In inertial confinement fusion, it's a very small volume for a very short time. But this is magnetic confinement, which means it's a pretty large volume (probably cubic meters in this one), and they want to keep it going as long as possible (I think 30 seconds is the record so far). But the plasma is very thin; the atoms move really fast but there aren't many of them, so the actual amount of heat isn't remarkable.
