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ALBUQUERQUE, N.M. — A pulsed power accelerator at Sandia National Laboratories has in recent weeks quadrupled its output and demonstrated its ability to pump out 85 terawatts of power — more than 50 times the output of the U.S. utility grid — causing scientists to revise their estimates of the power levels that can ultimately be achieved.
Such power levels are important in ensuring the safety of the nation’s nuclear weapon stockpile. Data generated inside Saturn and other Sandia accelerators are used to test three-dimensional computer codes that simulate what happens inside a nuclear weapon when it detonates. Simulations such as this are increasingly important as the United States seeks a worldwide ban on nuclear testing.
The record-breaking outputs were achieved after Sandia researchers tinkered with target arrays that produce the magnetic implosions. Though predicted analytically, the high output levels were only recently achieved experimentally.
The breakthrough has “altered the mindset we’ve been operating with” about Saturn’s capabilities, says Don Cook, Director of Sandia’s Pulsed Power Sciences Center. “Controlling the symmetry of the implosion was the key.”
In simplest terms, a pulsed power accelerator such as Saturn uses a series of capacitors, or electrical dams, to build and store electrical charges over a period of time, then simultaneously discharge them in fractions of a second. As the electrical floodgates open, the resulting electrical pulse drives very high currents through a target area. Collectively these currents — something like a tight bundle of lightning bolts — generate a magnetic field that “squeezes” objects in the target area.
In recent Saturn experiments, circular arrays of fine tungsten wires placed in the target area are ionized into a hot plasma and their masses driven toward their axis in billionths of a second by the compressive force of the electromagnetic field. This magnetic implosion in turn causes the split-second release of hundreds of thousands of joules of X-ray energy. The faster and more symmetrically the masses are squeezed together, the more X-ray power is generated in the implosion.
More wires means greater symmetry
Earlier Saturn experiments employed target arrays of as many as 24 tungsten wires arranged in a circle, one every 15 degrees. What the Sandia researchers have shown in the last six months is that using more wires with smaller diameters results in an implosion with much better symmetry.
Beginning last September, the researchers upped the number of tungsten wires in the circular arrays from 24 to 40, which increased Saturn’s X-ray outputs from 20 to 30 terawatts (trillion watts). In January, 70 wires netted 40 terawatts. By early February, the researchers were using circular arrays containing as many as 192 very fine wires, each only a few microns in diameter. As it turns out, an array of about 120 wires works best for producing X-rays, says Cook.
Why do more wires mean better symmetry and more powerful X-ray pulses? It has to do with irregularities, or “perturbations,” in the plasma field, he says. When the wires ionize and begin to move inward, having fewer wires causes greater irregularities in the imploding plasma’s boundaries. By decreasing the diameter of each wire and adding more wires, each wire causes less of a perturbation in the plasma field, which results in greater symmetry.
Seem technical? It is, unless you compare it to squeezing a glob of Jell-O with your fingers. “Even if you had twenty fingers, the Jell-O would just squirt out between your fingers,” he says. “But if you had webbed duck’s feet, which is analogous to having an infinite number of fingers, you could squeeze it from all sides much more uniformly.”
And, he adds, if the inward force of Saturn’s magnetic pulse is timed so that it peaks precisely when the plasma field is most symmetric, you get a more uniform implosion. That’s how the higher X-ray outputs are achieved. When the plasma stagnates on the target area’s axis, a more uniform plasma produces a shorter — and more powerful — X-ray pulse. Saturn shot times are down from about 20 nanoseconds (billionths of a second) six months ago to about 4 nanoseconds today.
Terawatts plus teraflops
The recent results were not a complete shock to Sandia’s pulsed power team. The high output levels were predicted analytically but only recently achieved experimentally. It was a breakthrough, though, because the researchers had expected to reach only about 20 terawatts experimentally on Saturn and then strive for 80 terawatts on Particle Beam Fusion Accelerator II (PBFA II), Sandia’s most modern fusion accelerator. (See sidebar: “A Brief History of Saturn,” below.)
“We’ve already passed the expected PBFA II power output levels on Saturn,” Cook says. “The recent data make us think that maybe we can go higher on PBFA II. Our codes tell us we could get to 150 terawatts. We’ll see.”
