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
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 =
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