Phase-correcting Mirrors Research Articles

Quasi-optical mode converter for a high power TE8,3-mode gyrotron

We successfully developed a Gyrotron HS-III (28-GHz/400-kW/5-s) for electron cyclotron resonance heating in a spherical tokamak EXL-50, which operated in the TE8,3 mode. An internal quasi-optical-mode converter was designed to separate the spent electron beam from the outgoing power and convert the operating mode to a fundamental Gaussian wave beam. The particle swarm optimization algorithm was used to reduce diffraction loss from the launcher in such a low-frequency band. This enabled us to optimize the perturbation distribution of the launcher and beam waist of the target function to obtain the largest possible scalar correlation coefficient. The launcher was followed by three mirrors: one quasi-parabolic mirror and two phase-correcting mirrors. These were also optimized based on the scalar diffraction theory and Katsenelenbaum–Semenov algorithm to allow us to change the beam direction and increase the purity of the Gaussian output beam. Simulation results revealed a transmission efficiency of 96% and vector Gaussian mode purity of 99.8% at the window plane.

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.

Hybrid Transmission Line for ECRH in the Helically Symmetric Experiment

The HSX oversized, mode-converting ECRH transmission line has been upgraded to a hybrid system to increase launched microwave power and reduce electrical arcing. Filtering of high-order, spurious modes ensures efficient coupling to a Gaussian beam for optimal electron heating. A Vlasov mode converter and two phase-correcting ellipsoidal mirrors convert the TE02 gyrotron output mode to a symmetric, linearly polarized, microwave beam. A swappable twist reflector plate rotates beam polarization for 2nd-harmonic X-mode or fundamental O-mode ECRH. Long distances are traversed by coupling the beam to a dual-mode (TE11 + TM11), smooth, circular cross-section waveguide. This system has been successfully tested without arcing for 50 ms pulses and over 100 kW of launched power. Analysis of the microwave beam for 50 kW, 2 ms microwave pulses reveals agreement with predicted beam shapes at two beam locations. The new system has also demonstrated increased plasma stored energy for ECRH plasmas with equal launched power.

The New Multifrequency Electron Cyclotron Resonance Heating System for ASDEX Upgrade

A new multifrequency electron cyclotron resonance heating (ECRH) system is currently under construction at the ASDEX Upgrade tokamak experiment. This system will, for the first time in a fusion device, employ multifrequency gyrotrons, step-tunable in the range 105 to 140 GHz. In its final stage the system will consist of four gyrotrons with a total power of up to 4 MW and a pulse length of 10 s. The variable frequency will significantly extend the operating range of the ECRH system both for heating and current drive. The matching optics unit includes a set of phase-correcting mirrors for each frequency as well as a pair of broadband polarizer mirrors. The transmission line consists of nonevacuated corrugated HE11 waveguides with inner diameter of 87 mm and has a total length of ˜70 m. A fast steerable launcher enables the steering of the beam over the whole plasma cross section poloidally. The first two-frequency gyrotron has been installed recently. It is equipped with a single-disk diamond window. The next gyrotrons will be step-tunable with two additional frequencies between 105 and 140 GHz. They will require a broadband output window, which will be either a Brewster or a double-disk window.

Quasi-Optical Mode Converter/Mirror System for a High-Power Coaxial-Cavity Gyrotron

This paper presents the investigation of a mirror system of a quasi-optical (QO) mode converter for a high-power coaxial-cavity gyrotron. The mirror system consists of three mirrors. The first mirror is a quasi-elliptical one. Based on the Katsenelenbaum-Semenov Algorithm (KSA), the second and the third mirrors are iteratively optimized as adapted phase-correcting mirrors to transform the outgoing wave beam into a fundamental Gaussian structure. The investigation shows that the focal length of the quasi-elliptical mirror has a great influence on the optimized conversion efficiency, and, hence, it should be chosen to match the asymptotic beam growth (ABG) angle well in order to obtain high conversion efficiency. The design of a mirror system has been performed for a 2 MW, continuous wave (CW), 170 GHz, and TE34,19 -mode coaxial-cavity gyrotron, which is under development at Forschungszentrum Karlsruhe, Germany. Taking into account the size of the mirrors and the conversion efficiency, a mirror system has been designed with a conversion efficiency of 98.3%

