So I assume you are talking about magnetic devices: the core temperature is a lot higher than the outer edges but the answer is, nothing. No material can withstand those temperatures plus if you did let your plasma touch an outer wall it would cool down. So that's why they contain the plasma with magnets, this holds it away from the walls (which are either carbon in old machines or beryllium/tungsten in new machines) and stops it from destroying the vacuum vessel.

Even then though when there are large instabilities or runaway electron beams the walls get damaged sometimes.