There's most likely not even a risk to the experiment itself from runaway plasma. Experiments like these use very low-density plasma (mBar) so despite being extremely hot, it's a fairly low mass overall of heated plasma. These experiments regularly lose deconfinement of the plasma within the magnetic fields and it collides into the inner walls of the tokamak and cools back down. This is the main thing these experiments are designed to study: how to keep the plasma stable for longer. Even if there were a larger and hotter mass, we don't have the pressure from gravity here that makes fusion work in the center of stars, so it will just kinda melt things and then cool off, if that.