Dept of Energy CSGF magazine 2007-8 -- DOE Lab Research

Sandia National Laboratories

Feeling Z Pinch

There’s a homegrown brand of fusion science Sandia and Los Alamos researchers want to help model with their algorithms for extended magnetohydrodynamics.

Sandia National Laboratory’s New Mexico location houses the Z machine, the world’s largest controlled X-ray generator. It’s designed to test materials under extreme conditions of radiation and pressure as part of the U.S. nuclear weapons stockpile stewardship program, but it’s also being studied as a route to creating the clean energy of nuclear fusion.

Through a system of huge capacitors and wires, and with 20 million amperes of electrical power, the Z machine can generate pulses of 200 trillion watts of power for short periods. The pulse vaporizes an array of metal wires around a housing the size of a spool of thread, turning them into plasma. The powerful magnetic field generated by the current compresses the plasma to about the thickness of a pencil lead. Under compression, the moving ions and electrons suddenly stagnate, releasing energy as X-rays and reaching temperatures of billions of degrees.

The current passing through the wires travels vertically, or along the Z axis. Since magnetic fields “pinch” the plasma, the process has been called Z-pinch confinement.

Sandia scientists have used the Z machine to fuse tiny amounts of deuterium, producing thermonuclear neutrons — a step toward creating a self-sustaining fusion reaction.

Sandia researcher John Shadid says some of his group’s algorithms may be useful in helping model hydromagnetic Rayleigh-Taylor instabilities in the plasma implosion. Such instabilities are an important limiting mechanism for the amount of radiation energy produced by a Z-pinch and are therefore critical to understand and control. The algorithms the groups are developing may find their way into production simulation codes like Sandia’s ALEGRA, which is used in modeling the Z machine.

“There’s a large program already at Sandia in terms of modeling this kind of physics, but there are a number of open issues still, and one of them is handling these complex, interacting, multiple time scales” — a focus for his group’s research, Shadid says. Luis Chacón, Shadid’s collaborator at Los Alamos National Laboratory, says the powerful machine is ripe for simulations using the group’s approach. It’s been hypothesized that electron physics plays a role in the Z machine’s operation “and that brings all the tools we’re talking about in that regard.” Computer models of the Z machine use operator-split time-stepping techniques. These methods can sometimes introduce instabilities and lead to significant error accumulation. Chacón and Shadid believe more effective implicit coupling could cut errors and increase efficiency, provided the faster time scales electron physics brings to the problem can be managed.