Given the extremely high plasma temperatures that are required to sustain a nuclear fusion reaction in a device such as a tokamak reactor, how can such high temperatures be maintained in the presence of radiative heat transfer from the plasma to the walls of the reactor vessel?
This is a really big problem, for at least two reasons.
First, when trying to get the plasma energetic enough to support fusion, the plasma in response tries very strongly to radiate its heat away. That radiated heat has to be continuously replentished in order to hold the desired temperature.
Second, the plasma temperature is sufficient to rip apart anything you might want to build the containment vessel out of, which means the plasma has to be held away from the walls so they do not instantly vaporize. Magnetic fields are used for this purpose.