Collaborations

Physicist Jong-Kyu Park with diagram showing beneficial distortions of the KSTAR plasma

Discovered: Optimal magnetic fields for suppressing instabilities in tokamaks

Fusion, the power that drives the sun and stars, produces massive amounts of energy. Scientists seeking to replicate this process face a long-time puzzle: how to lessen or eliminate a common instability called edge localized modes (ELMs) that occurs in the plasma that fuels fusion reactions. Just as the sun releases huge bursts of energy in the form of solar flares, so flare-like bursts of ELMs can slam into the walls of doughnut-shaped tokamaks that house fusion reactions, potentially damaging the walls of the reactor.

To control these bursts, scientists distort the symmetrically shaped plasma with small magnetic ripples that release excess pressure to lessen or prevent ELMs from occurring. The hard part is producing just the right amount of distortion to eliminate the ELMs without triggering other instabilities and releasing too much energy that that can lead to a major disruption.

Making the task exceptionally difficult is the fact that a virtually limitless number of magnetic distortions can be applied to the plasma, causing finding precisely the right kind of distortion to be an extraordinary challenge.

But no longer. PPPL physicist Jong-Kyu Park, working with a team of collaborators from the United States and the National Fusion Research Institute (NFRI) in Korea, have successfully predicted the entire set of beneficial distortions for controlling ELMs without creating more problems. Researchers validated these predictions on the Korean Superconducting Tokamak Advanced Research (KSTAR) facility, one of the world's most advanced superconducting tokamaks.

The result was a precedent-setting achievement. “For a long time we thought it would be too computationally difficult to identify all beneficial symmetry-breaking fields,” said Park, but our work now demonstrates a simple procedure to identify the set of all such configurations."

Researchers reduced the complexity of the calculations when they realized that the number of ways the plasma can distort is far fewer than the range of possible ripples that can be applied to the plasma. By working backwards, from distortions to ripples, the authors calculated the most effective fields for eliminating ELMs.

These findings, reported in Nature Physics, provide new confidence in the ability to control ELMs on ITER, the international tokamak under construction in France. Such control will be vital for ITER, which aims to create a “burning plasma” that produces 10 times more energy than it will take to heat the plasma. ☀︎

PPPL physicist Novimir Pablant, right, and Andreas Langenberg of the Max Planck Institute in front of the housing for the x-ray crystal spectrometer prior to its installation in the W7-X

“The spectrometer provided the primary measurement”

PPPL diagnostic is key to world record of German fusion experiment

When Germany’s Wendelstein 7-X (W7-X) fusion facility set a world record for stellarators, a finely tuned instrument built and delivered by PPPL helped prove the achievement. The record strongly suggested that the twisty design of the stellarator can be developed to capture the fusion that drives the sun and stars to generate a virtually unlimited supply of electric energy here on Earth.

The record achieved by the W7-X, the world’s largest and most advanced stellarator, was the highest “triple product” that a stellarator has created. The product combines the temperature, density and confinement time of a fusion facility’s plasma to measure how close the device can come to producing self-sustaining fusion power.

Measuring the temperature was an x-ray imaging crystal spectrometer (XICS) built by PPPL physicist Novimir Pablant, now stationed at W7-X, and engineer Michael Mardenfeld, formerly at PPPL. “The spectrometer provided the primary measurement,” said PPPL physicist Sam Lazerson, who also collaborates on W7-X experiments.

Researchers at the Max Planck Institute of Plasma Physics (IPP), which operates the stellarator in Greifswald, Germany, welcomed the findings. “Without XICS we could not have confirmed the record,” said Thomas Sunn Pedersen, director of stellarator edge and divertor physics at IPP. Concurred physicist Andreas Dinklage: “The XICS data set was one of the very valuable inputs that confirmed the physics predictions.”

PPPL’s Pablant worked with IPP scientists and engineers to implement the instrument. “It has been a great experience to work closely with my colleagues here on W7-X,” Pablant said. “The initial results from these high-performance plasmas are very exciting, and we look forward to using new measurements to further understand the confinement properties of W7-X, which is a truly unique magnetic fusion experiment.”

PPPL has designed and delivered additional components installed on the W7-X. These include a set of large trim coils that correct errors in the magnetic field that confines W7-X plasma, and a scraper unit that will lessen the heat reaching the divertor that exhausts waste heat from the fusion facility. ☀︎

New testing of model improves confidence in the performance of ITER

Scientists seeking to bring fusion — the power that drives the sun and stars —to Earth must first make the state of matter called plasma superhot enough to sustain fusion reactions; the task calls for heating the plasma to many times the temperature at the core of the sun. ITER, the international fusion facility being built in France, will heat both the free electrons and the atomic nuclei — or ions — that make up the plasma, raising a key question: How will the double heating affect the temperature and density of the plasma, which are crucial to fusion reactions?

