ALBUQUERQUE, N.M. — Researchers at Sandia National Laboratories’ Z machine have produced a significant output of fusion neutrons, using a method fully functioning for only little more than a year.
At the heart of
Sandia National Laboratories’ Z machine, Matt Gomez, left, presents an idea to
Steve Slutz, right, while Adam Sefkow looks on.(Photo by Randy Montoya) Click on the thumbnail for a
high-resolution image. The experimental work is described in a
paper to be published in the Sept. 24 Physical Review Letters online.
A theoretical PRL paper to be published on the same date helps
explain why the experimental method worked. The combined work demonstrates the
viability of the novel approach. “We are committed to shaking this [fusion]
tree until either we get some good apples or a branch falls down and hits us on
the head,” said Sandia senior manager Dan Sinars. He expects the project,
dubbed MagLIF for magnetized liner inertial fusion, will be “a key piece of
Sandia’s submission for a July 2015 National Nuclear Security Administration
review of the national Inertial Confinement Fusion Program.” Inertial confinement fusion creates
nanosecond bursts of neutrons, ideal for creating data to plug into
supercomputer codes that test the safety, security and effectiveness of the
U.S. nuclear stockpile. The method could be useful as an energy source down the
road if the individual fusion pulses can be sequenced like an automobile’s
cylinders firing. MagLIF uses a laser to preheat hydrogen
fuel, a large magnetic field to squeeze the fuel and a separate magnetic field
to keep charged atomic particles from leaving the scene. It only took the two magnetic fields and
the laser, focused on a small amount of fusible material called deuterium
(hydrogen with a neutron added to its nucleus), to produce a trillion fusion
neutrons (neutrons created by the fusing of atomic nuclei). Had tritium (which
carries two neutrons) been included in the fuel, scientific rule-of-thumb says
that 100 times more fusion neutrons would have been released. (That is, the
actual release of 10 to the 12th neutrons would be upgraded, by the more
reactive nature of the fuel, to 10 to the 14th neutrons.) Still, even with this larger output, to
achieve break-even fusion — as much power out of the fuel as placed into it —
100 times more neutrons (10 to the 16th) would have to be produced. The gap is sizable, but the technique is a
toddler, with researchers still figuring out the simplest measures: how thick
or thin key structural elements of the design should be and the relation
between the three key aspects of the approach — the two magnetic fields and the
laser. The first paper, “Experimental
Demonstration of Fusion-Relevant Conditions in Magnetized Liner inertial
fusion,” (MagLIF) by Sandia lead authors Matt Gomez, Steve Slutz and Adam
Sefkow, describes a fusion experiment remarkably simple to visualize. The
deuterium target atoms are placed within a long thin tube called a liner. A
magnetic field from two pancake-shaped (Helmholtz) coils above and below the
liner creates an electromagnetic curtain that prevents charged particles, both
electrons and ions, from escaping. The extraordinarily powerful magnetic field
of Sandia’s Z machine then crushes the liner like an athlete crushing a soda
can, forcefully shoving atoms in the container into more direct contact. As the
crushing begins, a laser beam preheats the deuterium atoms, infusing them with
energy to increase their chances of fusing at the end of the implosion. (A
nuclear reaction occurs when an atom’s core is combined with that of another
atom, releasing large amounts of energy from a small amount of source material.
That outcome is important in stockpile stewardship and, eventually, in civilian
energy production.) Trapped energized particles including fusion-produced alpha
particles (two neutrons, two protons) also help maintain the high temperature
of the reaction. “On a future facility, trapped alpha
particles would further self-heat the plasma and increase the fusion rate, a
process required for break-even fusion or better,” said Sefkow. The actual MagLIF procedure follows this
order: The Helmholtz coils are turned on for a few thousandths of a second.
