M. Thumm 1, 2*, J. Franck 1, P.C. Kalaria 1, K.A. Avramidis 1, G. Gantenbein 1, S. Illy 1, I.G. Pagonakis 1, M. Schmid 1, C. Wu 1, J. Zhang 1, and J. Jelonnek 1, 2,
Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131 Karlsruhe, Germany
1 Institute for Pulsed Power and Microwave Technology (IHM)
2 Institute of High Frequency Techniques and Electronics (IHE)
* Email: email@example.com
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Abstract: Current design studies on electron cyclotron heating and current drive (ECH&CD) systems for a DEMO fusion reactor demand gyrotron frequencies of above 200 GHz for efficient CD and a total gyrotron efficiency above 60 % to achieve an efficient fusion power plant operation. Considering the mandatory total heating power for DEMO and the space and maintenance requirements per tube, gyrotrons with a unit power of above 1 MW will be needed. Furthermore, for plasma stability control using a simple fixed ECCD launcher, fast frequency tunability (in a few seconds) in steps of about 2-3 GHz is necessary. Such tubes require broadband or tunable output windows and their quasi-optical mode converter must support the conversion of the various modes to a fundamental Gaussian wave beam. Slow tunability of gyrotrons (within a few minutes) in leaps of about 30–40 GHz is considered advantageous (multi-frequency gyrotrons), e.g. for DEMO plasma start-up, for DEMO variants with a relatively low magnetic field or for multi-purpose use in other fusion devices such as ITER. A multi-frequency gyrotron can operate with a simple, single-disk synthetic-diamond output window which is transparent at the relevant frequencies.
Physical design studies towards DEMO-compatible 2 MW-class coaxial-cavity gyrotrons are being performed at KIT. A well suited coaxial-cavity mode series with good multi-frequency properties is:
- TE35,21 (170 GHz) – TE42,25 (203.8 GHz) – TE49,29 (237.5 GHz) – TE56,33 (271.3 GHz).
At 237.5 GHz a coaxial-cavity design for the TE49,29 mode has been found and optimized with quite promising results (1.9 MW, 33% electronic efficiency, without depressed collector (DC)). This frequency has been chosen such that the respective mode TE35,21 exactly matches the ITER frequency of 170 GHz, allowing the gyrotron to be used for a later upgrade of the ITER ECH&CD system. The azimuthal neighboring modes of TE49,29 have a frequency separation of 2 GHz and are well-suited fast frequency tuning. The design of the magnetron injection gun (MIG) has been finished, based on a realistic 10 T gyrotron magnet system.
As a backup solution, conceptual design studies on a hollow-cavity 1 MW-class gyrotron have also been performed. The current design permits multi-frequency operation with the following modes:
- TE31,11 (170 GHz) – TE37,13 (203 GHz) – TE43,15 (236.1 GHz) – TE49,17 (269.1 GHz).
Numerical simulations with realistic electron beam velocity spread and beam width show, that about 1 MW output power at 36 % electronic efficiency (without DC) can be achieved.
The development of electron guns with high beam quality and of multi-stage depressed collectors (MSDC) for energy recovery is necessary to achieve the required overall gyrotron efficiency of above 60 %. KIT is installing a new gyrotron high-voltage power supply which will allow the operation of high-power gyrotrons with MSDCs.
Keywords: DEMO, Electron cyclotron heating and current drive (ECH&CD), Multi-frequency gyrotrons, Step-tunable gyrotrons, Mode selection, High-order modes, Magnetron injection gun, Quasi-optical mode converter, Broadband synthetic diamond Brewster and tunable windows.
Acknowledgments: This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
Parts of the simulations presented in this work have been carried out using the HELIOS supercomputer at IFERC-CSC.
Cite this article:
M. Thumm, J. Franck, P.C. Kalaria, K.A. Avramidis, G. Gantenbein, S. Illy, I.G. Pagonakis, M. Schmid, C. Wu, J. Zhang, and J. Jelonnek. Towards a 0.24-THz, 1-to-2-MW-class gyrotron for DEMO[J]. International Journal of Terahertz Science and Technology, 2015, Vol.8, No.3: 85-100. DOI:10.11906/TST.085-100.2015.09.09