Creating and controlling on Earth the fusion energy that powers the sun and stars is a key goal of scientists around the world. Production of this safe, clean and limitless energy could generate electricity for all humanity, and the possibility is growing closer to reality. A landmark report released by the American Physical Society Division of Plasma Physics Community Planning Process has proposed immediate steps for the United States to take to accelerate U.S. development of this long-sought power. The report also details opportunities for advancing our understanding of plasma physics and for applying that understanding to benefit society.

The report, the Community Plan for Fusion Energy and Discovery Plasma Sciences, delivered a major contribution to the long-range strategic plan that the U.S. Department of Energy’s (DOE) Fusion Energy Sciences Advisory Committee (FESAC) released last December. That document, prepared by a FESAC subcommittee on which PPPL physicist Rajesh Maingi sat, “provides a decade-long vision for the field of fusion energy and plasma science, presenting a path to a promising future of new scientific discoveries, industrial applications, and ultimately the timely delivery of fusion energy,” the preface said.

The landmark community report “reflects the enthusiasm among the U.S. fusion and plasma physics community to take bold steps to make fusion energy a reality, to expand our understanding of plasma physics, and to use that understanding to benefit society,” said PPPL physicist Nathan Ferraro, a co-chair of the plan the community assembled over a year.

The 199-page document, put together with input from hundreds of U.S. scientists and engineers from many professional societies, makes numerous recommendations ranging from enabling construction of a fusion pilot plant (FPP) that produces net electricity to advancing theory and modeling capabilities needed to understand and sustain burning plasmas, in which the plasma is chiefly heated by fusion reactions, a chief goal of the research.

The report is designed to help the Fusion Energy Sciences Advisory Committee (FESAC) fulfill a U.S. Department of Energy charge for the development of a long-range strategy for the Fusion Energy Sciences program of the DOE Office of Science. The document calls for partnerships with other offices and governmental agencies, as well as with private industry and international partners, to enact the full recommendations of the strategic plan.

All efforts to replicate in tokamak fusion facilities the fusion energy that powers the sun and stars must cope with transient heat bursts that can halt fusion reactions and damage the doughnut-shaped tokamaks. These bursts, called edge localized modes (ELMs), occur at the edge of hot, charged plasma gas when it kicks into high gear to fuel fusion reactions.

To prevent such bursts, researchers at the DIII-D National Fusion Facility that General Atomics (GA)operates in San Diego previously pioneered the injection of ripples of magnetic fields into the plasma to cause heat to leak out controllably. Now PPPL scientists have developed a control scheme to optimize the levels of these fields for maximum performance without ELMs.

The research, led by PPPL physicist Florian Laggner and developed on DIII-D with researchers from GA and other institutions, reveals a path to suppressing ELMs and maximizing fusion power on ITER, the international tokamak under construction in France that is designed to demonstrate the practicality of fusion energy.

The technique addresses the inherent conflict between optimizing fusion energy and controlling ELMs. The scheme focuses on the “pedestal,” the thin, dense layer of plasma near the edge of the tokamak that increases the pressure of the plasma and thus fusion power. However, if the pedestal grows too high it can create ELM heat bursts by suddenly collapsing.

So the key is controlling the height of the pedestal to maximize fusion power while preventing the layer from becoming so high that it triggers ELMs. The combination calls for real-time control of the process.

Unlocking the zig-zagging dance of hot, charged plasma particles that fuel fusion reactions can help to harness on Earth the fusion energy that powers the sun and stars. An experimentalist and two theorists at PPPL have worked together to develop a new algorithm, or set of computer rules, for tracking volatile particles — an algorithm that could advance the arrival of safe, clean and virtually limitless source of energy.

“This is a success story about close interaction between theorists and experimentalists that shows what can be done,” said Hong Qin, a principal theoretical physicist at PPPL. He and Yichen Fu, a theoretical graduate student whom he advises, collaborated on the algorithm with Laura Xin Zhang, an experimental graduate student and lead author of a paper that reports the research.

The new algorithm helps track fast charged particles in the plasma. Such particles could, for example, stem from the injection of high-energy neutral beams that are broken down, or "ionized," in the plasma and collide with the main plasma particles.

