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Oxygen Friend Not Foe to Fusion if Lithium is Involved

By Rebekah Marcarelli r.marcarelli@hngn.com | Nov 13, 2013 02:07 PM EST

"The lithium coating on graphite, which appears mountainous in this graphic done at microscopic scale, provides the perfect setting for oxygen to control the hot deuterium plasma."
"The lithium coating on graphite, which appears mountainous in this graphic done at microscopic scale, provides the perfect setting for oxygen to control the hot deuterium plasma." (Photo : C.N. Taylor and J.P. Allain)

Oxygen may be essential for living, but it "sucks the life out of" fusion; new research suggests lithium could help.

Oxygen radiates power away from high-temperature plasma, so most fusion facilities go through great lengths to reduce its concentration, an American Physical Society news release reported. 

New research suggest oxygen could actually be a friend instead of a foe to fusion if it is bound to the walls of fusion devices with lithium.

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Conditioning fusion device walls with lithium has been a common method for improving plasma performance at the National Spherical Torus Experiment (NSTX).

"These improvements include elimination of otherwise virulent edge plasma instabilities, and an improvement in the energy confinement of the plasma, both of which are correlated with a reduction of neutrals that 'recycled' at the plasma facing components," the news release stated.

The walls of NSTX are composed of graphite tiles made from carbon.

Researchers have long-thought they could thank lithium for these benefits, but this new research suggests otherwise. Lithium tends to "seep into" graphite, so the researchers were confused as to how there could be any left to aid in the fusion process. It turns out that when the lithium sinks into the carbon it react with the oxygen that also exists there, the elements combine to form a new "plasma-facing wall" containing the trio. 

This new surface has been shown to improve plasma performance. A strong reaction occurs when deuterium (a hydrogen isotope used in the plasma) interacts with this wall. 

The team used a measuring technique called "X-ray photoelectron spectroscopy" to observe the elements' reactions, they also looked at the process using a computer program. When they ran the simulation without oxygen, the deuterium retention was much lower and the carbon erosion was higher than it would have been if oxygen were present. When they tried it using only graphite and oxygen it was much higher than in the simulations (although this would be extremely difficult to make a reality in practice).

"The combination of these simulations and experiments leads to the conclusion that lithium forms the 'glue' that allows the carbon-lithium-oxygen surface layer to very effectively retain deuterium and reduce recycling. Without the lithium, the high levels of oxygen in the surface layers needed to see this beneficial effect would likely contaminate and cool the main plasma. Our results show how lithium should be prepared and maintained to yield optimum plasma performance," "physicist Chase Taylor, who led the experimental portion of the surface physics research at Purdue University with PI Prof. Jean Paul Allain who recently joined University of Illinois Urbana-Champaign," said, the news release reported. 

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