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Non-nuclear energy


Fission and radiation protection

JET is the largest tokamak in Europe © Image: JET
In tokamaks, plasma is confined in a toroidal vessel by a magnetic field with two main components. 

The first (toroidal) field is produced by a set of coils equally spaced around the reactor vessel, and keeps the plasma away from the walls. However, this field is not enough to confine the plasma by itself. 

A second (poloidal) field is added to counteract the natural pressure inside the plasma which tries to make it expand. In a tokamak, the poloidal field is generated by the large plasma current which is also used to heat the plasma. In a large device like JET, this toroidal current is about 3 to 6 million amps and is induced by transformer action – the plasma acts as a secondary winding. The resulting total magnetic field is ‘helical’ around the toroidal direction. Other magnetic field components are generated by further coils to shape and position the plasma in the reactor.

Tokamaks work in a pulsed mode but are capable of reaching steady state conditions by using additional current drive systems. Once the fusion burn is initiated external heating can be reduced as the desired operating value for power amplification Q (= fusion power out / input power to the plasma) is reached. The heating systems can then be used to ‘push’ the plasma electrons in the toroidal direction, creating a current independent of the transformer.

When fusion conditions are reached, alpha particles resulting from the fusion reaction will also heat the plasma. In a commercial power reactor, nearly all of the heating needed to compensate plasma energy losses will come from the alpha particles, with just a low level of external heating to fine-tune the plasma performance.

The tokamak design, developed by Sacharov and Tamm in Moscow in the 1960s, has been most successfully demonstrated in JET, the Joint European Torus experiment. JET has a plasma volume of approximately 85m³, the central part of which reaches fusion temperatures of 100–200 million °C. In 1997, over 16 MW of fusion power was generated by JET.

Complex magnetic fields confine the plasma

In addition to JET, the world’s largest tokamak, there are a number of medium - sized and small tokamaks  in Europe being used to provide additional important experimental data.