The main functions of RF windows in fusion reactors are: to act as a first barrier for tritium and radioactive dust, and to provide vacuum containment. They must have low RF losses. The lifetime must be satisfactory (>5 years); nevertheless maintenance by remote handling must be foreseen. Valves or shutters are needed to improve safety. The tokamak windows will be placed in a screened area where they will receive a maximum dose of about 1016-17 n/cm2 over their life, to avoid degradation of their dielectric and mechanical properties. The coolant must be an inert fluid to avoid damages to other materials of the machine in case of leakage. In ITER , the most demanding working conditions are those of the windows of the Electron Cyclotron (EC) system which should support 1 MW, CW in the 120 - 140 GHz range (probably at fixed frequency) in a gaussian or HE11 mode. In the Lower Hybrid (LH) system the windows will have to transmit 1 MW, CW at about 5 GHz.
L. Rebuffi, L. Rebuffi,
"High power RF windows in fusion reactors", Proc. SPIE 1576, 16th International Conference on Infrared and Millimeter Waves, 157666 (1 October 1991); doi: 10.1117/12.2297935; https://doi.org/10.1117/12.2297935