How can the energy density be low while the heat is high?
When such a structure collapses, the air will rush in and the heat will create a raging fireball storm, would it not?
Not sure what you're referring to.
If you're referring to the earlier discussion of "what would happen if the plasma got out," which I was addressing, the temperature in the plasma (that is, the velocity of the ion cores that are flying around in the plasma) is quite high, a hundred million degrees C worth. But their density is low, much lower than that of the ambient atmosphere.
The total amount of matter involved is small, just a few milligrams. If the plasma could "get out," it would immediately encounter ordinary cold matter (the air, or the walls of the fusion chamber) and be cooled down to ordinary temperatures. Any nuclear reactions would immediately cease.
That's one of the appealing differences between thermonuclear energy sources and ordinary nuclear power reactors.
In an ordinary nuclear reactor, the amount of fuel present is measured in pounds, or even in tons, in the case of commercial, utility reactor plants. The amount of energy in the reactor vessel is enormous, and it requires careful control and monitoring to make sure that it is only released in a (comparative) trickle.
But in a "hot fusion" reactor (like a tokamak, for example) the amount of fuel present in the reaction chamber is a tiny amount of gas (deuterium, tritium, etc.) at a fairly high vacuum. The amount of energy that can be released on any one "shot" is not that much; as I said, on the order of that which would be released by a few pounds of gasoline. A large explosion, but the containment vessels are also large and very well built.
I have seen pictures of serious (like melting) damage to the inner surfaces of tokamak reactors, but I'm not sure if those were caused by the plasma "getting away" (which I doubt) or by a sudden "quench" of the superconducting solenoid coils, which can cause a violent (explosive) release of energy. But that would be energy from a collapsing magnetic field (the plasma confinement field), not from any sort of thermonuclear plasma escape.