Bending beams: Focusing ions with ceramic

Gilson is part of the Virtual National Laboratory for Heavy-Ion Fusion, a partnership between the PPPL, E.O. Lawrence Berkeley Lab and Lawrence Livermore National Laboratory in California. According to Joe Kwan, the project manager of the Berkeley Lab, the PPPL’s job is to make the plasma source; the machine itself is located in Berkeley, and the Livermore lab helps with computer simulations. HIF-VNL has existed since 1999 and is funded by the Office of Fusion Energy at the U.S. Department of Energy, with the primary goal of developing ion beams as drivers for fusion energy and heavy ion physics research.

Fusion energy involves stripping an atom of its electrons, leaving a nucleus of protons behind. Under normal circumstances, these nuclei repel each other. But if a machine forces the nuclei together, they will coalesce, releasing energy in the process. In theory, this energy could be harnessed as commercial electricity, reducing dependence on fossil fuels.

One approach to fusion, called inertial confinement fusion, requires a driver to crush down fuel to force the nuclei together.

“If you remember those old Superman movies, you could almost imagine coming at [the fuel] with Superman’s hand crushing it. The driver, whether it’s Superman or ion beams or lasers, is kind of a separate question of can you take a fusion fuel and crush it and make it fuse,” Gilson explained.

But for a beam of ions to effectively fuse, it must be compressed into as small a point as possible for efficiency. Gilson’s device provides a source of negatively charged space for the positively charged beam to travel through. The ion-rich plasma neutralizes the repulsive forces of the ion beam, allowing it to hit its target in its most condensed form.

But how does a 43-inch-long cylinder, made of copper and barium titanate ceramic, turn into a swarm of neutralizing plasma? The answer lies in the ceramic itself, a material recommended to Gilson by former PPPL colleague Alexander Dunaevsky.

“When you pulse a high voltage across it, all the microscopic electric dipoles buried inside it all spin around and line up. It builds up a charge on its surface that is so strong that the like charges on the surface repel each other and blow off the surface ... It all comes off in the form of a plasma.”

Members of the Berkeley National Laboratory, which received the ceramic cylinder, completed the machine in late March and will begin their first experiments with the completed machine this summer, according to Kwan.

Both Gilson and Kwan emphasized the importance of this machine for understanding how materials behave in the conditions created by fusion energy, as opposed to an explicit connection to commercializing fusion energy.

“This machine does not have the power to create fusion ignition,” Kwan explained. “It’s only trying to create some conditions [in which] we can study the behavior of the matter during fusion conditions.”

“The shorter term goal is to use these beams, which aren’t quite ready for fusion energy ... in the near term to do other physics tests,” Gilson added. “People go through a lot of trouble to go through the ‘equation of state’ of things ... I thought we understood the equation of state for everything, but apparently not.”

In 2005, the HIF-VNL created the Neutralized Drift Compression Experiment-I, a linear accelerator that would produce a beam to allow for these studies of how materials behave under fusion-like conditions. Since July 2009 the team has been working to improve the NDCX-I by creating an entirely new machine, NDCX-II, which was given $11 million by the American Recovery and Reinvestment Act in 2009.

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Thus far, only Berkeley students have interacted with the machine on site, although Princeton astrophysics graduate students have worked in the lab group at the PPPL. Both Gilson and Kwan confirmed that Princeton students would be allowed to visit the site and possibly use it for their own materials science research, taking advantage of the higher power that the NDCX-II can offer thanks to Gilson’s device.

“It’s not a new invention compared to the NDCX-I; it’s similar in terms of the principles of how to produce neutralization. The improvement is by increasing the capabilities, the plasma densities and the length of the neutralizer,” Kwan said.