S. Ruess

An Inverse Magnetron Injection Gun for the KIT 2-MW Coaxial-Cavity Gyrotron

An inverse magnetron injection gun (MIG) has been designed for the 2-MW, 170-GHz, coaxial-cavity gyrotron built at the Karlsruhe Institute of Technology. The inverse gun design could offer the possibility for the implementation of a larger emitter ring without the need for a bigger bore hole in the magnet compared with the conventional type of MIGs. Considering the fundamental beam parameters, an excellent beam quality has been achieved in numerical simulation. Electron-trapping suppression criteria were considered during the design phase of the MIG.

A design proposal for an optimized inverse Magnetron Injection Gun for the KIT 2 MW / 170 GHz modular coaxial cavity gyrotron

In this work, the design of an inverse Magnetron Injection Gun (MIG) for the 2 MW / 170 GHz modular coaxial cavity KIT gyrotron is presented. The gun design criteria related to the suppression of the electron trapping mechanisms are considered. In addition, the proposed gun design satisfies the demanded electron beam criteria set at the start of the project.

Experimental results and outlook of the 2 MW 170 GHz coaxial-cavity gyrotron towards long pulse operation

The development of the modular short-pulse pre-prototype as base for the 2 MW, 170 GHz, CW coaxial-cavity gyrotron for international fusion project ITER and beyond is in progress at Karlsruhe Institute of Technology (KIT). The current modular pre-prototype configuration allows the generation of > 2 MW RF output power in short-pulses with a reasonable electronic efficiency of approximately 30 % without depressed collector operation. Recently, experiments with a new glidcop resonator has been performed. In depressed operation the overall efficiency of the gyrotron has been increased to ~48 %. In addition, an advanced water cooling system for a long pulse 2 MW gyrotron was developed. In this paper the experimental results, the suppression of the beam-halo and future plans for the 2 MW gyrotron will be presented.

Choice of material composition for a high-performance inverted Magnetron Injection Gun

An inverse Magnetron Injection Gun (IMIG) has been designed for the KIT 2 MW, 170 GHz coaxial cavity gyrotron. The ambition is the possibility for implementation of a larger emitter ring compared to the “conventional” MIGs used in todays fusion gyrotrons. Considering the fundamental beam parameters, in the theoretical analysis an excellent beam quality has been achieved which results in a very low velocity spread. In order to ensure a stable operation under hot condition, the thermomechanical behavior and material compositions were investigated and optimized.

From W7-X towards ITER and beyond: Status and progress in EU fusion gyrotron developments

In Europe, significant progress in gyrotron research, development and manufacturing has been made in 2014, starting from the successful continuation of the 1 MW, 140 GHz gyrotron production for the stellarator Wendelstein 7-X (W7-X) at Greifswald, Germany and the accelerated development of the EU 1 MW, 170 GHz conventional cavity gyrotron for the ITER tokamak at Cadarache, France. Based on that, a physical design activity was started which shall lead to a dual frequency gyrotron for TCV, Lausanne, Switzerland. Within the European fusion development consortium (EUROfusion), advanced gyrotron research and development has started towards a future gyrotron design which shall fulfil the needs of DEMO, the nuclear fusion demonstration power plant that will follow ITER. Within that research and development, the development of advanced design tools, components, and proper test environment is progressing as well. A comprehensive view over the status and prospects of the different development lines shall be presented.

Considerations on the selection of operating modes for future coaxial-cavity gyrotrons for DEMO

At KIT, a modular 170 GHz, 2 MW TE34, i9-mode coaxial-cavity gyrotron with advanced water cooling is ready for tests. The successful operation of this tube will be a first important step towards a possible future DEMO gyrotron. Nevertheless, looking forward, there are two questions to be answered: (i) what potential does the existing coaxial cavity offer with regards to MW-class multi-frequency operation also at higher frequencies, and (ii) what could be a different mode selection to achieve an even higher output power in a more compact gyrotron design. To provide an answer to (i), based on the 170 GHz, 2 MW pre-prototype the multi-frequency operation at multiples of the resonance frequency of the diamond disc RF output window was carried out. Additionally, a slightly modified cavity design was introduced. To answer the question (ii), the TE25,22-mode was chosen and compared with the results got for the TE34, i9-mode. The extreme volume TE25,22-mode allows to reduce the beam radius by around 25 % and to increase the RF output power of the gyrotron by up to 30 %.

Tolerance Studies on an Inverse Magnetron Injection Gun for a 2-MW 170-GHz Coaxial-Cavity Gyrotron

The magnetron injection gun (MIG) is the most critical part of any gyrotron. Small tolerances in the manufacturing process and alignment of the subcomponents directly affect the electron beam quality and therefore the beam wave interaction. At the Karlsruhe Institute of Technology (KIT), an innovative new Inverse MIG (IMIG) is proposed for the European 2-MW 170-GHz coaxial-cavity gyrotronwhich is under the developmentand test at KIT. The design of the IMIG has been done under strict consideration of the gun design criteria published earlier. In order to find the maximum allowed tolerances of that IMIG which allows operation within the allowed gun design criteria, systematic theoretical studies have been done. Commonly used “conventional” MIGs have been shown that a small emitter, anode, and cathodemisalignment have a significant influence to the electron beam quality.

Evaluation and Influence of Gyrotron Cathode Emission Inhomogeneity

In order to evaluate the emission inhomogeneity of thermionic gyrotron cathode emitters, a novel definition of emission inhomogeneity and evaluating method from the current–voltage characteristics is described in this paper. Results for three different types of gyrotron oscillators at different emitter temperatures based on experimental data and mathematical treatment are considered in the investigation. The influence of emission inhomogeneity on gyrotron efficiency is numerically calculated using the 3-D codes ARIADNE and EURIDICE. The emitter inhomogeneity upper limits for the distributions of work function, temperature, and electric field are given at the end of this paper.