How hot will be the wall in a fusion power plant?
The ignition and burn temperatures of a fusion power plant are about 100 million degrees. This is how hot it is in the core of the plasma which, at the same time, is of extremely low density. The insulating effect of the magnetic cage causes the temperature to decrease outwards from the core. The plasma ring still has a temperature of 100,000 degrees at its edge.
The temperatures of the wall of the plasma vessel will, however, be very much lower. Depending on the wall material, they are cooled to 300 to 600 degrees by a coolant, viz. water or helium. Consequently, the temperature of the heat exchanger will also be between 300 and 600 degrees. The hot water or gas behind the heat exchanger acts as a conventional drive for a turbine and generator producing power. The higher the temperature of the coolant, the higher the energy conversion efficiency.
It is, moreover, remarkably cold behind the plasma vessel and shielding. This is where the coils are located, which build up the magnetic cage confining the plasma. These coils are superconductive and are therefore thermally well insulated from the rest of the device and cooled to a low temperature of almost absolute zero.