Progress in the Development of a 1-MW, CW Gyrotron

at 140 GHz for Fusion Plasma Heating

G. Dammertz^1a, S. Alberti², A. Arnold^1a,3, E. Borie^1a, V. Erckmann⁴, G. Gantenbein⁵, E. Giguet⁷, R. Heidinger^1b, J.P. Hogge², S. Illy^1a, W. Kasparek⁵, K. Koppenburg^1a, M. Kuntze^1a, H. Laqua⁴, G. Le Cloarec⁷, F. Legrand⁷, Y. Le Goff⁷, W. Leonhardt^1a, C. Lievin⁷, R. Magne⁶, G. Michel⁴, G. Müller⁵, G. Neffe^1a, B. Piosczyk^1a, M. Schmid^1a, M. Thumm^1a,3, M.Q. Tran²

¹Forschungszentrum Karlsruhe, Association EURATOM-FZK, ^aInstitut für Hochleistungsimpuls- und Mikrowellentechnik, ^bInstitut für Materialforschung I, Postfach 3640, D-76021 Karlsruhe, Germany,

²Centre de Recherche en Physique des Plasmas, Association Euratom-Confédération Suisse, EPFL Ecublens, CH-1015 Lausanne, Suisse

³Universität Karlsruhe, Institut für Höchstfrequenztechnik u. Elektronik, D-76128 Karlsruhe, Germany

⁴Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald, Association EURATOM,

Wendelsteinstr. 1, D-17491 Greifswald, Germany

⁵Institut für Plasmaforschung, Universität Stuttgart, D-70569 Stuttgart, Germany

⁶CEA/Cadarache, 13108 Saint Paul-lez-Durance Cédex, France

⁷Thales Electron Devices, 2 Rue de Latécoère, F-78141 Vélizy-Villacoublay, France

e-mail: guenter.dammertz@ihm.fzk.de; phone: (+49) 7247-82 4160; fax: (+49) 7247-82 4874

1. Introduction

High frequency gyrotrons with high output power are mainly developed for microwave heating and current drive in plasmas for thermonuclear fusion experiments. Electron cyclotron resonance heating (ECRH) has proven to be an important tool for plasma devices especially for stellarators, as it provides both net current free plasma start up from the neutral gas and efficient plasma heating [1]. The development of high power gyrotrons (118 GHz, 140 GHz) in continuous wave operation (CW) has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. The major problems of high-power high-frequency gyrotrons are given by the Ohmic heating of the cavity surface, by internal stray radiation, by the dielectric losses in the output window and by the power capability of the collector.

2. Design

A few design parameters of the gyrotron are summarized in Table 1. A magnetron injection gun of diode type (without intermediate anode) is used. The cylindrical cavity is designed for operating in the TE_28,8 mode. It is a standard tapered cavity with linear input downtaper of 2.5° and a non-linear uptaper with the initial angle of 3°. The length of the cylindrical part is 15 mm, its diameter 40.96 mm. The transitions between tapers and straight section are smoothly rounded over a length of 4-6 mm to avoid mode conversion.

The TE_28,8 cavity mode is transformed to a Gaussian TEM_0,0 output mode by a mode converter consisting of a tapered rippled-wall waveguide launcher followed by a three mirror system. The output window unit uses a single, edge cooled CVD-diamond disk with an outer diameter of 106 mm, a window aperture of 88 mm and a thickness of 1.8 mm corresponding to four half wavelengths.

The collector is at ground potential, and a depression voltage for energy recovery can be applied to the cavity and to the first two mirrors.

3. Experiment

The short and long-pulse experimental results of the pre-prototype tube “Maquette” have been reported in [2,3]. The first prototype had been built with several modifications with respect to the “Maquette”: a corrected quasi-optical mirror system, an improved cooling system of the mirror box, an additional relief window for the stray radiation and a tilt of the output window disk of 1.5°. A newly developed high temperature braze (about 820°C) for brazing the CVD-diamond disk was used.

The RF output beam of the gyrotron is injected into an RF-tight microwave chamber which is equipped with water-cooled mirrors directing the beam towards the 1 MW water load. The difficulties concerning the limitation of power absorption inside the RF-load had been overcome by installing two polarisers in the RF-beam converting the linearly polarized beam into a circularly polarized one.

Fig. 1 shows the experimental output powers for the prototype tube. These values are measured calorimetrically by the temperature increase of all the water circuits inside the microwave chamber. An output power of 0.97 MW had been obtained in pulse lengths of about 11.8 s with an efficiency of 44% with single stage depressed collector. For a pulse length of 180s (limited by the HV power supply), an RF output power of 0.89 MW was obtained with an efficiency of 42%. For all pulses the internal stray radiation did not exceed 1.5 % of the output power. The undirected power inside the microwave chamber was measured to about 1.6% of the output power. These values indicate the excellent behaviour of the quasi-optical mode converter.

At high power, the HV power supply limits the pulse lengths to 180s. At reduced power levels, corresponding to beam currents not exceeding 30 A, continuous wave operation is possible. To see limitations of the tube caused by increase in tube pressure, operation at a reduced power level of 0.54 MW was performed. Despite the very low internal stray radiation, an exponential pressure increase was observed limiting the pulse length to 937 s corresponding to an energy content of more than 500 MJ. For an output power of 0.26 MW the gyrotron had to be switched off after about 1300 s (Table 2). Infrared measurements showed a strong temperature increase of the internal ion getter pumps during operation indicating that those pumps are not cooled sufficiently for long pulse operation.

References

[1] V. Erckmann et al., Plasma Phys. Controlled Fusion, Vol. 28 (1986) 1277-1290

[2] G. Dammertz et al., IEEE Trans. Plasma Science 30 (2002) 808-818

[3] G. Dammertz et al., Conf. Dig. 27^th Int. Conf. on Infrared and Millimeter Waves,

San Diego, CA, 2002, pp. 3-4.