I've been looking for an answer to this question for some time, and I apologize in advance for my lack of general knowledge on the subject matter. I can't wrap my head around how we can have temperatures in a fusion reactor or high-energy density experiments without it creating some sort of catastrophe. How can these temperatures exist on Earth and not destroy everything within a thousand miles?
So first of all, in an experimental fusion nuclear reactor, even though the plasma reaches temperatures of millions of degrees, the amount of material at this temperature is very low, so that in case of the worst possible scenario, the heat will be absorbed by the reactor with not much problem. Also, when the hydrogen or gas in the reactor reaches high temperatures, the gas transforms to a plasma state, where each atom loses the electrons. This plasma contains both ions and electrons that they cannot recombine due to the high temperatures. Because the plasma is positively charged, with the help of magnetic and electric fields, the plasma can be contained at the center of the reactor, without touching the walls or any material, just vacuum with the plasma flowing through the center of the reactor
High temperatures do not by themselves mean highly destructive. Temperature is simply a statistical parameter for a population of molecules. If that population is relatively small (and the amounts of the plasma in question are small compared to everyday, kitchen-cooking-recipe amounts of molecules), then their total energy and destructive power can still be small.
Moreover, not everything of high temperature transfers its potentially destructive energy to its surroundings with the same efficiency. High speed gas or plasma molecules don't tend to transfer much of their energy to solid containers when they collide; the effect is rather like an elastic collision between a small body and a very massive one - negligible energy gets transferred to the latter. Plasmas tend to be like highly diffuse gases. Energy coupling between high temperature solids is much swifter - you could poke your head for a few seconds into a flask of air at 500C and as long as you didn't breathe in, your face would simply feel slightly singed. In contrast, if you firmly grasp an iron rod at 200C for even a fraction of a second, you're going to need to urgently visit the local burns unit at your hospital with deep burns that may even cost you your hand.