Researchers at Sandia National Laboratories in Albuquerque, New Mexico, using the lab’s Z machine, a colossal electric pulse generator capable of producing currents of tens of millions of amperes, say they have detected significant numbers of neutrons—byproducts of fusion reactions—coming from the experiment.
For enough reactions to take place, the hydrogen nuclei must collide at velocities of up to 1000 kilometers per second (km/s), and that requires heating them to more than 50 million degrees Celsius.
Sandia’s technique is one of several that fall into the middle ground between the extremes of laser fusion and the magnetically confined fusion of tokamaks. It crushes fuel in a fast pulse, as in laser fusion, but not as fast and not to such high density. Known as magnetized liner inertial fusion (MagLIF), the approach involves putting some fusion fuel (a gas of the hydrogen isotope deuterium) inside a tiny metal can 5 millimeters across and 7.5 mm tall. Researchers then use the Z machine to pass a huge current pulse of 19 million amps, lasting just 100 nanoseconds, through the can from top to bottom. This creates a powerful magnetic field that crushes the can inward at a speed of 70 km per second.
Crushing the plasma also boosts the constraining magnetic field, from about 10 tesla to 10,000 tesla.
They heated the plasma to about 35 million degrees Celsius and detected about 2 trillion neutrons coming from each shot. (One reaction of fusing two deuteriums produces helium-3 and a neutron.) Although the result shows that a substantial number of reactions is taking place—100 times as many as the team achieved a year ago—the group will need to produce 10,000 times as many to achieve breakeven.
A schematic representation of the three critical components of the MagLIF concept. An axial current creates a Jz×BΘ force that is used to implode a gas-filled, premagnetized, cylindrical target. Near the start of the implosion, the fuel is heated by the laser. The liner compresses and further heats the fuel to fusion-relevant conditions at stagnation.
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.
This Letter presents results from the first fully integrated experiments testing the magnetized liner inertial fusion concept [S. A. Slutz et al., Phys. Plasmas 17, 056303 (2010)], in which a cylinder of deuterium gas with a preimposed 10 T axial magnetic field is heated by Z beamlet, a 2.5 kJ, 1 TW laser, and magnetically imploded by a 19 MA, 100 ns rise time current on the Z facility. Despite a predicted peak implosion velocity of only 70 km/s, the fuel reaches a stagnation temperature of approximately 3 keV, with Te≈Ti, and produces up to 2 trillion thermonuclear deuterium-deuterium neutrons. X-ray emission indicates a hot fuel region with full width at half maximum ranging from 60 to 120 μm over a 6 mm height and lasting approximately 2 ns. Greater than 10 billion secondary deuterium-tritium neutrons were observed, indicating significant fuel magnetization given that the estimated radial areal density of the plasma is only 2 mg per square centimeter.
SOURCES – Sandia National Labs, Physical Review Letters, Science
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