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

and distributes them via CVD-diamond vacuum barrier windows
to individually movable antennas (launchers) in the
W7-X torus. Two symmetrically arranged MBWGs are used
to transmit the power of all gyrotrons. In 2007, a major work
package was the design completion and manufacturing of
the transmission system near the torus. This comprises the
reflectors type M13 and M14, which will be installed in two
“towers” in front of the W7-X ports.
The fabrication of both towers was finished, and the installation
of control systems for reflectors and launchers, support
structures, and granite absorbing plates has started.
Both towers, one of them is seen in figure 28, have to be
stored until W7-X assembly is completed and access is provided
for the installation at their final position in the torus
hall. In the meantime, retro-reflectors were mounted in the
beam-duct in the image plane at half distance of the MBWG
transmission line. Full distance transmission can be simulated
and tested by this arrangement. First calorimetric high
power measurements of the two-pass transmission efficiency
of the first part of the MBWG were performed, yielding a
loss of about 3 % for 10 reflections on the 2×3 MBWG-mirrors
and the 4 additional guiding mirrors, which is in good
agreement with calculations and low power measurements.
This result confirmes the high quality of the quasi-optical
concept for high power, long distance and low loss microwave
transmission.
8.1.4 In-vessel-components (IPP)
The design of the front steering ECRH-antennas in the Aand
E-type ports is compatible with full power cw requirements
and was finished in 2007. The CAD-drawing is
shown in figure 29. Most of the antenna components are
already manufactured, the assembly has started.
Figure 29: ECRH front steering antenna with bi-axially and individually
movable front mirrors
For the two optional high-field side launchers to be installed
in the narrow N-ports, it was decided to adopt the
remote steering scheme, which is based on the imaging
Wendelstein 7-X
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properties of a square corrugated waveguide. A movable
reflector at the waveguide entrance controls the direction of
the beam injected into the waveguide; at the exit of the
waveguide near to the plasma edge, the beam is launched at
the same angle. For the N-ports, a solution with a bent
waveguide was chosen. This eases the integration of the
waveguides and leads to a lower antenna beam divergence
due to increased waveguide cross-section, but requires
thorough optimization of the position of the mitre bend and
the vacuum valve in the waveguide. The conceptual design,
as well as first optimization calculations have been started.
The ECRH operation range can be strongly extended if
heating scenarios with the second harmonic ordinary mode
(O2) and the third harmonic extraordinary mode (X3) can
be applied. The first enables high plasma density, which is
necessary for efficient divertor operation, the second allows
operation at a lower magnetic field. An incomplete single
pass absorption in the range of 40 % to 80 % has to be
accepted for W7-X parameters. Specially shaped reflectors
made from TZM (titanium, zirconium, molybdenum alloy)
will be installed to avoid the thermal overload of the high
field side heat shield by the microwave shine-through.
After first preliminary high power test in 2006 the reflectortile
was redesigned and its surface was polished to reduce
losses. The watercooled reflector tile was retested in the
ECRH-installation at Greifswald with about 0.5 MW incident
power, which is the expected “worst case” loading for
W7-X. The calorimetrically measured absorption reached
values of 0.3 %, which is consistent with the theoretical
value deduced from the material properties and with previous
low power tests. The surface temperature reaches
470 °C and 390 °C are measured at the cooling plate,
respectively, after 200 s for the worst case, which is acceptable.
The knowledge of the beam position and power load
at the reflector tiles is essential for reliable X3- and O2operation.
An array of small pick-up holes will therefore be
integrated into the tiles to measure the exact beam position
and the single pass absorption. The microwave signal is
transferred by 4 mm copper waveguides towards the B-type
port in the W7-X vacuum vessel. The positions of the
120 pick-up horns and the routing of the related waveguides
were defined in close cooperation with the W7-X
KIP group.
8.1.5 Advanced components and ITER-related R&D
8.1.5.1 Improved e-beam power dissipation at the Gyrotron
collector
The peak power load of the electron beam at the gyrotron
collector is a limiting factor for the next generation cw high
power gyrotrons with 1.5-2 MW output power. The W7-X
Gyrotrons are equipped with a conventional vertical sweep
coil system, which moves the electron beam intersection with
the collector surface up and down in the vertical direction.