Development of Multimegawatt Gyrotrons
for Fusion Plasma Heating and Current Drive
G. Dammertz1a,
S. Alberti2, A. Arnold1a,3, E. Borie1a, V.
Erckmann4, G. Gantenbein5, E. Giguet7,
R. Heidinger1b, J.P. Hogge2, S. Illy1a,
W. Kasparek5, K. Koppenburg1a, M. Kuntze1a,
H. Laqua4, G. Le Cloarec7, F. Legrand7, Y. Le
Goff7, W. Leonhardt1a, C. Lievin7,
R. Magne6, G. Michel4,
G. Müller5,
G. Neffe1a, B. Piosczyk1a, T. Rzesnicki1a, M.
Schmid1a, M. Thumm1a,3, M.Q. Tran2
1Forschungszentrum Karlsruhe, Association EURATOM-FZK, aIHM,
bIMF I,
Hermann-von-Helmholtz-Platz 1, D-76344
Eggenstein-Leopoldshafen, Germany,
2Centre de
Recherche en Physique des Plasmas, Association Euratom-Confédération Suisse,
EPFL Ecublens, CH-1015 Lausanne, Suisse
3Universität Karlsruhe, IHE, 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
High frequency gyrotrons with high output
power are mainly used 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. The development of high power
gyrotrons (118 GHz, 140 GHz and 170 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. This paper describes the
work of the Forschungszentrum Karlsruhe for the development of
conventional-cavity 1-MW CW gyrotrons, coaxial cavity 2-MW short-pulse
gyrotrons and a frequency step-tunable gyrotron in the frequency range between
105 –140 GHz.
The development
of gyrotrons with 1-MW output power for CW operation is mainly linked with the
construction of the new superconducting stellarator Wendelstein 7-X at IPP
Greifswald, Germany. For this facility a total power of 10 MW of electron
cyclotron waves (ECW) is planned and will be produced by ten gyrotrons each
with a power of 1-MW. The gyrotrons are equipped with a diode-type magnetron
injection gun, a conventional TE28,8
mode cavity, an advanced quasi-optical (q.o.) mode converter system, an
output RF-window with a single edge-cooled CVD-diamond disk and a depressed
collector for energy recovery. After the construction and test of two
prototypes, the series gyrotrons have been ordered. The second prototype
yielded an output power of 890 kW for 180 s, and at reduced beam current a
power of 540 kW for 937 s. After these tests, the tube had been disassembled
for visual inspection. It was rebuilt with a new cathode emitter ring in order to increase the uniformity of the
electron beam. Further the cooling for the internal ion getter pumps was
improved. The tube is now ready for tests.
Coaxial Cavity
Gyrotron
In
previous experiments, an output power as high as 2.2 MW was achieved during
short-pulse operation at 165 GHz in the TE31,17 mode. For longer
pulse operation, the built up of a Penning discharge in the rear part of the
electron gun occurred. This could be suppressed by a modification of the gun
geometry. At a beam current of 50 A, a pulse length of 22 ms could be achieved
with an RF-output power of about 1 MW. The pulse-length was limited due to the temperature rise of the
collector. At reduced currents, a pulse length of more than 100 ms could be
achieved without Penning discharge.
A leakage current
to the insert has been measured which increased linearly with the depression
voltage (retarding voltage of the collector). This leakage current reaches
about 37 mA at a depression voltage of 27 kV. However, no influence of the
accelerating voltage was found. It is assumed that the leakage current is
created by trapped electrons generated by ionization of the background gas. In
case of an existing depression voltage, the trapped electrons only can escape
by diffusion across the magnetic field.
Based on the
results of these measurements a conceptual design of an industrial prototype CW
gyrotron with a coaxial cavity for an RF-output power of 2 MW at 170 GHz has
been completed. The manufacturing process has been launched. The design of the
cavity has been verified by using two different time-dependent, multimode and
self-consistent codes. To reduce the internal stray radiation, an advanced
RF-output system has been designed. A single disk CVD-diamond window with a
thickness of 1.852 mm and a novel single-stage depressed collector will be
used.
In order to
verify the design of the critical components as electron gun, cavity, and
output system, the 165 GHz coaxial gyrotron will be modified for operation at
170 GHz in the TE34,19 mode. The tests will allow to prove the RF
generation, the mode competition and as well as the efficiency of the RF output
system and to measure the amount of internal stray radiation. The tests are
expected to start soon.
The
availability of MW gyrotrons with fast frequency step tunability permits the
use of a non-movable antenna for local electron cyclotron current drive and
plasma stabilization in thermonuclear fusion devices.
The
existing TE22,6 mode gyrotron was modified to be operated in the TE22,8
mode. This mode is suitable for long-pulse operation at 1MW level. The
resonator was optimized for a series of modes from 105-140 GHz. A q.o.-mode
converter system has been designed, fabricated and tested in low-power
measurements. The converter consists of a dimpled-wall antenna (in order to
minimize the internal stray radiation) and a beam-forming mirror system with a
large quasi-elliptical mirror and two additional phase-correcting mirrors. The
field distribution measurement showed very good agreement with the calculated
one. The power measurements with a quartz Brewster window gave an output power
of 700 kW for different modes.
The
use of a Brewster window from CVD-diamond even with a diameter of 140 mm needs
a special construction of the window and an optimized RF beam to fit into this
construction. A new q.o.-mode converter to be used with a diamond Brewster
window has to be designed and will be constructed.