The silvery metal lithium helps treat bipolar disorders and powers smartphones. It could even play a crucial role in the global effort to harvest the clean, safe, and almost unlimited fusion energy on Earth. This fusion energy is known to power the stars and the Sun.
At the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory, the elaborately upgraded Lithium Tokamak Experiment-Beta (LTX-β) provided initial results that demonstrated that the significant improvements not only operate as intended but also enhance the performance of the hot, charged plasma that is believed to power upcoming fusion reactors.
The upgrade, which took three years to complete, has turned the present LTX-β into a denser, hotter, and more fusion-applicable device that will test whether optimal coating of all plasma-facing walls with liquid lithium would enhance the confinement and boost the plasma temperature.
“We achieved many of our initial engineering goals,” stated Drew Elliott, a physicist from Oak Ridge National Laboratory and also a major collaborator of the LTX-β. Elliott is on long-term assignment to Princeton Plasma Physics Laboratory and is the lead author of the initial results published in the IEEE Transactions on Plasma Science journal.
During fusion reactions, light elements are combined in the form of plasma—the state of matter made up of atomic nuclei and free electrons and constituting up to 99% of the visible universe—to emit large amounts of energy. Globally, physicists are looking for ways to replicate and manage fusion reactions to produce safe, unlimited carbon-free power to create electricity.
The main aspects of the LTX-β, which is a smaller version of the extensively employed doughnut-shaped magnetic tokamak facilities that accommodate fusion reactions, comprise these factors: a virtually doubled magnetic field compared with the earlier device, a strong neutral beam injector to fuel and heat the plasma, and a twin evaporation system to completely coat all the plasma-facing surfaces with liquid lithium.
Beam operation corresponded well with the predictions of the small amount of power that it would deposit into the plasma, instead of merely illuminating through it.
“We’re looking to increase the power deposition toward 100% so that all the power we inject goes into the plasma,” added Elliott, who headed the optimization of the neutral beam built on the technology pioneered at Ridge National Laboratory in the 1970s. “That will be a big scientific push, in future campaigns.”
The aim of these significant improvements is to test whether the LTX-β is capable of improving the performance of plasma beyond the extraordinary achievements of its predecessor. These involve the demonstration of temperatures that continued to stay flat, or constant, all the way from the hot plasma core to the usually cool exterior edge.
These gradient-free temperature profiles, which were observed in a magnetic fusion facility in the earlier device for the first time, stem from the potential of lithium to hold onto wandering particles leaking from the plasma core and prevent them from recycling back and cooling the core and edge of the plasma.
When the hot edge is sustained, it expands the amount of plasma that is available for fusion, and the creation of flat temperature avoids instabilities that decrease plasma limitation from developing.
Goals of the Upgrade
The goals of the upgrade are to determine whether very low recycling lithium walls can improve plasma confinement in a tokamak with neutral beam heating. If LTX-β is successful, we can move on to experiments on liquid lithium in the National Spherical Torus Experiment-Upgrade [NSTX-U].
Dick Majeski, Principal Investigator, Lithium Tokamak Experiment-Beta, Princeton Plasma Physics Laboratory
NSTX-U is the flagship fusion experiment at Princeton Plasma Physics Laboratory.
The preliminary run of the LTX-β showed enhancements that involved the following:
- Better deposition of liquid lithium across more than 90% of the interior walls of the LTX-β
- Better density and fueling of the plasma, which are key objectives of the neutral beam injector
- Higher plasma current is a crucial element that causes the magnetic field to spiral, which is required to limit the plasma
- Longer plasma pulses, or discharges, facilitated by the reinforced magnetic field
New plasma diagnostics have also been installed in the upgrade. They will further define the expanded operating regime of the facility. Advanced diagnostics, which are yet to be commissioned, will quantity the accurate profile of a number of plasma parameters.
The addition of the neutral beam increases the input power to the plasma by an order of magnitude and has the potential of creating a fusion-relevant plasma regime with enhanced performance. Dick Majeski and the entire LTX-β team should be commended for completing this aggressive upgrade on budget and schedule.
Phil Efthimion, Head, Plasma Science & Technology Department, Princeton Plasma Physics Laboratory
The department includes the LTX-β.
Experts Across the United States
The three-year upgrade pulled from experts across the United States, including association from Princeton Plasma Physics Laboratory, Oak Ridge National Laboratory, Princeton University, the University of California, Los Angeles (UCLA), and the University of Tennessee, Knoxville, and offers a substantial tool for fusion studies.
ORNL and PPPL have been partners in fusion science and technology for many years, and this continues that strong union. LTX-β will allow the fusion community to dig deeper into the promise of lithium and what it could unlock in enabling practical fusion energy.
Mickey Wade, Director of Fusion Energy Division, Oak Ridge National Laboratory
Majeski has made big plans ahead.
“In the future, we’d like to increase the pulse length of the neutral beam to provide a longer period of heating and fueling for the plasma,” he said. “The beam adds a lot of flexibility to the experiment, and we want to take advantage of the new capabilities,” Majeski concluded.
The study was funded by the DOE Office of Science.
Elliott, D., et al. (2020) Initial Results from the Newly Upgraded LTX-β. IEEE Transactions on Plasma Science. doi.org/10.1109/TPS.2020.2983854.