Design of a Quasi-Optical Mode Converter for a Dual-Frequency Coaxial-Cavity Gyrotron

A quasi-optical mode converter is under development for an 170/204 GHz coaxial-cavity gyrotron at KIT. It is operated in the TE 34,19mode at 170 GHz and the TE 40,23mode at 204 GHz. A mirror-line launcher should be used for such modes with the ratio of caustic to launcher radius of approximately 0.32. The optimum value of the launcher radius has been found to allow for a high Gaussian-mode content at both frequencies

Progress of the Methods for Optimum of Quasi-Optical Mode Converters at KIT

The paper reports the progress of the methods for the synthesis of Quasi-Optical (QO) mode converters at KIT in the period from 2018 until today. Typically, the QO mode converter consists a waveguide launcher and a mirror system. The first progress done is the development of the spectrum reconstruction method for smoothing the launcher wall. The second is the improvement of the method for the design of quasi-parabolic mirrors. The third progress considers the synthesis of a Denisov-type launcher for the conversion of co- and counter-rotating modes

Experimental Classification and Enhanced Suppression of Parasitic Oscillations in Gyrotron Beam Tunnels

High-power gyrotrons may suffer from parasitic oscillations that are excited in the electron-beam compression zone. Different damping structures are proposed in the literature that reduce the possibility of parasitic excitation by increasing the starting currents of the modes. In this work, we focus on a dielectric-loaded (stacked) beam tunnel. Based on our previous theoretical studies, we make targeted modifications to the beam tunnel in order to classify the parasitic signals and localize the areas where they are excited. After two successive modifications, the beam tunnel exhibits improved behavior with higher starting currents of the parasitic modes. The experiments are performed by using a modular 170-GHz, 1-MW short-pulse gyrotron, which due to its flanged construction gives the possibility to modify the beam tunnel without affecting the rest of the tube

Plasma Spraying of a Microwave Absorber Coating for an RF Dummy Load

The European fusion reactor research facility, called International Thermonuclear Experimental Reactor (ITER), is one of the most challenging projects that involves design and testing of hundreds of separately designed reactor elements and peripheric modules. One of the core elements involved in plasma heating are gyrotrons. They are used as a microwave source in electron–cyclotron resonance heating systems (ECRH) for variable injection of RF power into the plasma ring. In this work, the development and application of an alumina-titania 60/40 mixed oxide ceramic absorber coating on a copper cylinder is described. The cylinder is part of a dummy load used in gyrotron testing and its purpose is to absorb microwave radiation generated by gyrotrons during testing phase. The coating is applied by means of atmospheric plasma spraying (APS). The absorber coating is deposited on the inner diameter of a one-meter cylindrical tube. To ensure homogeneous radiation absorption when the incoming microwave beam is repeatedly scattered along the inner tube surface, the coating shows a varying thickness as a function of the tube length. By this it is ensured that the thermal power is distributed homogeneously on the entire inner tube surface. This paper describes a modeling approach of the coating thickness distribution, the manufacturing concept for the internal plasma spray coating and the coating characterization with regard to coating microstructure and microwave absorption characteristics

Status and First Operation of Gyrotron Teststand FULGOR at KIT

FULGOR, the new KIT gyrotron teststand for megawatt-class gyrotrons, will be presented. Results of initial experiments using a 1.5 MW 140 GHz short pulse pre-prototype gyrotron will be discussed

Development of a CUSP-Type Electron Gun for a W-Band Helical Gyro-TWT

To drive a broadband gyro-TWT with a helically corrugated interaction region, a high-quality axis-encircling electron beam is required. In this publication, a CUSP-type electron gun, capable of generating such a beam, is developed for a 94 GHz helical gyro-TWT. The design was optimized using the electron-beam-optics code ESRAY [1]. The final electron gun is optimized for the generation of an electron beam with a 50 kV beam voltage, 1.5 A current, and a pitch factor of α=1.0 with an RMS spread as low as 3.49 %. Additionally, tolerance studies, including the influence of deviations in the emitter position and the surface roughness of the emitter, are performed

The double-disk diamond window as backup broadband window solution for the DEMO Electron Cyclotron System

The second variant of the electron cyclotron heating and current drive system in DEMO considers the deployment of 2 MW power Gaussian microwave beams to the plasma by frequency steering. Broadband optical grade chemical vapor deposition diamond windows are thus required. The Brewster-angle window represents the primary choice. However, in the case of showstoppers, the double-disk window is the backup solution. This window concept was used at ASDEX Upgrade for injection of up to 1 MW at four frequencies between 105 and 140 GHz. This paper shows computational fluid dynamics conjugated heat transfer and structural analyses of such a circumferentially water-cooled window design aiming to check whether it might be used for DEMO microwave beam scenarios. This design was then characterized with respect to different parameters. Temperature and thermal stress results showed that it is a feasible window solution for DEMO, but safety margins against limits shall be increased by introducing design features able to make the fluid more turbulent. A first design change is proposed, showing that, in combination with a higher inlet flow rate, the maximum temperature in the disks can be reduced from 238 to 186 °C, leading, therefore, to lower thermal gradients and stresses in the window

The Double-Disk Diamond Window as Backup Broadband Window Solution for the DEMO Electron Cyclotron System

The second variant of the electron cyclotron heating and current drive system in DEMO considers the deployment of 2 MW power Gaussian microwave beams to the plasma by frequency steering. Broadband optical grade chemical vapor deposition diamond windows are thus required. The Brewster-angle window represents the primary choice. However, in the case of showstoppers, the double-disk window is the backup solution. This window concept was used at ASDEX Upgrade for injection of up to 1 MW at four frequencies between 105 and 140 GHz. This paper shows compu- tational fluid dynamics conjugated heat transfer and structural analyses of such a circumferentially water-cooled window design aiming to check whether it might be used for DEMO microwave beam scenarios. This design was then characterized with respect to different parameters. Temperature and thermal stress results showed that it is a feasible window solution for DEMO, but safety margins against limits shall be increased by introducing design features able to make the fluid more turbulent. A first design change is proposed, showing that, in combination with a higher inlet flow rate, the maximum temperature in the disks can be reduced from 238 to 186 °C, leading, therefore, to lower thermal gradients and stresses in the window