One of the more challenging obstacles to creating a “star in a jar” on Earth is finding a material that can contain and facilitate the massive amount of energy produced in a fusion device. PPPL scientists are experimenting with unique ways to cover the interior walls of fusion reactors that can handle the heat, absorb the battering of atomic collisions, and help produce energy.

The unique Lithium Tokamak Experiment (LTX), the most advanced facility of its kind in the world, is set to test this key issue in the development of fusion energy — whether liquid lithium can serve as an ideal substance for covering the walls of fusion reactors to facilitate production of the energy. The light, silvery metal acts like a sponge, soaking up particles that stray from the hot core of the plasma that fuels fusion reactions, and keeps the particles from bouncing back to cool the plasma and halt the reactions.

Now to be tested is whether lithium can continue this process under hotter, denser and more fusion-relevant conditions. To find out, researchers have installed a powerful neutral beam injector that will further heat and fuel the magnetically confined plasma — a gas composed of electrons and atomic nuclei, or ions — inside the facility. The beam will move the device a step closer to a commercial fusion reactor, says Richard Majeski, principal investigator of the experiment.

Collaborating with PPPL on this state-of-the-art research are Oak Ridge National Laboratory, UCLA, the University of Tennessee, Knoxville, and Princeton University.

The spherical LTX tokamak, now known as the LTX-β, racked up an impressive list of firsts prior to installation of the injector. The tokamak, roughly the size of a sport utility vehicle, was the first to coat its walls with lithium by evaporating it from a pool at the bottom of the facility, and the first to show that the liquid metal improves plasma performance. Most notably, lithium has kept the plasma temperature flat, a crucial condition technically known as “isothermal” to prevent it from cooling.

However, the density of the plasma, a crucial ingredient in fusion reactions, has tended to drop off fast in previous LTX experiments. The neutral beam fueling will raise the density and show whether lithium can maintain the temperature of hotter plasmas.

PPPL has long been a global leader in the use of lithium as a plasma-facing first wall to maintain fusion reactions. The Laboratory has used the metal in powdered form in the predecessor to its flagship, the National Spherical Torus Experiment-Upgrade (NSTX-U), in the DIII-D tokamak that General Atomics operates in San Diego, and in the EAST tokamak in China. It has also helped apply lithium in liquid form to extend the performance of plasma on EAST, and collaborated with solid lithium pellet injection on the ASDEX-Upgrade tokamak in Germany. Plans now call for liquid lithium to be tested in experiments on the NSTX-U, for which research on the LTX-β could serve as a positive example.

“If we find that liquid lithium walls lead to much smaller, cheaper, better performing tokamaks,” Majeski said of the LTX-β research, “then fusion power will be a lot closer to reality."