Status of the 1-MW, 140-GHz, CW Gyrotron for W7-X
G. Gantenbein1a,
G. Dammertz1a, S. Alberti2, A. Arnold1a,3, V.
Erckmann4, E. Giguet6, R. Heidinger1b,
J.P. Hogge2, S.
Illy1a, W. Kasparek5, K. Koppenburg1a, H.
Laqua4, F. Legrand6, W. Leonhardt1a, C.
Liévin6, G.
Michel4, G. Neffe1a, B. Piosczyk1a, M. Schmid1a,
M. Thumm1a,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 und Elektronik,
Kaiserstr. 12, D-76128
Karlsruhe, Germany
4Max-Planck-Institut für
Plasmaphysik Teilinstitut Greifswald,
Association EURATOM,
Wendelsteinstr. 1, D-17491
Greifswald, Germany
5Institut für
Plasmaforschung, Universität Stuttgart, Pfaffenwaldring 31, D-70569 Stuttgart,
Germany
6Thales Electron Devices, 2 Rue de Latécoère, F-78141 Vélizy-Villacoublay,
France
e-mail:
gerd.gantenbein@ihm.fzk.de
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. For the stellarator Wendelstein 7-X
now under construction at IPP Greifswald, Germany, a 10 MW ECRH system is
foreseen. A European collaboration has been established between
Forschungszentrum Karlsruhe (FZK), IPP Garching / Greifswald, IPF Stuttgart, CRPP
Lausanne, CEA Cadarache and TED Vélizy, to develop and build the 10 gyrotrons
each with an output power of 1 MW for continuous wave (CW) operation.
2. Design
The design parameters of the
gyrotron are summarized in Table 1. A temperature limited magnetron injection
gun without intermediate anode (diode type ) is used.
The
cylindrical cavity is designed for operation in the TE28,8 mode. It
is a standard cavity with a linear input downtaper of 2.5° and a non-linear
uptaper with the initial angle of 3°. The transitions between tapers and
straight section are smoothly rounded over a length of 4-6 mm to avoid mode
conversion. The TE28,8-cavity mode is transformed to a Gaussian TEM00
output mode by an advanced rippled-wall mode converter and a three mirror system.
The output window unit uses a single, edge cooled CVD-diamond disk with an
outer diameter of 106 mm, a aperture of 88 mm and a thickness of
1.8 mm.
3. Experiment
The output power
of the first series tube turned out to be almost linearly dependent on the
electron beam current at constant
magnetic field. An output power of 1 MW at 40 A and 1.15 MW at
50 A was measured in short pulse operation (~ms) with efficiencies of 31 %
and 30%, respectively (without depressed collector operation). This behaviour
shows the good quality of the electron emitter which had been proven by an
optical inspection and by the homogeneous temperature distribution before
installation of the emitter ring into the gyrotron.
RF-field
distribution measurements (perpendicular to the output RF-beam direction) were
performed at different positions with respect to the window. The beam is
shifted by about 12 mm in horizontal direction. The Gaussian content was
calculated from the measurements to be 97.5 %.
In a range
between 5.52 – 5.56 T of the magnetic field at the cavity, no
maximum for the output power was found. The power increased slightly with
increasing magnetic field. For maximum output power, the accelerating voltage
(corresponding to the energy of the electrons inside the cavity) was adjusted
and followed nicely the law that the ratio between magnetic field and the
relativistic factor g has to be constant.
Increasing the voltage beyond this value leads to an excitation of neighbouring
modes. The measurements were performed at a constant beam current of 40 A,
but with optimising the electron beam radius.
A strong
dependence of the output power has been found for different electron beam radii
in the cavity. The desired mode can only be excited in a small range between
10.25 mm and 10.43 mm. At lower beam radii, arcing occurs, at higher radii a
wrong mode is excited. The optimum value of the beam radius decreases slightly
with decreasing cavity field.
In long pulse
operation, the power was measured calorimetrically with a RF-load which was
placed about 6 m away from the gyrotron window. The highest output power
inside the load for a three minute pulse was measured to be 906 kW.
Including the calorimetrically measured stray radiation the total output power
was 922 kW with an efficiency of 45 % (depressed collector
operation).
At FZK, the
pulse length at full power is limited to three minutes, but at reduced electron
beam current (< 30 A) longer pulses can be achieved. The pressure
increase during the 30 minute pulse (1839 s) is less than a factor of two
ending up at about 6·10-9 mbar.
After the
successful tests at FZK, the tube was delivered to IPP Greifswald for tests at
highest output power and a pulse length of 30 minutes. A directed output power
of 865 kW was measured inside the load, and a total output power of about 920
kW was estimated taking the losses in the transmission line into account (world
record in energy content).
The second
series tube arrived at FZK end of November. First very short pulse experiments
(1 ms) yielded an output power of 960 kW.
4. Conclusions
The first series
tube of the 1-MW, 140 GHz, CW gyrotrons for W7-X passed the site
acceptance test successfully. In short pulse operation the power was measured
to be 1 MW, and for three minutes the power was 922 kW at an
efficiency of 45 %. At IPP the pulse length could be extended to 30
minutes with the same output power.