Evaluation of the airborne molecular contamination inside the LIL

Abstract Several instruments are sensitive to gaseous contamination. Indeed, telescopes, LIDAR instruments or high power laser optics may suffer from optical performance degradation due to the deterioration of the optics coating itself and to the accumulation of the contamination on the optics surfaces. The Ligne d’Integration Laser (LIL) is a prototype installation at scale 1 of one of the 30 lasers of the future Laser Mega Joule (LMJ). It is intended to validate technological choices made for LMJ and to prepare its exploitation. The facility will contain nearly 10,000 optical components and over 4000 m 2 of mirrors. Cleanliness will be an essential matter in the facility since contamination of optics can reduce their laser damage threshold. However it is possible to limit the negative effects of such a contamination, by identifying and controlling the sources: atmospheric pollution, outgassing from assembly materials, contamination during the storage and the assembly process and quality of the gas system. Specific operating and cleaning procedures can then be defined. This paper reports the first actions carried out in order to assess the airborne molecular contamination inside the LIL.