Mode Converter System Research Articles

Optimal Design and Experimental Investigation of a Compact Ka-Band Mode Converter System

A novel synthesis method is used to design the 32-GHz high-power microwave (HPM) mode converter system in this paper. The mode converter system includes a TE01 right-angle bend transition, a TE01 mode taper, and a TE01–TE11 mode converter. The new synthesis method provides more compact structure dimensions and higher mode conversion efficiency. The transmission efficiencies for three parts above are 99.21%, 99.87%, and 99%, respectively. Last, the mode converter system is fabricated, assembled, and tested on gyrotron experimental platform. The test results indicate the validity and the accuracy of the novel synthesis method.

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.

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.

Development of 28 GHz/35 GHz Dual-Frequency Gyrotron for Fusion Research

The initial design study of 28 GHz/35 GHz dual frequency gyrotron is presented. The design is based on 28 GHz 1 MW gyrotron which we have developed. And it is studied whether 35 GHz oscillation is possible or not using the present 28 GHz gyrotron cavity. From the numerical simulation results, the cavity can form 35.45 GHz TE9,4 mode oscillation of over 1 MW oscillation power with high efficiency. However, in the present mode converter and transmission mirror system, the radiated RF power through the output window is only about 70 % of the 35.45 GHz oscillation power because of the radiation angle difference of 15.85°. For more high efficiency operation for 35 GHz, we will re-design the cavity in which there are good candidate modes such that there is no radiation angle difference.

Mode-converters for rectangular-core fiber amplifiers to achieve diffraction-limited power scaling

A rectangular-core (ribbon) fiber that guides and amplifies a single higher-order-mode (HOM) can potentially scale to much higher average powers than what is possible in traditional circular-core large-mode-area fibers. Such an amplifier would require mode-conversion at the input to enable interfacing with seed sources that typically output TEM(00) mode radiation and at the output to generate diffraction-limited radiation for end-user applications. We present the first simulation and experimental results of a mode conversion technique that uses two diffractive-optic-elements in conjugate Fourier planes to convert a diffraction limited TEM(00) mode to the HOM of a ribbon fiber. Mode-conversion-efficiency is approximately 84% and can theoretically approach 100%. We also demonstrate a mode-converter system that converts a single HOM of a ribbon fiber back to a diffraction-limited TEM(00) mode. Conversion efficiency is a record 80.5%.

A High-Efficiency Quasi-Optical Mode Converter for a 140-GHz 1-MW CW Gyrotron

A highly efficient quasi-optical mode converter system with several novel features has been designed and tested at Forschungszentrum Karlsruhe (FZK). The converter consists of a dimpled-wall waveguide launcher, one quasi-elliptical mirror and two toroidal mirrors. The coupled-mode theory has been used to analyze the operation of the prebunching waveguide launcher; the radiated fields from the cut of the launcher have been calculated by the scalar diffraction integral. Simulation results show that the advanced dimpled-wall launcher generates a well-focused Gaussian radiation pattern with low diffraction losses. In this case, toroidal mirrors are sufficient to obtain a desired output beam pattern. An efficiency of more than 98% has been achieved to convert the rotating TE/sub 28,8/ cavity mode at 140 GHz into a fundamental Gaussian beam. Experimental measurements show close agreement with theoretical predictions.

Development of Long Pulse and High Power 170GHz Gyrotron

Recent activities on the research and development of 170 GHz gyrotron were presented. In the 170 GHz gyrotron experiment in JAERI, pulse duration had been limited by the beam current decrease due to the emission cooling of the electron emitter. A preprogramming control of the heater power was introduced to keep the beam current constant and to avoid the oscillation mode shift to a lower mode during the operation. And, a built-in mode converter system was improved to reduce the stray radiation. In the preliminary experiment results of 0.2 MW/480 sec and 0.13 MW/600 sec were obtained. The pulse extension experiment at higher power will be followed. In parallel, a study of the high order mode oscillation was carried out using a short pulse gyrotron (∼1 msec). The oscillation mode is TE31,12, which allows 1.5 MW level CW operation. A stable oscillation of TE31,12 was demonstrated at the power of 1.56 MW. The maximum efficiency was 30% at 1 MW output. This result indicates that the high order mode up to the level of TE31,12 will be acceptable to increase the power or to reduce the heat load on the cavity wall at 1 MW output.

