Cornell researchers and their partners lead the quest to achieve a viable fusion energy source.

By Jason Gorss

Buried in the high desert of New Mexico southeast of Albuquerque, surrounded by concentric pools of de-ionized water and oil, sits a machine that everyone simply calls “Z.” When an operator hits the switch, electric current pulses toward a single spot in the center. The current generates unimaginably high forces on parallel wires that form a cylindrical cage about the size of a spool of thread. The wires are vaporized and ionized, creating a searing plasma that can be, for an instant, many times hotter than the interior of the sun.

Housed at Sandia National Laboratory, Z is a gymnasium-sized affair that produces the world’s most energetic X-ray pulses. Supporters hope it will someday crush a tiny pellet of fuel, ignite a controlled thermonuclear fusion reaction, and provide nothing less than a limitless supply of clean energy to the world.

Lower part of a 16-wire cylindrical 8-mm arrayshowing the contacts of the wires with the negative electrode. Robert Barker/University Photo

Cornell researchers are recreating the mayhem of Z in the basement of Grumman Hall, albeit on a smaller scale. Funded by the U.S. Department of Energy’s National Nuclear Security Administration—the same group that funds the Z machine—these researchers and their partners are studying the fundamental nature of high energy density plasmas, a sizzling topic the National Research Council recently dubbed the “X Games” of contemporary science.

In 2002, the National Nuclear Security Administration began providing $2 million per year to establish the Center for the Study of Pulsed-Power-Driven High Energy Density Plasmas, which is led by Cornell and includes partners at Imperial College, London; the University of Nevada, Reno; the University of Rochester; the Weizmann Institute of Science in Israel; and the P.N. Lebedev Physical Institute in Moscow. The bulk of the center’s research is focused on understanding the physics behind imploding arrays of wires—the objects at the heart of Sandia’s Z machine. A number of potential applications and insights could stem from the work, but the overarching goal is to support Sandia and other national laboratories in the quest to understand high energy density plasmas and, ultimately, to achieve a viable fusion energy source.

Pinch me, I’m dreaming

In a fusion reaction, two atoms are smashed together to form a single heavier atom. The total mass of the new atom is less than the combination of the original two; the extra mass is converted into energy in accordance with Albert Einstein’s legendary equation, E=mc2. Since the speed of light (c) is a very big number, even a tiny mass (m) can release an enormous amount of energy (E). This is the hope of fusion: One gram of fusion fuel could potentially produce more energy than burning 10 tons of coal.

Clayton Myers, senior in Engineering Physics

All nuclear power plants operating today are based on the concept of fission, or splitting atoms apart. The original versions of the atomic bomb were also based on fission. While fission reactions release two million times as much energy per kilogram as burning traditional fuels like gasoline or coal, they release only 25 percent of the energy per kilogram of fusion reactions. In addition, the fuel for fusion reactors is more abundant in nature and the byproducts cleaner than is the case for fission. But in order to get nuclei of the atoms close enough to combine, they must have enough energy to overcome the repulsive forces that want to keep them apart.

To get an idea of how much energy, consider the hydrogen bomb—a fusion-boosted fission explosive device that makes an ordinary fission bomb look like a firecracker. In a hydrogen bomb, fusion reactions are ignited by exploding a fission bomb first.

Needless to say, scientists are pursuing other avenues to produce fusion in the laboratory. In stars like our sun, fusion is achieved by heating gravitationally confined hydrogen gas to temperatures above 10 million degrees, at which the hydrogen atoms are stripped of their electrons, forming a plasma. Achieving such high temperature plasmas on earth is no mean feat, of course, but researchers have found two viable approaches. The most well known of these is magnetic confinement fusion, which is soon to be implemented as the gargantuan fusion experiment known as ITER. (ITER was originally an acronym standing for International Thermonuclear Experimental Reactor.) This approach uses huge magnets in a toroidal configuration called a tokamak to confine the hot plasma long enough to generate energy. After 10 years of construction, ITER is expected to run its first tests to produce significant energy by 2020.

The other main approach is called inertial confinement fusion. In these experiments, a tiny spherical capsule of fusion fuel is bombarded with intense radiation. The pellet implodes and ignites the fuel, spurring fusion reactions. The radiation can be furnished using extremely high-powered lasers, such as the one being built at the National Ignition Facility at Lawrence Livermore National Laboratory. It can also be achieved by generating X-rays with a wire array “Z-pinch,” which is the approach taken by Sandia’s Z machine.

With a current pulse of about 20 million amperes, soon to be increased by 35 percent, Z heats a cylindrical arrangement of fine metal wires until it becomes a plasma. At the same time, the flow of electrical current creates a strong magnetic field, essentially pinching the plasma toward the axis of the cylinder—the “Z axis.” For a few billionths of a second, the plasma produces a burst of X-rays with peak powers reaching almost 300 million watts.

