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SANDIA LAB NEWS

Lab News -- June 24, 2005

June 24 , 2005

LabNews 06/24/2005PDF (650KB)

‘Active Denial System’ could boost security of DOE nuclear assets

By Michael Padilla

A multi-organizational team has developed technology that can be used to put the “heat” on adversaries and help protect DOE nuclear assets.

The DOE Office of Security and Safety Performance Assurance (SSA) is exploring the potential to use directed energy weapons technology sponsored by the Department of Defense (DoD), named Active Denial Technology (ADT), to help protect DOE nuclear assets.

SSA is sponsoring Sandia to investigate how this technology can be used on adversaries by developing a new small-sized Active Denial System (ADS) to meet the unique and rapidly evolving security needs of DOE. To help solve the many technical issues associated with this challenge, Sandia has partnered with Raytheon and the Air Force Research Laboratory (AFRL), because both organizations have significant experience with earlier ADS system developments.

ADS systems are a new class of nonlethal weaponry using 95 GHz-millimeter-wave directed energy. This technology is capable of rapidly heating a person’s skin to achieve a pain threshold that has been demonstrated by AFRL human subject testing to be very effective at repelling people, without burning the skin or causing other secondary effects.

In the mid 1990s, the Air Force funded development of an ADT system demonstrator that was led by AFRL and built by Raytheon in partnership with Communications & Power Inc. (CPI) and Malibu Research. The success of this demonstration system has resulted in several ongoing DoD-sponsored projects, such as the Joint Non-Lethal Weapons Directorate’s Vehicle Mounted Active Denial System (VMADS) and the Office of Force Transformation’s (OFT's) project SHERIFF.

Steve Scott, Sandia’s Access Delay Technology (4122) department manager, says, “DOE and Sandia have been closely tracking ADT developments and have recognized its potential to enhance the protection of DOE nuclear facilities. This has been confirmed this by conducting a feasibility study in 2002, under the supervision of researcher Jim Pacheco [4122].”

Acting on the feasibility study’s conclusions, SSA’s Carl Pocratsky (SO-20) initiated an effort at Sandia to explore and develop a small Active Denial System (ADS) that is more suitable for DOE fixed-site applications. To date, DoD efforts have focused on larger systems, considered by many to be better suited for military applications at extended ranges.

In 2004, the AFRL’s Human Effectiveness Directorate (HEDR) completed a study that analyzed preexisting test data to estimate the potential effectiveness of what could be achieved with an ADS that has a smaller beam. Also in 2004, Sandia conducted simulations of how the smaller ADS might be used and how it would perform against adversary attack scenarios within a DOE facility using the Joint Conflict and Tactical Simulation (JCATS) software modeling tool.

“The results of the AFRL small beam ADS effectiveness study and the JCATS study were very encouraging and provided a strong basis for continuing the development of a comparitively small ADS for DOE fixed-site applications,” says Jim.

“Recently there has been significant progress with this project,” says Willy Morse, Sandia’s principal investigator. “On May 5 we took acceptance of the SSA ADS prototype system built by Raytheon’s Advanced Electromagnetic Technologies (AET) Center in partnership with CPI and Malibu Research. Initial characterization and performance tests were completed at the end May.”

On May 19 a memorandum of understanding was completed between DOE-SSA, Sandia, DoD-OFT, and AFRL. This memorandum establishes a formal partnership between the DoD and DOE in developing small-sized ADSs. During the next six months the AFRL’s Human Effectiveness Directorate, Brooks City-Base, is being funded by the OFT to complete human effects testing. This testing will use the SSA ADS system to determine its effectiveness for DOD applications and validate the conclusions of the 2004 small-beam-size effectiveness study sponsored by SSA. Testing results from Sandia, AFRL, and OFT will guide the operational concept and design of a second-generation small-size ADS system expected to be fielded at several DOE nuclear facilities as early as 2008. DOE-SSA and Sandia will continue to actively seek opportunities to collaborate with other government agencies on technical issues associated with developing and deploying ADS systems. -- Michael Padilla

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Silly Putty probe yields non-silly results about time-dependent material properties

By Neal Singer

Leave it overnight on your papers and it’s sticky as bubblegum. Pat it into a sphere and it bounces like a tennis ball. Hit it sharply with a hammer, and it separates with edges flat as crystal.

