Karlsruhe Institute of Technology (KIT) is doing research and development in the field of megawatt-class radio frequency (RF) sources (gyrotrons) for the Electron Cyclotron Resonance Heating (ECRH) systems of the International Thermonuclear Experimental Reactor (ITER) and the DEMOnstration Fusion Power Plant that will follow ITER. In the focus is the development and verification of the European coaxial-cavity gyrotron technology which shall lead to gyrotrons operating at an RF output power significantly larger than 1 MW CW and at an operating frequency above 200 GHz. A major step into that direction is the final verification of the European 170 GHz 2 MW coaxial-cavity pre-prototype at longer pulses up to 1 s. It bases on the upgrade of an already existing highly modular short-pulse (ms-range) pre-prototype. That pre-prototype has shown a world record output power of 2.2 MW already. This paper summarizes briefly the already achieved experimental results using the short-pulse pre-prototype and discusses in detail the design and manufacturing process of the upgrade of the pre-prototype toward longer pulses up to 1 s.

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Gyrotron R&D within EUROfusion Work Package Heating and Current Drive is addressing the challenging requirements posed on gyrotrons by the European concept for a demonstration fusion power plant (EU DEMO). The paper reports on the progress of these activities, on the recent results, and on near-term planning.

The generation of a specific high order mode with excellent mode purity in a highly oversized cylindrical waveguide is mandatorily required for the verification of high power components at sub-THz frequencies. An example is the verification of quasi-optical mode conversion and output systems for fusion gyrotrons. A rotating high order mode can be excited by taking a low power RF source (e.g. RF network analyser) and by injecting the RF power via a horn antenna into a specific adjustable quasi-optical setup, the so-called mode generator. The manual adjustment of the mode generator is typically very time consuming. An automatized adjustment using intelligent algorithms can solve this problem. In the present work the intelligent algorithms consist of five different mode evaluation techniques to determine the azimuthal and radial mode indices, the quality factor, the scalar mode content and the amount of the counter-rotating mode. Here, the implemented algorithms, the design of the computer-controlled mechanical adjustment and test results are presented. The new system is benchmarked using an existing TE28,8 mode cavity operating at 140 GHz. In addition, the repeatability of the algorithms has been proven by measuring a newly designed TE28,10 mode generator cavity. Using the described advanced mode generator system, the quality of the excited modes has been significantly improved and the time for the proper adjustment has been reduced by at least a factor of 10.

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.

At KIT, a modular 170 GHz, 2 MW TE34,19-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 preprototype 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 TE34,19-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 %.

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