While this amounts to the most powerful laboratory source of X-rays in the world, it still falls short of the level needed to achieve a significant number of fusion reactions.

The adventures of COBRA and MAGPIE

Bruce Kusse

Enter the Cornell-led research team. “We are working on experiments and theoretical support of experiments similar to the larger ones at the national laboratories,” says Bruce Kusse, professor of applied and engineering physics at Cornell and director of the center. “Our experiments are at a smaller scale and consequently we can collect data at a higher rate than they can.”

By operating at less severe conditions, the university researchers can more easily make measurements of basic physical phenomena. Goals, in addition to achieving a better understanding of Z-pinch physics, include improving the efficiency of wire array implosions and developing new diagnostic techniques for the larger national laboratory programs. “Centers like the one at Cornell play an important role in developing the science behind the applied research at Sandia,” says Daniel Sinars, a researcher with Sandia’s Z machine program.

The heart of the Cornell program is COBRA. This pulsed-power machine, which was commissioned in July 2004, is capable of generating extremely short bursts of current in excess of one million amperes. This offers scientists from around the world—as well as Cornell undergraduate and graduate students—a critical tool to study the high energy density plasmas similar to those produced by Sandia’s Z machine, but on a much more manageable scale.

The center also boasts several other user facilities at locations across the globe, including Imperial College’s MAGPIE. “It would be a waste of money to make two identical facilities, so COBRA was designed to complement MAGPIE,” says Simon Bland, a research fellow in the plasma physics group at Imperial College. The main difference between the machines is the current pulse: COBRA has a flexible current pulse shape that reaches one million amperes in as little as 100 nanoseconds; MAGPIE takes 240 nanoseconds to reach its peak current, but this can be up to 1.4 million amperes at present—and it will soon be upgraded to about two million amperes.

“These differences allow different physics to be explored,” Bland says. With its higher current and longer pulse, MAGPIE can drive heavier arrays made of more wires, allowing the researchers to examine how symmetry affects the implosion of an array. COBRA, on the other hand, can explore how faster rates of rising current affect array dynamics, according to Bland.

Bland spent more than a week in Ithaca last summer, giving seminars and exchanging data with Cornell researchers. “I came over to Cornell mainly to share information,” Bland says. “This is one of the actual purposes of the center, and it has proven to be incredibly useful. Based on this, we are now working on two or three joint publications.”

Why focus on wire array Z-pinches? Bland believes ITER will eventually succeed in demonstrating a controlled fusion reaction, but in the end, he fears that a tokamak-based

approach will be “so costly as to never see the light of day.” Likewise, he expects the National Ignition Facility at LLNL to demonstrate the feasibility of using lasers to ignite fusion, and this knowledge will help move the whole field of inertial confinement fusion forward. But even the best lasers are not very efficient—5 to 10 percent at best.

“Wire arrays to drive inertial confinement fusion have a number of potential advantages,” Bland says. “They produce incredible powers and energies of X-rays, and this is done at amazing efficiencies. Approximately 15 percent of the energy available to power an array is converted to X-rays.” And the generators required to implode arrays are relatively inexpensive, he adds.

David Hammer

One of the center’s primary goals is to increase the efficiency of wire arrays even further, according to David Hammer, the J.C. Ward Professor of Nuclear Energy Engineering at Cornell and the center’s associate director. “If we understand the processes of wire array Z-pinch explosions better, to the point that we can reduce some instability or get a little extra radiation out because it implodes more uniformly, then Sandia can pick up what we learn and apply it to their machine and hopefully get the same improvement,” he says.

This has happened in the past, Hammer notes. Some fundamental Cornell experiments in the late 1990s had a significant effect on the way researchers expected wires to explode, causing people to rethink the initial conditions used in computer codes they were using to help to understand the experimental results. And studies performed on both MAGPIE and COBRA have provided valuable measurements of the dynamics of wire arrays. “The resulting understanding . . . has given us important tools for ‘tailoring’ the radiation produced by wire arrays to match the requirements for inertial confinement fusion, and thus has helped us make a credible case for our ability to achieve fusion driven by Z-pinch-produced radiation,” Sinars says.

True believers

Of course, fusion research has its detractors. The running joke among cynics is that a viable fusion energy source is about 20 years away, and it will always be about 20 years away. But Hammer is, by his own admission, an incurable optimist. And he has been for more than 40 years.

“I started in plasma physics because of fusion, and I have been in it for 42 years now,” he says. “In 1964, after my senior year at Caltech, I was offered a summer job at Oak Ridge National Laboratory in a group that was carrying out experiments in magnetohydrodynamics and plasma physics, neither one of which I had heard of.” After only two months of working at the lab, the research team’s enthusiasm for controlled fusion had permanently rubbed off on the young intern. Hammer came to Cornell a year later to begin graduate work in the burgeoning plasma physics program.

