Laboratory fusion is a rather delicate process, once described as 'containing a blob of jello with rubber bands'. The typical Tokamak fusion plasma is orders of

Laboratory fusion is a rather delicate process, once described as 'containing a blob of jello with rubber bands'. The typical Tokamak fusion plasma is orders of magnitude less dense than air, and if it hits the wall of the containment chamber, it rapidly loses heat and stops fusion-ing. The challenge is really to keep it going long enough and strong enough for it to be useful; there is no runaway mechanism.