ArcJet Systems is a large military base located on a hill in the center of the Fort Hagen Sector. It consists of a small area on the surface and a complex of a few dozen rooms within the bunker. The whole base is filled with synths.
The best way to visit the base is with Paladin Danse as a part of the Call to Arms quest. He'll help you get through all of the rooms and get rid of the enemies. Meanwhile you'll be able to search the rooms, chests and bodies.
The complex contains quite a lot of safes and terminals with very high levels of security. You won't, however, find anything interesting inside, just ammo and drugs. Usually there's a terminal near a difficult safe that lets you open it. You have a choice - use lockpicking or hacking.
One of the rooms contains Tesla Science magazine that increases energy weapon critical hits by 5%.