Plasma Sprayed Tungsten-based Coatings and Their Usage in Edge Plasma Region of Tokamaks

J. Matejicek (Sp), V. Weinzettl, E. Dufkova, V. Piffl, V. Perina, Academy of Sciences of the Czech Republic, Praha (Czech Republic) 
Tungsten is a candidate material for plasma facing components for ITER and other fusion devices. Plasma spraying is among prospective fabrication technologies, thanks to its ability to coat large areas and the possibility of in-situ repair. Several types of tungsten-based coatings were produced at IPP Prague, using water-stabilized plasma spraying. Their usage as plasma facing materials (covering different diagnostic tools) in the edge plasma region was tested at the CASTOR tokamak at IPP. Behavior of 0.4-2 mm thick plasma-sprayed surfaces and their influence on the discharge quality was studied. Plasma sprayed tungsten-based samples, solid tungsten and graphite were inserted into the plasma at various radii and exposed to tokamak discharges (30 ms pulse length, 30 kW ohmic heating). Moreover, positive and negative voltages were applied on inserted samples (edge plasma biasing) to increase electron and ion interaction with the sample surface. Broad scans over the biasing voltages (from -300 V to +200 V) and surface locations (from the plasma edge up to the core region) were performed. Afterwards, the sample surfaces were imaged by electron microscopy. Small craters in graphite and “bright branches” on sprayed tungsten samples were seen, probably as a consequence of unipolar arcing. Compositional changes in sprayed surfaces were also measured by RBS and ERDA methods. Hydrogen and Carbon deposition, and Oxygen content decrease were observed. Generally, pure tungsten coatings are suitable for covering different edge plasma diagnostics because of their low surface erosion and negligible influence on the discharge parameters.

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