FOR half a trillionth of a second, the Petawatt generates over a thousand
times the power output of the US electrical grid. Focus this on a target just a
few micrometres across and the results are literally explosive. Anything in the
laser鈥檚 way is instantly vapourised into plasma of ions and electrons鈥攍ots
of them. The Petawatt is the world鈥檚 most powerful laser and this is based at
the Lawrence Livermore National Laboratory in California.
It would be easy to think that the products of such an action would be
useless. Far from it. In the early 1990s, Max Tabak, a theorist at Livermore,
found an extraordinary use for the electrons produced in these plasmas. He
showed that the surfeit of electrons could trigger tiny thermonuclear fusion
reactions in an entirely new way.
This new trick, known as fast ignition, has many scientists hugely excited.
They believe that it neatly sidesteps many of the devilishly difficult problems
that have plagued fusion research for years. If they are right, the technique
could lead to a new generation of fusion research in which scientists are able
to study the nuclear and plasma physics involved with unprecedented accuracy.
Some even whisper about producing energy this way, albeit many years into the
future.
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Now scientists at Livermore are busy using the Petawatt to study the
processes at work in fast ignition. And they hope to build an even more powerful
laser to achieve it on a larger scale.
Livermore鈥檚 researchers are no strangers to thermonuclear fusion. After all,
the lab鈥檚 goal is to manage America鈥檚 ageing stockpile of nuclear weapons to
ensure that these weapons function properly should ever they be needed. From a
national security standpoint, it is essential to understand what goes on in a
nuclear explosion and how the condition of materials making up stockpiled
weapons can affect their safety, both in storage and use. 鈥淚n many cases these
studies are critical to answering questions about nuclear weapons,鈥� says Bill
Hogan, a senior researcher working on Livermore鈥檚 next generation laser, the
National Ignition Facility or NIF.
Inspecting stored weapons inevitably reveals subtle changes in their
component materials, but how important is this? 鈥淭he epoxy in a glued joint may
have turned to green cheese,鈥� says Hogan, 鈥淭he question is whether that weapon
is safe and reliable . . . the weapons scientists don鈥檛 know the properties of
green cheese!鈥� However, put a piece of green cheese in a fusion reaction and you
can scrutinise its behaviour. For instance, does green cheese epoxy absorb or
emit radiation in a way that might compromise the safety or performance of a
weapon?
But Livermore鈥檚 scientists have one hand tied behind their backs. Because of
the international ban on nuclear weapons testing, they cannot detonate the
weapons for real to ensure they work. Instead, they use lasers to create
thermonuclear fusion reactions on the tiniest scales and study these
instead.
In principle, igniting a fusion reaction is a simple process. Take nuclei of
tritium and deuterium, the heavy isotopes of hydrogen, heat them to a hundred
million degrees Kelvin and they will fuse, producing highly energetic neutrons
and helium nuclei. While the temperature is important to start the reaction,
density is important to continue it so that it becomes a chain reaction. For
this to happen, these helium nuclei must collide with more tritium and deuterium
nuclei, accelerating them to the energies at which they fuse, and so on. This is
only possible if the tritium and deuterium nuclei are present in high enough
densities to absorb this bombardment. For ignition, researchers require a few
millionths of a cubic centimetre to be compressed to a density roughly 20 times
that of lead. Such a chain reaction can release a million times the energy
involved in straightforward chemical reactions such as burning oil or coal.
But achieving these two critical conditions of temperature and density is not
easy. The approach taken so far in laser labs all over the world is to use a
powerful laser pulse to both heat and compress a tiny pellet of tritium and
deuterium, a technique known as inertial confinement fusion. To achieve this the
pellet is zapped from several directions at once. The capsule鈥檚 outer shell
vaporises and expands, squeezing the contents to huge densities and creating at
its centre temperatures high enough to start the reaction.
