Proposal of a testing procedure to qualify ITER window assemblies and absorbing coatings exposed to high microwave stray radiation

In any large device for research on magnetic confinement thermonuclear fusion, the window assemblies are essential to preserve the conditions for successful experiments and to guarantee adequate access for inspection and measurement. In ITER, the materials traditionally used for the windows will be exposed to an exceptionally harsh environment. Moreover, the systematic use of tritium as fuel would make any failure of the primary vacuum containment a particularly dangerous accident. It is therefore essential to understand the potential threats to the integrity of the windows assemblies and define a series of tests to ensure their properties and quality before installation. One specific hazard to the windows is the microwave radiation due to either heating schemes or specific diagnostics. A fraction of the stray radiation incident on the window leads to dielectric heating, which causes a thermal load. The potentially harmful consequences of such a thermal load on the window assemblies are: (i) excessively high temperature (risk of bonding melting) and (ii) excessive, heating rate (risk of cracking due to a high temperature gradient at the location of the bonding). In this paper the main causes of degradation, which could lead to failures under microwave loads, have been identified. A series of laboratory tests have been defined, to assess the quality of the materials and the assemblies, including the coatings for the absorption of the microwave radiation in the ducts leading to the windows. Complete testing procedures and an overview of the main facilities, where the assemblies and materials could be qualified, are also provided.