A promising experiment that encloses hot, magnetically confined plasma in a full wall of liquid lithium is undergoing a $2 million upgrade at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL). Engineers are installing a powerful neutral beam injector in the laboratory’s Lithium Tokamak Experiment (LTX), an innovative device used to test the liquid metal as a first wall that enhances plasma performance. The first wall material faces the plasma.

“This will bring us one step closer to demonstrating this particular approach to fusion,” said Dick Majeski, principal investigator of the LTX. The experiment is a collaborative effort that includes researchers from Oak Ridge National Laboratory, UCLA, the University of Tennessee, Knoxville, and Princeton University, as well as PPPL. Funding comes from the DOE Office of Science.

Energetic beams

The neutral beam injector, a Russian-built device on loan from the Tri Alpha fusion firm in California, will shoot energetic beams into the small spherical tokamak to fuel the core of the plasma and increase its temperature and density — key factors in fusion reactions. “The beams will maintain the density and raise the temperature to a more fusion-relevant level,” said Philip Efthimion, PPPL head of the Plasma Science and Technology Department that includes the LTX.

The experiment recently became the first device in the world to produce flat temperatures in a magnetically confined plasma. Such flatness reduces the loss of heat from the plasma that can halt fusion reactions. The LTX also has provided the first experimental evidence that coating a large area of walls with liquid lithium can produce high-performance plasmas.

However, without fueling from the neutral beam the density of an LTX plasma tends to drop off fast. The beam upgrade will keep the density from dropping, and test whether the liquid lithium coating can continue to maintain flat temperatures in much hotter plasmas.

A leader in use of liquid lithium

PPPL has long been a leader in the use of liquid lithium to coat and protect the plasma-facing components inside tokamaks. The predecessor to LTX, the Current Drive Experiment-Upgrade (CDX-U), ran with a circular pool of liquid lithium at the bottom of the plasma. The CDX-U operated from 1999 to 2007 before it was disassembled for installation of heatable shells composed of thin stainless steel and thick sheets of copper that form the tokamak’s inner walls. LTX made its first plasma in 2008 and first used liquid coatings in 2010.

Researchers have also explored the use of liquid lithium in the National Spherical Torus Experiment-Upgrade (NSTX-U), the laboratory’s flagship fusion experiment, prior to its recent upgrade. PPPL will continue to investigate use of the liquid metal in the revamped machine.

The value of lithium as a first-wall material comes from its ability to sponge up particles that stray from the core of the plasma and keep them from recycling back and cooling down the edge and then the core. Lithium is a highly reactive material that combines with other elements and doesn’t let go.

In LTX experiments, researchers use an electron beam to evaporate a pool of liquid lithium at the base of the tokamak. The evaporated metal then coats the shells. Keeping the temperature of the shells above the melting point of lithium sustains its liquid state.

Differs sharply from a heavy metal first wall

This approach differs sharply from the use of a heavy metal such as tungsten for a tokamak’s first wall. While tungsten resists erosion, has a high melting temperature and conducts heat well, heavy impurities kicked up by contact with the plasma can rapidly cool down the hot core. The Joint European Torus (JET) in the United Kingdom experiments with tungsten. ITER, the international tokamak under construction in France, also plans to use it.

The LTX upgrade, scheduled for completion later this year, marks the latest PPPL format for studying the liquid metal. Experiments could resume next spring and plasma operations with the neutral beam by fall. The performance of the LTX upgrade could then provide new evidence of the ability of liquid lithium to serve as a first wall.

PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.