Why Are Fusion Reactors Not Common Yet?

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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.


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