But the perfect symmetry required is tough to create. 鈥淚t鈥檚 like squashing a
tomato in your hand-it oozes out between your fingers,鈥� says Mike Perry, head of
the team that built the Petawatt to study Tabak鈥檚 idea. Nevertheless,
researchers using Nova, Livermore鈥檚 largest laser, have used this technique to
persuade a tiny fraction of the nuclei of tritium and deuterium to fuse. To go
further, they need to heat more of the plasma to higher temperatures and for
this they鈥檒l need a laser even more powerful than Nova. The lab is busy building
one. The National Ignition Facility will be a giant laser capable of delivering
1.8 megajoules of energy in single pulse. This is 40 times the energy and 10
times the power of Nova.
But although this energy will heat more of the plasma to the critical
temperature, the perfect symmetry required to compress the plasma to the correct
density will be even more difficult to achieve. This is partly because of the
number of optical components in the laser beam鈥檚 path. A big laser may contain
hundreds of optical surfaces, each of which must be close to perfect, with no
distortions or imperfections to affect the beam. 鈥淭he number of square feet of
polished optical surface is a critical issue,鈥� says Hogan. And for more
energetic lasers, the size of individual components becomes a problem. As well
as being difficult to manufacture, big lenses and mirrors can sag under their
own weight, making it even tougher to produce a well-focused beam. The heat from
the laser can also distort the optics. All in all, with the very high powered
lasers, it becomes almost impossible to create a uniform beam.
Fast ignition takes a different approach. It turns out that the required high
density occurs more easily in a relatively low temperature plasma. If only a
small part of this plasma is then heated to the ignition temperature, the fusion
reaction should then spread through it like wildfire.
Perry draws an analogy between the conventional method of triggering fusion
with a laser and the way a diesel engine works. Compress air and diesel together
and the mixture combusts spontaneously. This is essentially the way Livermore
scientists are approaching fusion with Nova.
Fast ignition, on the other hand, works more like a petrol engine: after the
fuel and air are compressed, a spark ignites the mixture. Fast ignition uses
relatively low power beams to compress the mixture and a hugely powerful one to
ignite it. Significantly, the most powerful beam does not need to be
symmetrical.
Normally a plasma at the critical density would reflect the more powerful
beam preventing it from triggering fusion. So fast ignition uses another
beam鈥攁 precursor pulse鈥攖o carve a channel through to the heart of
the plasma. At that point the Petwatt kicks in, delivering a huge amount of
energy very quickly through this channel to the heart of the plasma.
This is where electrons become important. Like any electromagnetic wave, the
Petawatt pulse consists of electric and magnetic fields. But unlike other waves,
these fields are powerful enough to accelerate electrons in the plasma close to
the speed of light. These high-energy electrons smash into the nuclei of tritium
and deuterium giving them the energy to fuse. In theory, the reaction should
spread because the rest of the plasma has already achieved the critical density.
With fast ignition, chain reactions should be possible at much lower energies
and with a higher yield of fusion energy than had previously been thought.
Nobody is quite sure if it will work, however, because the extreme physical
processes involved have never been studied before. The job of working out
exactly what is going on has fallen to a team headed by Mike Key, a physicist at
Livermore who leads the fast ignition research. He would like to know how the
precursor beam carves a channel through the plasma, how this channel behaves and
how long it lasts. In particular, he wants to study the way laser-accelerated
electrons trigger the fusion reaction. Tabak and his colleagues are attempting
to model the process on computer but need real data to calibrate their
results.
One of the big challenges is to understand the effect of so called
Rayleigh-Taylor instabilities which make the plasma difficult to control.
鈥淚magine turning a cup of tea upside down,鈥� says Key. If the surface of the tea
were perfectly flat, air pressure would hold it in the cup. But small ripples
quickly develop on the surface and rapidly become larger. 鈥淪oon gloopy fingers
of liquid loop down while pockets of air push up,鈥� he says. 鈥淏efore long, the
entire surface becomes so unstable that the tea falls out of the cup.鈥� These
ripples are Rayleigh-Taylor instabilities, says Key, and they are exactly
analogous to the way the plasma behaves as the precursor pulse ploughs through
its centre. 鈥淭his is very new science so we are not sure our simulations are
correct,鈥� says Key. Nevertheless, earlier this year, the team got the first
fleeting glimpses of a fusion reaction triggered by accelerated electrons this
way late last year.
