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

3.5.1 Target modules
The ten divertor units of the high heat flux target plates are
designed to remove 10 MW convective stationary power
load. Each unit consists of an area of 1.9 m² which is loaded
up to 10 MW/m² and an intermediate area of 0.54 m² which
is loaded only up to 1 MW/m 2 . Each divertor unit is assembled
from several differently shaped target modules. The high
loaded target modules are made up from sets of bar-like target
elements which are supplied by cooling water in parallel and
fixed on a common frame. The supports of these modules
are adjustable within a range of a few millimetres to allow
for compensation of manufacturing and assembly tolerances
of the plasma vessel or uneven heat-loading during plasma
operation. Armouring of the higher-loaded area requires
890 target elements. Their surface must closely follow the
3-D shape of the plasma boundary and will be machined
before assembly of a module. 8 mm thick CFC tiles made of
SEPCARB® NB31, produced by SNECMA Propulsion
Solide, are joined to a water-cooled CuCrZr heat sink.
During the pre-series activities, two types of bonding technologies
were investigated: electron beam welding or hot
isostatic pressing (HIP). The delivered SEPCARB® NB31
material did not meet the specified mechanical properties;
the material was, however, accepted based on the results of
the tests of an additional pre-series. In order to optimize the
manufacturing route and to reduce the stress at the interface
between the CFC and the cooling structure, additional
18 target elements were manufactured by Plansee SE applying
different technologies. The different technologies comprised
the addition of a compliant copper layer between the
AMC ® interlayer and the CuCrZr heat sink by HIP (see
figure 11), the reduction of the size of the CFC tiles to 1/2
and 1/3 of the original tile width, and the rotation of CFC
tiles and hence of the orientation of the ex-PAN fibres by
90°. All these modifications were supported by numerical
simulations. Extensive high heat flux tests in the GLADIS
facility at IPP (see further information in section “Plasmafacing
Materials and Components”) confirmed the benefit of
the introduction of the compliant copper layer whereas the
other modifications did not bring a significant advantage.
However, metallographic examinations of the CuCrZr cooling
structure revealed that a central blind weld between the
cooling channels of the heat sink and the back plate gradually
degrades during the cyclic load tests in GLADIS. This failure
results in a by-pass flow between the cooling channels and
hence reduced heat transfer. As a consequence the design of
the cooling structure was improved and additional eight heat
sinks and ten target elements were manufactured. High heat
flux tests with the heat sinks of this pre-series in GLADIS
were successful; the temperature distribution along the target
elements fully agrees now with numerical simulations.
The procedure for repairing of damaged target tiles shall be
demonstrated by Plansee SE and will be qualified by testing
Wendelstein 7-X
36
in GLADIS in 2008. The results of the high heat flux tests
will also be used to correlate the GLADIS results with the
predictions from infrared measurements carried out in the
ARGUS facility of Plansee SE and in the SATIR facility of
CEA-Cadarache. The correlation shall be the basis for the
acceptance criteria of the target elements of the serial fabrication.
The design of the divertor modules of the lower
loaded area uses graphite tiles clamped on a CuCrZr heat
sink with a graphite paper (Sigraflex) interlayer. Cooling is
provided by stainless steel tubes brazed onto the backside of
the heat sink.
3.5.2 Baffle modules
The baffle modules prevent back-streaming of the neutralised
gas from the target plates. The design uses graphite
tiles which are clamped on a cooling structure made of
CuCrZr. Water cooling is achieved by stainless steel tubes
which are brazed to the backside of the heat sink. The fixing
screws for the graphite tiles are made from titanium zirconium
molybdenum (TZM) alloy. So far about 30 % of the
cooling structures have been manufactured by the IPP workshop
in Garching, and the second lot of graphite tiles has
been ordered.
3.5.3 Wall protection
About 70 m² of the plasma vessel surface is covered by
double-wall stainless steel panels with integrated watercooling.
The second series of 90 panels has been formed by
MAN DWE and is in the final stage of assembly. The channels
within the panels are formed by hydraulic pressing with
approximately 100 bar. In the areas, where ports have been
omitted the plasma vessel will be closed by appropriate steel
plates. Since manufacture of the wall panels is already well
advanced and most of the panels are already in different
stages of manufacture, some areas of the plasma vessel will
not be protected by wall panels during the first operation
phase. During later replacement of the divertor, also some
wall panels have to be replaced in order to also protect the
open spaces. . The inner wall of the plasma vessel is protected
by heat shields. These heat shields use graphite tiles which
are clamped to a cooling structure in a similar same way as
for the baffles. The design has to integrate also several plasma
diagnostic components as well as an NBI beam stopper and
a mirror for ECR heating. By the end of 2007 50 % of the
cooling structures have been fabricated in the IPP workshops.
During steady state and full power plasma operation,
the inner surfaces of the ports need to be protected in the
same way as the plasma vessel. For budgetary reasons, construction
of the port protection panels has been postponed to
a later date. Nevertheless, the design of these protection
elements was continued to fix their interfaces and define the
rooting of the cooling water lines. Since the space behind the
wall protection is very restricted, all port protection panels