compatibility. It was optimized for high vacuum integrity, low reflection, high fundamental Gaussian mode content, low sidelobe levels, and high directivity over the frequency bandwidth of 90–100 GHz. Over this operating frequency band the reflectivity was better than −37 dB and the coupling to a fundamental Gaussian mode was above 97%. The far-field pattern showed a directivity of approximately 27 dB in the measurement with sidelobes lower than −30 dB.

A Numerical Synthesis Method for Hybrid-Type High-Power Gyrotron Launchers

A numerical synthesis method for the optimization of launchers with hybrid-type wall surface for high-power fusion gyrotrons is proposed in this paper. The profile of such a launcher is built from a novel combination of a periodic harmonic wall perturbation followed by a mirror-line-type deformation, so we named it hybrid-type launcher. The synthesis method has been tested to design a hybrid-type launcher to transform the TE32,9 mode of the ITER EU 1-MW gyrotron operating at 170 GHz. The simulation results show that the fundamental Gaussian mode content (FGMC) of the millimeter-wave beam at the radiating aperture of the hybrid-type launcher is approximately 99.1%. In comparison with the corresponding harmonically deformed launcher, the simulations results reveal that the launcher with hybrid-type surface contour provides higher conversion efficiency combined with low diffraction loss (0.7%). The computation time for the design of hybrid-type launchers is just half of that for mirror-line launchers. In the case of not very short launchers, hybrid-type launchers can provide high FGMCs of RF beams with low diffraction losses that are comparable with those of mirror-line launchers.

Experimental verification of the European 1 MW, 170 GHz industrial CW prototype gyrotron for ITER

The EU 1MW, 170GHz gyrotron with hollow cylindrical cavity have been designed within the European GYrotron Consortium (EGYC) in collaboration with the industrial partner Thales Electron Devices (TED) and under the coordination of Fusion for Energy (F4E). In the frame of the EU program, the short-pulse (SP) version of this tube was designed and manufactured by KIT in collaboration with TED. The experimental verification of the SP gyrotron prototype was successfully completed in 2015. The achieved experimental results show a very stable gyrotron operation with RF output power above 1MW at good cavity interaction efficiency around 35%. The gyrotron was operated up to 10ms pulse length; the nominal cavity mode TE32,9 has been excited at the frequency 170.1GHz in agreement with the corresponding ITER specification. The Gaussian mode content of the output RF beam was about 98% and the total level of internal stray radiation was in the range of 2–3%. The manufacturing of the first industrial continuous-wave (CW) prototype gyrotron, based on the SP gyrotron design, was completed in November 2015 at TED. The tube was delivered to KIT and the experimental verification started in February 2016. Operation of the gyrotron with ms pulse duration resulted in stable excitation of the nominal mode at 170.22GHz for a wide range of operating parameters and with a level of stray radiation comparable with the SP version. The maximum RF power achieved up to now in short-pulse operation at a level of 1MW. The measured Gaussian mode content of the RF beam is 97%. Currently, further preparation for the CW operation of the gyrotron is in progress.

RF Behavior and Launcher Design for a Fast Frequency Step-tunable 236 GHz Gyrotron for DEMO

AbstractAs part of the EUROfusion project, the conceptual design of a 1 MW 236 GHz hollow-cavity gyrotron is ongoing at IHM, KIT for a DEMOnstration Power Plant (DEMO), along with a 2 MW coaxial-cavity design concept. Fast frequency-tunable gyrotrons (tuning within a few seconds) are recommended for plasma stabilization using a non-steerable antenna. In this work, the mode-selection approach for such a frequency-tunable gyrotron is presented and suitable operating modes for fast frequency tunability are suggested. Magnetic field tuning has been studied as an effective technique to tune the gyrotron operating frequency. The step-tunability of the 236 GHz gyrotron within the frequency range of ±10 GHz in steps of 2–3 GHz is demonstrated in numerical simulations. A hybrid-type Quasi-Optical Launcher (QOL) has been designed for a step-frequency tunable gyrotron with sufficiently high Fundamental Gaussian Mode Content (FGMC).

Quasi-Optical Mode Converter for a 0.42-THz TE26Mode Pulsed Gyrotron Oscillator

A quasi-optical (QO) mode converter for a 0.42-THz, TE26 mode, pulsed gyrotron is designed, fabricated, and tested. Based on the geometric optics principle, vector diffraction theory, and the iterative Stratton–Chu method, a computer code is developed to design and optimize the QO mode converter. The scalar Gaussian mode content is about 93.7%, which agrees well with that obtained by the electromagnetic simulation software. The preliminary experiment of the 0.42-THz, TE26 mode QO mode converter shows that a well-focused Gaussian-like beam with 90.4% scalar Gaussian mode content has been observed on the output window by using an infrared video camera. The experimental results agree well with the theoretical prediction.

