New Paths to Fusion Energy
A quick, easy way to suppress instabilities in fusion devices
Scientists have discovered a remarkably simple way to suppress a common instability that can halt fusion reactions and damage the walls of reactors built to create a “star in a jar.” The findings stem from experiments on the National Spherical Torus Experiment-Upgrade (NSTX-U) at PPPL.
The suppressed instability is called a global Alfvén eigenmode (GAE) — a common wave-like disturbance that can cause fusion reactions to fizzle out. Suppression was achieved with a second neutral beam injector recently installed as part of the NSTX-U upgrade. Just a small amount of highly energetic particles from this second injector was able to shut down the GAEs.
Such instabilities are akin to a snake or dragon that swallows its own tail. The neutral beam particles become fast ions — or atomic nuclei — inside tokamaks and stir up the GAEs. Once triggered by these fast ions, the GAEs can rise up and drive the ions out, cooling the plasma and halting fusion reactions.
Suppressing this arousal were beams from the second injector, which flow through the plasma in a direction roughly parallel to the magnetic field that confines the hot gas. Physicists call such beams “outboard” to distinguish them from the “inboard” beams that the original NSTX-U injector produces, which flow through the plasma in a more perpendicular fashion.
Injection of the outboard beam suppressed GAEs in milliseconds by halting their ability to form and ripple through the plasma. “This research demonstrates suppression of GAEs with just a small population of energetic particles,” said physicist Eric Fredrickson, who headed the study. “It gives confidence that by using this code, reasonable predictions of GAE stability can be made for ITER,” the international fusion experiment under construction in France.
Physicist Eric Fredrickson in the NSTX-U control room
Sketch of neutral beam geometry with original NSTX beams in green and new beams installed on the upgrade in red
How lithium and fuzz affect fusion plasmas
Everyone knows that the game of billiards involves balls careening off the sides of a pool table — but few people may know that the same principle applies to fusion reactions. How plasma interact with the walls of tokamaks helps determine how efficiently fusion reactions occur. In a phenomenon known as secondary electron emission (SEE), electrons strike the wall and cause other — or secondary — electrons to be emitted that cool the edge of the plasma and dampen its overall performance.
Researchers at PPPL have studied SEE for decades, since the behavior of secondary electrons could affect the performance of future fusion machines. Two visiting physicists — Marlene Patino, a graduate student at the University of California, Los Angeles, and Angela Capece, a professor at the College of New Jersey —have recently investigated how different wall materials affect secondary electrons and have made important advances.
Angela Capece in the Surface Science and Technology Laboratory at PPPL
Marlene Patino
Capece studied how electrons interact with lithium, a wall material that bonds easily with other elements such as oxygen to form molecules like lithium oxide. She found that when electrons strike lithium oxide on a tokamak wall, many more secondary electrons are released into the plasma than from non-lithium wall material. Capece then quantified exactly how the amount of lithium oxide in the wall affects the number of secondary electrons that can enter the plasma and alter its performance.
Patino studied SEE from a different perspective, investigating tiny structures called “fuzz” that form when the nuclei of helium atoms bombard tungsten linings of tokamak walls. Her research found that tungsten with fuzz can reduce SEE by 40 percent to 60 percent compared with tungsten without fuzz, and the reduction could directly affect performance of the plasma.
Correlation analysis of three plasma discharges on NSTX for five different locations near the plasma edge. The red regions marked with a blue cross have a high positive correlation — or connection — with turbulence, while the blue regions marked with a yellow cross have a high negative correlation with the red region of turbulence
A deep dive into plasma turbulence
A big hurdle for fusion researchers is understanding turbulence, the ripples and eddies that can cause the superhot plasma that fuels fusion reactions to leak heat and particles and keep fusion from taking place. PPPL scientists have now delved beyond previously published characterizations of turbulence and found clues to the causes of such leakage.
“This study is an incremental step toward a fuller understanding of turbulence,” said physicist Stewart Zweben, lead author of the research published in the journal Physics of Plasmas. “It could help us understand how turbulence functions as the main cause of leakage of plasma confinement.”
