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Plasma physicists combine three parameters – temperature, density and time – by multiplying them together to form what is known as the fusion product or triple product. At a certain value of the fusion product, called ignition, the reaction becomes self-sustaining: the heat generated by the reaction is enough to keep the plasma hot and so the external heating systems can be turned off. For deuterium-tritium fusion this value is about: nτT ≥ 5×1021 m-3 s KeV. JET has reached values of nτT of over 1021 m-3 s KeV.
Deuterium-tritium fusion reactions require temperatures in excess of 100 million degrees. To achieve these remarkable temperatures, three separate heating systems are usually used in tokamaks, each capable of delivering well over a million watts of power to the fuel. Together they generate and sustain plasma that is easily hot enough for the high energy collisions required for fusion to occur.
The constituents of a 100 million-degree plasma are moving about really fast, and, if left alone would soon be so far apart as to render collisions extremely unlikely. To keep the density of the plasma high enough to ensure collisions do actually occur, the plasma vessel is surrounded by huge electromagnets. These create magnetic fields 10,000 times stronger than the Earth’s magnetic field and confine the plasma to perpetually circulate within the ring-shaped vessel. However if the plasma gets too dense then collisions of a different kind – between nuclei and electrons – begin to create large amounts of radiation. This radiation, called bremsstrahlung, saps energy from the plasma and prevents fusion from occurring – the optimum density value is around one millionth of the atmosphere.
80% of the fusion energy is carried away by the neutrons, but the 20% carried by the helium nuclei remains in the plasma. The newly formed helium ricochets around the vessel colliding with unburnt fuel nuclei, heating them up, thereby reducing the need for the external heating systems. However it takes time for this to happen – depending on the density and temperature of the plasma. The length of time for which particles are confined within the plasma is denoted by τ (the Greek letter tau). Typical current values at JET are on the order of a second, and at ITER should be around four seconds.