Z machine on another dramatic climb toward fusion conditions

Z has achieved three of four milestones, the fourth reachable

Researchers at Sandia’s Z – the former dark horse among accelerators meant to produce conditions required for nuclear fusion (Lab News, Aug. 1, 1997) – have increased the machine’s X-ray power output by nearly 10 times in the last two years.

SUPER STRINGER – Dolores Graham (a contractor in 9573) uses tweezers to build an array of wires, each 1/10 the diameter of a human hair, that form a target about the size of a spool of thread (between horizontal metal rings) for Sandia’s Z accelerator. (Photo by Randy Montoya)

This achievement resulted from advances in theory and experiments by a team involving DOE labs, universities, and DoD labs.

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 needed for each trial is only enough to provide electricity to about one hundred 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 reach velocities that would fly a plane from Los Angeles to New York in a second. (Most targets are about the size of a spool of thread.)

Z’s advance in power is expected to make a major contribution to DOE’s 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.

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 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/Albuquerque.

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 compress a capsule containing fusion fuel symmetrically until it ignites and achieves high-yield fusion.

Milestones Z was expected to achieve were an X-ray energy of 1.5 megajoules – achieved is 2.0 megajoules. The X-ray 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 compressions 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 the four milestones we set for Z, and we are very close to meeting the fourth – a radiation temperature of 1.7 million degrees," says Don Cook, Director of Pulsed Power Sciences Center 9500.

These results show that X-1, a larger accelerator 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 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 resorting to underground testing.

Collisions produce intense radiation

In experiments with the basic vacuum hohlraum (see "An affordable approach to high yield" below) led by John Porter (9573), the discharge of energy through the wires creates a magnetic field that compresses the exploded wire array at a speed equivalent to traveling from Los Angeles to San Francisco 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) cylinders or solid materials inside the wire array (a concept called a dynamic or internal hohlraum), even higher temperatures can be achieved inside the rapidly compressed volume. Mark Derzon, Gordon Chandler, and Tom Nash (all 9577) experimented with wire arrays imploding onto annular and solid-core cylinders made of plastic foam, with two-dimensional computer simulation support from Los Alamos National Laboratory researchers Richard Bowers, Walter Matuska, and Darrell Peterson.

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. Both the "useful" and peak temperatures are expected to increase in upcoming experiments because better design will help control instabilities in the interior, heated region. Controlling these instabilities 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, several 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.

"A key feature of our design is that the pressure from the imploded beryllium plasma keeps the radiation channel open long enough to extract 20 kilojoules of energy, which is approximately the optical energy of the Nova laser," says Toor.

In both the Sandia and 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, Sandians Rick Spielman, Chris Deeney (both 9573), Tom Nash, and Melissa Douglas (9571), with Los Alamos’ 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 US. 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 simulation by Melissa Douglas and Peterson indicated the route to optimization, 290 trillion watts was achieved on Z in January by Sandia researchers.

"We have substantially more current available than earlier researchers did, so we have more flexibility in designing the wire arrays," says Rick Spielman.

Need full 3-D simulations

Future experiments will involve optimizing parameters and models.

"The complex physics in these experiments has been remarkably well modeled by Darrell Peterson using their 2-D simulation codes," says Gerry Yonas, VP of Systems, Science, and Technology Div. 9000, "but we will need full 3-D simulation to prepare for the ultimate high-yield experiments on X-1 we will be capable of doing in the next ten years."

Gerry points out that the teraflops computer at Sandia, 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."

Computational Physics Research and Development Dept. 9231 under James Peery is making steady progress toward the objective of simulating the implosion of a wire array in three dimensions.