Putting the heat on components
High-power test facility now in operation at IPP / Loading tests for Wendelstein 7-X and ITER
Europe’s most modern facility for testing large, heat-resistant components used in fusion devices is now in operation at Max-Planck-Institut für Plasmapyhsik (IPP) in Garching near Munich. The special feature of GLADIS (Garching Large Divertor Sample Test Facility): The heat test facility is suitable for investigating not only small samples but also large components with their own water cooling.
The energy for the heat tests is supplied by two powerful particle beams: Fast hydrogen ions deposit powers of up to 90 megawatts per square metre of test sample in pulses of up to 30 seconds. The reaction of components to high loads is recorded by numerous measuring facilities. The first task of GLADIS will be to test components for the Wendelstein 7-X fusion experiment, now being built at Greifswald Branch Institute of IPP. The facility can subsequently be used for preparing the ITER international test reactor.
The aim of fusion research is to develop a power plant which, like the sun, derives energy from fusion of atomic nuclei. In order to ignite the fusion fire the hydrogen plasma fuel has to be heated to temperatures exceeding 100 million degrees. As the plasma is confined by a magnetic field to avoid contact, it is only cooled fuel that impinges on just a few specially equipped areas of the vessel wall. However, in fusion devices of the next generation, such as Wendelstein 7-X and ITER, the thermal load on these wall regions will be appreciable: Powers of up to 10 megawatts per square metre, with transients much higher, are expected. The development of special water-cooled cladding for these wall regions, the so-called divertor, is to be supported by tests on the GLADIS heat test facility at Garching.
“We had the good fortune,” states project head Henri Greuner, “of being able to enlist existing powerful components for building GLADIS and thus save costs.” For example, the beam sources forming the core of the test facility were taken from the heating system for the plasma of the Wendelstein 7-AS fusion device, shut down three years ago. That is also where the power supply came from; the water-cooled acceleration grids originate back to ASDEX, shut down in 1990. The approx. 350,000 euros invested therefore essentially applies to the vessel and the vacuum, diagnostic and cooling facilities.”
The powerful ion sources form the core of GLADIS: From neutral hydrogen gas they produce positively charged hydrogen ions and accelerate them with grid-shaped electrodes to high velocities. Three metres behind the acceleration grids the two arm-thick particle beams impinge on the test sample to be investigated and deposit heat powers of 90 megawatts maximum in pulses lasting up to 30 seconds. The configuration is enclosed in a vacuum chamber. The front section of the separable steel vessel accommodates the technical equipment – ion sources, vacuum pumps, water cooling, and the infrared and video cameras – while the sample chamber is located at the back. Connected through numerous ports are measuring facilities which record up to 40 measuring signals, including temperature profiles of the sample and the velocity and temperature of the cooling water. Any damage to a component can thus be identified whenever it arises and – in conjunction with metallographic investigations or electron microscopy in the laboratory – the mechanical strength, material fatigue and thermohydraulic behaviour can be exactly determined. “The possibility of investigating actively cooled components up to two metres long that are subjected to high cyclic loads makes the IPP test facility the most modern of its kind at present in Europe.”
The main task in the next few years will be heat load tests for the Wendelstein 7-X fusion experiment, now being built at Greifswald Branch Institute of IPP: The entire construction phase is to be accompanied by series testing of the protective elements for the plasma vessel – individual tiles and complete, water-cooled divertor modules – under the later operating conditions. Besides providing quality control, it will thus be possible to specify exactly the requirements to be met by the cooling. For the tests one ion beam with heating powers of 5 to 12 megawatts per square metre will be sufficient. When it comes to the larger ITER components, which can be shifted in the beam path by remotely controlled manipulators, both beams can then be used.