I forgot to address this in my earlier comment, but you don't worry about the plasma escaping -- it is very delicate, generally orders of magnitude less dense t

I forgot to address this in my earlier comment, but you don't worry about the plasma escaping -- it is very delicate, generally orders of magnitude less dense than air. Rather, keeping it alive is the difficult part. If the plasma hits the wall of a fusion device in an uncontrolled manner, it will dump all its energy into it. If the plasma picks up too many impurities, it will radiate energy away in the form of bremsstrahlung. It is hard to keep the ions hot enough to fuse while there are many phenomena conspiring against you.