The Livermore scientists would now like to build an even more powerful laser
that can achieve fast ignition on a larger scale. One possibility is to modify
NIF for this purpose. To set off the fusion burn, this laser must release tens
of thousands of joules of laser energy into an area of compressed fuel less than
20 micrometres across, in around 20 picoseconds (10-12 seconds).
The short high-energy pulses used in fast ignition research are so powerful
that they can damage the sensitive optics that created them. To prevent this,
many powerful lasers use a technique known as chirped pulse amplification, so
called because it uses a technique to vary the pulse鈥檚 frequency like the song
of a twittering bird.
The technique is straightforward. A pair of diffraction gratings disperses
the different wavelengths, stretching out the pulse so its intensity is low. The
pulse is then amplified and recompressed using another pair of diffraction
gratings to create the hugely powerful pulse.
One of the biggest challenges facing Perry is in building diffraction
gratings capable of handling more powerful pulses. And he has stumbled across
important spin-offs in laser machining in the process. Normally, lasers destroy
solid materials by heating them rapidly until they vaporise. Essentially, the
electric fields generated by the laser light shake the atomic lattice until it
flies apart.
But Perry鈥檚 team has discovered that very short pulses destroy materials in
an entirely different way. The electric fields in the pulse blast electrons away
from the surface of the material. This cloud of electrons then sucks any ions on
the surface with it. But because the pulse is so short, all this happens before
the lattice has time to heat up. The significance of this new mechanism is that
the entire process occurs without thermal stresses building up in the
material.
Always on the look out for spin-offs, Perry says this non-thermal mechanism
could be useful in other areas such as laser dentistry. The thermal stresses
generated by laser machining usually create tiny cracks in enamel which then
become sites for decay. Perry points to a tooth in his laboratory that has been
precisely machined by picosecond pulses without creating these cracks. 鈥淭his is
an entirely new field of laser machining,鈥� he says.
But building a diffraction grating that can resist the picosecond bombardment
is not easy. Conventional gratings are made from metal with narrow grooves that
diffract light. Since the angle of diffraction depends on its frequency, light
is spread out into its spectrum of colours. But metals are particularly
susceptible to laser damage because electrons are free to move around inside
them. When the laser pulse hits, these electrons are easily blown away by the
electric fields sucking ions with them.
So Perry plans to make gratings not out metal but out of layers of ceramic
dielectric materials in which there are no free electrons. And instead of
reflecting the beam off the surface of this grating, he hopes to reflect it
between these layers so that it interferes with itself. The idea is that these
internal reflections will spread the beam into its spectral range in the same
way that a thin film of oil on water divides white light into its component
colours.
Since electrons are bound more tightly in ceramic materials, such a grating
would have a significantly higher damage threshold than a metal version. Small
prototypes鈥攐nly 15 centimetres across鈥攁re performing well. What
remains is to build the full-scale, gratings over a metre across for the more
powerful laser.
The fusion reactions that should be possible with such a device will have
enormous potential as research tools for astronomers, plasma physicists and
materials scientists. The fusion burn lasts only a fraction of a second, but
even in that short time it has all the characteristics of its giant stellar and
nuclear counterparts. Obviously, travelling to a star鈥檚 core to find out what
goes on there is impossible. Terrestrial fusion reactors are an ideal way of
throwing light on knotty problems such as the age of the universe and what
really happens at the heart of a supernova
(see 鈥淪upernova鈥�, New 杏吧原创, 18 April, p 30).
The very newness of fast ignition adds to its allure. But that newness means
that it must rely on physics and computer models that are still untested. Now
the race is on to explore these new areas of physics.
