Co2 laser galvo needs a protective window

Hello!!!

I have a 150W RF Co2 Galvo Marking System, the galvo head is Gsi Lumonics fitted with a F-Theta Lens 140x140mm marking field and 200mm focal distance
and i what to use this system cu make a SLS/SLM 3D Printer.
I made a project of the hole setup i bought almost all the component's.
If your curios of my project you can see detail's here 403 - FORBIDDEN and here 403 - FORBIDDEN
Here is a YouTube video of a similar machine so you better understand what I mean https://www.youtube.com/watch?v=9E5MfBAV_tA

Ok.... to go back to why I'm here!

As a start, I'm a novice when it comes to laser optics.

The hole process of selective laser sintering/melting of plastic/metals needs to take place in a gas tight chamber (oxygen free, av' can oxidase or worse explode)and filed with nitrogen or argon, so my problem is how to deliver the laser marking beam in the chamber. the answer is a ZnSe Window Lens or ZnSe Protective Window Lens (don't know if it's a difference, maybe someone from here can enlighten me). So i think!!!
My question is what tip of ZnSe Window is better and what diameter, regarding the fact that this window will be mounted in the top plate of the chamber and after the F-Theta Lens (i have the possibility to place it at almost 5mm from the F-Theta) [laser ¬> galvo mirrors ¬> f-theta ¬> znse window ¬> marking zone].
One more question: It will influence the laser beam? Considering that will enter through ZnSe Window at different angles.
More question to come!

Thanks in advance!!!!