100M seems insanely high, beyond what anything man made would be able to contain. Is it extremely short lived? Or over a very small area? Very interesting.
It is incredibly high. We hold it in not by any material, but by magnetic fields. Plasma has the handy feature of being magnetic. The Tokamak[1] design used in ITER and this example uses a whole heap of magnets to hold this plasma in a doughnut-shaped area.
Fundamentally it doesn't have to, because like a weight resting on the top of a ladder magnetic fields store energy and do not dissipate it. In practice it does because our ways of producing and maintaining strong magnetic fields require a lot of upkeep. This, like most things in fusion, is an example of where the realities of present day engineering are far cruler than the laws of physics.
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.
