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

Abstract

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 %.

Overview of recent gyrotron R&D towards DEMO within EUROfusion Work Package Heating and Current Drive

“…In order to first validate the construction and assembly of the new components, the upgraded gyrotron was tested in short pulses (< 10 ms) using the diode electron gun of the short-pulse gyrotron with the goal to reproduce the previous behavior of the short-pulse tube [14]. The validation was fully successful, since, at nominal parameters, the operating TE 34,19 mode was stably excited at 169.9 GHz with an output RF power close to 2.1 MW and an efficiency slightly above 30 % in non-depressed collector operation. After further parameter optimization, the RF power was increased to 2.2 MW with ~33 % efficiency.…”

“…To keep the development path towards the DEMO gyrotron as fast and cost-effective as possible, the design of a 2 MW, 170/204 GHz coaxial gyrotron has been initiated using the existing 170 GHz coaxial gyrotron as starting point [19][20]. The simulation results show that a good performance can already be achieved with only minor modifications of the existing modular gyrotron.…”

Theoretical Study on the Operation of the EU/KIT TE34,19-Mode Coaxial-Cavity Gyrotron at 170/204/238 GHz

“…In initial studies, the KIT pre-prototype cavity without the non-linear uptaper was considered and already presented in [12]. However, the simulation results will be repeated within this paper for completeness.…”

“…Relative starting current of competing modes at 8.15 T and Ibeam = 70 A (filled bars: co-rotating; empty bars: counter rotating modes)[12].…”

Towards Advanced Fusion Gyrotrons: 2018 Update on Activities within EUROfusion

“…On the other hand, it would be much simpler, if one could operate a modified version of the existing 2 MW 170 GHz TE34,19-mode pre-prototype at 204 GHz and at 238 GHz even. In [15] it is shown by simulations that the existing KIT TE34,19mode coaxial-cavity pre-prototype gyrotron is capable to operate in the new KIT FULGOR gyrotron test stand using a 10.5 T super-conducting magnet if the gyrotron construction will be slightly adapted. The modified gyrotron will be capable to operate as a multi-purpose / multi-frequency device.…”