Machine set to see if lithium can help bring fusion to Earth

Lithium, the light silver metal used in all applications, pharmaceutical applications to the batteries of your smart phone or your electric car, could also help exploit on Earth the fusion energy that illuminates the sun and the stars. Lithium can maintain heat and protect the walls inside donut-shaped tokamaks harboring fusion reactions. It will be used to produce tritium, the isotope of hydrogen that, coupled with its cousin, deuterium, will fuel fusion in future reactors.

At the Princeton Plasma Physics Laboratory (PPPL) at the Department of Energy (DOE), researchers have completed upgrading the Tokamak Lithium experiment, a three-year operation now called Lithium Tokamak Experiment-Beta (LTX-β ), a unique device test the ability of the metal to maintain heat and protect the walls of the tokamak now more powerful.

Neutral beam injector

The upgrade, funded by DOE's Office of Science, installed a Triangle Energy Neutral Beam – a long-term injector, which became TAE Technologies – to heat, fuel and increase plasma density. Among other improvements include the increase of the magnetic field confining the plasma and the installation of new lithium systems. The improvements bring the conditions of the experiment closer to those of a fusion reactor, said Dick Majeski, the principal investigator of the experiment.

The new device, which uses a lithium coating to cover the inner wall of the small tokamak, was, before the upgrade, the first to maintain a constant temperature from the hot central core of the plasma to the normally cold outer edge. "The machine is now ready to exploit all the capabilities of the upgrade," said Phil Efthimion, head of Plasma Science and Technology Unit at PPPL, who oversees the experiment.

The fusion combines light elements in the form of plasma – the hot, charged state of matter composed of free electrons and atomic nuclei – generating enormous amounts of energy. Scientists seek to replicate the fusion on Earth to obtain a virtually inexhaustible source of energy for the production of electricity.

To complete the upgrade, the team produced a neutral beam power of 500 kilowatts while increasing the strength of the magnetic field that contained the plasma to two-thirds and covering the walls of the tokamak with a coating lithium; the seemingly magical metal absorbs lost plasma particles and prevents them from bouncing back into the heart of the plasma and cooling it. The team then increased the power of the neutral beam to more than 600 kilowatts, multiplying by 10 the heating power of the machine.

Always maintain a good confinement?

The next test is to determine whether the upgraded machine can maintain good containment and constant temperature in much hotter plasmas with stronger magnetic fields. The improved beam will prevent the density from falling and will demonstrate whether the hotter and more energetic plasma can still be controlled.

The construction of the upgrade required steps including the installation of a more powerful power supply and a new lithium evaporator and was "a difficult task to accomplish," Majeski said. "Everyone worked very hard, we have a lot of help from the NSTX-U laboratory [National Spherical Torus Experiment-Upgrade] team of engineers. "Tom Kozub of the team oversaw the engineering work and the physicist Dennis Boyle directed the unit when it met the operating requirements.

Scientists from eight research centers across the country are collaborating on LTX-β: Oak Ridge National Laboratories and Lawrence Livermore; Princeton University; University of California at Los Angeles; The University of Wisconsin-Madison; University of Washington; and University of Tennessee, Knoxville.

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