JET’s fuel is hydrogen isotopes. Most often it is deuterium only, but occasionally it is mixed with tritium. Tritium is very rare, expensive and complicated to handle because it is radioactive, but the most potent fusion fuels is a 50:50 mix of deuterium and tritium. Occasionally other related fuels are used as well – for example hydrogen, which is lighter, and helium which is heavier.
The fuel is injected into the vessel in a number of ways. Firstly simply as gas at the beginning of the pulse, just before the discharge is created, which heats the gas until it becomes a plasma.
The second method of fuelling JET is with the neutral beam heating system, which sends extremely hot fuel into the plasma. Doubly effective, it provides fuel and heat to the plasma.
Strangely the third way the plasma is fuelled is with frozen pellets, at around -260 degrees Celsius. The reason these are useful is that they penetrate further into the centre of the plasma before being ionised and swept into the plasma flow. A serendipitous side effect is that the pellet entering the plasma causes a small burst of turbulence, which is key to controlling larger, less predictable turbulent events.