BBC_Knowledge_2014-06_Asia_100p

factor is pressure. The Sun is a massive object and the

sheer gravitational pressure on the particles inside it is

immense. And finally there is the strange behaviour

of quantum particles, like these nuclei. They undergo

a process called quantum tunnelling that means they

can jump through a barrier, like the repulsive force,

and appear close to another particle. A fusion reactor

has to simulate these intense conditions.

One approach, adopted by some of the contenders

in the fusion race, is to go all out for heat. Without

intense pressure accompanying it, this means that

astonishing temperatures in excess of 100 million °C

are required. Inevitably it brings sizeable challenges

in getting the fuel up to that temperature, and

making sure that it doesn’t come into contact with

anything else. That might seem an impossible

restriction in itself. How can you prevent the fuel

from touching the reactor? Luckily, the difficulty

that makes fusion near-impossible in the first

INSIDE THE JET TOKAMAK

In a Tokamak reactor

such as JET, a plasma is

created by letting a small

puff of gas into a vacuum

chamber and then heating

it by driving a current

through it using a powerful

primary magnet. This hot

plasma is then confined in

the chamber by a series

of magnetic fields. Two

sets of magnets, known

FUSION
A fusion reactor typically
brings together deuterium
nuclei (hydrogen with
one extra neutron) and
tritium nuclei (hydrogen
with two extra neutrons)
under high temperatures
and pressures. This forms
helium and a free neutron.
Kinetic energy produced
by a loss of mass, as
illustrated in Einstein’s
famous equation E=mc2,
generates heat.

Even this has its good side. Unlike a fission reactor,

a fusion power station is not going to go critical or

melt down or explode. Unless everything is just right,

the reaction simply stops. But this reluctance to keep

working makes the whole process a huge challenge.

The problem is that the positively charged nuclei

of atoms really don’t want to fuse together. Bring

together two such particles and they repel each other.

The closer they get, the stronger the repulsion. But to

enable them to fuse, they have to be incredibly close

before the nuclear force that binds them together,

which operates over tiny distances, cuts in. Anything

over 2.5 femtometres (2.5/1,000,000,000,000,000

metres) and the force hardly exists. In a star, like the

Sun, three factors combine to make this possible.

One is high temperature. The core of the Sun is

around 15 million °C. This means that the nuclei that

are going to fuse have a lot of kinetic energy and take a

lot of stopping as they fly towards each other. A second

How plasma is confined to reach the

temperatures necessary for fusion Toroidal magnets

Primary magnet

Poloidal magnets

Plasma stream

as toroidal and poloidal,

are used to create a field

in both the vertical and

horizontal directions. These

fields act as a magnetic

‘cage’ to hold the plasma

in the desired shape.