IPP Annual Report 2007 - Max-Planck-Institut für Plasmaphysik ...

diagnostic neutral beam injection system, KFKI/RMKI in
Budapest, Hungary, is developing and constructing the
video diagnostic systems for W7-X, IPPLM, Warsaw is
developing a neutron activation system and performing
MCP calculations for W7-X, the university of Opole (Poland)
is preparing a C/O monitor diagnostic and the Akademia
Morska, Szczechin and the Szczechin University of Technology
are investigating the sightline of the Interfero-
Polarimeter and different microwave based polarimeter and
interferometer methods, CIEMAT investigates potential and
components for CO 2 -Intefererometry, IST participates in
developing fast tomographic inversion methods (P. Carvalho
4 Months stay in Greifswald) and is developing ADC/DAQ
stations being linked to XDV, PTB, Braunschweig is performing
preparatory work for a contract on the development
of a Neutron-Counter System for W7-X and IOFFE Institute
St. Petersburg, the Culham Science Centre (UKAEA) and
CIEMAT in Madrid are collaborating in the field of CX-
Neutral Particle Analysis.
8 Heating
8.1 Project Microwave Heating for W7-X (PMW)
The Electron Cyclotron Resonance Heating (ECRH) system
for W7-X is being developed and built by FZ Karlsruhe
(FZK) as a joint project with IPP and IPF Stuttgart. The
“Project Microwave Heating for W7-X” (PMW) coordinates
all engineering and scientific activities in the collaborating
laboratories and in industry and is responsible for the entire
ECRH system for W7-X. ECRH is designed for a microwave
power of 10 MW in continuous wave (CW) operation
(30 min) at 140 GHz, which is resonant with the W7-X magnetic
field of 2.5 T. It will consist of ten Gyrotrons with
1 MW power each, a low loss quasi-optical transmission
line and a versatile in-vessel launching system. It was
shown recently, that the gyrotrons operate also at 103.6 GHz
with about half the output power, which would extend the
flexibility of the ECRH significantly. PMW is strongly
involved in advanced and ITER related R&D activities.
8.1.1 The W7-X Gyrotrons (FZK)
The TED-Gyrotrons SNo. 2 and 3 failed to meet the specified
output power in the acceptance tests and were sent back
to TED for inspection. Strong defects in the electron beam
tunnel between gun and cavity were identified, which were
believed to be the reason for the measured power limitation.
TED presented a failure analysis indicating a fabrication
problem during brazing of these parts. An improved procedure
for the manufacture was qualified and the critical parts
were replaced in both, the SNo. 2 and 3 Gyrotrons. The
SNo. 2 (repair) Gyrotron was delivered to FZK by end of
July, where an output power of 0.55 MW/30 min and
0.85 MW/3 min were achieved, respectively. Then a water
Wendelstein 7-X
55
leak opened at the cw dummy load in the test stand, which
prevented further long pulse testing. The gyrotron was
shipped to IPP, where the FAT will be continued in January.
As short-pulse testing is still possible in the FZK test stand,
the Gyrotron SNo. 3 (repair) was installed by fall of the year
and short pulse testing has started. The Gyrotron SNo. 4 was
also equipped with the improved beam tunnel and passed
the Factory Acceptance Test (FAT) at FZK successfully
(0.5 MW/30 min, 0.91 MW/3 min). The gyrotron was then
transferred to IPP-HGW for the site acceptance test (30 min),
where 0.83 MW were achieved for 8 min, as the tests had to
be stopped, because the rf-beam parameters showed some
deterioration after a cooling failure. The gyrotron was
shipped to TED for inspection. The W7-X gyrotrons are
optimized for single frequency operation at 140 GHz. As the
gyrotron diamond window has a resonant thickness of 4λ/2
at 140 GHz, it is, however, also transparent at 105 GHz corresponding
to 3λ/2. Two modes, the TE 21,6 (103.6 GHz) and
TE 22,6 (106.3 GHz) exist in the vicinity of the desired frequency.
Both modes could be exited by tuning the resonant
magnetic field and adjusting the operation parameters
(I beam =40 A and U acc =62 kV). We have focused on the
TE 21,6 -mode operation, because the output beam was almost
perfectly centered on the output window, whereas the beam
from the TE 22,6 -mode was located somewhat off centre. As
seen from figure 27 (above), a maximum output power of
about 0.52 MW was achieved without collector voltage
depression corresponding to an efficiency η=21 %, which is
slightly higher than the theoretical prediction of η =17 %.
The output power drops with increasing depression voltage
while the efficiency increases from 21 % to 27 %. The corresponding
collector loading at 0 and 8 kV depression voltage
is 1.9 and 1.7 MW, respectively, which is incompatible
with the collector-loading limit of 1.3 MW. Thus only operation
with reduced beam current around 34 A (about 400 kW)
can be handled safely by the collector. The rf-beam was
transmitted through 7 mirrors of the quasi-optical transmission
line into a calorimetric cw-load. Transmission losses
of about 20 kW were measured, which compares well with
the transmission loss fraction at 0.9 MW, 140 GHz operation.
It is worth noting, that both the beam matching mirrors
as well as the set of polarizers can be used without modification.
The larger beam size at the lower frequency is expected
to introduce slightly higher losses as compared to
the nominal frequency. On the other hand the atmospheric
absorption is somewhat lower.
Assuming, that all series gyrotrons behave similar to the
Prototype, ECRH for W7-X will be operated as a two-frequency
system. As sketched in figure 27 the operation range
of experiments can then be extended towards different resonant
magnetic field of 1.86 T (X2 and O2 mode) and 1.25 T
(X3 mode), respectively. Once plasma start-up could be
achieved with X3-mode, which is not clear yet, operation at