Gyrotron Development in EU for Present Fusion Experiments

and for ITER

M. Thumm^1a,2, S. Alberti³, A. Arnold², D. Bariou⁴, G. Dammertz^1a, C. Darbos⁵, O. Dumbrajs⁶, G. Gantenbein⁷, V. Erckmann⁸, E. Giguet⁴, R. Heidinger^1b, J.-P. Hogge³, S. Illy^1a, J. Jin^1a, W. Kasparek⁷, C. Liévin⁴, R. Magne⁵, G. Michel⁸, B. Piosczyk^1a, O. Prinz^1a, T. Rzesnicki^1a, M.Q. Tran³, X. Yang^1a, I. Yovchev³

¹Forschungszentrum Karlsruhe, Association EURATOM-FZK, ^1aIHM, ^1bIMF-I

Postfach 3640, D-76021 Karlsruhe, Germany. e-mail: manfred.thumm@ihm.fzk.de

²Universität Karlsruhe, IHE, Kaiserstr. 12, D-76128 Karlsruhe

³Centre de Recherches en Physique des Plasmas, Association EURATOM - Confédération Suisse, EPFL Ecublens,

CH-1015 Lausanne, Switzerland

⁴Thales Electron Devices, 2 Rue de Latécoère, F-78141 Vélizy-Villacoublay, France

⁵Association EURATOM - CEA, CEA/DSM/DRFC, Centre de Cadarache, 13108 Saint-Paul-lez-Durance, France

⁶Helsinki University of Technology, Association EURATOM - TEKES, FIN-02150 Espoo, Finland

⁷Institut für Plasmaforschung, Universität Stuttgart, Paffenwaldring 31, D-70569 Stuttgart, Germany

⁸Max-Planck-Institut für Plasmaphysisk, Teilinstitut Greifswald, Association EURATOM - IPP, Wendelsteinstr. 1,

D-17491, Greifswald, Germany

Abstract

The long term strategy of the EU in the field of gyrotrons for magnetic confinement fusion plasma applications is based on two approaches: an R&D in laboratories to develop advanced concepts and industrial development of state-of-the-art tubes for use in present experiments like TCV, Tore Supra  (118 GHz, 0.5 MW, CW) and W7-X (140 GHz, 1 MW, CW). The results from these two approaches are then applied to the development of a coaxial-cavity gyrotron operating at 170 GHz and delivering 2 MW-CW for the electron cyclotron wave system of ITER. The paper will recall the main achievements of this program and will outline the present status of the 170 GHz gyrotron development. The main design characteristics of these EU tubes are summarized in the following Table.

Frequency	118 GHz	140 GHz	170 GHz
Cavity mode	TE22,6	TE28,8	TE34,19 coaxial cavity
Power, Pulse length	0.5 MW, 5 s / 0.4 MW, 600 s	1 MW, CW (1800 s)	2 MW, CW (3600 s)
Electronic efficiency	33%	35%	30%
Electrical efficiency	28%	48%	45%
Magnetron injection gun	Triode	Diode	Diode
Current density at cathode	1.6 A/cm^2	2.5 A/cm^2	4.2 A/cm^2
Electron beam radius at cavity	9.6 mm	10.1 mm	10.0 mm
Beam thickness at cavity	0.2 mm	0.2 mm	0.2 mm
Electron beam energy	81.5 kV	81 kV	90 kV
Electron beam current	22A	40 A	75A
Velocity ratio a	1.3	1.3	1.3
Collector depression voltage	-	27-30 kV	35 kV
Cavity heat load (Ideal Cu, s =5.9 10^7 S/m)	< 1kW/cm^2	< 1 kW/cm^2	1kW/cm^2
Window	Sapphire at LN2 temperature	CVD diamond	CVD diamond
Gaussian output	95.8%	98.8%	96%
Waist of output beam	20.4 mm	22 mm	20.4 mm
Collector magnetic field	DC + AC field (2 coils)	UC field (1 coil)	UC field (1 coil)

The record pulse length of the 118 GHz tube is 110 s with an average power of 300 kW. The 140 GHz gyrotron delivered an output power of 970 kW (efficiency: 44% with single-stage depressed collector) for a pulse length of 11.7 s, and 890 kW (42%) for 180 s (limit of the HV power supply at FZK). At reduced electron beam current (30 A), the pulse length could be increased to 939 s at 539 kW (506 MJ). For this case the limit was given by the increase of the internal pressure causing the interlock to switch off the gyrotron; at even more reduced electron beam current and lower output power of 257 kW a pulse length of 1300 s could be achieved. First short pulse (1 ms) experiments with the pre-prototype of the 170 GHz coaxial-cavity gyrotron gave 1.15 MW output power with 20% efficiency without collector depression.