In recent Z Machine experiments, two Helmholz coils unexpectedly altered and slowed the growth of the magneto Rayleigh-Taylor (RT) instabilities, a plasma distortion that usually spins quickly out of control and has sunk past efforts to achieve controlled fusion. Radiographs of the process showed that the Helmholz coils’ field altered and slowed magneto-RT instability growth, which had been thought to occur unavoidably. (Unavoidably, because even the tiniest differences in materials turned to plasma are magnified by pressures applied over time.)

An RT instability is an instability of an interface between two fluids of different densities that occurs when one of the fluids is accelerated into the other. As the instability develops, the initial perturbations progress from a linear growth phase into a nonlinear or “exponential” growth phase, eventually developing “bubbles” flowing upwards (in the gravitational buoyancy sense) and “spikes” falling downwards.

“Our experiments dramatically altered the nature of the instability,” said Sandia physicist Tom Awe (High Energy-Density Experiments Dept.). “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 left-hand image demonstrates the instabilities that grow on a liner’s surface without a secondary magnetic field. This “bubble-and-spike” structure is caused by the magneto Rayleigh-Taylor instability, which commonly plagues z-pinch experiments. The bubble regions give up their mass to the spike regions; eventually, the bubbles break through the liner wall, and the liner’s ability to compress fuel is lost. By contrast, the right-hand image shows the unique and unexpected helical instability structure that forms when the liner is premagnetized with a 7 Tesla field. Preliminary evidence suggests that this modified instability structure may work its way through the liner more slowly, enabling it to more effectively compress the fuel within.

The left-hand image demonstrates the instabilities that grow on a liner’s surface without a secondary magnetic field. This “bubble-and-spike” structure is caused by the magneto Rayleigh-Taylor instability, which commonly plagues z-pinch experiments. The bubble regions give up their mass to the spike regions; eventually, the bubbles break through the liner wall, and the liner’s ability to compress fuel is lost. By contrast, the right-hand image shows the unique and unexpected helical instability structure that forms when the liner is premagnetized with a 7 Tesla field. Preliminary evidence suggests that this modified instability structure may work its way through the liner more slowly, enabling it to more effectively compress the fuel within.

In previous attempts to use Z’s huge field without the Helmholtz coils, radiographs showed instabilities appearing on the exterior of the liner. These disturbances caused the liner’s initially smooth exterior to resemble a stack of metallic washers. Such instabilities increase dramatically in mere 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—which 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.

Two Helmholz coils were added to Z Machine fusion experiments to demonstrate that the secondary field would create a magnetic barrier that, like insulation, would maintain a Z-created plasma’s charged-particle energy. The coils indeed buffered the particles.

Sandia physicist Thomas Awe examines the Helmholz coils that reduce plasma instabilities in the quest for controlled nuclear fusion at Sandia’s Z Machine. (Photo by Randy Montoya)

Sandia physicist Thomas Awe examines the Helmholz coils that reduce plasma instabilities in the quest for controlled nuclear fusion at Sandia’s Z Machine. (Photo by Randy Montoya)

But unexpectedly, radiographs of the process showed that the coils’ field had altered and slowed the growth of the magneto-RT instabilities. The strength of instabilities seen in hundreds of previous z-pinches was reduced, possibly significantly. Firing with the secondary field up and running clearly altered and slowed formation of the magneto-RT 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—stacks of washers—into a helical pattern that more resembles the slanting patchwork of a plaid sweater.

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 preset 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 RT instabilities.

Read the Sandia news release.