An apparatus and method is disclosed which may comprise a high power excimer or molecular fluorine gas discharge laser DUV light source system which may comprise: a pulse stretcher which may comprise: an optical delay path mirror, an optical delay path mirror gas purging assembly which may comprise: a purging gas supply system directing purging gas across a face of the optical delay line mirror. The optical delay path mirror may comprise a plurality of optical delay path mirrors; the purging gas supply system may direct purging gas across a face of each of the plurality of optical delay line mirrors. The purging gas supply system may comprise: a purging gas supply line; a purging gas distributing and directing mechanism which may direct purging gas across the face of the respective optical delay path mirror.