That addresses the concern about the fusion reaction. But when the plasma melts a hole in a building-sized vacuum chamber and air suddenly rushes in, I expect

That addresses the concern about the fusion reaction. But when the plasma melts a hole in a building-sized vacuum chamber and air suddenly rushes in, I expect that you'll get a big implosion, which will ricochet off itself and become an explosion, like if you punched a hole in a CRT with a nail. The article talks about a solenoid that exerts more force than the Space Shuttle does to lift itself to orbit. When the vacuum chamber deforms the solenoid, that energy is going to go somewhere. Plus, they mention gigajoules of electricity going through superconducting wires. I expect that the vacuum chamber implosion would destroy them before they heated up and no longer became superconducting. So instead of gigajoules going through essentially giant resistors and heating up something fierce, that energy will do something else, not sure what. I don't think this will destroy a mountain, but I would expect the reactor and building to be catastrophically destroyed by the vacuum chamber implosion, along with some, shall we say, interesting, but certainly powerful, effects from the solenoid and other magnets.