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Aluminum oxide films are used as low refractive index layers in multilayer dielectric mirrors, which are expected to be used in ITER plasma diagnostic optical systems. In such mirrors, a film with a low refractive index is the outer layer. In addition to the normal operating modes of ITER, a number of emergency modes are assumed, one of which is the destruction of the water cooling system of the first wall, divertor or blanket. In this case, the vacuum chamber is filled with steam, the parameters of which depend on the location of the destruction and on the stage of operation of the reactor at which the accident occurs. The maximum steam parameters are set by a special system that limits the pressure to 150 kPa, and the maximum temperature of 250°C can be the case when an accident occurs during vacuum training of the chamber. The work examines the interaction of steam with amorphous films of aluminum oxide deposited on glass of the K-9 grades by reactive magnetron sputtering. It has been shown that exposure of a film 300 nm thick at a temperature of 250°C and a steam pressure of 150 kPa for 2 hours is accompanied by intense hydroxylation of the film and the transformation of the entire oxide film into hydroxides. This leads to severe degradation of light transmission, which may cause a change in the optical properties of the dielectric mirror. As a next step, similar steam exposures are planned of the mirrors in which with a low refractive index layers will alternate with layers of oxides with a high refractive index, such as hafnium, tantalum, and zirconium oxides.
Gorodetsky et al. (Mon,) studied this question.