Brightly glowing plasma inside the MAST tokamak.

Most tokamaks are not attempting to ignite fusion. They create plasma from hydrogen, deuterium or helium and explore its properties, but for most experimental devices, fusion itself is not the primary goal. Deuterium-tritium fusion requires radioactive tritium and produces high-energy neutrons that create radiation hazards and activate materials in the experimental machine. After significant D–T operation, activated in-vessel components may no longer be accessible for direct human maintenance, so remote handling systems are required. Additional radiation shielding and protective systems are also needed, increasing the complexity and cost of the facility.

For experiments without fusion, obtaining fuel for tokamaks is straightforward — plasma can be created from hydrogen, deuterium or helium.

Hydrogen isotopes.

Hydrogen

Hydrogen is an abundant element that can easily be obtained from water by electrolysis. Water also contains a small amount of the heavy hydrogen isotope deuterium, whose nucleus consists of one proton and one neutron. About one in every 6,400 hydrogen atoms is deuterium.

Helium

Helium is also readily available and can be used as a working gas in plasma experiments. In any case, tokamak fuel consumption is very low — even in large machines, only a few grams of fuel are present in the plasma at any given time.

Deuterium and Tritium

For D–T fusion, deuterium and tritium are used in approximately equal proportions. Of these two isotopes, tritium is much more difficult to obtain. The present global tritium inventory is only of the order of a few tens of kilograms. Tritium is a radioactive isotope of hydrogen with one proton and two neutrons in its nucleus and a half-life of about 12.3 years. It is extremely rare in nature, and any stored inventory gradually decreases through radioactive decay.

Existing tritium resources would be insufficient to supply a fleet of D–T fusion power plants, so future reactors will have to breed their own tritium. Neutrons produced by D–T fusion can react with lithium in a breeding blanket to form tritium. Some blanket concepts also use neutron-multiplying materials, such as beryllium or lead, to increase the number of neutrons available for tritium production. Tritium will be bred in a lithium-containing blanket surrounding the plasma, then extracted and recycled as fuel. The development of reliable breeding blankets is therefore a key area of fusion research; ITER will test four different Test Blanket Module concepts during its deuterium-tritium operation.

Fuelling

Tritium.

Fuel is commonly introduced into the vacuum vessel as a gas. Before fuelling, the vessel is evacuated to minimise impurities. The gas is then ionised and heated by the plasma current and external heating systems to form hot plasma. During a discharge, additional fuel can be supplied by gas puffing or by pellet injection, depending on how deeply the fuel needs to penetrate into the plasma.

Neutral gas introduced from the vessel wall is rapidly ionised near the plasma edge, so gas puffing mainly fuels the outer plasma. To deposit fuel deeper inside, small cryogenic pellets of frozen hydrogen isotopes can be injected at high velocity. As a pellet penetrates the plasma it gradually ablates, releasing fuel particles well inside the plasma column.

Deuterium–Tritium Fusion

So far, only two experimental tokamaks have conducted deuterium–tritium campaigns: TFTR in the United States and JET in the United Kingdom. These experiments provided important information about the behaviour of fusion-born alpha particles, plasma self-heating, high-energy neutron production and the activation of machine components.

ITER, the world’s largest experimental tokamak under construction, will also operate with deuterium-tritium fuel. Its D–T experiments are expected to provide essential data on burning plasmas, alpha-particle self-heating and reactor-relevant technologies for the development of future demonstration fusion power plants.