The fifteen meters long experiment Magnum- PSI. The white chamber on the left hand side is the in-vacuo analysis chamber, which will dock onto the target chamber enclosed by the wooden mock-up of the future superconducting magnet. The manipulator arm carrying the wall material will reach through the analysis chamber into the target chamber. Credit: FOM Institute for Plasma Physics Rijnhuizen

Researchers and engineers at the Dutch FOM-Institute for Plasma Physics Rijnhuizen have recently reached an important milestone: They completed the vacuum system of the new experiment on Plasma Surface Interaction, Magnum- PSI, and found that it behaves exactly as it was designed. Scientists want to use Magnum-PSI to investigate the interaction between the reactor wall and hot plasma in future fusion reactors, such as ITER.

In designing a fusion reactor, it is essential to understand the effects that the hot, magnetised plasma has on the structure of the wall material. Furthermore, the wall surface will erode under these stressful conditions and possibly release material into the plasma. This will affect the plasma conditions and behaviour. The Magnum-PSI experiment is unique because it is specifically designed to study plasma-wall interactions in the ITER-relevant regime. It allows scientists to create specific plasma conditions and to use diverse wall materials. The experiment will allow studies of dust formation, re-deposition, migration, and the possible absorption of hydrogen into the wall material (retention).

In the linear steady state experiment, the plasma is also heated by microwaves and focused by a superconducting magnet into a 10 cm wide beam. This beam size is necessary to ensure that the experiment captures the various effects, which may occur near the wall in a fusion reactor – for instance, ensuring that the eroded wall-material has sufficient time to interact with the plasma before leaving the beam. A manipulator arm brings the wall material samples into the plasma beam. An analysis chamber is connected to the target chamber and is equipped with various tools used to investigate the structure and condition of the wall material without releasing the vacuum. The plasma itself is analysed through viewing ports just in front of the target area. The following factors are of interest: Its general condition, density and temperature as well as, for example, potential areas with different concentrations of ions and electrons. The aim is both to investigate the proposed wall materials for ITER, and to experiment with more advanced materials that could be used in DEMO, ITER’s planned successor. Furthermore, in the Magnum-PSI facility, numerical models will be validated against controlled and well-diagnosed experiments. The research is intended to contribute to the design of the divertor, the part of the reactor in which plasma comes into direct contact with the wall material and where the fusion product helium leaves the vacuum vessel. Construction of Magnum-PSI will be completed at the end of 2009, and the first experiments are expected to start early 2010.

Contribution of Gieljan de Vries, FOM-Institute
for Plasma Physics Rijnhuizen