“I had convinced myself that I wanted to be an ‘applied physicist,’ meaning that I wanted to work on ‘useful applications’ of physics, rather than things like elementary particle physics and cosmology,” Hammer remembers. “As a graduate student, I helped start the intense electron beam experimental program starting in 1967 and did a thesis on intense electron beam propagation. Although my thesis was not directly relevant to fusion research, that application was enough in the back of my mind to be part of the conclusion chapter in my thesis.”

Kate Bell, graduate student in electrical and computer engineering

Kusse was also drawn to the field more than 40 years ago, and he is similarly optimistic. “I believe that nuclear power, whether fusion, fission, or a hybrid fusion/fission process, is the most likely energy source once fossil fuels are no longer widely available,” he says.

Both men’s careers are success stories that researchers involved with the center hope to repeat. In fact, one of the most important aspects of the center is that it attracts talented students into the high energy density physics community and prepares them for careers in the field, providing a valuable supply of Ph.D.s to national laboratories and universities. Sinars is a case in point: He received a Ph.D. in applied physics from Cornell in 2001, studying under Hammer and Kusse. Now he spends his days working directly with the Z machine. He and his colleagues are hoping to take the next step with a refurbished facility called ZR that could potentially double the X-ray power output of Z. They expect ZR to come online in Spring 2007.

Some of the “bad press” about fusion stems from unrealistic assessments of the technical difficulties that needed to be overcome, according to Kusse. “The fusion process is much more difficult than fission and may be a more expensive energy source,” he suggests. “Two things have to happen before fusion power plants are a reality: more work needs to be done on the practical aspects of fusion plants, and the costs of energy production from fossil fuels will probably have to increase significantly above what they are now.”

As for that running joke: “People have been saying we’re going to have fusion in 20 years, but a device to do it has never been built,” Hammer says. “We finally are doing that with ITER. In effect the clock is starting as soon as they dig the first spade of dirt.”

When opportunity knocks

Fusion is not the only game in town. The groups at Imperial College and the University of Rochester have been pioneers in another application of the Z-pinch technology: laboratory astrophysics. The idea is to simulate in the laboratory the processes thought to occur near black holes, rotating neutron stars, and newly forming stars. The results are then compared to computer simulations and observations of actual astrophysical events.

Richard Lovelace

On its face, “laboratory astrophysics” is something of an oxymoron. How is it possible to replicate absurdly energetic events near black holes in the confines of a laboratory? The topic is not uncontroversial, but Richard Lovelace defends it. “I think these experiments are very revealing of mechanisms,” says Lovelace, professor of applied and engineering physics and astronomy at Cornell and a co-investigator at the center. “It’s necessary to consider how the properties scale. In the lab you are dealing with an experiment that is 10 centimeters across, whereas in astrophysics it is 10 to the 14th power centimeters across. The lab experiments can shed light on these mechanisms when scaled properly.”

Outer space has many different systems with astrophysical jets: active galaxies, binary black holes, early versions of solar systems. Each has different properties, and little is known about the mechanisms behind their formation. Astrophysical jets are very high energy phenomena, which makes them perfect for studying in the high energy environment produced by a Z-pinch. The majority of Lovelace’s work to date has been theoretical, but he is planning to begin some experimental work on COBRA within the next year.

Another area of research that has produced surprising results involves an alternative configuration called the X-pinch. Instead of the Z-pinch’s wire array, the X-pinch is formed by passing about 100,000 amperes of current through two or more thin wires that cross at a single point. Just as in a Z-pinch, the wires vaporize and form a plasma that emits X-rays. But the radiation is concentrated at one tiny spot, making the machine an excellent point source of X-rays.

Hammer’s original interest in the X-pinch was as an X-ray source for lithography in microelectronics manufacturing. “We demonstrated that you could get adequate power out of it to be able to run a production line, but industry did not decide to go that direction,” he says. The X-pinch showed a great deal of promise, but chip manufacturers stuck with optical methods that were already familiar to them.

The X-pinch also has shown promise as a “camera” for producing extremely high-resolution radiographs of very small objects. At a 2001 meeting of the American Physical Society, the Cornell team received a good deal of attention for showing off images of house flies and beetles with micrometer-scale detail. And because the X-pinch produces relatively low total energy in the radiation bursts, it could be useful for imaging of biological tissue: the radiation is benign enough to image even live objects, such as ants. The X-ray pulse happens so quickly—in less than a nanosecond—that it produces stop-action radiographs of extremely fast-changing events, like exploding wires.

Hammer is happy to highlight some of these X-pinch experiments because they excite people and help attract more students. “Not very many people are interested in the fundamental instabilities of exploding wires, so when opportunity knocks, we try not to ignore our bug-imaging capability,” he says. “But ultimately, no matter what the immediate goal of an experiment is, off in the distance there is the idea that we could contribute to producing energy from the fusion process.”