Within that relatively large amount of time, a 19-megaAmpere electrical pulse
from Z, with its attendant huge magnetic field, fires for about 100 nanoseconds
or less than a millionth of a second with a power curve that rises to a peak
and then falls in intensity. Just after the 50-nanosecond mark, near the
current pulse’s peak intensity, the laser, called Z-Beamlet, fires for several
nanoseconds, warming the fuel. According to the paper’s authors, the unusual arrangement of using magnetic forces both to collapse the tube and simultaneously insulate the fuel, keeping it hot, means researchers could slow down the process of creating fusion neutrons. What had been a precipitous process using X-rays or lasers to collapse a small unmagnetized sphere at enormous velocities of 300 kilometers per second, can happen at about one-quarter speed at a much more “modest” 70 km/sec. (“Modest” only comparatively; the speed is about six times greater than that needed to put a satellite in orbit.)
Sandia National
Laboratories researchers Paul Schmit, left, and Patrick Knapp discuss equations
and graphs that describe aspects of Sandia’s Z machine. (Photo by Randy Montoya) Click on the
thumbnail for a high-resolution image. The slower pace allows more time for
fusible reactions to take place. The more benign implosion also means fewer
unwanted materials from the collapsing liner mix into the fusion fuel, which
would cool it and prevent fusion from occurring. By analogy, a child walking
slowly in the ocean’s shallows stirs less mud than a vigorously running child. Sandia senior scientist Mike Campbell said,
“This experiment showed that fusion will still occur if a plasma is heated by
slow, rather than rapid, compression. With rapid compression, if you mix
materials emitted from the tube’s restraining walls into the fuel, the fusion
process won’t work; also, increased acceleration increases the growth of
instabilities. A thicker can [tube] is less likely to be destroyed when
contracted, which would dump unwanted material into the deuterium mix, and you
also reduce instabilities, so you win twice.” Besides the primary deuterium fusion
neutron yields, the team’s measurements also found a smaller secondary
deuterium-tritium neutron signal, about a hundredfold larger than what would
have been expected without magnetization, providing a smoking gun for the
existence of extreme magnetic fields. The question remained whether it was indeed
the secondary magnetic field that caused the 100-fold increase in this
additional neutron pulse, or some other, still unknown cause. Fortunately, the
pulse has a distinct nuclear signature arising from the interaction of tritium
nuclei as they slow down and react with the primary deuterium fuel, and that
interaction was detected by Sandia sensors. The secondary magnetic field is the subject
of the second, theoretical paper, “Understanding fuel magnetization and mix
using secondary nuclear reactions in magneto-inertial fusion.” Using
simulations, Sandia researchers Paul Schmit, Patrick Knapp, et al confirmed the
existence and effect of extreme magnetic fields. Their calculations showed that
the tritium nuclei would be encouraged by these magnetic fields to move along
tight helical paths. This confinement increased the probability of subsequently
fusing with the main deuterium fuel. “This dramatically increases the
probability of fusion,” Schmit said. “That it happened validates a critical
component of the MagLIF concept as a viable pathway forward for fusion. Our
work has helped show that MagLIF experiments are already beginning to explore
conditions that will be essential to achieving high yield and/or ignition in
the future.” The foundation of Sandia’s MagLIF work is
based on work led by Slutz. In a 2010 Physics of Plasmasarticle, Slutz showed that a tube enclosing
preheated deuterium and tritium, crushed by the large magnetic fields of the
27-million-ampere Z machine and a secondary magnetic field, would yield
slightly more energy than is inserted into it. A later simulation, published January 2012 in Physical
Review Letters by Slutz and Sandia researcher Roger Vesey, showed that
a more powerful accelerator generating 60 million amperes or more could reach
“high-gain” fusion conditions, where the fusion energy released exceeds by more
than 1,000 times the energy supplied to the fuel. A paper led by Sefkow et al, published July
24, in Physics of Plasmas, further explicated and designed the
experiments based on predictions made in Slutz’s earlier paper. But, said Campbell, “there is still a long
way to go.” Sandia National Laboratories is a
multi-program laboratory operated by Sandia Corporation, a wholly owned
subsidiary of Lockheed Martin Corp., for the U.S. Department of Energy’s
National Nuclear Security Administration. With main facilities in Albuquerque,
N.M., and Livermore, Calif., Sandia has major R&D responsibilities in
national security, energy and environmental technologies and economic
competitiveness. Sandia
news media contact: Neal
Singer, nsinger@sandia.gov, (505) 845-7078 |
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