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Materials that line the fusion chamber walls must withstand extreme temperatures of more than 100 million degrees and be robust enough to maintain structural homogeneity when coming into contact, or interaction, with the plasma. The effect of the plasma on the wall components, or plasma wall interaction (PWI), is an important focus of fusion research. While some progress has been made, continued support for research and development (R&D) will be needed to ensure that a solution is found in time for the operational period of the ITER experiment.
The co-operative research experiments conducted under the auspices of the Implementing Agreement for a Programme of Research and Development on Plasma Wall Interaction in TEXTOR (PWIT IA) seek to understand the PWI phenomena and to identify materials. Specifically, this includes erosion and deposition behaviour of wall materials; fuel recycling, retention and removal from deposited material layers; control techniques to limit peak heat loads; transport physics; modelling the transport in the edge plasma within three-dimensional magnetic structures; and development of plasma wall diagnostics relevant to the ITER project. There are currently three Contracting Parties.
Wall components made of tungsten will play a central role in future fusion reactors as it is the element with the highest melting point (3 422 °C). Most importantly, tungsten wall materials retain much less fuel gas compared with graphitic wall components. Nevertheless, tungsten interacts with the plasma and creates impurities that need to be minimised.
The chamber wall design for the ITER device includes tungsten tiles covered by a special magnetic field configuration known as the divertor. The divertor removes the outer boundary layer of the plasma away from the hot core and deposits it on collector plates, removing impurities which would have otherwise reacted with the walls of the vessel and resulted in structural damage.
TEXTOR1 is a highly specialised tokamak device that served as a test facility for the PWIT IA to run experiments on the plasma wall phenomena. On the basis of these experiments, a bulk tungsten divertor was designed for the Joint European Torus (JET). The JET divertor replicates the design of the ITER wall and is in the second important phase of testing. The JET tungsten divertor has been successfully in operation since August 2011, performing closely to the design specifications for ITER.
The JET experimental campaign showed that oxygen and residual carbon were greatly reduced. The erosion of tungsten from the new wall structure was found to be substantially lower than traditional wall component materials such as graphite. Despite the absence of active cooling, the solid tungsten tiles were able to withstand a power density of 7 megawatts per square metre (MW/m2) to 10 MW/m2 and a total deposited energy of 60 megajoules per square metre (MJ/m2) in spite of the absence of cooling.
These results show a positive experience with the ITER wall, an important step forward for a successful operation of ITER.
1. The Tokamak Experiment for Technology Oriented Research (TEXTOR), operated by Forschungszentrum Jülich (Germany).
* Photo courtesy of the European Fusion Development Agency Joint European Torus.
For more information: www.fz-juelich.de/fusion
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