MICHAEL Perry鈥檚 research team at the Lawrence Livermore National Laboratory
never intended to turn gold into platinum. They were only shining a beam of
light on a piece of metal. But strange things happen when you鈥檙e fooling with
the world鈥檚 most powerful laser beam. Such effects are hard to avoid when you
take a device that generates 1200 times as much power as the entire electrical
grid of the US and direct its energy onto a slice of gold foil just half a
millimetre thick.
Perry鈥檚 laser beam, called the Petawatt, flashes out of a souped-up arm of
Livermore鈥檚 mighty Nova laser, the world鈥檚 biggest. Nova鈥檚 ultraviolet rays
pulse through a stadium-sized building on Livermore鈥檚 campus near San Francisco,
where they surge to tremendous energies and converge to a spot, obliterating any
specks of matter put in the way. Built 13 years ago to help design nuclear
weapons, Nova is central to the US effort to keep its nuclear arsenal safe and
viable without nuclear testing. It is also the leading test-bed for laser-driven
inertial confinement fusion (ICF), the ongoing attempt to turn heavy hydrogen
isotopes into helium and harness for humankind the energy source that powers a
star. The hope is that the laser can heat and compress a pellet of fuel so
violently that its nuclei will undergo fusion, as in a thermonuclear
explosion.
These are the better known uses for Nova. But scientists at Livermore have
discovered that creating miniature nuclear fireballs in the lab has all sorts of
other intellectual payoffs. Physicists like Perry are going beyond military and
fusion research. Using Nova鈥檚 outlandish bursts of energy to create conditions
that have never before existed on Earth, they are finding answers to questions
that you would never imagine would be accessible to experiment. Such as: how
does hydrogen behave under the crushing pressures of Jupiter鈥檚 core? And what do
the shock fronts of exploding supernovae look like?
Advertisement
Re-creating such conditions in the lab requires rather a lot of power. In
Nova鈥檚 ordinary configuration, 10 ultraviolet laser beams meet inside a
鈥渉olraum鈥�, a hollow gold cylinder no bigger than a few pellets of bird seed. A
minuscule target placed in the holraum gets zapped by a 45-trillion-watt pulse
lasting a billionth of a second. Nova鈥檚 Petawatt extension is not as energetic,
but with pulses some 2 000 times shorter than Nova鈥檚, it reaches far greater
power. 快猫短视频s can direct this burst into a holraum or onto a different
material鈥攕uch as the gold foil鈥攊n a special Petawatt target
chamber.
Even in this era of inflated adjectives, 鈥減eta-鈥� may yet be an unfamiliar
prefix. It signifies 1015鈥攁 quadrillion. In 1996, Petawatt became the
first laser to cross the quadrillion-watt threshold (This Week, 8 June 1996, p
22). And yet its intense beam has a modest beginning. Each pulse starts out at
very low power in a room flecked by dizzying green strobes (see Diagram).
It is then stretched out until it is 30 000 times longer. At this stage, the
light would not harm a fly. But then one of Nova鈥檚 optical amplifiers joins the
fray, and pumps up the pulse, making it 1011 times more intense. Without the
initial stretching, the amplified beam would blow up the laser optics. But with
it, the pulse flows through smoothly.
Creating light with immense power means either giving it lots of energy, or
packing the energy into an exceedingly short pulse. And colossal compression is
Petawatt鈥檚 trick. Working with about 1 per cent of Nova鈥檚 overall energy,
Petawatt packs that energy into a pulse lasting half a trillionth of a second.
The compression happens in a vacuum chamber as big as a trailer that holds the
world鈥檚 largest diffraction gratings, nearly a metre across. Peering at the
gratings on a maintenance day, you can see why Christopher Barty, director of
ultrafast science at the University of California, San Diego, describes them as
鈥渋ncredibly beautiful鈥�. These shimmering discs boost the laser pulse鈥檚 power 10
000 times. At its peak, it reaches 1.3 quadrillion watts. If your home burnt
energy at that rate, your monthly bill would be more than a trillion pounds.
Fortunately for the taxpayers who foot Livermore鈥檚 power bill, the pulse is so
fleeting that it contains only enough energy to light a 100-watt bulb for 6
seconds.
Still, the light is so intense that it vaporises metal on impact, turning the
surface layer of a foil instantly to plasma. 鈥淚ts pressure is greater than the
pressure in a nuclear explosion,鈥� says Barty. 鈥淛ust about anything you put in
front of it will result in something you don鈥檛 expect.鈥� And the pulse鈥檚 brevity
only adds to its fury. The pulses flicker on and off so quickly that they
subject electrons to mind-numbing forces, much like those the particles would
feel just centimetres outside a black hole. Yanked about by a Petawatt pulse,
electrons accelerate 1019 times faster than an apple during its fall to
Earth.
