Fusion instabilities lessened by unexpected effect
A surprising effect created by a 19th-century device called a Helmholtz coil offers clues about how to achieve controlled nuclear fusion at Sandia National Laboratories’ powerful Z machine.
A Helmholtz coil produces a magnetic field when electrified. In recent experiments, two Helmholtz coils, installed to provide a secondary magnetic field to Z’s huge one, unexpectedly altered and slowed the growth of the magneto-Rayleigh-Taylor instabilities, an unavoidable, game-ending plasma distortion that usually spins quickly out of control and has sunk past efforts to achieve controlled fusion. “Our experiments dramatically altered the nature of the instability,” said Sandia physicist Tom Awe. “We don’t yet understand all the implications, but it’s become a different beast, which is an exciting physics result.”
The experiments were reported in Physical Review Letters.
The purpose of adding two Helmholtz coils to fusion experiments at the Z machine, which produces a magnetic field 1,000 times stronger than the coils, was to demonstrate that the secondary field would create a magnetic barrier that, like insulation, would maintain the energy of charged particles in a Z-created plasma. Theoretically, the coils’ field would do this by keeping particles away from the machine’s walls. Contact would lower the fusion reaction’s temperature and cause it to fail.
Researchers also feared that the Helmholz field might cause a short in Z’s huge electrical pulse as it and its corresponding magnetic field sped toward the target, a small deuterium-stuffed cylinder.
Z’s magnetic field is intended to crush the cylinder, called a liner, fusing the deuterium and releasing neutrons and other energies associated with nuclear fusion. Anything hindering that “pinch” or “z-pinch,” would doom the experiment.
In preliminary experiments by Awe’s group, the coils indeed buffered the particles and didn’t interfere with the pinch.
Enter, the coils
But unexpectedly, radiographs of the process showed that the coils’ field had altered and slowed the growth of the magneto-Rayleigh-Taylor instabilities. Those distortions had been thought to occur unavoidably because even the most minute differences in materials turned to plasma are magnified by pressures applied over time.
The strength of instabilities seen in hundreds of previous z-pinches was reduced, possibly significantly.
The typical distortion pattern also changed shape from horizontal to helical.
The unexpected results occurred in a series of experiments to study a concept called Magnetized Liner Inertial Fusion, or MagLIF.
Experimental process like French toast
Researchers placed the Helmholtz coils around a liner containing deuterium so the coils’ magnetic field lines soaked both container and fuel over a period of milliseconds. The relatively slow process, like soaking bread in beaten eggs and milk to make French toast, allowed time for the magnetic field lines to fully permeate the material. Then the liner was crushed in tens of nanoseconds by the massive magnetic implosion generated by Sandia’s Z machine. In previous attempts to use Z’s huge field without the Helmholz coils, radiographs showed instabilities appearing on the exterior of the liner. These disturbances cause the liner’s initially smooth exterior to resemble a stack of metallic washers, or small sausage links separated by horizontal rubber bands. Such instabilities increase dramatically in nanoseconds, eating through the liner wall like decay through a tooth. Eventually, they may collapse portions of the inner wall of the liner, releasing microrubble and causing uneven fuel compression that would make fusing significant amounts of deuterium impossible.
The disturbances are a warning sign that the liner might crumple before fully completing its fusion mission.
But firing with the secondary field running clearly altered and slowed formation of the instability as the liner quickly shrank to a fraction of its initial diameter. Introducing the secondary magnetic field seemed to realign the instabilities from simple circles—stacked washers, or rubber bands around sausages—into a helical pattern that more resembled the slanting patchwork of a plaid sweater.
Like a kayak crossing a river
Researchers speculate that the vertical magnetic field created by the helical coils, cutting across Z’s horizontal field, may create the same effect as a river slanting a kayak downstream rather than straight across a channel. Or it may be that the kayak’s original direction is pre-set by the secondary magnetic field to angle it downstream. Whatever the reason, the helical instability created does not appear to eat through the liner wall as rapidly as typical horizontal Rayleigh-Taylor instabilities.
Flashes of x-rays that were released when material from the horizontal instabilities collided in the liner’s center no longer appeared, suggesting more uniform fuel compression occurred, possibly a result of the increasing resistance of the implanted vertical magnetic field to the compression generated by the Z horizontal field.
The overall approach of Awe and his colleagues uses a method described in two papers by Sandia theorist Steve Slutz. In a 2010 article in Physics of Plasmas, Slutz suggested that the magnetic field generated by Z could crush a metallic liner filled with deuterium, fusing the atoms. Slutz and co-authors then indicated, in a 2012 paper in Physical Review Letters, that a more powerful version of Z could create high-yield fusion—much more fusion energy out than the electrical energy put in.
The apparently simple method—turn on a huge magnetic field and wait a few nanoseconds—takes for granted the complicated host of engineered devices and technical services that allow Z to function. But, those aside, the process as described by Slutz needed only two additional aids: a powerful laser to preheat the fuel, making it easier for the compressed fuel to reach fusion temperatures, and Helmholtz coils above and below the target to generate a separate, weaker magnetic field that would insulate charged particles from giving up their energy, thereby lowering the temperature of the reaction.
Ongoing experiments on Z will determine how well reality bears out Slutz’s predictions.
Source: Sandia National Laboratories