A Step Toward Fusion Energy

A small but important step has been made toward understanding one of physics' biggest mysteries -- the creation of controlled fusion energy. But it was not taken by one of the trillion-watt-generating giant lasers at a national laboratory. Instead, it was thanks to the less assuming Z-pinch located at the Nevada Terrawatt Facility.

The creation of controlled fusion energy is still decades away, but because of the recent research conducted at the Nevada Terawatt Facility (NTF), a part of the physics department at the University of Nevada, Reno, researchers have a better understanding of some fundamental processes required to achieve it.

Scientists stand above the Z-pinch target chamber at the University of Nevada, Reno's Nevada Terawatt Facility.

The Z-pinch is a type of plasma confinement system that uses a fast electrical current in the plasma to generate a magnetic field. "Shots" of fast, 100-ns pulses exceeding 20 million amps are fired through tungsten wires on the order of tens of microns at the Sandia National Laboratory Z-pinch, and the huge lasers at national laboratories can generate up to 40 trillion watts of x-ray power.

"With our 1-million-amp NTF Z-pinch, we can explore some very interesting physics that can be applied to the bigger pinches at the national laboratories," said Vladimir Ivanov, whose research with wire array Z-pinches at NTF led to the publication of a recent article in the journal Physical Review Letters.

In Ivanov's article, "Dynamics of Mass Transport and Magnetic Field Fields in Low Wire Array Z-Pinches," he and a team of students and researchers found the microscopic effects that cause inefficiencies limiting the conversion of electrical energy required for implosion energy.

The implications of Ivanov's work are important, said Tom Cowan, NTF director. "This is the fundamental stuff, the physics if you will, that is limiting the transfer of the electrical energy into the implosion energy which is responsible for the heating and x-ray production that will eventually lead to a fusion energy reaction in a laboratory," he said.

Ivanov's experiment used the 1-million-amp "Zebra" Z-pinch generator along with plasma diagnostics that included five-frame laser probing of the z-pinch in three directions. This measured mass transport during implosion.

Previously, laser probing work in this area proved difficult to read. Images from the "shots" often suffered from poor resolution, blurring, or lack of contrast.

The images created by Ivanov's technique were vivid. They showed not only plasma "bubbles" rising on breaks in the wires used, but what Cowan calls the "fingers" of matter left behind from the implosion.

Sandia's Z machine has produced plasmas that exceed temperatures of 2 billion degrees kelvin -- hotter than the interiors of stars. The electromagnetic pulse when the machine is discharged causes impressive lightning, referred to as "arcs and sparks" or "flashover", which can be seen around many of the metallic objects in the room. (Sandia National Lab photo by Randy Montoya)

"With these trailing fingers of mass, some of the current is left behind," Cowan said. "The current drives the implosion process, so these inefficiencies are very important. The sequence of these little failure modes, these little fuse effects happening on the wires, is what is limiting the big experiments. By understanding this better, we can come up with new ways at looking at how the current flows into the plasma, and how the mass interacts."

Ivanov was able to use his previous research in laser plasma physics to his advantage in dealing with his experiment in Z-pinch plasma physics. He approached the experiment suspecting that previous Z-pinches, working in deliveries of 100-ns pulses, could be improved if one could understand the dynamics on a shorter time scale. For typical laser plasmas, a delivery of a nanosecond or even faster, such as a thousandth of a nanosecond, would be more common.

"So he decided that he would like to look at this with shorter laser pulses, and that was the enabling piece of the technology to get the useful information out," Cowan said. <br.>

For more information, visit: www.unr.edu

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