January 29, 2010

Defense Systems Assessments VP Jerry McDowell (5000), center right, and Gordon Leifeste, manager of Z Diagnostics Dept. 1675, center left, discuss capabilities of Sandia’s Z machine with a delegation of senior technical staff from the University of Texas at Austin. The UT delegation visited Sandia last week to get an overview of Sandia’s technical program and to explore joint collaboration projects of mutual interest.

In addition to a tour of the Z facility, the delegation visited the Center for Integrated Nanotechnologies (operated jointly by Sandia and Los Alamos National Laboratory) and received a briefing about Sandia’s science, technology, and engineering (ST&E) capabilities from Div. 1000 VP Steve Rottler.

Energy Systems Center 6200 Director Margie Tatro provided an overview of the Labs’ interests and direction in energy research and Jerry led a discussion about issues around national security. Technical hosts for the visit were Engineering Sciences Center Director 1500 Art Ratzel and National Security Studies Dept. 0545 Manager Mark Ladd.

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How do Z pinches contribute to a variety of scientific research projects?

Z provides the fastest, most accurate, and cheapest method to determine how materials will react under high pressures and temperatures, characteristics that can then be expressed in formulas called “equations of state.” Equations of state tell researchers how materials will react if basic conditions like pressure and temperature are changed by specific amounts. Dovetailing theoretical simulations with laboratory work, Sandia researchers have been able to perform equation-of-state measurements more precisely than ever before. Exposing targets to the high power levels of Z also allows scientists to study extreme states of matter, such as plasmas, and it may produce unexpected reactions and generate responses of great interest to many areas of science.

Fusion research on Z, too, contributes to broader scientific insight. Because near-perfect symmetry is necessary to ignite fusion (so the imploding particles will be forced to collide by not having room to escape), a persistent challenge in fusion science has been to heat the target evenly, so it will implode symmetrically. The capsule and container holding the target have to work together to produce the desired outcome, and their configurations and interactions have been the focus of intense theory and experimentation.

Diamond, for example, has been the object of much study as a potential capsule material. In melting diamond to a puddle, Z scientists have been able to understand the material’s various states – from solid to liquid, with a mixed state in-between. Thanks to Z, researchers now have a better understanding of the mixed state, which is not ideal to ignite a fusion reaction, and they can avoid it as they continue to experiment with diamond. In this and other ways, research on Z provides a roadmap for potential problems and opportunities on the path to fusion.

Beyond the fabrication of fusion pellets and the careful design of targets, achieving fusion requires work on many other interdependent elements including the machines, the mechanisms for delivering power onto a target, implementing detailed diagnostics for experiments, and creating computer codes to understand and then predict what the diagnostics revealed. Fusion is conducted in extremely complicated systems that involve complicated radiation dynamics as well as densities and temperatures not otherwise seen in nature. Trying to understand all the elements involved requires large computer codes, and tests conducted on the Z machine are very useful for testing and refining those codes. All of this work is crucial especially in conjunction with the National Ignition Campaign, which is the program to reach ignition of an inertial confinement fusion target at the National Ignition Facility.

How do Z pinches work?

The Z machine uses electricity to create radiation and heat, which are both applied to a variety of scientific purposes ranging from weapons research to the pursuit of fusion energy.

The process starts with wall-current electricity, which Z uses to charge up large capacitors (structures designed to store an electric charge). The electricity is supplied by a local utility company, and in every shot the machine consumes only about as much energy as it would take to light 100 homes for a few minutes. Metal cables arranged like the spokes of a wheel connect the capacitors to a central vacuum chamber, 10 feet in diameter and 20 feet high. The cables, some insulated by water and some by oil, are each as big around as a horse and 30 feet long.

When the accelerator fires, powerful electrical pulses strike a target at the center of the machine. Each shot from Z carries more than 1,000 times the electricity of a lightning bolt, and it finishes 20,000 times faster. The target is about the size of a spool of thread, and it consists of hundreds of tungsten wires, each thinner than a human hair, enclosed in a small metal container known as a hohlraum (German for hollow space). The hohlraum serves to maintain a uniform temperature.

The flow of energy through the tungsten wires dissolves them into a plasma and creates a strong magnetic field that forces the exploded particles inward. The speed at which the particles move is equivalent to traveling from Los Angeles to New York – about 3,000 miles – in slightly less than one second. The particles then collide with one another along the z axis (hence the name “Z pinch”), and the collisions produce intense radiation (in physics terms, 2 million joules of X-ray energy) that heats the walls of the hohlraum to approximately 1.8 million degrees Celsius.

As fast and powerful as the implosion is, the instruments that measure it must be even faster in order to record the details of the process. This is an added challenge for researchers who rely on the accuracy of these measurements to understand Z results and nuclear events in general. Accurate measurements also allow scientists to experiment with a variety of target arrangements to create conditions useful for many different purposes.

How do Z pinches contribute to the development of clean-energy technologies?