Alchemical energy
With such power at its disposal, Petawatt regularly creates conditions more
extreme than in any other lab in the world. After the light turns the gold
foil鈥檚 top layer to plasma, electrons in the plasma slam into gold atoms in the
remainder of the foil, spewing what is called bremsstrahlung radiation.
The resulting X-ray photons are so energetic that they are nearly not X-rays at
all, but gamma rays, the most piercing radiation there is. 鈥淭o my knowledge鈥�,
says Perry, 鈥渢his is the brightest point source of such X-rays ever produced on
the planet.鈥�
Late last year, the team realised that the gold foil became radioactive after
Petawatt blasted through. The researchers deduced that the X-rays had knocked a
neutron out of a small fraction of the gold nuclei. Most of the affected nuclei
decayed to gold鈥檚 lighter neighbour on the periodic table, platinum. But oddly,
7 per cent of the unstable atoms changed into mercury, the next heaviest
element. This makes Petawatt the only device capable of transmuting elements
with mere light. With such power it is a fabulous tool for research.
鈥淥nce we understand how to use it,鈥� says Perry, 鈥渋t will generate new
applications.鈥� He hopes to use Petawatt as an X-ray source to illuminate the
nuclei of atoms. 鈥淲e may be able to image the motions of atoms within a
crystal,鈥� he says.
Firmer results are already flowing from Nova in its non-Petawatt
configuration. For instance, Luiz Da Silva, another Livermore physicist, is
using Nova to squash hydrogen to pressures far exceeding anything on Earth. He
and his collaborators are chasing after the equation of state for hydrogen.
That鈥檚 a description of the element鈥檚 different forms鈥攇as, liquid or
solid, insulator or metal, and so on鈥攁t various temperatures and
pressures. The Nova laser has been supplying such information for fusion and
weapons research for years. Understanding how hydrogen behaves under extreme
pressures and temperatures could be the key to fathoming the guts of giant
planets such as Jupiter.
Jupiter鈥檚 outer layers consist mostly of hydrogen gas, along with a dash of
other elements, which paint the planet鈥檚 clouds with a distinctive palette. As
on Earth, the hydrogen here is molecular鈥攖wo atoms joined by a single
bond. Deeper down, as the pressure and temperature mount, the gas starts
behaving differently. Planetary theorists think that it first turns into a
liquid, and then starts acting like a metal. That is, it becomes a good
conductor of electricity. Even deeper in, the molecular hydrogen becomes atomic:
single atoms press together, their bonds ruptured by the overlying weight.
The problem is that no one knows where鈥攐r if鈥攖hese transitions
occur, especially the deepest ones. 鈥淚t鈥檚 a raging controversy,鈥� says Da Silva.
鈥淗ydrogen is the simplest element in the Universe, but it鈥檚 still a big
challenge.鈥� The Nova laser, if only for an instant, comes close to recreating
the conditions at Jupiter鈥檚 core. The planet analogue is a tiny copper vault
filled with a liquid hydrogen isotope, deuterium, and capped by an aluminium
piston. The laser sears off the top of the piston, launching the rest of the
aluminium into the vault like a rocket and compressing the hydrogen to a million
times Earth鈥檚 atmospheric pressure. The temperature soars above 15 000
掳颁.
Heavy metal
In the past few months, Da Silva鈥檚 team has observed X-rays reflecting off
the deuterium鈥檚 surface. This shininess is tantalising. It means the deuterium
is so highly compressed that electrons become dislodged from atoms and flow
freely through the sample, conducting electricity. That sounds awfully like a
metal, although no one utters the M word without qualifying it to the hilt. But
one of Da Silva鈥檚 colleagues, physicist Gilbert Collins, is less equivocal about
another notable milestone. 鈥淲e do think we鈥檝e produced the transition from a
molecular phase to a purely atomic phase,鈥� he says. 鈥淣o one else has seen that
in the laboratory.鈥�
Nor have other groups mimicked the interior of Jupiter to such great depths,
notes Neil Ashcroft, a condensed matter physicist at Cornell University. 鈥淭he
Livermore experiment is exciting to planetologists, for one would expect to see
metallic hydrogen under these conditions,鈥� Ashcroft says. 鈥淚t鈥檚 an important
step toward understanding matter in the Universe in all its forms.鈥�
Unfortunately, even the Nova laser lacks the energy to reproduce all the
conditions found deep down in Jupiter, or in even denser planets and stars. But
Nova鈥檚 successor鈥攖he $1.2 billion National Ignition Facility
(NIF)鈥攊s already being built at Livermore. Whereas Nova has 10 independent
beams that pump energy into a target, NIF will have 192 and will produce 100
times as much energy (鈥淚gniting laser fusion鈥�, 快猫短视频, 21 May
1994, p 23). Due to be completed by 2003, NIF is, like Nova, intended to help
secure the safety of nuclear weapons without testing and bring the elusive goal
of fusion power a little closer. NIF鈥檚 peak power of 500 trillion watts will be
less than half Petawatt鈥檚, but its pulses will last a thousand times as long.
