Steins;Gate: Youen no Valhalla
遥遠のヴァルハラ
The Distant Valhalla is a side story set in the Alpha Attractor Field. It is set 1 year and a half after the Rounder Raid on the lab, and is about "Operation Valhalla", Okabe's plan to escape from SERN's captivity.
(Source: Steins;Gate Wikia)