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Next week, top officials at the US Department of Energy will hear an
extraordinary appeal for funds. The plea will come from beleaguered weapons
designers at the US government’s most renowned military research establishments,
where shrinking defence spending has put many jobs at risk.

The designers want $900 million for a laser that will simulate nuclear
explosions in the laboratory more accurately than ever before. They in-sist
that while there’s a moratorium on the detonation of nuclear bombs for research,
the laser is essential for maintaining the country’s expertise in weapons
design and for assuring the safety of its existing arsenal.

Known as the National Ignition Facility, the proposed laser is a giant,
much more powerful than any laser that has ever been built before. It would
be capable of generating 500 million megawatts, or nearly two hundred times
the capacity of all the world’s power stations, for more than 3 billionths
of a second. As a spin-off, say the designers temptingly, the laser could
make fusion power a reality. Although even their optimistic scenario puts
the first demonstration reactor of this elusive source of cheap energy at
least three decades away, the proposition has encouraged civilian proponents
of fusion power to back the military establishment’s case for the giant
laser.

Testing time

If the energy department officials endorse the proposal – and the researchers
seem confident that they will – the military laboratories will have cleared
the first major hurdle in a race to secure their future. The department
would then include the proposal in its own bid for funds, which it is due
to submit to the federal government next month in the form of a draft budget
for 1996. But if the officials turn down the idea, or if Washington opposes
it or cannot find the money to fund it before the fiscal year begins in
October 1995, the prospects for many defence workers look bleak.

Most at risk are jobs at the Lawrence Livermore National Laboratory
in California, principally because Livermore staked its future on the Star
Wars programme and became a big loser as the programme has shed futuristic
space-based defence systems over the past few years. The laboratory’s research
had included work on X-ray laser weapons powered by nuclear bombs and investigations
into an array of anti-missile satellites known as Brilliant Pebbles.

Livermore has tried to diversify since then, but without much success.
Ambitious management plans to redirect the laboratory’s efforts toward civilian
research are opposed by private high-technology companies that object to
government-funded competition. To make matters worse, as budgets tighten,
the Department of Energy must justify operating three nuclear weapons establishments
– and the national laboratories at Sandia and Los Alamos are considered
to have more bankable military expertise than Livermore.

Now the threatened laboratory has dusted off plans for a giant laser,
which has been part of its long-term development programme for many years,
and is leading the campaign to see the delayed project operational by 2002.
Besides winning the backing of weapons designers and both military and civilian
fusion experts, Livermore has lined up support from leading Californian
politicians, inclu-ding Senator Barbara Boxer.

Earlier this month Bruce Tarter, Livermore’s acting director, told Congress
that the NIF was of the ‘highest priority’. His words are echoed by Donald
Correll, deputy leader of the fusion programme at Livermore: ‘The technical
credibility and need have been documented.’ If all this lobbying pays off,
the laboratory’s expertise in laser research could ensure that it reaps
the greatest benefits. Although no site has yet been earmarked for the giant
laser, Livermore’s senior scientists are bubbling with confidence. Mike
Campbell, associate director for lasers, predicts that the NIF ‘will be
the centrepiece of the laboratory in 10 years’ time’.

Livermore already houses what is currently the most powerful laser in
the world, albeit a machine with barely one-twentieth of the NIF’s generating
capacity. Nova, which was completed in 1985 for $176 million, can generate
around 30 million megawatts of power for one billionth of a second. Researchers
at Livermore have been directing this power at tiny frozen spheres, about
1 millimetre across, of the hydrogen isotopes deuterium and tritium. The
idea is to heat up the surface of a pellet so quickly that it explodes uniformly,
thereby creating a reactive implosion that causes the nuclei of the isotopes
to fuse, yielding helium nuclei and high-energy neutrons to sustain the
reaction. It is these micro-explosions of the fusion reaction that so interest
weapons designers; they are similar to what goes on inside a thermonuclear
bomb when it is detonated.

But the fusion process is fraught with difficulties. The particles being
forced together inside the pellet are all positively charged, and so naturally
repel each other. Fusion researchers use deuterium and tritium, which have
one and two neutrons in addition to hydrogen’s single, positive proton,
because these isotopes repel each other less than any other known particles.
Nevertheless, the repulsive forces between them are still huge. Another
difficulty is supplying the implosive force uniformly over the surface of
the pellet.

Ignition threshold

In attempting this type of fusion, the key to success is to constrain
energetic particles at high densities, because nuclei fuse only if they
collide at speeds fast enough to overcome the repulsion of their positive
charges – conditions that exist naturally only in the cores of stars. This
will be possible with the NIF, say the giant laser’s backers. Its longer
and more powerful pulses could compress larger targets (of around 1 centimetre
across) more energetically, and so produce more fusion reactions. They calculate
that the giant laser would supply enough energy, known as the ‘ignition’
threshold, to trigger a self-sustaining reaction in which the fused particles
would deliver more energy than they absorbed.

At the moment, a blast from Nova delivers around 30 kilojoules of energy
but generates no more than a few tens of joules of fusion energy in return.
An NIF pulse delivering 1800 kilojoules is expected to trigger a fusion
reaction that generates 10 times the energy supplied. Weapons designers
say such success would enhance studies of the physics of thermonuclear bombs
by allowing them to verify predictions of elaborate computer models. They
could also assess the effects of nuclear radiation on military equipment,
which would help to ensure the safety of existing arsenals. Furthermore,
they insist, it would give civilian researchers a head start in turning
fusion power into a commercial proposition.

