International Journal of Terahertz Science and Technology
  TST >> Vol.11, No.1, March 2018: PP. 1-20

(Invited Paper)Design studies and analysis of operational limits of 0.24-THz gyrotrons for DEMO

M. Thumm 1, 2*, K. A. Avramidis 1, J. Franck 1, G. Gantenbein 1, S. Illy 1, J. Jin 1, P. C. Kalaria 1, I. Gr. Pagonakis 1,   S. Ruess 1, 2, C. Wu 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)
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Abstract: Electron Cyclotron Heating and Current Drive (ECH&CD) systems using gyrotrons as RF sources play a key role in present controlled thermonuclear fusion plasma experiments and are also planned for the future European DEMOnstration power plant (DEMO). Following the development of 1 MW continuous wave (CW) 140 GHz and 170 GHz gyrotrons for the W7-X stellarator and the ITER tokamak, respectively, the conceptual designs of tubes with frequencies up to approximately 270 GHz are ongoing at KIT. Along with a 237.5 GHz, 2 MW coaxial-cavity gyrotron design, a 236 GHz, 1 MW hollow-cavity approach is under investigation, as backup solution. In both cases, operating modes have been selected considering multi-frequency operation at around 170 GHz, 204 GHz, 237 GHz and 270 GHz for multi-purpose applications and fast-frequency tunability in steps of 2-3 GHz within the frequency range of \10 GHz around the operating center frequency for plasma stability control.

At 237.5 GHz, a coaxial-cavity design for the TE49,29-mode (eigenvalue ~ 158) has been found and optimized using realistic electron beam parameters with quite promising 1.9 MW output power and 33% interaction efficiency at a maximum cavity wall loading of 2 kW/cm². At 203.8 GHz, oscillating in the TE42,25-mode (eigenvalue ~ 136), the same cavity could deliver 1.9 MW output power with 32 % interaction efficiency at reduced maximum cavity wall loading of 1.7 kW/cm². For 170 GHz operation in the TE35,21-mode (eigenvalue ~ 113), the corresponding parameters would be 1.8 MW, 31% and 1.3 kW/cm².

In the case of a TE43,15-mode (eigenvalue ~ 103) hollow-cavity gyrotron operating at 236.1 GHz, again considering realistic electron beam parameters in the cavity (rms velocity spread: 6%, radial width: λ/4) and a realistic conductivity of the anticipated cavity material Glidcop, the results suggest stable output power of 0.92 MW with an interaction efficiency of 36% at a maximum cavity wall loading of 2 kW/cm². For the TE37,13-mode (eigenvalue ~ 89) at 203.0 GHz and the TE31,11-mode (eigenvalue ~ 74) at 170.0 GHz the corresponding values are 1.15 MW and 1.55 MW and 35% and 33%, respectively, at the same wall loading.

The development of magnetron injection guns (MIGs) with high electron beam quality and of multi-stage depressed collectors (MDCs) for energy recovery is necessary to achieve the required total gyrotron efficiency of  > 60 %.

Keywords: DEMO, Electron cyclotron heating and current drive (ECH&CD), Multi-frequency gyrotrons, Frequency step-tunable gyrotrons, Coaxial-cavity gyrotrons, Mode selection, High-order modes, Magnetron injection gun, Quasi-optical mode converter, Broadband synthetic diamond Brewster and tunable windows.

Received: 2018-3-6

Published: 2018-3-30

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 super- computer at IFERC-CSC and the Marconi-Fusion supercomputer facility.

The authors are glad to acknowledge very helpful discussions with Minh Quang Tran from EPFL-SPC in Lausanne, Switzerland.

Cite this article:
M. Thumm, K. A. Avramidis, J. Franck, G. Gantenbein, S. Illy, J. Jin, P. C. Kalaria, I. Gr. Pagonakis, S. Ruess, C. Wu and J. Jelonnek. (Invited Paper)Design studies and analysis of operational limits of 0.24-THz gyrotrons for DEMO[J]. International Journal of Terahertz Science and Technology, 2018, Vol.11, No.1: 1-20.  DOI:10.11906/TST.001-020.2018.03.01




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