Output System for a 170-GHz/1.5-MW Continuous Wave Gyrotron Operating in the TE28,12Mode

In this paper, design aspects of the output system of a 170-GHz/1.5-MW continuous wave gyrotron operating in the TE28,12 mode is presented for testing the latest version of gyrotron design suite (GDS.V3.2014), which has been equipped with new features. The output system has been incorporated and showcased here considering the design aspects of the output system of this specific gyrotron as a case study. A nonlinear up-taper has been designed using a multiobject particle swarm evolutionary algorithm and the design of an advanced dimpled-wall launcher is carried out using launcher optimization tool. Single-disk chemical vapor deposition (CVD) diamond and CVD boron nitride windows have been designed to take out the output power. Besides, the reconstruction of the wave phase at different planes outside the tube is calculated by an iterative phase retrieval algorithm. The Gaussian mode content achieved by the launching system is 99.5%. In addition, design of a matching optics unit has also been done considering a very recent approach for off-axis incident beams.

High-Efficiency Quasi-Optical Mode Converter for a 1-MW &lt;formula formulatype="inline"&gt; &lt;tex Notation="TeX"&gt;${\rm TE}_{32,9}$&lt;/tex&gt;&lt;/formula&gt;-Mode Gyrotron

A 1-MW, continuous wave, 170-GHz, TE32,9-mode gyrotron for use in International Thermonuclear Experimental Reactor (ITER) is under development within the European Gyrotron Consortium. A quasi-optical mode converter is employed in the gyrotron to transform the high-order cavity mode into a fundamental Gaussian wave beam. The quasi-optical mode converter contains a launcher and a mirror system. The launcher is numerically optimized to provide Gaussian mode content of 98.43% at the launcher aperture. The mirror system consists of three mirrors. The first mirror is a quasi-elliptical mirror, the second and third mirrors are beam-shaping mirrors, which are used to change the beam parameters, such as the beam waist and the position of the focusing plane. The field distribution in the mode converter has been analyzed. The simulation results show that the fundamental Gaussian mode content of the wave beam is 98.6% at the window plane. A first numerical estimation of the stray radiation generated by the mode converter is 1.75%, to be verified in future measurements. The proper synthesis of the quasi-optical mode converter has been verified by comparison of the simulation results from TWLDO with results obtained using the commercial 3-D full-wave vector analysis SURF3D code.

Vector Method for Synthesis of Adapted Phase-Correcting Mirrors for Gyrotron Output Couplers

In this paper, a method for the synthesis of phase-correcting mirrors for gyrotron output couplers is presented. A computer code for vector analysis of the electromagnetic fields in mirror systems has been developed in terms of the vector diffraction theory. Based on the vector analysis of the fields on mirror surfaces, a method for the synthesis of phase-correcting mirrors has been developed. As an example, the mirror system of a quasi-optical (QO) mode converter for a 94-GHz TE6,2-mode gyrotron has been designed. The QO mode converter consists of a simple, easily manufacturable Vlasov launcher, and two mirrors. Both are adapted phase-correcting mirrors and are optimized using the proposed synthesis method. The simulation results show that the conversion efficiency of the operating cavity mode to a fundamental Gaussian distribution at the window aperture is 87.19% (97.38% fundamental Gaussian mode content) including power transmission losses and depolarization.

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%.

2.2-MW Record Power of the 170-GHz European Preprototype Coaxial-Cavity Gyrotron for ITER

A 2-MW continuous-wave (CW) 170-GHz coaxial-cavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development within the European Gyrotron Consortium (EGYC1), a cooperation between European research institutions. To support the development of the industrial prototype of a CW gyrotron, a short-pulse tube (preprototype) is used at KIT Karlsruhe (former FZK) for experimental verification of the design of critical components, like the electron gun, beam tunnel, cavity, and quasi-optical RF output coupler. Significant progress has been achieved recently. In particular, RF output power of up to 2.2 MW with 30% output efficiency has been obtained in single-mode operation at 170 GHz. Furthermore, a new RF output system has been designed, with an efficient conversion of the generated RF power into a Gaussian RF output beam. The results have been successful, yielding a Gaussian mode content ~96%.