When you're looking under the Ark installation, like the city management screen, it's an option under the "Decisions" tab. If you have the activation index tech researched, you can make use of it.

Originally posted by Sly’sMind:

Same as title. I have the ark and a few halo rings built, but I cant seem to get the research to come up. Am I missing something

The technology to use the Halo Array from the Ark requires the Clarion Network research which is separate from the Activation Index research. You can only get the Clarion research option by having the Activation Index tech researched as a prerequisite and owning the Ark (the weight modifier for the research is tied to the Ark Citadel deposit).

To sum it up: Own the Ark > Research Mega Engineering (if you don't have it already) > Research Halo Activation Index (requires mega engineering tech researched and at least one Halo Installation controlled) > Research Clarion Network for remote activation from the Ark.

For reference, the Ark origin should provide you with all the necessary tech to build and activate individual Halos except for the Clarion Network research. The decision to fire Halos is in the planet decisions tab on that Halo Ring or Ark megastructure.

Both Halo Array research options are rare Tier 5 physics research, so it'll take some time for it to come up.

im having an issue where the rings in half the galaxy will go off, but the other half won't. know anything i can do to fix it. its just a bummer because i was going to use it to kill the flood, but it won't turn on in their side of the galaxy or in the territory of the forerunner as well.