Similar work is either planned or already under way in Europe and Japan.
French fusion researchers expect to receive government approval to build
a laser as big as Livermore’s proposed facility, says Michel Andre, head
of the laser division at the French Atomic Energy Commission in Limeil-Brevannes
near Paris. In Britain, the Atomic Weapons Establishment at Aldermaston
wants to replace its 1-kilojoule laser with one capable of generating a
hundred times as much energy, says Paul Roper, director of nuclear science
at the Ministry of Defence. And the Japanese government, which does not
have a nuclear weapons programme, is considering a proposal from Osaka University
to build a 100-kilojoule laser for fusion research.

The fusion technique that interests them all is known as inertial confinement
fusion because the inertia of material pushed inwards confines the fusion
fuel. This is unlike most fusion energy research, which uses a strong magnetic
field to confine a hot but not very dense mixture of particles for a comparatively
long time, a second or more. After more than forty years of research, the
leading magnetic fusion design is the tokamak, a doughnut-shaped vessel.

Last December, the Princeton University Plasma Physics Laboratory produced
6.2 megawatts of fusion power for about 4 seconds in a tokamak, breaking
the previous record by researchers in Europe (This Week, 18 December 1993).
But this is not enough to sustain the fusion reaction, and Princeton’s machine
is to shut down at the end of the year and a larger version built. American
researchers hope to complete a demonstration power plant in 2025. Inertial
confinement fusion has the same timetable for producing energy commercially,
but military applications are much closer to realisation and have paid for
most of the research. Even when underground testing of nuclear weapons
was allowed, the DOE backed the laboratory work because the underground
tests cost around $100 million apiece.

When weapons designers simulate bomb explosions, they need to apply
the laser’s energy to the fusion target as uniformly as possible. So, instead
of irradiating a pellet of fusion fuel directly, they often focus the laser’s
energy onto the inside of a metal shell called a hohlraum, which surrounds
the fusion target (see Figure). The laser pulse vaporises the metal to produce
a burst of X-rays that, like X-rays from the fission explosion in a full-scale
thermonuclear bomb, heat and compress the target to cause fusion. By adjusting
the shape and composition of hohlraums, designers can simulate different
bomb designs.

Using a hohlraum may seem cumbersome because it drives fusion only indirectly.
But military researchers have found that the approach allows existing lasers
to mimic bomb explosions more realistically and cause more particles to
fuse. A hohlraum enhances the yield by converting a non-uniform laser pulse
into a more even burst of X-rays for compressing the target. Variations
in the distribution of energy on a pellet’s surface are bad news because
they prevent a target com-pressing efficiently, even when the variation
is only a few per cent, a level that Nova cannot achieve.

Outside the military laboratories, however, other groups of researchers
in the US are working on lasers that illuminate targets evenly enough to
drive fusion directly, without hohlraums. ‘If everything works right, direct
drive offers higher gain for civilian fusion,’ says John Soures of the
University of Rochester. But there’s a disadvantage for military researchers.
While such fusion experiments can simulate the effects of nuclear explosions
on military equipment, they cannot model the physics of the explosions themselves.

More power

The NIF will show what happens to both direct-drive and hohlraum targets
when laser power is turned up dramatically. It takes advantage of many refinements
since Nova was designed. As in Nova, light comes from the rare earth element
neodymium, embedded in a glass host. A neodymium laser is the most powerful
solid-state source yet developed, but the near-infrared wavelength it generates
when its ions are excited by bright flash lamps transfers energy poorly
to targets. To get around that problem, the beam from the neodymium will
pass through crystals of potassium dihydrogen phosphate that will convert
it to ultraviolet light. Although this conversion absorbs much of the beam’s
energy (more than half in the case of Nova, which generates 100 million
megawatts in the infrared), the greater efficiency of ultraviolet light
in transferring energy to the fusion target more than offsets the loss.

The big new system will also benefit from advances in laser design.
Nova has 10 parallel arms that together direct its 30-kilojoule pulses
of energy at a target. The NIF will have between 192 and 240 parallel arms,
called ‘beamlets’, delivering a total of 1.8 megajoules. The much greater
number means that NIF should illuminate targets more evenly than Nova. However,
the giant laser may still need hohlraums to enhance that uniformity.

Livermore has just begun to test the NIF’s first beamlet, which has
a design radically different from Nova’s arms. In Nova, a laser pulse passes
only once through a series of glass rods and disks doped with neodymium,
extracting energy from the element’s excited ions as it does so, thereby
increasing its power along the way. Pulses in the new laser will make many
passes through each block of laser glass embedded with neodymium, gaining
power each time. This is a more efficient arrangement because it enables
the pulses to extract more of the available energy from the element.

Thanks to these improvements, the NIF will cost only five times as much
as Nova, even though the difference in the energy each supplies is much
greater than that – around 1800 kilojoules as against 30 kilojoules.

Researchers in the field of inertial confinement fusion have talked
about new lasers for several years, but a 1990 review by the National Academy
of Sciences said they should first learn more about how to compress targets
uniformly. The panel, headed by Steve Koonin of the California Institute
of Technology, also sought better laser aiming, improved hohlraum design,
and an assurance that the physics would work as expected with larger lasers
and targets. Livermore says those questions have been answered.

But if commercial fusion power is to become a reality, researchers
will need to replace the glass lasers used by Nova and the NIF. Nova can
fire no more than one shot an hour, and works better if several hours elapse.
The NIF will be able to fire only one shot each eight-hour shift. While
that’s enough to test nuclear effects, or to conduct other military research,
it is a long way from the requirements of power generation.