Post by plutronus on Sept 24, 2013 18:13:21 GMT -6
Released into the public domain...
Source: www.eurekalert.org/pub_releases/2013-09/aiop-fa092313.php, released into the public domain
Not quite yet, but in the journal 'Physics of Plasmas,' researchers from National Ignition Facility show just how close we've come.
WASHINGTON D.C. Sept. 24, 2013 --
The dream of igniting a self-sustained fusion reaction with high yields
of energy, a feat likened to creating a miniature star on Earth, is getting
closer to becoming reality, according the authors of a new review article
in the journal Physics of Plasmas.
Researchers at the National Ignition Facility (NIF) engaged in a
collaborative project led by the Department of Energy's Lawrence
Livermore National Laboratory, report that while there is at least one
significant obstacle to overcome before achieving the highly stable,
precisely directed implosion required for ignition, they have met many
of the demanding challenges leading up to that goal since experiments
began in 2010.
The project is a multi-institutional effort including partners from
the University of Rochester's Laboratory for Laser Energetics, General
Atomics, Los Alamos National Laboratory, Sandia National Laboratory, and
the Massachusetts Institute of Technology.
To reach ignition (defined as the point at which the fusion reaction
produces more energy than is needed to initiate it), the NIF focuses 192
laser beams simultaneously in billionth-of-a-second pulses inside a
cryogenically cooled hohlraum (from the German word for "hollow room"), a
hollow cylinder the size of a pencil eraser. Within the hohlraum is a
ball-bearing-size capsule containing two hydrogen isotopes, deuterium
and tritium (D-T). The unified lasers deliver 1.8 megajoules of energy
and 500 terawatts of power—1,000 times more than the United States uses
at any one moment—to the hohlraum creating an "X-ray oven" which
implodes the D-T capsule to temperatures and pressures similar to those
found at the center of the sun.
"What we want to do is use the X-rays to blast away the outer layer
of the capsule in a very controlled manner, so that the D-T pellet is
compressed to just the right conditions to initiate the fusion
reaction," explained John Edwards, NIF associate director for inertial
confinement fusion and high-energy-density science. "In our new review
article, we report that the NIF has met many of the requirements
believed necessary to achieve ignition—sufficient X-ray intensity in the
hohlraum, accurate energy delivery to the target and desired levels of
compression—but that at least one major hurdle remains to be overcome,
the premature breaking apart of the capsule."
In the article, Edwards and his colleagues discuss how they are using
diagnostic tools developed at NIF to determine likely causes for the
problem. "In some ignition tests, we measured the scattering of neutrons
released and found different strength signals at different spots around
the D-T capsule," Edwards said. "This indicates that the shell's
surface is not uniformly smooth and that in some places, it's thinner
and weaker than in others. In other tests, the spectrum of X-rays
emitted indicated that the D-T fuel and capsule were mixing too much—the
results of hydrodynamic instability—and that can quench the ignition
process."
Edwards said that the team is concentrating its efforts on NIF to
define the exact nature of the instability and use the knowledge gained
to design an improved, sturdier capsule. Achieving that milestone, he
said, should clear the path for further advances toward laboratory
ignition.