“We care about this because we want to understand how these fast particles influence the plasma,” Zhang said. “We use these particles to do all sorts of things. They can heat and drive current in the plasma. Sometimes they create plasma instabilities and sometimes they reduce them. Our simulations are all part of understanding how these particles behave.”

When Zhang first tried simulating the fast particles she ran into a problem. She used a classic algorithm that failed to conserve energy when the electrons in plasma collide with the ions, which are roughly 2,000-times heavier, in collisions akin to ping-pong balls bouncing off basketballs.

Conserving that energy is critical, said Qin. Otherwise, “the simulation cannot be trusted.” He therefore devised an explicitly solvable algorithm that conserves the energy of the particles, which Zhang found worked better than the classic algorithm but could not be proven theoretically. "I’m an experimentalist at heart, so I ran a bunch of simulations and did all kinds of numerical experiments and showed that the algorithm did work better,” she said.

Qin next handed the problem to graduate student Fu, who put together a clever mathematical proof of the correctness of the algorithm that could become a step to further solutions. “The algorithm we developed is for a simplified model,” Zhang said. “But I am charging ahead and aiming to apply the new algorithm to new plasma physics problems.”

Researchers have found that injecting pellets of hydrogen ice rather than puffing hydrogen gas improves fusion performance at the DIII-D National Fusion Facility, which General Atomics operates for the U.S. Department of Energy. The studies by physicists based at PPPL and Oak Ridge National Laboratory (ORNL) compared the two methods, looking ahead to the fueling to be used in ITER, the international fusion experiment under construction in France.

The research showed that icy pellets of hydrogen improve the temperature of the fusion plasma when compared with the gas-fueling method now typically used in doughnut-shaped fusion facilities called tokamaks. Higher temperatures are beneficial for the fusion reactions. The results on DIII-D are encouraging for ITER, which plans to use pellet injection to fuel its hot inner core.

The joint research effort on DIII-D compared the two fueling methods in high-performance plasmas planned for ITER. The experiments revealed a significantly higher pressure of plasma — a key to fusion reactions — using hydrogen ice compared to gas injection when the rate of fueling is roughly evenly matched between the two methods.

“The fueling plays a big role in the edge plasma performance,” said Andrew “Oak” Nelson, a graduate student in the Program in Plasma Physics at Princeton University and first author of the Nuclear Fusion article describing these results. The technology for injecting the ice pellets was developed by scientists at ORNL.

The research also demonstrates how graduate students can make important contributions to fusion energy by working on these large national research facilities. “For a graduate student to play an important role in this experimental study on DIII-D is impressive,” said Egemen Kolemen, a PPPL and Princeton University physicist who was an advisor for the project. “Oak's success shows how large fusion experiments provide significant leadership opportunities for students and early career scientists.”

Permanent magnets much stronger than those used on refrigerators could speed the development of fusion energy – the same energy produced by the sun and stars. Such magnets could in principle greatly simplify the design and production of twisty fusion facilities called stellarators, say scientists at PPPL and the Max Planck Institute for Plasma Physics in Germany. PPPL founder Lyman Spitzer Jr. invented the stellarator in the early 1950s.

Most stellarators use a set of costly and complex twisted coils that spiral like stripes on a candy cane to produce magnetic fields that shape and control the plasma that fuels fusion reactions. Refrigerator-like permanent magnets could produce the difficult-to-create part of these essential fields, the researchers say, allowing simple, non-twisted coils to create the remaining part in place of the complex coils.

Simplifying stellarators, which run without the risk of damaging disruptions that more widely used tokamak fusion devices face, can hold great appeal. “I am extremely excited about the use of permanent magnets to shape the plasma in stellarators,” said Steve Cowley, PPPL director. “It leads to much simpler engineering design.”

The novel idea for permanent magnets is an offshoot of a science fair project that Jonathan Zarnstorff, the son of PPPL Chief Scientist Michael Zarnstorff, put together in junior high school. Jonathan wanted to build a rail gun, a device that usually uses high-voltage current to generate a magnetic field that can fire a projectile. But the high-voltage current would be dangerous to use in a classroom.

The solution that father-and-son arrived at was to use rare earth permanent magnets to safely produce the magnetic field. Rare earth magnets have surprising and useful properties. Such magnets generate quite powerful fields for the magnets’ small size, and they are almost unaffected by other fields nearby. The magnets could thus provide what physicists call the “poloidal” part of a spiraling stellarator field, while simple round coils could provide the “toroidal” part that makes up the rest of the field.

