FUSION DESIGN #2
electrical current through the plasma – a ‘disruption’
or contact with the metal vessel, which both stops the
process and causes considerable damage.
It’s difficulties like these that have set back progress.
The early experimenters on fusion machines expected
a similar development timescale to that of nuclear
fission, which went from early experiments to the
first practical power generation in around 10 years.
In reality, more than 50 years in, we are still decades
from a tokamak reactor joining a power grid. The
most advanced of the existing reactors, JET ( Joint
European Torus), based at Culham in Oxfordshire,
has made the biggest leap forward so far. It’s given
us an understanding of how to get consistent
performance out of a fusion device.
The next big step, ITER, will still not be a usable
power plant, but it should crack the break-even
barrier. Clearly, to be useful as a generator, a fusion
reactor has to provide more energy than is put in to
keep the plasma contained, and at high temperatures.
JET has never achieved this, but ITER should by a
wide margin, providing the experimental foundations
for the first true fusion generator. ITER is a vast
project that suffers from the inevitable bureaucratic
difficulties of managing input from seven different
countries with their own agendas. Timescales
have slipped and the cost has tripled to around € 16
billion, while a recent external assessment has slated
its management. But despite these problems, ITER
remains an essential step on the path to fusion power.Practical power
A second device at Culham, the Mega Amp Spherical
Tokamak (MAST) is being used to find out how to
make a practical generator. “We hope that MAST
will show us how to drive down the cost and size of a
practical fusion reactor,” said Professor Steve Cowley,
director of Culham. “Going with the ITER model
for electricity generation could result in machines that
are too big and expensive. We can’t expect the first
commercial reactors to be competitive on price, but
they need to be in the ballpark. And it’s important
they aren’t too big, or a failure would have too big
an impact, and the grid couldn’t cope with the input.
Around 1GW is best.”
The spherical tokamak is helping scientists learn
how to produce a smaller, more cost-effective device.
A true spherical machine would not be suitable for a
production reactor as it wouldn’t have enough space
to stop the heat-generating neutrons and harness
their energy. But a hybrid between the MAST-style
spherical design and the traditional D cross-section
tokamak may well be the pattern for commercial
machines in the future. In the meantime, though,
another contender has been working on beating the
tokamak to the prize – a fusion device that is straight
out of a Bond villain’s armoury.
At the Lawrence Livermore National Laboratory
in California two vast, 10-storey halls contain
the mechanism of the National Ignition Facilityfrom a blast of radio waves, and supercharging by
firing a stream of high-speed neutral particles into
the chamber. These collide with the fuel and give it
extra kinetic energy and hence temperature. Rather
than the high temperatures, the biggest factor slowing
the development of tokamak reactors has been the
intransigence of plasma.
Plasma is the fourth state of matter after solid, liquid
and gas. Just as a gas is what you get when you heat a
liquid, a plasma is the result of heating a gas. Unlike a
gas, which is made up of atoms or molecules, a plasma
consists of charged particles, known as ions, produced
when atoms gain or lose electrons. The positively
charged nuclei that are the fuel of the fusion reactor
form a plasma. And plasmas behave terribly. Inside
the magnetic confinement they writhe and pulsate as
if they were alive. This can lead to a collapse of the
6’30”
is the world record
duration of plasma in
a tokamak, in ITER’s
predecessor at
Cadarache, France.TECHNOLOGY – ITER is a
scaled up version of JET
that can hold 10 times the
volume of plasma. ITER
is still an experimental
machine, but should easily
be able to produce more
energy than is put in. The
facility will be used to test
the technologies required to
make a commercial fusion
power station.ODDS OF WINNING RACE
3 to 1ITER
(INTERNATIONAL THERMONUCLEAR
EXPERIMENTAL REACTOR)
DATE STARTED 2007
METHOD D cross-section magnetic
confinement plasma tokamak
SIZE 12m diameter
LOCATION Cadarache, France
ESTIMATED COMPLETION DATE 2020A cut-away model
of ITER’s massive
tokamak chamber