The importance of Z in solving the world’s energy challenges is directly connected to its potential in the realm of fusion. Inertial confinement fusion for peaceful production of electricity has always been of interest to Sandia’s Pulsed Power Sciences. But today, in light of growing concern about the health of our planet and considering our escalating energy needs, the development of fusion technology is especially promising for several reasons.

First, the fuel needed for fusion is virtually limitless – deuterium, an isotope of hydrogen, is abundant in seawater; tritium is bred in the fusion power plant process. In addition to being abundant, seawater as fusion fuel is incredibly energy-rich. Half a bathtub full of seawater in a fusion reaction could produce as much energy as 40 train cars of coal. The fusion reaction is a good alternative to combustion because fusion doesn’t involve burning fuel, which means it doesn’t contribute to air pollution. Finally, the fusion energy production process creates virtually no radioactive waste, which makes fusion a good alternative also to fission, the method currently used in nuclear power plants, which does produce long-lived radioactive waste.

To put it simply, the ability to get useable energy out of a fusion reaction is the Holy Grail in fusion physics. In order to produce and use fusion energy, a process involving very high temperatures and densities must be contained and controlled. Beyond the challenges faced by researchers trying to achieve inertial confinement fusion, scientists working on fusion energy have to figure out a way to produce internal confinement fusion events at a high enough rate and with enough efficiency to be useful as a power source. At the moment, the Z machine fires approximately once a day. A fusion energy machine would have to fire around 10 times per second, capture the energy from the shots and transmit it to a power-producing system.

How do Z pinches contribute to groundbreaking research?

Fusion is the process by which two atomic nuclei are joined together.

In order to fuse the atoms, the force that repels them as they come together must be overcome. Accelerators accomplish this by forcing molecules to collide with one another at very high temperatures (high temperatures are simply molecules moving at high speeds).

When light nuclei are involved, fusion can produce more energy than was required to start the reaction. This process is at work in nature in the Sun, for example, whose source of energy is an ongoing fusion chain reaction.

As an unconfined event, fusion has long been used in the development of weapons. But its great potential as a new source of energy, which depends on scientists’ ability to harness its power in confined laboratory events, continues to be explored. The Z machine is central to that effort.

The major challenge for fusion researchers is to figure out a way to contain a hot plasma – hot enough to melt any container – for long enough to get energy out. Scientists are attempting to do this mainly in one of two ways. One approach involves confining a low-density plasma for a long time using magnetic fields, which don’t melt. This approach is known as magnetic confinement fusion. The other major approach works under the assumption that a plasma cannot be contained for long. The goal, then, becomes creating a series of bursts of energy, getting as much energy as possible out of a small, high-density plasma before it expands and cools. This is what is known as internal confinement fusion, and this is the main approach used at Z.

In their study of fusion, Z researchers work with every aspect of a carefully designed process. Inertial confinement fusion involves a BB-sized fuel capsule placed inside a container about the size of a spool of thread. An enormous pulse of power is focused for a few nanoseconds on the fuel capsule containing a mixture of hydrogen isotopes (deuterium and tritium). The source of power is often a laser, or in the case of the Z machine, a Z pinch. Whatever the source, the intense burst of power causes the target to implode, compressing the material in it and heating it to temperatures near those at the center of the sun. If the heat and pressure are intense enough, the conditions should ignite a fusion reaction.

For years fusion work with Z has been sought primarily for weapons effects simulations, weapons physics, and other scientific purposes, but there is a synergy between that kind of research and the potential energy applications of fusion. Fusion research for weapons and fusion research for energy share many of the same basic physics issues, and high-yield fusion in the laboratory would translate into progress in both areas.

How do Z pinches help keep the world safe?

The detonation of nuclear weapons may affect equipment even at great distances from the explosion. This means electronic weapons systems and related equipment may malfunction when exposed, even at a distance, to radiation from an opponent’s weapons. Because of the wide variety of materials used to build weapons and military equipment, studying the effects of nuclear radiation on a variety of materials and under varying conditions is key to understanding the vulnerability of U.S. weapons.

In this area of research, simulations had always been an attractive alternative to live tests because live tests were more expensive and produced more explosive power and radiation than laboratory tests. But since 1992 the U.S. has had a moratorium on nuclear testing anyway, so in 1994 the Department of Energy established the Stockpile Stewardship Program to allow for continuing understanding of the stockpile in the absence live tests.

Today simulations based on giant computer models and laboratory experiments remain the only available method to assess the reliability and safety of our nuclear stockpile as it ages. Simulation work involves testing of existing systems to assess their vulnerabilities to radiation, predicting problems as the stockpile ages, developing ways to harden future systems, and remanufacturing weapons and components as necessary.

Z has been crucial to the effort in all areas – it has allowed scientists to study materials under conditions similar to those produced by the detonation of a nuclear weapon, and it has produced key data used to validate physics models in computer simulations. Also, by contributing to the remanufacturing of weapons components as replacements become necessary, Z plays an important part in supporting the engineering and research facilities that underpin the country’s deterrence policy.