He says the recent successes may accelerate Sandia’s plans to build a new accelerator that can achieve even higher energy output levels. (See sidebar: “Turn-of-the-Century X-1 Advanced Radiation Source to Succeed PBFA II,” below.) But primarily the results will boost the pulsed power program’s ability to support Sandia’s stockpile stewardship mission, he says.
Data generated inside Sandia’s accelerators are being used to test three-dimensional computer codes being developed within the nuclear weapons complex that simulate what happens inside a nuclear weapon when it detonates, such as radiation flow and radiation coupling between the primary and secondary portions of a nuclear weapon. Such simulations are becoming increasingly important as the US seeks a worldwide ban on nuclear testing.
“Nuclear weapons are huge sources of X-rays,” Cook says. “Part of Sandia’s science-based stockpile stewardship mission requires us to scale up both our laboratory X-ray sources and our computation capabilities so that we can do the job we used to do in Nevada, but must now learn to do without ever actually detonating a weapon. That will require simulations on very good computer codes, tested against data produced in part inside Sandia accelerators.”
The increased radiation outputs will also help Labs weapons researchers study radiation-hardness issues and ensure that electronics and other components in nuclear weapon arming, fuzing, and firing systems will continue to work as expected in hostile environments.
SIDEBAR 1:
Turn-of-the-Century X-1 Advanced Radiation Source to Succeed PBFA II
Particle Beam Fusion Accelerator II (PBFA II), Sandia’s most powerful pulsed power accelerator, was originally designed to produce peak X-ray outputs of 50-100 trillion watts (terawatts). Because recent experiments on the Saturn accelerator already have generated 85 terawatts, it now appears possible to reach 150 terawatts on PBFA II, says Don Cook, Director of Pulsed Power Sciences Center.
In coming months, as Labs researchers achieve those output levels on PBFA II, Sandia will begin preparations to build its next-generation X-ray source, the X-1 Advanced Radiation Source (ARS). The X-1 ARS would provide four times the peak X-ray output of PBFA II, quadrupling Sandia’s X-ray-generation capabilities from 2 million joules per shot (PBFA II’s peak output) to 8 million joules (megajoules). “Until recently, our objective with the X-1 was to produce up to 200 trillion watts in a lab here at Sandia,” Cook says. “But the recent breakthrough on Saturn suggests that the X-1 might go even higher, possibly to 400 trillion watts.”
X-1 an intermediate step
By continuing to scale up Sandia’s accelerator capabilities, he says, researchers in the “laboratory” can continue to close in on approximating the extreme conditions found inside a weapon as it detonates. Future accelerators also will help validate three-dimensional computational models that simulate nuclear detonations.
Construction of the X-1 ARS would begin in 2000 and be completed by 2003. The pulsed power team hopes to reach the accelerator’s maximum output levels by about 2005. Then, if results on the X-1 warrant scaling up further, construction of a new super accelerator currently known as Jupiter would produce as much as 32 megajoules of energy by around 2015.
Because of the amount of X-ray energy it would produce, 16 times that of the building-booming PBFA II, Jupiter would likely be located at the Nevada Test Site (NTS). “Not because of the boom, though,” says Cook.
New accelerator switch technologies are being developed that are more efficient and produce less of a boom than previous versions. Jupiter might be located at NTS to explore the possibility of “high gain” with Inertial Confinement Fusion (ICF) at the level of 100 to 1,000 megajoules of fusion energy, the output level needed to make fusion energy sources practical in a power plant.
Originally Sandia had planned to complete scale-up tests on PBFA II this summer, then skip right to Jupiter. “We have good theory and good experimental data now on Saturn, and we’re eagerly looking forward to additional PBFA II experiments later this year,” he says. “But a series of technical steps is required before we get to Jupiter. The X-1 represents an additional step.”
No formal decision has been made to build the X-1. Sandia has begun setting technical specifications and performing feasibility studies and cost determinations.
SIDEBAR 2:
A brief history of Saturn
Today’s Saturn accelerator was originally commissioned in 1980 as Particle Beam Fusion Accelerator I (PBFA I). When PBFA II was commissioned in 1986, PBFA I was reconfigured as a high-energy X-ray source. PBFA I was recommissioned as Saturn in 1987 and has been used as a laboratory X-ray source since.
Currently the Labs operates four major pulsed power accelerators: PBFA II, Saturn, Hermes, and SABRE (for Sandia Accelerator and Beam Research Experiment).