Development of a Prototype of a 1-MW 105-156-GHz Multifrequency Gyrotron

We present the results of studying the possibility of numerical and experimental optimization of the helical electron beam (HEB) formation system and the processes of mode interaction in the electron beam – cavity system for a prototype of a 1-MW 105–156-GHz step-tunable gyrotron with various operating modes. The system parameters are optimized to achieve the maximum efficiency of the gyrotron with an acceptable ohmic load in the cavity. We also analyze the influence of mode competition and depression of the electron-beam potential as well as the possibility of energy recovery of the collector electron beam. The possibility of optimizing the mode converter system for various operating modes is demonstrated.

A 1.5-MW, 140-GHz, TE/sub 28,16/-coaxial cavity gyrotron

The design of a 1.5-MW, 140-GHz, TE-/sub 28,16/-coaxial cavity gyrotron is presented and results of experimental operation are given. A cavity with a cylindrical outer wall and a radially tapered inner rod with longitudinal corrugations was used. A maximum output power of 1.17 MW has been measured in the design mode with an efficiency of 27.2%. Single-mode operation has been found over a wide range of operating parameters. The experimental values agree well with the results of multimode calculations. Frequency-step tuning has been performed between 115.6 and 164.2 GHz. In particular, an output power of 0.9 MW has ben measured in the TE/sub 25,14/ mode at 123.0 GHz and 1.16 MW in the TE/sub 32,18/ mode at 158.9 GHz. At frequencies its with strong window reflections the parameter range for which stable operation is possible is reduced significantly. In order to obtain results relevant for a technical realization of a continuously operated gyrotron, a tube with a radial radio frequency (RF)-beam output through two output windows and a single-stage depressed collector has been designed and is under fabrication. A two-step mode conversion scheme-TE-/sub 28,16/ to Te/sub +76.2/ to TEM/sub 00/-which generates two narrowly directed (60/spl deg/ at the launcher) output wavebeams has been chosen for a quasioptical (q,o) mode converter system. A conversion efficiency of 94% is expected.

Generation of the Gaussian-like HE11 mode from gyrotron TEOn mode mixtures at 70 GHz

This work reports calculations and measurements on mode converters for the transformation of TEOn gyrotron mode mixtures (primarily TEO2) into the linearly polarized HE11 hybrid mode at 70 GHz. This mode is ideal for quasi-optical launching systems for ECRH of plasmas. Mode transducers with axisymmetric radius perturbations convert the gyrotron TEOn mode composition into the TEO1 mode. Proper matching of the phase differences between the various modes and of the perturbation amplitudes of the several converter sections is required. The TEO1 wave is used for long-distance transmission through smooth-walled overmoded waveguides. A mode converter with constant diameter and periodically perturbed curvature transfers the unpolarized TEO1 mode into the polarized TE11 wave. The experimentally determined ΣTEOn-to-TEO1 conversion efficiency is (98.5±1)% (99% predicted) while the TEO1-to-TE11 transformer has a (94±2)% efficiency (93% theoretically); ohmic losses are included. The “Gaussian-like” HE11 mode with axisymmetric power distribution and almost no cross polarization is produced in a circumferentially corrugated TE11-to-HE11 mode transducer with a measured conversion efficiency of (98.3±1.5)% (98.5% predicted). The overall efficiency of the complete mode converter system in the desired mode was determined to be (91±2.5)%. High-power operation (200kW, 100ms) has been successfully demonstrated.