The research, which looked at the correlation of turbulent ripples and eddies at the edge of tokamak plasmas, studied 20 discharges as representative samples of those created in the National Spherical Torus Experiment (NSTX) at PPPL prior to its upgrade. A fast camera reported the discharges at the rate of 400,000 frames per second, capturing the correlations within the turbulence.
Researchers performed computational analysis of the data from the camera, showing how turbulence in one part of the plasma varied with respect to turbulence in another part. The findings could help lead to improved understanding of turbulence and methods to reduce it, facilitating the development of fusion as a safe, clean and abundant source of energy for generating electricity from power plants around the world.
PPPL Physicist Masayuki Ono
Loops of liquid metal could improve future fusion power plants
PPPL physicists and collaborators have proposed an innovative new design that could improve the ability of future fusion power plants to generate safe, clean and abundant energy in a continuous, or steady state. The design would use loops of liquid lithium to clean and recycle tritium, the radioactive hydrogen isotope that fuels fusion reactions, eliminate dust and other impurities from the reactor chamber, and safeguard the divertor plates that exhaust waste heat from the plasma.
Lead designer of the lithium concept is Masayuki Ono, a principal research physicist at PPPL. The silvery metal readily combines with other elements and would handle these functions:
- Recycle tritium, a key fuel that will fuse with deuterium to produce fusion reactions in future power plants. Only approximately one percent of the tritium that is injected into the plasma is expected to be consumed in this fusion reaction process. The remaining unconsumed tritium must be removed and recycled back to maintain fueling.
- Remove dust and unwanted elements. If left unchecked, many tons of dust could accumulate in a year from interactions between the plasma and the fusion chamber walls. The same loop that recycles tritium would deliver the accumulating dust to a filter. The flowing liquid lithium would also carry away impurities such as nitrogen and oxygen that arise from contact between the plasma and the tokamak walls.
- Covering divertor plates. Injection of liquid lithium into the tokamak divertor chamber would coat the plates with the liquid substance, protecting them from heat and particles that rise up from the core of the plasma
Addressing these ideas are groups around the world that are testing flowing liquid lithium concepts. “We are looking to the future to come up with solutions,” said Ono. “These issues must be dealt with if we are to realize practical and attractive fusion power plants.”
Lithium could be nearly ideal for the walls of fusion reactors
Lithium is a light, silvery metal that is used in products as varied as cell phone batteries and drugs to treat clinical depression. At PPPL, researchers are uncovering a growing body of evidence that lithium is also a near-ideal wall material for a fusion reactor. The Lithium Tokamak Experiment (LTX), is the only facility in the world to employ fully lithium covered walls, and the first to test a full liquid metal wall in a fusion device.
Experiments with lithium wall coatings at PPPL date back to the Tokamak Fusion Test Reactor (TFTR) in the 1990s. Recently, LTX became the first device to demonstrate that the unique properties of lithium produce a boundary which does not cool the edge of the hot plasma in a tokamak at all. The resulting plasma in LTX had a uniform temperature of nearly 3,000,000 °C, completely contained within a lithium-coated wall. Some researchers have predicted that such an “isothermal” plasma may lose energy at a very slow rate, and would be much easier to heat to fusion conditions than conventional tokamak plasmas, which are strongly cooled by the walls to very low temperatures.
The LTX experiments were only able to maintain isothermal conditions for a short time. But now the device has been upgraded with a neutral beam injector, and higher magnetic fields, which will help to maintain the density of the plasma for longer periods, and greatly increase the temperature. The injector, to begin operation in late summer, will test whether lithium walls can continue to maintain isothermal conditions for longer times with a much hotter, fusion-relevant plasma in the upgraded device, LTX-β. The next step would be to continue tests of liquid lithium walls in the more powerful National Spherical Torus Experiment-Upgrade (NSTX-U). Ultimately, the goal is to learn to use liquid lithium walls to enable much smaller, better-performing – and cheaper – tokamaks, to speed the development of fusion power.
Lithium coating on the inner wall of LTX