Recent collaboration between physicists Brian Grierson at PPPL and Gary Staebler at General Atomics (GA) suggests an answer. Their research tested a model created by Staebler and reported in Physics of Plasmas indicating that dual heating produced turbulence with short-to-long wavelengths. This turbulence modified the leakage of heat from the plasma and altered the gradient — or spatial rate of change — in the density of the plasma.

“This shows what happens when electron heating is added to ion heating,” said Grierson, who led testing of the model that projected to ITER results of experiments on the DIII-D National Fusion Facility, which GA operates for the U.S. Department of Energy. Diagnostics supplied by the University of Wisconsin-Madison and the University of California, Los Angeles, measured the resulting plasma.

The model looked specifically at the impact of electron heating on the overall heating mix and showed that studying multiscale turbulence will be essential to understanding how to deal with the transport of heat, particles and momentum in ITER and other next-generation fusion facilities. “We need to understand transport under ion and electron heating to confidently project to future reactors,” Grierson said, “because fusion power plants will have both types of heating.” ☀︎

Interior of the DIII-D tokamak at General Atomics

Physicist Rajesh Maingi

“So, the question was whether lithium will have the same effect on tungsten walls as it does with carbon walls”

Lithium — it’s not just for batteries: The powdered metal can reduce instabilities in fusion plasmas

You may be most familiar with the element lithium as an integral component of your smart phone’s battery, but scientists have found that the element also plays a role in the development of clean fusion energy. The silvery metal can reduce instabilities in plasma that can damage the walls of reactors that house fusion reactions.

Experiments by physicists at PPPL and collaborators on China’s Experimental Advanced Superconducting Tokamak (EAST) have found that lithium powder can eliminate instabilities known as edge-localized modes (ELMs) when used to coat a tungsten-lined plasma-facing component called the “divertor” — the unit that exhausts waste heat and particles from the plasma that fuels fusion reactions.

The results are good news for future devices that plan to use tungsten for their own divertors that are designed to work with lithium.

Past experiments with lithium powder on EAST successfully tested the element when the upper and lower divertors were coated with light and porous carbon rather than the heavy metal tungsten. “So, the question was whether lithium will have the same effect on tungsten walls as it does with carbon walls,” said PPPL physicist Rajesh Maingi, lead author with Jiansheng Hu of the Institute of Plasma Physics at the Chinese Academy of Sciences (ASIPP) of a description of the results in the journal Nuclear Fusion.

Recent experiments show that lithium injected into plasma in contact with tungsten reduces ELMs just as much as lithium does when the plasma exhausts its heat on carbon. Physicists now have increased confidence that the techniques used to reduce ELMs in current fusion machines will also work in future larger machines that are designed to be compatible with lithium. ☀︎

New model sheds light on key physics of magnetic islands that halt fusion reactions

Magnetic islands, bubble-like structures that form in fusion plasmas, can grow and disrupt the plasmas and damage the doughnut-shaped tokamak facilities that house fusion reactions. Recent PPPL research has used large-scale computer simulations to produce a new model that could be key to understanding how the islands interact with the surrounding plasma as they grow and lead to disruptions.

The findings, which overturn long-held assumptions of the structure and impact of magnetic islands, come from simulations led by visiting physicist Jae-Min Kwon during a year-long sabbatical from the Korean Superconducting Tokamak Advanced Research (KSTAR) facility. Kwon and physicists at PPPL modeled the detailed and surprising experimental observations recently made on KSTAR.

“The experiments intrigued many KSTAR researchers including me,” said Kwon, first author of a paper selected as an Editor’s Pick in the journal Physics of Plasmas. “I wanted to understand the physics behind the sustained plasma confinement that we observed. Since I knew that the powerful XGC code at PPPL had the capabilities that I needed to study the problem, I decided to spend my sabbatical at the Laboratory.”

Kwon worked with C.S. Chang, a principal research physicist at PPPL and leader of the XGC team, and PPPL physicists Seung-Ho Ku, and Robert Hager. When the researchers modeled magnetic islands using plasma conditions from the KSTAR experiments, the structure of the islands proved markedly different from standard assumptions — as did their impact on plasma flow, turbulence, and plasma confinement during fusion experiments.

The simulations found that plasma confinement can be maintained even as the islands grow.

These findings contradicted past models, which assumed that magnetic islands simply degraded the confinement, and agreed with the experimental observations made on KSTAR. Said Chang:. “These findings could lay new groundwork for understanding the physics of plasma disruption, which is one of the most dangerous events a tokamak reactor could encounter.” ☀︎

The Korean Superconducting Tokamak Advanced Research facility

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