Another dramatic climb toward fusion conditions for Sandia Z accelerator

ALBUQUERQUE, N.M. — Researchers at Sandia National Laboratories’ Z machine — the former dark horse among accelerators meant to produce conditions required for nuclear fusion — have increased the machine’s X-ray power output by nearly 10 times in the last two years.

The most recent advance resulted in an output X-ray power of about 290 trillion watts — for billionths of a second, about 80 times the entire world’s output of electricity.

The figure represents almost a 40 percent increase over the 210 trillion watts — itself a world record — reported last summer.

Strangely, the power used in each trial is only enough to provide electricity to about 100 houses for two minutes. Electricity is provided by ordinary wall current from a local utility company.

Yet particles imploded in the accelerator’s tiny targets — about the size of a spool of thread — reach velocities that would fly a plane from Los Angeles to New York in a second.

Z’s advance in power is expected to make a major contribution to the Department of Energy’s (DOE) science-based approach to stockpile stewardship, which must use giant computing and laboratory experiments to provide the basis to sustain the nation’s nuclear stockpile without above- or below-ground tests. This achievement resulted from advances in theory and experiments by a team involving DOE and Department of Defense labs, and universities.

Achieved 1.6 million degrees C
In a different series of experiments, the accelerator achieved a temperature of approximately 1.6 million degrees Celsius (140 electron volts) in a container the size of a spool of thread.

Other experiments in a still smaller volume target suggest temperatures may eventually be achieved on Z in the range of 2.0 to 2.2 million degrees. The now-realistic goal of reaching 2.0 million degrees is so significant because radiation temperatures in the range of two million to three million degrees are generally considered an essential condition for nuclear fusion.

This potential for the Z facility was demonstrated in experiments performed by a group from Lawrence Livermore National Laboratory in California and a group from Sandia in New Mexico.

These small-volume experiments have thus far been limited by implosion instabilities, but the most recent results indicate that these instabilities can be controlled. A year and a half ago, Z could achieve a radiation temperature of only 0.7 million degrees. Each doubling of temperature theoretically results in a 16-fold increase in intensity of the radiation, necessary to drive a fusion capsule.

Other requirements for fusion, besides temperature, include adequate energy and power in X-rays to symmetrically compress a capsule containing fusion fuel until it ignites to achieve high-yield fusion.

Milestones Z was expected to achieve were an X-ray energy of 1.5 megajoules — achieved is 2.0 megajoules. The power milestone was 150 terawatts — achieved, 290 terawatts. There were two milestones in temperature: the first for weapons physics configurations was 100 eV (1.2 million degrees). The achieved value was 140 eV (1.6 million degrees). The second temperature milestone in a configuration suitable for target compression experiments was 150 eV (1.7 million degrees). Sandia has achieved 140 eV (1.6 million degrees).

Close to fourth milestone
“We have now met three of four milestones we set for Z, and are very close to meeting the fourth — a radiation temperature of 1.7 million degrees,” says Don Cook, director of Sandia’s Pulsed Power Sciences Center. These results show that X-1, a larger accelerator scheduled to follow Z, should be able to produce 16 million joules of energy, more than 1,000 trillion watts of power, and temperatures of more than 3 million degrees. Because Sandia’s concept for X-1 is based on the high efficiency already demonstrated on the Z — 15 percent from energy going into the accelerator to energy coming out in X-rays — the cost of X-1 is expected to be modest. Sandia is seeking support from DOE for embarking on the conceptual design of X-1 and plans to make a formal request to do so this spring. If DOE approves start of a conceptual design this year, X-1 should be able to contribute to DOE’s science-based stockpile stewardship program in a timely way. X-1 will provide laboratory data on the physics of nuclear weapons implosions and their effects. The data are necessary to validate the increasingly sophisticated computational models of weapon performance, without underground testing.

An affordable approach to high yield
Z relies upon wall-current electricity to charge giant capacitor banks and upon enormous metal cables that connect to a central vacuum chamber, 10 feet in diameter and 20 feet high.

When the accelerator fires, powerful electrical pulses are delivered by 36 transmission cables protected by insulation techniques developed over the last 30 years. Highly synchronized laser-triggered switches allow the stored energy to be discharged simultaneously through the 36 cables, each as big around as a horse and 30 feet long, arranged like spokes of a wheel and insulated by water. The enormous electrical pulse of 50 trillion watts strikes a complex target about the size of a spool of thread. (The machine is named Z because the current passing directly into the target travels vertically — a direction conventionally labeled “z” by mathematicians and physicists to distinguish it from the x and y directions, both horizontal.)

