Working with two magnetic fields and a laser, all at low points of their power outputs, Sandia’s Z machine has released neutrons in an amount surprisingly close to ‘break-even’ fusion.
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.
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.
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