H. Braune*,
P. Brand**, R. Krampitz*, W. Leonhardt ***, D. Mellein***, G. Michel*,
G. Mueller**, M. Winkler* and the W7-X ECRH teams at IPP, IPF and FZK
* Max-Plank-Institut für Plasmaphysik,
EURATOM Association
Teilinstitut Greifswald,
Wendelsteinstraße 1, D-17491 Greifswald, Germany
** Universität Stuttgart, Institut für
Plasmaforschung
Pfaffenwaldring 31, D-70569 Stuttgart,
Germany
*** Forschungszentrum Karlsruhe, Association EURATOM –FZK, IHM
FZK,
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany,
Electron Cyclotron Resonance Heating (ECRH) is the main heating method
for the Wendelstein 7-X Stellarator (W7-X), which is under construction at
IPP-Greifswald.
A 10 MW ECRH plant with CW-capability at
140 GHz is under construction to meet the scientific objectives. The general
features of the ECRH-plant such as frequency, power, cw‑capability,
flexibility and the experimental experience are of high relevance for the ITER
system. The microwave power is generated by 10 gyrotrons with 1 MW each, two
gyrotrons are operational at IPP in Greifswald. The tubes are equipped with a
single-stage depressed collector for energy recovery. At the nominal depression voltage, the cathode
is typically
at -55 kV. The beam
tunnel, cavity, the quasi-optical launcher and the first two mirrors have the depression potential of +25kV. The
corresponding values for an ITER gyrotron are -55 kV and +35 kV. For
heat wave experiments, the gyrotrons will operate with an output power
modulation between 0.3 and 1 MW with a sinusoidal frequency of up to 10
kHz, which is achieved by modulating the depression voltage.
Each gyrotron is fed by two high-voltage sources. A high-power supply
for driving the electron beam and a precision low-power supply for beam
acceleration. The high-power facility consists of modular solid state
HV-supplies (-65 kV, 50 A each) providing fast power control and high
flexibility. The low-power high-voltage source for beam acceleration is
realized by a high-voltage servo-amplifier driving the depression voltage such
that the influence of the voltage noise of the main high-power supply on the
acceleration voltage is counteracted by feed-back control of the amplifier. In
addition, a protection system with a thyratron crowbar for fast power removal
in case of gyrotron failure by arcing is installed. The low-power high-voltage
source is capable of generating up to 38 kV, 1 A peak current and could supply an 2 MW ITER gyrotron without any modification.
Analog electronic devices control the fast functions of the high-voltage system
for each gyrotron and a hierarchy of PLC 's and computers supervise the whole
ECRH-plant.
An overview of high-voltage system and fast control requests is presented.