The target consists of a metal can containing several hundred nearly invisible tungsten wires, each much smaller in diameter than a human hair. The metal can, called a hohlraum, functions like an oven, confining radiation energy released when the wires first explode and then subsequently collapse on themselves. Several variations of this assembly are responsible for increasing the temperature and power of Z.

Collisions produce intense radiation
In experiments with the basic vacuum hohlraum, the discharge of energy through the wires creates a magnetic field that compresses the exploded wire array at a speed equivalent to traveling 3,000 miles in one second. The vaporized particles, pushed inward by the magnetic field, collide with each other at the magnetic axis. The collisions produce intense radiation, enough to heat the surrounding walls of the hohlraum to 140 eV (approximately 1.6 million degrees). The X-ray radiation emitted from the walls is then used to study the properties of materials at high temperatures and pressures.

By placing coreless (annular) or solid cylinders inside the wire array (an arrangement called a dynamic or internal hohlraum), even higher temperatures can be achieved inside the rapidly compressed volume. Researchers experimented with wire arrays imploding onto cylinders made of plastic foam, with two-dimensional computer simulation support from Los Alamos National Laboratory.

The group has achieved 1.6 million degrees C (140 eV) in useful form — that is, able to drive a fusion capsule — and 2.2 million degrees (190 eV) as a peak temperature. Increases in “useful” and peak temperatures are expected in upcoming experiments because better design will help control instabilities in the interior, heated region. This is the next challenge faced by all Z researchers.

In collaborative experiments among Lawrence Livermore and Sandia scientists, led by Arthur Toor of Lawrence Livermore, foam layers surrounding a beryllium tube are used inside the wire array, thereby providing a slower, more precise collapse of the imploding plasma. This arrangement produced a hohlraum temperature of 170 eV — 2 million degrees — in a configuration useful for studies of weapons physics.

In both the Sandia and Lawrence Livermore dynamic hohlraums, there were serious problems with integrity of the central region due to instabilities, but recent results show promise in controlling these instabilities.

Nested wire arrays achieve 290 TW
To achieve a radiated power of about 290 terawatts, Sandia researchers with Los Alamos researcher Darrell Peterson designed a nested wire array, which consists of two concentric wire arrays.

Incoming power vaporizes the outer wire array. Its magnetic field drives the vaporized ring of wire material inward until the faster moving portions strike the inner wire array and are slowed. The deceleration allows the slower moving material at the back of the outer ring to catch up and, together, sweep up the material in the inner array and drive it forward. This reduces instabilities in the implosion. The vaporized materials then slam into each other at the central axis, converting the energy of motion into radiation energy with a much shorter pulse than if a single wire array had been used.

Theory for the nested wire array experiment was worked out more than 10 years ago inside and outside the U.S. Experiments on Z with nested wire arrays began in October. In these initial experiments, an output X-ray power of roughly 250 trillion watts was achieved, compared to the 210 trillion watts that had been obtained with a single wire array. After computer simulations indicated the route to optimization, 290 trillion watts was achieved on Z in January. “We have substantially more current available than earlier researchers did, so we have more flexibility in designing the wire arrays,” says Sandia researcher Rick Spielman, leader of the effort.

3-D simulations needed
Future experiments will involve optimizing parameters and models. “The complex physics in these experiments has been remarkably well modeled by Darrell Peterson using the 2D simulation codes,” says Gerry Yonas, Sandia’s vice-president of Systems, Sciences and Technology, “but we will need full 3D simulation to prepare for the ultimate high-yield experiments on X-1 we will be capable of doing in the next ten years.” He points out that Sandia’s teraflops computer, capable of a trillion operations a second, and the other advanced computers being developed for DOE’s science-based stockpile stewardship program “will be needed to reach our goals. We will succeed with creating high-yield fusion when we can fully harness the power of both our teraflops and terawatts.”

For more information:

• Z, which reaches temperatures of the Sun, to help astronomers interpret Chandra data (November 5, 1999)
• Concept for rapid-fire thermonuclear explosions proposed by Sandia scientists (September 13, 1999)
• Sandia researchers push Z machine to new limits to test radiation effects (June 16, 1999)
• Sandia formally proposes to design accelerator expected to produce high-yield fusion (April 8, 1998)
• Output of Sandia Z Accelerator Climbs Closer to Fusion (August 1, 1997)
• High-Output Sandia Accelerator Able to Predict Nuclear Blast Physics (December 2, 1996)

Technical contacts:
Gerold Yonas, gyonas@sandia.gov, (505) 845-9819
Don Cook, dlcook@sandia.gov, (505) 845-7446