10 MW, 140 GHz, CW GYROTRON AND OPTICAL TRANSMISSION SYSTEM FOR
MILLIMETER WAVE HEATING OF PLASMAS IN THE STELLARATOR W7-X
M. Thumm*, G. Dammertz, G.
Gantenbein, S. Illy,
W. Leonhardt, G. Neffe, B. Piosczyk, M. Schmid
Forschungszentrum
Karlsruhe, Association Euratom-FZK, IHM, Postfach 3640,
76021
Karlsruhe, Germany
*and Universitaet
Karlsruhe, Institut fuer Hoechstfrequenztechnik und Elektronik,
Kaiserstr. 12, 76131 Karlsruhe, Germany
manfred.thumm@ihm.fzk.de
H. Braune, V. Erckmann,
H.P. Laqua, G. Michel, M. Weissgerber
Max-Planck-Institut fuer Plasmaphysik (IPP),
Wendelsteinstr. 1, 17491 Greifswald, Germany
P. Brand, W. Kasparek, C. Lechte
Institut fuer Plasmaphysik, Universitaet Stuttgart,
Pfaffenwaldring 31, 70569
Stuttgart Germany
During the last years
electron cyclotron heating (ECH) has proven to be one of the most attractive
heating schemes for stellarators, as it provides net current free plasma start
up and heating. Both, the stellarator Wendelstein 7-X (W7-X), which is under
construction at IPP-Greifswald, Germany, and the ITER tokamak, which will be
built at Cadarache, France, will be equipped with a strong EC-heating and
current drive system. Both systems are similar in frequency and have CW
(continuous wave) capability (140 GHz, 10 MW for W7-X and 170 GHz, 24 MW for
ITER). The commissioning of the ECH plant for W7-X is well underway, the status
of the project and first integrated full power test results from two modules
are reported and may provide valuable input for the ITER plant.
The ten 1 MW gyrotrons at
W7-X are arranged in two sub-groups symmetrically to a central beam duct in the
ECH hall. The RF-waves of each subgroup are combined and transmitted by a
purely optical multibeam waveguide transmission line (copper mirrors) from the
gyrotrons to the plasma torus. The combination of the five gyrotron beams to
two beam lines each with a power of 5 MW reduces the complexity of
the system considerably. The single-beam- as well as the multi-beam waveguide
mirrors and the grating polarizers have been manufactured. Cold tests of a full
size uncooled prototype line delivered an efficiency exceeding 90%. The
microwave power will be launched to the plasma through ten CVD-diamond barrier
windows and in-vessel quasi-optical plug-in launchers allowing each 1 MW
RF-beam to be steered independently. The polarization as well as the poloidal
and toroidal launch angle can be adjusted individually to provide optimum
conditions for different heating and current drive scenarios.
The first series gyrotron
was tested and yielded a total output power of 0.98 MW, with an efficiency of
31% (without a single-stage depressed collector) in short-pulse operation and
of 0.92 MW in pulses of 1800 s (efficiency of almost 45% at a depression
voltage of 29 kV). The Gaussian mode output power was 0.90 MW and the power
measured in a calorimetric load after a 25-m-long quasi-optical transmission
line (seven mirrors) was 0.87 MW.
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*Work supported by the
European Communities, EFDA