There's also the problem of getting rid of the ash. Conventional tokamaks have divertors at the base (and sometimes the top) to get the spent plasma out of the chamber. These plates inevitably come into contact with the plasma, and we just don't have the materials to withstand that yet. We're not unimaginably far off, but it's unsolved. To add to the fun, some of the more promising materials have the rather entertaining property of being explosive under standard conditions...