sides of the laser beam in this embodiment it may be possible to use an even smaller clearance many of the above described design features of window mount assembly 14 are designed to minimize the frequency of window changes needed during the operation of laser 10 however window mount assembly 14 is designed not only to minimize the frequency of window changes needed but also to facilitate the window changes themselves in order to minimize the down time of laser 10 caused by window maintenance to this end window 62 is mounted within a removable window cassette 58 as previously described the procedure for changing windows will now be described when a window change is to be performed pressure seal assembly 38 is moved from the open position to the closed position as indicated by directional arrows in fig4 rod 39 is then rotated to press o ring 42a against the surface of window mount 36 forming a pressure tight seal the gas contained in plenum 46 and cavity 48 is then vented to the atmosphere by means of feedthrough 36d preferably through a hood vent or some other containment mechanism due to the poisonous nature of the gas mixture.