PPPL is now developing permanent magnets with an outside developer. The work could become the forerunner for a program that takes the risk out of designing and building these twisty machines.

Picture an airplane that can only climb to one or two altitudes after taking off. That limitation would be similar to the plight facing scientists who seek to avoid instabilities that restrict the path to clean, safe and abundant fusion energy in doughnut-shaped tokamak facilities. Researchers at PPPL and General Atomics (GA) have published a breakthrough explanation of this tokamak restriction and how it may be overcome.

Doughnut-shaped tokamaks are prone to intense bursts of heat and particles, called edge localized modes (ELMs) that can damage the reactor walls and must be controlled to develop reliable fusion power. Fortunately, scientists have learned to tame these ELMs by applying spiraling rippled magnetic fields to the surface of the plasma that fuels fusion reactions. However, the taming of ELMs requires specific conditions that limit the operational flexibility of tokamak reactors.

Now, researchers at PPPL and GA have developed a model that, for the first time, accurately reproduces the conditions for ELM suppression in the DIII-D National Fusion Facility that GA operates for the U.S. Department of Energyy. The model predicts the conditions under which ELM suppression should extend over a wider range of operating conditions in the tokamak than previously thought possible. The work presents important predictions for how to optimize the effectiveness of ELM suppression in ITER, the massive international fusion device under construction in the south of France to demonstrate the feasibility of fusion power.

PPPL physicists Qiming Hu and Raffi Nazikian are the lead authors of a paper describing the model in Physical Review Letters. They note that under normal conditions the rippled magnetic field can only suppress ELMs for very precise values of the plasma current that produces the magnetic fields that confine the plasma. The authors show how, by modifying the structure of the helical magnetic ripples applied to the plasma, ELMs should be eliminated over a wider range of plasma current with improved generation of fusion power. Hu said he believes the findings could provide ITER with the wide operational flexibility it will need to demonstrate the practicality of fusion energy. “This model could have significant implications for suppressing ELMs in ITER,” he said.

Blobs can wreak havoc in plasma required for fusion reactions. This bubble-like turbulence swells up at the edge of fusion plasmas and drains heat from the edge, limiting the efficiency of fusion reactions in doughnut-shaped tokamaks. Researchers at PPPL have now discovered a surprising correlation of the blobs with fluctuations of the magnetic field that confines the plasma fueling fusion reactions in the tokamak core.

“These results add a new aspect to our understanding of the heat loss at the edge of the plasma in a tokamak,” said physicist Stewart Zweben, lead author of a paper in Physics of Plasmas that editors have selected as a featured article. “This work also contributes to our understanding of the physics of blobs, which can help to predict the performance of tokamak fusion reactors.”

Researchers discovered the surprising link when re-analyzing experiments made in 2010 on PPPL’s National Spherical Torus Experiment (NSTX) — the forerunner of today’s National Spherical Torus Experiment-Upgrade (NSTX-U). The blobs and fluctuations in the magnetic field develop in all tokamaks and have traditionally been seen as independent of each other.

The scientists then analyzed several possible causes of the unexpected correlation but could find no single compelling explanation. To understand and control this phenomenon, Zweben said, further data analysis and modeling will have to be done. Further investigation of the correlation and its role in the loss of heat from magnetic fusion reactors could help to produce on Earth the fusion energy that powers the sun and stars.

Turbulence — the unruly swirling of fluid and air that mixes coffee and cream and can rattle airplanes in flight — causes heat loss that weakens efforts to reproduce on Earth the fusion that powers the sun and stars. Now scientists have modeled a key source of the turbulence found in a fusion experiment at PPPL, paving the way for improved experiments to capture and control fusion energy.

The research, led by Juan Ruiz Ruiz while a graduate student at the Massachusetts Institute of Technology (MIT) who worked with PPPL researchers Walter Guttenfelder and Yang Ren, used state-of-the-art simulations to zero-in on the source of the turbulence that produces heat loss. The findings predicted results consistent with experiments on the National Spherical Torus Experiment (NSTX) fusion device at PPPL, pinpointing the source as microscopic turbulent eddies. Driving these eddies is the gradient, or variation, in the electron temperature in the core of the plasma, the so-called electron temperature gradient (ETG).

