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Understanding Fusion


JET-Inside the vessel
JET - Inside the vessel (Source: EFDA-JET)

In order to produce a self-sustaining fusion reaction, the tritium and deuterium plasma must be heated to over 100 million °C – this requires powerful heating devices and minimal thermal loss. To sustain such a temperature the hot plasma must be kept away from the walls of the reactor. However, because the plasma is an electrically-charged gas it can be held or contained by magnetic fields. This allows the plasma to be held, controlled and even heated by a complex cage of magnets, whilst enabling the neutrons to escape as they have no electric charge.

In a tokamak the plasma is held in a doughnut shaped vessel. Using special coils, a magnetic field is generated, which causes the plasma particles to run around in spirals, without touching the wall of the chamber.

‘Toroidal magnetic confinement fusion’ is the advanced technology that is the main approach for European fusion research and is at the heart of the ITER experiment. The reactions take place in a vessel that isolates the plasma from its surroundings it has a torus or ‘doughnut shape’ – essentially a continuous tube. The confining magnetic fields (toroidal and poloidal fields ) are generated by electromagnets located around the reactor chamber and by an electrical current flowing in the plasma itself. This current is partly induced by a solenoid at the centre of the torus which acts as the primary winding of a transformer. The resulting magnetic field keeps the plasma particles and their energy away from the reactor wall.

View of the plasma inside the tokamak MAST in the United Kingdom. (Source: UKAEA-Culham)

To achieve net fusion power output in a deuterium tritium reactor, three conditions must be fulfilled: a very high temperature greater than 100 million °C; a plasma particle density of at least 1022 particles per cubic metre; and an energy confinement time for the reactor of the order of 1 second. In order to control the plasma we need to understand fully its properties.

For example: how it conducts heat, how particles are lost from the plasma, its stability, and how unwanted particles (impurities) can be prevented from remaining in the plasma.

One of the major challenges in fusion research has been to maintain plasma temperature. Impurities cool the plasma and ways must be found to extract them. Plasma is heated by the electrical current induced by the transformer arrangement, but additional heating is needed to reach the high temperatures required. This includes the injection of beams of highly energetic fusion fuel particles (deuterium and or tritium) which, on collision with plasma particles, give up their energy to them, and radio-frequency heating where high-power radio waves are absorbed by the plasma particles.

Fusion Research in Europe
Fusion research in Europe

Europe has a large track record in fusion. Europe’s JET (Joint European Torus) located at Culham (UK) is the world’s largest fusion facility and the only one currently capable of working with a Deuterium-Tritium fuel mixture. JET has reached all its originally planned objectives and in some cases surpassed them. In 1997 it achieved a world record fusion power production of 16 MW and a Q = 0.65.

Europe has also been building on the knowledge accumulated through the Tore Supra tokamak in France , the first large tokamak to use superconducting magnets; the ASDEX device in Germany with ITER-shaped plasmas; the reversed pinch device RFX in Italy and the stellarators TJ-II in Spain and W7-X in Germany.