Development of a 2 MW, CW, 170 GHz Coaxial Cavity Gyrotrons for ITER
B. Piosczyk1a, S. Alberti2, A. Arnold1a,3, A. Beunas4, E. Borie1a,
H. Budig1a, G. Dammertz1a,
D. Bariou4, O.
Dumbrajs5, O. Drumm1a,3, V. Erckmann6a, D.
Fasel2, E. Giguet4, T. Goodman2,
R. Heidinger1b, M. Henderson2, J.P. Hogge2,
S. Illy1a, J. Jin1a,
W. Kasparek7, G. LeCloarec4, C. Lievin4,
G. Michel6a, G. Müller5, M. Thumm1a,3,
M.Q. Tran2 and D. Wagner6b,
I. Yovchev2
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 und Elektronik, Kaiserstr. 12, D-76128 Karlsruhe, Germany
4Thales Electron Devices, 2 Rue de Latécoère, F-78141 Vélizy-Villacoublay,
France
5 Department
of Engineering Physics and Mathematics, Helsinki University of Technology,
Association
EURATOM TEKES, FIN-02150 Espoo, Finland
6 Max-Planck-Institut für Plasmaphysik,
aTeilinstitut Greifswald, Association EURATOM, Wendelsteinstr. 1, D-17491 Greifswald, Germany
7Institut für Plasmaforschung, Universität Stuttgart, Pfaffenwaldring 31, D-70569 Stuttgart, Germany
e-mail:
bernhard.piosczyk@ihm.fzk.de
A 170 GHz coaxial cavity gyrotron
with an RF output power of 2 MW in continuous wave (CW) operation as can
be used in ITER is under development in a collaboration between European
research centers and European tube industry. A conceptual design of such a
coaxial gyrotron is in progress and will be finished within this year followed
by a fabrication of a first prototype. An experimental coaxial tube at FZK is
now under modification for operation at 170 GHz in order to be used for an
experimental verification of the design of critical components as gun, cavity
and the RF output system.
The decision for development of a
2 MW, CW coaxial cavity gyrotron has been encouraged by the impressive
results obtained in the development of the 1 MW, 140 GHz long pulse
(up to 30 min) conventional gyrotron for use at the W7-X stellarator.
Within this development already with a second prototype an RF output power of
0.89 MW (output efficiency of 42 %, with depressed collector) has
been achieved for a pulse length of 180 s limited by the performance of
the high voltage power supply of the gyrotron test facility at FZK. At a reduced
beam current (< 30 A), for which the power supply is not limiting
anymore, the pulse length has been extended up to 937 s and 1284 s at an
RF output power of 0.54 and 0.26 MW, respectively. In both cases the
increase of the pressure inside the tube was limiting the pulse length. It has
been found that the pressure rise is due to heating up of some parts inside the
tube by microwave stray radiation. This gyrotron is now under modification and
experimental tests with the improved tube will be performed within the next few
months.
The development of the coaxial cavity
gyrotron is based on recently finished experimental and theoretical
investigations on a coaxial gyrotrons operated at 165 GHz at short pulses
(up to ~20 ms) limited by the power loading at the collector surface.
Within this work all problems specific to the coaxial arrangement have been
investigated and the feasibility of manufacturing a 2 MW, CW coaxial
gyrotron has been demonstrated. The usability of the components for CW
operation and their compatibility with technical constraints has been proven.
The expected technical problems are comparable with conditions of the above
mentioned 140 GHz conventional gyrotron. The availability of low loss CVD
diamond discs allows the transmission of 2 MW through one output window
with a great safety margin.