In order for fusion to occur in the very hot gas – or plasma –created inside JET, the plasma must be heated to temperatures in excess of 150 million degrees Celsius. In order to achieve this, the plasma is actively held away from the walls of the tokamak container by using powerful magnetic fields.
However, it does sometimes touch the walls: what material could withstand that temperature? The key here is that there is only a very small amount of plasma there ( ~0.1 g). So although it is exceptionally hot, this is counteracted by the very small amount, compared with the wall, which is many hundreds of tonnes in mass. Therefore the wall can withstand impact without getting seriously damaged. Earlier, carbon inner walls were employed because of their heat resistance. However, now beryllium and tungsten tiles have been installed inside JET, which do not contaminate the plasma as much as carbon.