With such tremendous energy, NIF will be able to re-create the conditions found
within Jupiter鈥檚 core and deep within the small failed stars called brown
dwarfs, which may drift through our Galaxy in huge numbers. Using
pulse-compression tricks like those at work in Petawatt, Livermore physicists
will vary the durations and intensities of NIF鈥檚 laser pulses to extend the
range of accessible temperatures and pressures. 鈥淭he only clues we have about
the density profile of hydrogen in a brown dwarf are theoretical,鈥� Collins says.
鈥淎ny experimental data will be enlightening.鈥�
And yet, even before NIF gets going, Nova is allowing physicists to study
some of the Universe鈥檚 most cataclysmic explosions in the lab. In 1995, Bruce
Remington, a physicist at Livermore, noticed a striking likeness between images
of a fusion capsule being compressed and simulations of how a supernova expands
into space. Each featured Medusa-like curls and vortices that are caused by a
basic fluid dynamical process called the Rayleigh-Taylor instability.
The Rayleigh-Taylor instability is what foils attempts to balance a heavier
fluid over a lighter one (see Diagram). The interface between the two is
鈥渦nstable鈥�, and the upper fluid always seeps through in 鈥渇ingers鈥� that curl into
the lower fluid. The same thing happens, without gravity, in a supernova, as a
hot cloud of heavier gases and metals rushes out from the centre of an exploding
star, pushing lighter hydrogen out of its way.
Nova, Remington realised, could replicate such conditions by incinerating a
layered target. For before its explosive end, a giant star consists of lighter
layers surrounding progressively heavier ones, from hydrogen to helium and so
forth. By analogy, Remington鈥檚 millimetre-wide target was made of plastic on top
of a copper base. Precisely machined ripples at the interface echoed the
roughness鈥攃aused by stirrings of hot gases鈥攂etween a star鈥檚 layers.
The idea was to place this package in a small window in the holraum, fire up
Nova, and take pictures of the ensuing conflagration. However, scepticism
abounded.
鈥淎t first, you鈥檇 think this had to be a sales job for a big laser,鈥� Remington
admits. 鈥淏ut we can show a rigorous mathematical relationship between the two
processes, even though the scale differs by 13 orders of magnitude.鈥� Most
astrophysicists now agree. 鈥淭his approach is very good for understanding how
Rayleigh-Taylor instabilities develop in a supernova,鈥� says Stan Woosley, a
theoretical astrophysicist of the University of California, Santa Cruz. Nova is
now helping to solve the riddle posed by Woosley鈥檚 favourite cosmic object:
Supernova 1987A, the nearest exploding star since the invention of the
telescope.
Six months after this supernova was spotted, astronomers detected the core
elements iron, cobalt and nickel poking through the growing hydrogen bubble.
They appeared twice as fast as models predicted. However, says graduate student
Jave Kane of the University of Arizona in Tucson, these models considered just
two dimensions. By contrast, Nova鈥檚 three-dimensional blasts produce spikes of
copper that, it appears, explain the discrepancy. 鈥淲ith a 3D spike, overlying
material can flow out of the way on all sides more quickly than in two
dimensions,鈥� Kane says.
Supernova shock waves
Now an intriguing new round of Nova tests has begun. The shock waves from
Supernova 1987A will soon slam into rings of matter cast off by the star before
it exploded. Astrophysicists predict these collisions will unleash bright
cascades of energy, from X-rays to radio waves. Already, the Hubble Space
Telescope has detected the first stages of the encounter, which is expected to
last several years.
Paul Drake, a physicist at the University of Michigan in Ann Arbor, designed
a three-layer Nova target to model this event. The laser vaporises a layer of
plastic, creating a hot plasma that resembles the shock wave of the supernova.
The plasma rips through a smoky 鈥渁erogel鈥濃�攁n ultra-low-density
foam鈥攖hat mimics diffuse particles from the star鈥檚 solar wind. Finally,
the ensemble crashes into a thicker piece of plastic, a stand-in for the rings.
The results, Drake hopes, should help researchers interpret what they will see
in space. 鈥淲e鈥檒l get to watch the cosmic collision develop on human timescales
with our most modern instruments,鈥� Drake says. 鈥淚t will be amazing and
飞辞苍诲别谤蹿耻濒.鈥�
There is a sense of urgency about these projects, however, for Nova is
nearing the end of its life. Within two years, a plant will take its place to
make parts for NIF, being built next door. NIF will carry the torch and open
unimagined new doors in physics. Meanwhile, perhaps 900 times this year, Nova
flashes on.