Silly Putty™. The words should strike fear in the hearts of experimentalists dealing with visco-elastic materials. The polymeric stuff creeps forever under a static load, so conditions of the experiment continually change. Worse, the stuff strongly adheres to probes: Blind sensors register little data.

However, Sandia researcher Jack Houston (1114) sees the solid liquid merely as the most extreme representation of a common modern material: particles encased in a polymer matrix. These include golf club shafts and rocket fuel, polymers bonding steel to cement, and rubber bands. All these polymers (and many others), elastic at first, weaken over time, leaving golf clubs creaky, compressing rocket fuel into a bomb, weakening cement, and causing aged rubber bands to snap when stretched.

What’s needed, Jack thought, are better methods to locally measure the effect of time on these polymer chains and to set up a chart of expected deterioration rates. Methods used today either involve bulk probes or don’t have quick enough response to map time-dependent effects. Jack felt he had a better product: the interfacial force microscope, which he invented 15 years ago and on which he and Bill Smith (1114) recently obtained a patent for an updated sensor.

Like the show-business phrase about making it in New York, Jack figured if he could measure the time response of silly putty — the most experimentally difficult of all polymer matrices — he could make such measurements on any polymer matrix.

Characterizing matrix deterioration in the laboratory could alert manufacturers or users before unpleasant outcomes come to pass. And timetables of matrix decay could be established, if suitable measurements could be made to examine the pace at which a particular matrix changes.
“We started out wanting to study adhesion in a more fundamental way by actually watching the bond form and subsequently fail,” says Jack of his efforts of a decade ago. At that time no technique was available to make such measurements.

And it wasn’t as though anyone recently radioed Jack to say, “Houston, we have a problem.” But it occurred to him that his microscope could solve this increasingly widespread problem of measuring local changes in matrix behavior better than any other tool.
His results, supported by DOE’s Office of Basic Energy Sciences, have been accepted for publication by the Journal of Polymer Science B (Physics).

The IFM is unique in being able to obtain quantitative and mechanically stable data of both the adhesive interaction and a material’s time-dependent mechanical properties. It’s like the atomic force microscope, or AFM, but that popular technique suffers from being mechanically unstable, says Jack. It snaps in and out of contact, like trying to bring two kitchen magnets, one in each hand, together controllably.

The IFM, on the other hand, has a tip located on one end of a very small “teeter totter,” which is supported by torsion bars above two tiny capacitor pads. When a sample is brought very near the tip, the force of attraction between the tip and sample causes the teeter to totter, increasing the capacitance of one and decreasing the other. The key to the IFM concept is that this rotation is forced to zero by a feedback system, which places the proper voltages on the capacitor pads. Forces are thereby measured quantitatively by the amount of voltage necessary to achieve balance without tip motion.

By pushing on its target, the probe deforms the material. The measurable force changes with time and depends on the nature of the material. Suddenly advancing the tip into Silly Putty results in a spring-like deformation and a large initial force, which rapidly decays as the material creeps away from the tip in a viscous flow, leaving behind a dent.

“This tells you how much stress the material can tolerate and over what period of time the stress can be maintained, which can be translated into the material’s frequency response,” says Houston. The microscope measures this stress response in a few seconds, with results that matched 10 to 12 individual frequency tests by a classical rheometer.

Rheometer tests are done on bulk samples and consist of a series of measurements over a range of frequencies. Such measurements can take several minutes, during which time the sample can creep and change the experimental conditions.

The IFM measurement gives the details on how the material reacts to being deformed and is done in a time frame where the experimental conditions remain the same.

There are currently 17 IFM machines in use at Sandia and various universities around the US and in Canada.

Houston expects more next year when a newly patented laser interferometric measuring system replaces the simple radio-frequency bridge system that can’t be scaled to smaller sensor dimensions. The new system achieves greater sensitivity. -- Neal Singer

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Tom Sanford shares European physics prize for work on Z

By Neal Singer

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.” -- Neal Singer

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