Microwave response of ITER vacuum windows

Diagnostic systems are essential for the development of ITER discharges and to reach the ITER goals. Many of these diagnostics require a line of sight to relay signals from the plasma to the diagnostic, typically located outside the torus hall. Such diagnostics then require vacuum windows that isolate the torus vacuum and, crucially, ensure containment of hazardous substances. While such windows are routine in many fusion experiments, ITER poses new challenges. The vacuum windows are safety important components class 1 that must withstand all ITER loads. As a consequence, in many cases double disk windows are used with modified frequency response as compared to single disk windows. ITER is a long pulse machine with 20 MW microwave heating installed, giving rise to gradual heating of windows due to stray radiation. The particular microwave heating scheme at ITER may also – in case of an erroneous polarization setting – result in a refracted beam with much higher power density. This paper looks at microwave aspects of ITER windows. The microwave response as a function of frequency is calculated for proposed arrangements. From this response the impact on diagnostic performance may be assessed as well as the thermal load on the window itself.