Progress toward optimization of phase-correcting mirrors for a multifrequency 1-MW gyrotron

Due to the requirement of a newly designed ultra-broadband chemical vapour deposition (CVD)-diamond window for a multifrequency 1-MW short-pulse gyrotron at Forschungszentrum Karlsruhe (FZK), the existing beam-forming mirror system inside the gyrotron tube has to be modified. This paper describes the design scheme for shaping the output beams from the launching antenna waveguide into the desired fundamental Gaussian profile by using adapted phase-correcting mirrors with nonquadratic surface contour function. Simulation results show these phase-correcting mirrors can be used for broadband operation of a frequency step-tunable gyrotron, which operates in nine modes from the TE17,6 mode at the frequency of 105 GHz to the TE23,8 mode at 143 GHz. Further calculations predict that efficiencies of more than 94% can be obtained for converting the high-order cylindrical cavity modes into the usable fundamental Gaussian mode

Synthesis of gyrotron phase-correcting mirrors using irradiance moments

A new approach using the moments of field amplitudes has been applied for phase-correcting mirror synthesis of a 110-GHz gyrotron internal mode converter. The synthesized mirrors have smooth profiles in contrast to the mirrors synthesized using the previously employed iteration method. The design has been checked using a physical optics propagation code with the result of a Gaussian output amplitude at the gyrotron window position.

Evaluation of phase correcting mirrors for an 84GHz gyrotron based on direct phase measurements at low-power level

Abstract A low-power test system using a vector network analyzer is constructed to evaluate the performance of phase correcting mirrors in a matching optics unit (MOU). The MOU is used in order to facilitate the good coupling of a millimeter wave beam from a gyrotron into corrugated waveguides. The newly developed diagnostic system allows us to make measurements not only of intensity-, but also of phase-profiles of millimeter wave beams and opens a new range of possibilities for designing and testing high-precision quasi-optical systems. Low-power tests are carried out to evaluate the performance of the newly designed MOU by reversely injecting a Gaussian beam from the output port of the MOU. The output beam from the input port of the MOU is measured with the analyzer. In this inverse process, the injected Gaussian beam should be converted to the real beam pattern radiated from the gyrotron. Using correlation analysis, the measured- and calculated-electric fields are compared in their amplitude- and phase-profiles. Good agreement between the measurement and the calculation is obtained. Therefore, it is judged that the phase-correcting mirrors in the MOU will work correctly in upcoming high-power tests.

Investigation of a Broadband Quasi-Optical Mode Converter for a Multi-Frequency 1 MW Gyrotron

A broadband quasi-optical (QO) mode converter for a multi-frequency gyrotron has been designed and tested at Forschungszentrum Karlsruhe (FZK). The launcher is optimized for the TE/sub 22,8/ mode at 140 GHz, but the radiated beams present an almost identically focused pattern for all 9 considered modes between 105 GHz (TE/sub 17,6/) and 143 GHz (TE/sub 23,8/). Combining with a beam-forming mirror system, which consists of a quasi-elliptical mirror and two phase-correcting mirrors, further calculations show that efficiencies of more than 94% have been achieved for converting the rotating high-order cylindrical cavity modes into the usable fundamental Gaussian mode. Low power (cold) measurements show a good agreement with theoretical predictions. This QO mode converter can be used for the broadband operation of a multi-frequency 1 MW gyrotron.

Design of a quasi-optical mode converter for a frequency step-tunable gyrotron

The quasi-optical mode converter for a frequency step-tunable gyrotron which consists of a dimpled-wall antenna (Denisov-type launcher) and a beam-forming mirror system has been optimized for 9 modes from TE17,6 at 105 GHz to TE23,8 at 143 GHz. The first mirror is a large quasi-elliptical focusing one; the second and third are phase-correcting mirrors with a non-quadratic shape of the surface. The results of calculations show that for these modes the Denisov-type launcher has a well-focused beam with low diffraction losses, and the radiation pattern presents an almost identical field shape for all modes considered. A multi-mode optimization of the phase-correcting mirrors with two different methods has been tested. The simulations show that the phase-correcting mirrors can be used for broadband operation in the frequency range from 105 GHz up to 143 GHz in the various design modes. This quasi-optical mode converter can achieve efficiencies of 94%-98% for converting the rotating high-order cylindrical cavity modes into the usable fundamental Gaussian mode.