Europe is on track to build a huge laser fusion facility, a step
on the road to fusion-based power stations in the middle of the
century.
Announced in London yesterday and to be called
HiPER,
for High Power Laser Energy Research Facility, the football
stadium-sized building would contain the world's most powerful
lasers.
Research into pulsed fusion, also known as inertial fusion, lags
research into magnetically-confined continuous fusion of the type
under investigation at JET - the Joint European Torus - at the
Culham Lab
in Oxfordshire and its successor ITER
in Cadarache, France.
However, pulsed fusion should not be ignored, said Professor
Mike Dunne of the Oxfordshire's
Rutherford Appleton Laboratory, as even if
continuous fusion is a success "I believe the prize is so large we
can't put all of our eggs in one basket. It seems inconceivable
that we will need only one source of fusion."
The prize being a vast amount of carbon-free energy.
For the planet, "viable energy sources that meet the scale of
mankind’s future needs can be counted on the fingers of one finger:
fusion," said Dunne.
"The deuterium in a small glass of sea water contains as much
energy as a super tanker full of oil. A cubic kilometre of sea
water has more energy than the entire world's oil reserves."
Rather than magnetically supporting sustained plasma in a torus,
pulsed fusion involves dropping a hollow capsule containing the
hydrogen isotopes deuterium and tritium into a spherical chamber a
few metres across.
As it reaches the centre, a bank of lasers fire. "The outer
surface of the capsule heats up and expands outwards. Following
Newton's laws, the rest of the capsule undergoes an equal and
opposite reaction: it implodes," said a HiPER technical report -
hence the term inertial fusion.
Somewhere near the point at which the deuterium and tritium
reaches maximum density, a second bank of lasers fires to initiate
fusion - known as ignition.
Net energy gain with inertial fusion was demonstrated in the
1980s by US military researchers.
If the scheme works with high energy gain, a gigawatt-class
power station - up there with conventional nuclear power stations
and the largest coal-fired stations - will only consume five of
these marble-sized capsules per second.
HiPER is not a power station, but would be the one of the last
steps to a power station: consuming capsules at between one every
10s and one per second, in short bursts.
The compression bank with its amplifiers, pulse shapers and
wavelength shifters will fire 50 - 200 half metre diameter beams
focused down to a millimetre and containing a total of 250kJ at a
wavelength of 0.35µm over "multiple nanoseconds" said HiPER.
These will exert 109 bars of pressure. "That is
equivalent to 10 aircraft carriers or 90,000 London buses stacked
on your thumb," explained Dunne.
This compresses the plasma to 300g/cm3 - "20 times
the density of lead, or even gold, depending how efficient you
are", said Dunne.
A 1mm diameter gold funnel protects the ignition beam
from fragments of the glass capsule which holds tritium and
deuterium. Five of these per second would produce over
1GW.
Ignition is initiated by a 15ps 70kJ pulse, focussed to 100µm
diameter to match the size of the super-dense plasma, which heats
the compressed matter to 100 million Kelvin. "The total laser power
is 10,000 times the UK National Grid for a split second," said
Dunne.
To couple energy into the plasma more effectively, a tiny gold
target on the side of the capsule actually converts the ignition
beam into fast electrons as it enters.
The target is at the tip of a 1mm hollow gold cone, which acts
as a blast shield, lasting just long enough to direct fragments of
the outer capsule away from the ignition beam.
For the source lasers "we are looking at laser diodes. At the
moment they are under 10% efficient and produce 1kW average power,"
said Dunne. "We believe we need 10-20% efficiency and a factor of
10 more power."
If all goes well, somewhat more than 100 times the input energy
will emerge as fast neutrons, which are converted to heat by a
thick lithium lining on the inside of the spherical chamber.
In a power station, this heat would be taken away by water and
used to generate steam and drive turbines.
This neutrons-lining-heat-steam arrangement is the same as
proposed for magnetic fusion, so development of this will be shared
between the two research communities, said Dunne.
Lithium has been chosen as is has good neutron-thermal
characteristics and also produces tritium as a by-product, which
would be captured and used in the capsules. "This serendipity is
one of the events in science that gives you real hope," said
Dunne.
Some neutrons escape. The chamber and surrounding infrastructure
have to withstand the neutron flux and become the plants only
radioactive waste, becoming safe after around 100 years, said
Dunne.
HiPER is to some extent a pre-emptive project, betting on the
future success of two other projects: the National Ignition
Facility in California (which when completed in 2009 aims to first
laser-initiate fusion with 1.8MJ and 500TW of ultraviolet laser)
and the similar sized Laser Megajoule in France.
If these do work, which seems extremely likely said Dunne,
something like HiPER will be needed to take inertial fusion
onwards.
The London announcement marks the start of a three year
preparatory phase following a two year feasibility study.
Fundamental science will also benefit as HiPER will be the
worlds most powerful laser energy source allowing measurements on
matter that have not been made before.
"As well as the applications of the 70kJ picosecond ignitor
laser pulse, HiPER offers the longer-term potential to be
reconfigured in a mode that could generate 10kJ in 10fs providing a
power of 1EW," said HiPER.
"Focused to a spot of 1 micron the irradiance would be
1026 W/cm2."
How HiPER might look (click to expand). Compression
lasers with their amplifiers, pulse shapers and frequency shifters
are on the far side. The 15ps 100µm diameter 5PW ignition pulse
comes from lasers on this side.