Source of contamination in damage-test sample and vacuum

Contamination of optics observed in LFEX compression chamber was a critical problem for maintaining high damage threshold and high optical performance for mirrors and gratings in the vacuum environments. We conducted a study for understanding this problem, and we found important knowledge on the nature of contamination, namely materials of contaminants, source of contamination, the invasion mechanism, and removal method of contamination. We also found the samples for the damage test is easily contaminated in the storage environment. This means the optical coating accumulate contaminations even in the air. So we tested in-situ damage test in a gas flowing chamber with controlled contaminants. The degradation was time-dependent phenomena, and proportional to the vapor pressure of contaminants. Several materials were tested, and even in water vapor, the damage threshold was decreased about 10%. We also found out two methods for removing contaminations from the coatings. According to these studies, our conclusion is special treatments should be used for knowing the intrinsic damage threshold of the coatings.