Hello Commander. The GKP-3 Stealth Field Generator artifact increases the defense of your Interceptors by 20%. Imagine having two of them!! 40% extra defense and it doesn't matter where the Interceptors are located the extra def works everywhere. Also in Emitters so you will contribute great defense to your clan for example. The Offense power drops 60% but lets be honest NO ONE ever uses Interceptors in Offensive actions. So I think you know my answer by now :) 

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Oh wow, you can have MULTIPLE of them? Ha, nice! Thanks. :)

Yes Commander you have 2 slots to place them.

Good luck!!!

Yep.  It's the strongest artifact in the game by far, since interceptors are the main defensive unit that most high level players use.  For a player with a large army, having 2 of those artifacts can mean hundreds of millions of extra defense power.