Man-made star to unlock cosmic secrets By Jonathan Fildes
Science and technology reporter, BBC News
When the world's most powerful laser facility flicks the switch on its
first full-scale experiments later this month, a tiny star will be born
on Earth.
The National Ignition
Facility (NIF) in California aims to demonstrate the feasibility of
nuclear fusion, the reaction at the heart of the Sun and a potentially
abundant, clean energy source for the planet.
But whilst many eyes at the facility will be locked on
the goal of satisfying humanity's energy demands, many scientists hope
to answer other fundamental questions for mankind.
"In recreating the process of fusion it was always
understood that we could pursue three areas of interest and value,"
explained Dr Erik Storm of the Lawrence Livermore National Laboratory
(LLNL), the home of NIF.
First and foremost, NIF has been built for national
security purposes, to study the conditions that exist in nuclear
explosions and the way that nuclear weapons perform.
"That gives you an ability to maintain a credible
nuclear deterrent in the absence of underground nuclear testing," said
Dr Storm.
"Then, we can study the physics of fusion - can you
make a fusion power plant here on our planet? And we can do basic
physics and planetary science."
Right on time
It is this last area that has got the attention of scientists around
the world, who hope to use the machine to study distant phenomena, such
as the formation of planets or the violent explosions of supernovae,
from the comfort of the lab.
HOW NIF TRIGGERS FUSION
* A pea-sized spherical capsule is filled with fusion fuel
* This comprises a 150-microgram mix of deuterium and tritium
* The NIF laser set-up pulses for 20 billionths of a second
* For that time, it generates about 500 trillion watts
* That's equivalent to five million million 100-watt light bulbs
* All the laser power is focused on to the capsule's surface
* The fuel is compressed to a density 100 times that of lead
* It is heated to more than 100 million degrees Celsius
* Under these extreme conditions, fusion is initiated
"To understand where we find ourselves in the Universe and what we find
ourselves made of, one really needs to understand exploding stars,"
explained Professor Paul Drake of the University of Michigan.
He is just one of a number of researchers waiting in
the wings at NIF who hope to test their theories using the giant
facility.
"At NIF you can schedule a supernova explosion for
Thursday at nine in the morning instead of waiting for one to happen by
accident in the Universe," said Dr Storm.
"And you can change experiments each time. So you can do a supernova explosion
again, and again and again."
Other facilities, such as the Omega laser at the University of
Rochester in New York, are already used for this kind of test.
But NIF's 192 lasers will deliver more energy than any
facility has ever done, giving scientists an unprecedented glimpse into
what are usually distant cosmic processes.
During fusion experiments, the beams briefly focus 500
trillions watts of power - more than the peak electrical generating
power of the entire United States - on to a ball-bearing-sized pellet
of hydrogen fuel.
The intense energy creates temperatures of 100 million degrees and
pressures billions of times greater than Earth's atmospheric pressure,
forcing the hydrogen nuclei to fuse and a colossal amount of energy to
be released.
In the astrophysical experiments, however, the fuel
pellet would be substituted for a half-sphere of layered elements,
designed to mimic the core of a star.
“ The periodic table that we learn about when we first start chemistry is
fundamentally altered at pressures of a million atmospheres ”
Professor Ray Jeanloz UC Berkeley
"You choose the material and the structures between them to be relevant
to what happens when the star explodes," explained Professor Drake.
"The laser would strike the centre - the analogue of
the core of the star- launching a tremendously strong shock wave that
would blow the material apart."
The whole experiment may take just billionths of a
second, meaning the explosion has to be monitored in incredible detail
by a suite of sensors.
"The challenge is to do experiments that reproduce the
conditions that occur and then scale the results to the astrophysical
environment."
This should allow the researchers to probe the insides
of stars and supernovae in unprecedented detail and understand more
about how these astronomical objects came into being.
Diamond showers
But it is not just astrophysicists who are excited about getting their
hands on NIF. Planetary scientists also want to get hold of the machine
to test their theories about how planets and solar systems formed.
“ Hydrocarbons would actually decompose to a mixture of hydrogen and a
carbon - the end result being that diamonds would actually be hailing
out of the atmosphere. ”
Ray Jeanloz
"The architecture of the Solar System was very likely controlled to
some extent by the existence of planets like Jupiter," said Professor
David Stevenson of the California Institute of Technology.
The gravity of the giant planet controlled the position
of vast clouds of dust and debris in our cosmic neighbourhood and
therefore what building blocks were available to form the other
planets, including Earth.
And since 300 gas giants that have a similar mass or
are bigger than Jupiter have recently been found orbiting other distant
stars, understanding how and when these giants form could also help
shed light on the evolution of other planetary systems.
To do this, scientists have turned to NIF to try to
understand more about the extreme temperatures and pressures at the
heart of the planets and their effects on matter.
Previous generations of experimental facilities were
able to create pressures up to a million times that found at sea level
on Earth; NIF's lasers will be able to produce pressures up to billions
of atmospheres.
"These are conditions that exist inside these super
giant planets," Professor Ray Jeanloz, of the University of California,
Berkeley, told BBC News.
At these crushing pressures, he said, the conventional
understanding of chemistry and the behaviour of materials is turned on
its head.
"The periodic table that we learn about when we first
start chemistry is fundamentally altered at pressures of a million
atmospheres," he said.
"By a billion atmospheres, we expect even more dramatic changes."
For example, at millions of atmospheres, the bonds between atoms begin
to break down. At billions of pressures, the atoms themselves begin to
be crushed.
"This regime has never been explored before," Professor Jeanloz told BBC News.
Scientists hope to probe what happens to abundant elements such as
hydrogen and helium, as well as life-critical materials such as carbon
and water.
"It's only been in the last year that the theoretical
community has really pursued these calculations. We're just beginning
to get a more detailed sense [of what might come out of NIF]."
However, even lower pressure experiments hint that the results may range from
the exotic to the bizarre.
For example, tests have shown that hydrogen - the most abundant element
in the Universe - becomes a metallic fluid at pressures similar to
those found inside the Earth.
Whilst at higher pressures, such as those found on Jupiter, stranger things
begin to happen.
"Hydrocarbons would actually decompose to a mixture of hydrogen and a
carbon," explained Professor Jeanloz. "The end result being that
diamonds would actually be hailing out of the atmosphere.
"That's the kind of process you would never have guessed unless you had studied
the materials themselves."
Story from BBC NEWS:
http://news.bbc.co.uk/go/pr/fr/-/2/hi/science/nature/8044620.stm
Published: 2009/05/22 01:17:40 GMT
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