Sandia LabNews

Tom Sanford shares European physics prize for work on Z

Tom Sanford shares European physics prize for work on Z

In the late afternoon of July 6, 1995, Sandia researcher Tom Sanford (1677) looked at data representing the first Z-pinch implosion ever achieved with a large number of target wires — 90 — and saw a sight that stunned him.

What he saw became arguably the most important observation ever made in transforming Z into the most powerful laboratory X-ray source in the world.

The greater number of wires and subsequent implosions on the Saturn pulsed power generator increased the output radiation pulse from aluminum wire arrays to 40 terawatts, three times the X-ray power measured from Z-pinch implosions of similar wire materials.

The experiment on the Saturn facility showed it was possible to concentrate the X-ray output from a 100-nanosecond-long Z-pinch implosion into 3 nanoseconds. Formerly — using 24 wires or less, the standard for decades — the X-ray pulse was longer than 15 nanoseconds.

The result led to a furious burst of work by Sandia technical staff that produced nearly 80 terawatts (TW) of X-ray power from tungsten wire arrays on Saturn. The increase in X-ray output increased the excitement about the potential of the on-going project to convert the more powerful PBFA-II (a pulsed power machine that bombarded targets with lithium ions) into a high-current driver for Z-pinch implosions. Completed in September 1996 and using large numbers of wires, the accelerator — dubbed Z — soon produced more than 200 TW of X-rays for stockpile and fusion energy purposes.

For his observation, and for follow-up work by Tom with other Sandians, and for work by Russian and English colleagues, all of which continue to this day, Tom will share the European Physical Society’s Hannes Alfven Prize. Tom and the other recipients, Malcolm Haines, former director of London’s Imperial College Plasma Physics Dept., and Valentin Smirnov, director of the Institute of Nuclear Fusion at the Kurchatov Institute in Moscow were cited for “the remarkable achievements of the multi-filament Z-pinch development in the recent years.”

The three will share a prize of 5,000 Euros to be awarded on Monday, June 27, at this year’s annual meeting of the Society in Barcelona, Spain.
Both Smirnov and Haines, in separate interviews with the Lab News, described the considerable depth and longevity of their own contributions to Z-pinch development but graciously gave credit to Tom and Sandia for his observation and the Labs’ subsequent validating tests by a number of personnel.

“[Tom’s] technical observation was correct, but he had to be stubbornly persuasive to get resources transferred to this [multifilament] area,” observed Haines.

Said Smirnov, “The greatest achievement [in Z-pinch work] was made by Sandia in increasing the radiating material of the wires and in reconstructing PBFA II to Z.”

Says Tom, “To prove out an idea like this, you need community. And I had it at Sandia.” He particularly mentioned support from Gerry Yonas (16000), Wendland Beezhold (ret.), Ray Leeper (1677), John Maenchen (1645) , Tom Nash (1677), Barry Marder (ret.), George Allshouse (deceased), and Ray Mock (1677).

What Tom saw

A Z-pinch is so named because the electrical current that vaporizes slender wires hanging vertically — to mathematicians, the “z” direction of space — in a cylindrical pattern also creates a magnetic field that pinches the resultant ions into a much smaller volume.

Energy is emitted when ions stop suddenly upon arrival at the center of the cylindrical array.

The general assumption before Tom’s 1995 observation was that plasma — a field of ions — generated by cylindrical arrays containing 24 wires or less, themselves formed a cylindrical cloud or “shell” that compressed evenly by the action of the overall magnetic field.

That assumption led to a consensus that adding more wires would cause only marginal improvements in the X-ray energies generated by their plasmas. It was a convenient belief. Though theorists at the Naval Research Laboratory had advocated increasing the number of wires in the array, adding wires was a laborious process. The wires were only microns thick and snapped easily. And so the overall experimental consensus held for decades.

What startled Tom in January of that year was that pictures taken by a new pinhole camera showed a completely unexpected effect. The pinhole camera was state of the art: it had electronics that allowed a nanosecond exposure and no lens to shatter from the force of an explosion; its focal length alone, predetermined merely by the camera’s size, was enough to take pictures of unequalled clarity. Aided by protective devices, it could be placed close to the wire array.

What its film showed was that a single magnetic shell was not formed by the vaporized wire ions. Instead, individual shells formed around each wire. Each wire, in effect, was self-pinching. And each lurched inward, inharmoniously with its neighbors, in the grip of the overall magnetic field.
“If they’re clumping like this,” thought Tom, “[using only] a few wires seems like a bad idea.”

Installing many more wires in the array, he thought, might create the magnetic shell mistakenly thought to be already in place.

If the amount of energy already achieved were merely the result of individual wire shells in effect staggering inward, how much more energy could be obtained from an implosion involving many more wires that created a true shell that compressed coherently toward the center of the pinch?
The force created might exceed the simple addition of individual wire plasmas added to other wire plasmas.

Because of the complexity of building arrays with large numbers of wires, the experiment had never been tried.

Tom, with aid from other Sandians, proceeded to find out.

He had been trained by two high-energy physics Nobel laureates — Leon Lederman and Sam Ting — not to settle for inconclusive solutions.
In the tenacity of his experiments, says his manager Ray Leeper simply, “Tom’s a bulldog.”

Tom set up a series of experiments, using different radii of wires with spacing adjusted to keep the total wire mass constant, to determine whether wire size and spacing had any appreciable effect as his team painstakingly measured

X-ray output produced by arrays ranging from a very small number to hundreds of wires. The results were clear. A larger number of thinner wires with minimum spacing between them sent the output of Saturn, and then Z, skyrocketing, and eventually caused a change in the world scientific view of the Z-pinch process.

“When I saw the narrow radiation pulse emerging from a 90-wire array, I knew that by significantly increasing the number of wires we had cracked the instability nut that had plagued Z-pinches for a number of decades,” Tom said.

Said Haines, “When Tom got those spectacular results by increasing the number of wires and decreasing their separation and found the X-ray yield went up enormously, that’s what alerted us. We switched to tungsten wires from cryogenic deuterium fiber.”

The fibers at Imperial College were an attempt to achieve fusion directly, rather than the two-step process at Z of first bottling tungsten plasma-produced X-rays in a hohlraum to then attack a deuterium pellet.

“I knew it was going to be a big deal,” says Tom, “but I didn’t know how big.

“Where it ends up, we don’t know yet. But it’s regenerated a worldwide effort on Z-pinches.”

Tom, who has been “riding the tsunami of papers” generated by his discovery, has since then been “swimming in the ocean of Z-pinch physics,” (as he phrases these things), turning out more than 20 papers in the last 10 years on the phenomenon, and his work is ongoing.

Co-winner Malcolm Haines’ contributions to Z-pinch work began in the mid-1950s with his PhD thesis in 1957, when he predicted the conditions for the explosions of single wires and the amount of current necessary in Z-pinches to produce thermonuclear fusion. He continued over decades with theoretical explanations and practical experiments that relied upon results from the smaller pulsed power machine at Imperial College. Among his contributions was a theory that satisfactorily explained the increase in power generated by the increased number of wires of the Z-pinch.

In recent work with David Lepell (1646), Chris Deeney (1640), and Christine Coverdale (6744), he proposed a solution for why more energy is radiated than the energy of the implosion. “I like a mystery,” he said.

The contribution of co-winner Valentin Smirnov’s group stretches back to the 1980s, when gas puffs were considered a possible source of ions for Z-pinches. “Smirnov’s group provided incentive to us to push the pinch,” says Gerry Yonas. “They had insights into pinches before we did. We sent a team to measure their results, and they sent a team here.”