Why are fusion reactors not common yet?
Across the world, for over 50 years, a large number of physicists and engineers have tried
to build fusion reactors. Fusion reactors try to copy the mechanism of energy release
used by the Sun.The first machine that realized macroscopic energy production was the
Joint European Torus* (JET for short) located in Culham in the United Kingdom.
The idea of JET is to produce an extremely Ref. 156 hot plasma that is as dense as possible. At
high enough temperature and density, fusion takes place; the energy is released as a particle
flux that is transformed (like in a fission reactor) into heat and then into electricity.
To achieve ignition, JET used the fusion between deuterium and tritium, because this
reaction has the largest cross section and energy gain:
D + T → He4 + n + 17.6MeV . (61)
Because tritium is radioactive, most research experiments are performed with the much
less efficient deuterium–deuterium reactions, which have a lower cross section and a
lower energy gain:
D + D → T + H + 4MeV
D + D → He3 + n + 3.3MeV . (62)
Fusion takes place when deuterium and tritium (or deuterium) collide at high energy.
The high energy is necessary to overcome the electrostatic repulsion of the nuclei. In
* See www.jet.edfa.org.
Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net Copyright © Christoph Schiller November 1997–July 2010
the sun, the stars and the birth of matter 155
other words, the material has to be hot. To release energy from deuterium and tritium,
one therefore first needs energy to heat it up.This is akin to the ignition of wood: in order
to use wood as a fuel, one first has to heat it with a match.
Following the so-called Lawson criterion, published in 1957 by the English engineer
Ref. 157 John Lawson, (but already known to Russian researchers) a fusion reaction releases energy
only if the triple product of density n, reaction (or containment) time τ and temperature
T exceeds a certain value.Nowadays this criterion is written as
nτT > 3 ⋅ 1028 sK/m3. (63)
In order to realize the Lawson criterion, JET uses temperatures of 100 to 200MK, particle
densities of 2 to 3 ⋅ 1020 m−3, and confinement times of 1 s. The temperature is much
higher than the 20MK at the centre of the Sun, because the densities and the confinement
times are lower for JET.
Matter at these temperatures is in form of plasma: nuclei and electrons are completely
separated. Obviously, it is impossible to pour a plasma at 100MK into a container: the
walls would instantaneously evaporate.The only option is to make the plasma float in a
vacuum, and to avoid that the plasma touches the container wall. The main challenge of
fusion research in the past has been to find a way to keep a hot gas mixture of deuterium
and tritium suspended in a chamber so that the gas never touches the chamber walls.The
best way is to suspend the gas using a magnetic field. This works because in the fusion
plasma, charges are separated, so that they react to magnetic fields. The most successful
geometric arrangement was invented by the famous Russian physicists Igor Tamm and
Andrei Sakharov: the tokamak. Of the numerous tokamaks around the world, JET is the
largest and most successful. Its concrete realization is shown in Figure 75. JETmanages to
keep the plasma from touching the walls for about a second; then the situation becomes
unstable: the plasma touches the wall and is absorbed there. After such a disruption, the
cycle consisting of gas injection, plasma heating and fusion has to be restarted. As mentioned,
JET has already achieved ignition, that is the state were more energy is released
than is added for plasma heating. However, so far, no sustained commercial energy production
is planned or possible, because JET has no attached electrical power generator.
The successor project, ITER, an international tokamak built with European, Japanese,
US-American and Russian funding, aims to pave the way for commercial energy generation.
Its linear reactor size will be twice that of JET; more importantly, ITER plans
to achieve 30 s containment time. ITER will use superconducting magnets, so that it will
have extremely coldmatter at 4K only a fewmetres fromextremely hotmatter at 100 MK.
In other words, ITER will be a high point of engineering.The facility will be located in
Cadarache in France and is planned to start operation in the year 2016.