As more lithium is added (with increasing discharge number in yellow), the plasma light changes from red (from recycling of the deuterium fuel gas) to green (lithium emission from the edge of the plasma) and the overall (recycling) light levels decrease. Image: Princeton Plasma Physics Laboratory |
A
team of scientists working at the U.S. Department of Energy’s Princeton
Plasma Physics Laboratory (PPPL) has found that increasing the amount
of lithium coating in the wall of an experimental fusion reactor greatly
improves the ability of experimentalists to contain the hot, ionized
gas known as plasma. Adding more lithium also enhances certain plasma
properties aiding the reaction, the researchers found.
“The
lesson here for confining plasma is surprising and simple: When you use
more and more lithium, the plasma confinement gets better and better,”
said Rajesh Maingi, a physicist from the Oak Ridge National Laboratory
(ORNL) who is on long-term assignment to PPPL. “This is not what we
expected to see. We thought the effect would taper off at some point.
But it doesn’t. When it comes to fusion plasmas, it’s “the more, the
merrier.'”
If
plasma energy confinement is improved, a fusion reactor can, in
principle, be made smaller and, therefore, cheaper. Energy confinement
is a measure of how long (in seconds) power that is injected into the
plasma stays in the plasma before leaking out to the walls.
Spraying
lithium onto the inner surface of an experimental fusion device at PPPL
known as the National Spherical Torus Experiment (NSTX) improves
several other conditions for fusion, the experimenters found. The
lithium reduced recycling—the problematic ricocheting of particles into
the vessel wall and back into the plasma. Recycling leads to cooling,
contamination, and, ultimately, dissipation of the energy of the plasma.
In addition to reducing recycling, the lithium coating—and enhanced
coatings based on it—also enhanced the reaction by decreasing chaotic
instabilities both at the plasma’s edge, and also on a larger scale.
The
experiment’s results indicate that fusion machine designers may be able
to reduce the size and the level of heating in future devices designed
to use lithium coating technologies.
In
addition to Maingi, other researchers on the effort included: Stanley
Kaye and Charles Skinner from PPPL, D.P. Boyle from Princeton
University; and J.M. Canik from ORNL. This work recently appeared in
Physical Review Letters 107 (2011) 145004.
The
work is being presented at the 53rd Annual Meeting of the American
Physical Society Division of Plasma Physics, being held Nov. 14-18, in
Salt Lake City, Utah.
