Sky & Telescope - October 2019 - 39

closer to the X-ray source - right on top of one of the holes in
the cylinder, in fact - so that it reaches higher temperatures.
("They are closer to the campfire," quips Montgomery.)
Encasing the foil is a tamping material, to keep the superheated-foil-turned-plasma from expanding. As the temperature and pressure rise, the plasma reaches a density similar to
that within the Sun.
Bailey and his colleagues found that, depending on the
specific wavelength measured, iron opacities were off from
theory by anywhere between 30% and 400%. "Our experiment can't be matched by any known theory," he says. "We
spent the next three years doing more experiments and doing
every test that we could think of . . . to try to see if we could
find anything wrong with the experiment." But they found
no errors in their setup.
Bailey and his colleagues have now turned for insights
to two elements on either side of iron on the periodic table,
chromium and nickel, to see if the slight differences in the
configurations of their electrons from that of iron change the
results. The preliminary findings, published this year, suggest
there's indeed something particular about iron: Although the
data for chromium and nickel do disagree with the models,
the discrepancy is not as severe. It's possible that current
theoretical models aren't able to understand how the iron
atoms' configurations of bound electrons are affecting radiation transport.

expands rapidly. To conserve momentum, the inner surface
collapses and compresses whatever is inside the shell.
For nuclear astrophysicist Maria Gatu Johnson (MIT),
inside that shell is helium-3 gas. (He-3 is a helium nucleus
that's missing a neutron.) She and her colleagues are investigating a very specific reaction, which provides nearly half
the energy in our Sun right now. They fill a tiny glass shell
with He-3 and place it in the center of the 10-meter diameter
NIF target chamber. In some experiments, that shell is 1.6
mm wide, and in others it's 3 mm wide. When the lasers hit
the capsule's surface with about 2 megajoules of energy (the
energy of a 2-ton truck hitting you at 45 m/s, or 100 mph),
the gas collapses into a dense plasma and ignites. A burst of
nuclear reactions lasting just a few nanoseconds follows.
The researchers capture the protons that stream from
those reactions using a specialized detector placed 10 cm
from the outburst. From the measured data, they can calculate the reaction rate, which tells them about the fusion
outcomes, like the generated energy, the formed elements,
and the emitted neutrinos.

Evolve a Blast
Laboratory astrophysics projects at NIF aren't exclusively
focused on nuclear reactions. Other researchers use the
imploding plasma as a scaled-down model of what's happening in a supernova, to test how those stellar explosions
work. Even though observational astronomers have found

L AWRENCE LIV ER M ORE N ATION A L L A BOR ATORY (2)

Focus All the Lasers
About a thousand miles away, another powerful machine at
another Department of Energy laboratory creates star-like
conditions to test a crucial process: the nuclear reactions
occurring in stellar cores.
The fact that a DOE laboratory is studying nuclear fusion
isn't surprising. After all, both Lawrence Livermore National
Laboratory (LLNL) and Sandia are firmly rooted in nuclear
science as it relates to our country's security. Much of LLNL's
laboratory data wasn't declassified and made accessible to
civilian scientists until the early 1990s, four decades into its
history. Now, the LLNL's National Ignition Facility (NIF), the
world's largest and most energetic laser, opens 10% of its time
to researchers wanting to do basic science. Bruce Remington
(LLNL) leads this Discovery Science Program and says most
of the projects pertain to astrophysics and planetary science.
NIF and Z are two of the most-used facilities for laboratory astrophysics work, because they're both able to reach the
conditions of starstuff. "The details of how they heat and
compress the matter are different," says Remington, "but the
physics discussions are very similar."
When NIF switches on, 192 lasers aim at a point at the
center. What's at that center depends on what researchers
want to zap. Sometimes the lasers focus directly on a spherical shell only a few millimeters wide, and other times that
capsule is within a cylinder and the cylinder itself receives the
radiation. In both cases, when the energy hits the surface of
that shell, it vaporizes the shell's outer surface, which then

TINY TARGETS
In the NIF's setup,
laser beams enter
through a capsule,
called a hohlraum, and
compress and heat the
target within. Scientists
modify the exact setup
depending on what
they're testing.

sk yandtele scope.com * OCTOBE R 2 019

39


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Sky & Telescope - October 2019

Table of Contents for the Digital Edition of Sky & Telescope - October 2019

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Sky & Telescope - October 2019 - 1
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