The findings confirmed theories of when ETG can be a main driver of electron turbulence, technically known as electron thermal transport, that whips up the heat loss in spherical tokamaks such as NSTX. The consistency of the simulation with experimental data gives confidence “that the simulation contains the necessary physics to explain the loss of heat,” said Ruiz Ruiz, now a postdoctoral research assistant at the University of Oxford and first author of a paper reporting the results in Plasma Physics and Controlled Fusion.

Understanding the source of electron thermal transport is a top priority for confining heat in future fusion facilities, and particularly in spherical tokamaks, which lose most of their heat through such transport in high-performance H-mode plasmas. Further research could confirm the source of this heat loss on the upgraded NSTX, called the NSTX-U, and the Mega Ampere Spherical Tokamak (MAST) in the United Kingdom, Ruiz Ruiz said. “That could demonstrate the ability of the simulations to accurately forecast the loss of heat — and therefore the performance — of spherical tokamaks.”

PPPL scientist have bettered their understanding of a promising method for improving the confinement of superhot fusion plasma inside magnetic fields. Improved plasma confinement could enable construction of smaller and less expensive type of fusion reactor called a spherical tokamak, moving the world closer to reproducing on Earth the fusion energy that powers the sun and stars.

Creating the improved confinement is a condition called the enhanced pedestal (EP) H-mode, a variety of the high performance, or H-mode, plasma state that scientists have observed for decades in tokamaks around the world. This mode requires less heating to raise plasma to the superhot temperatures necessary for fusion reactions.

Scientists led by physicists at PPPL have discovered that EP H-mode improves upon H-mode in spherical tokamaks by lowering the density of the plasma edge. The reduced density occurs when small instabilities in the plasma edge eject relatively cold, low-energy particles. With fewer cold particles to bump into, the hotter particles in the plasma are less likely to leak out.

“As the higher energy particles stay in the plasma in larger quantities, they increase the pressure in the plasma, feeding the instabilities that throw out colder particles and further lowering the edge density,” said PPPL physicist Devon Battaglia, who led the research project. “Ultimately, the fortuitous interaction allows the plasma to stay hotter with the same heating and little change to the average plasma density.

“It’s like adding better insulation to your house,” Battaglia said. “The more the plasma holds on to its heat, the smaller you can make the device.” Moreover, he added, “by taking the next leap in our understanding of how the EP H-mode process comes about, we can have more confidence in being able to predict if it’s going to happen.”

Lithium, the silvery metal that powers smart phones and helps treat bipolar disorders, could play a significant role in the worldwide effort to harvest on Earth the safe, clean and virtually limitless fusion energy that powers the sun and stars. First results of the extensively upgraded Lithium Tokamak Experiment-Beta (LTX-β) at PPPL demonstrate that the major enhancements operate as designed and improve the performance of the hot, charged plasma that will fuel future fusion reactors.

The three-year upgrade turned what is now the LTX-β into a hotter, denser and more fusion-relevant device that will test how well coating all plasma-facing walls with liquid lithium can improve the confinement and increase the temperature of the plasma. “We achieved many of our initial engineering goals,” said physicist Drew Elliott of Oak Ridge National Laboratory, a major collaborator of the LTX-β on long-term assignment to PPPL who served as lead author of the first results paper.

Key features of the LTX-β include a powerful neutral beam injector to heat and fuel the plasma; a nearly doubled magnetic field compared with the previous device; and a twin evaporation system to fully coat liquid lithium on all the plasma-facing surfaces.

Operation of the beam matched well with predictions of the fraction of power that it would deposit into the plasma, rather than simply shining through it. “We’re looking to increase the power deposition toward 100% so that all the power we inject goes into the plasma,” said Elliott.

The substantial enhancements aim to test whether the LTX-β can improve plasma performance beyond the notable achievements of its predecessor. These include the demonstration of temperatures that remain constant, or flat, all the way from the hot core of the plasma to the normally cool outer edge. Sustaining the hot edge expands the volume of plasma available for fusion and the production of flat temperature prevents instabilities that reduce plasma confinement from developing.

“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,” said Dick Majeski, principal investigator for LTX-β. “If LTX-β is successful, we can move on to experiments on liquid lithium in the National Spherical Torus Experiment-Upgrade [NSTX-U],” the flagship fusion experiment at PPPL.