How much NCR infamy do I gain from arming ARCHIMEDES?

Solution 1:

Wearing a disguise will avoid the infamy. Toss on your finest toga, yell out your loudest "AVE CAESAR!" and let toast those NCR pigs.

I can smell the bacon from here.

Solution 2:

You can have access to the laser without killing the NCR at Helios One. When you configure the power grid, you can select from several different grid configurations, including one named "Archimedes II". Selecting this option arms the orbital laser for your use. Independent of which configuration you choose, you can also select the option "Arm Archimedes plant defense system". It is this second choice which kills all the NCR at Helios One.

Just choose "Archimedes II" for power output, and don't choose to arm the plant defenses, and then you can have your awesome orbital laser.