Electrons slide through the hourglass on surface of bizarre material (Nature)

Posted on by Catherine Zandonella
An illustration of the hourglass fermion predicted to lie on the surface of crystals of potassium mercury antimony. (Bernevig et al., Princeton University)
An illustration of the hourglass fermion predicted to lie on the surface of crystals of potassium mercury antimony. (Image credit: Laura R. Park and Aris Alexandradinata)

By Staff

A team of researchers at Princeton University has predicted the existence of a new state of matter in which current flows only through a set of surface channels that resemble an hourglass. These channels are created through the action of a newly theorized particle, dubbed the “hourglass fermion,” which arises due to a special property of the material. The tuning of this property can sequentially create and destroy the hourglass fermions, suggesting a range of potential applications such as efficient transistor switching.

In an article published in the journal Nature this week, the researchers theorize the existence of these hourglass fermions in crystals made of potassium and mercury combined with either antimony, arsenic or bismuth. The crystals are insulators in their interiors and on their top and bottom surfaces, but perfect conductors on two of their sides where the fermions create hourglass-shaped channels that enable electrons to flow.

The research was performed by Princeton University postdoctoral researcher Zhi Jun Wang and former graduate student Aris Alexandradinata, now a postdoctoral researcher at Yale University, working with Robert Cava, Princeton’s Russell Wellman Moore Professor of Chemistry, and Associate Professor of Physics B. Andrei Bernevig.

The new hourglass fermion exists – theoretically for now, until detected experimentally – in a family of materials broadly called topological insulators, which were first observed experimentally in the mid-2000s and have since become one of the most active and interesting branches of quantum physics research. The bulk, or interior, acts as an insulator, which means it prohibits the travel of electrons, but the surface of the material is conducting, allowing electrons to travel through a set of channels created by particles known as Dirac fermions.

Fermions are a family of subatomic particles that include electrons, protons and neutrons, but they also appear in nature in many lesser known forms such as the massless Dirac, Majorana and Weyl fermions. After years of searching for these particles in high-energy accelerators and other large-scale experiments, researchers found that they can detect these elusive fermions in table-top laboratory experiments on crystals. Over the past few years, researchers have used these “condensed matter” systems to first predict and then confirm the existence of Majorana and Weyl fermions in a wide array of materials.

The next frontier in condensed matter physics is the discovery of particles that can exist in the so-called “material universe” inside crystals but not in the universe at large. Such particles come about due to the properties of the materials but cannot exist outside the crystal the way other subatomic particles do. Classifying and discovering all the possible particles that can exist in the material universe is just beginning. The work reported by the Princeton team lays the foundations of one of the most interesting of these systems, according to the researchers.

In the current study, the researchers theorize that the laws of physics prohibit current from flowing in the crystal’s bulk and top and bottom surfaces, but permit electron flow in completely different ways on the side surfaces through the hourglass-shaped channels. This type of channel, known more precisely as a dispersion, was completely unknown before.

The researchers then asked whether this dispersion is a generic feature found in certain materials or just a fluke arising from a specific crystal model.

It turned out to be no fluke.

A long-standing collaboration with Cava, a material science expert, enabled Bernevig, Wang, and Alexandradinata to uncover more materials exhibiting this remarkable behavior.

“Our hourglass fermion is curiously movable but unremovable,” said Bernevig. “It is impossible to remove the hourglass channel from the surface of the crystal.”

Bernevig explained that this robust property arises from the intertwining of spatial symmetries, which are characteristics of the crystal structure, with the modern band theory of crystals. Spatial symmetries in crystals are distinguished by whether a crystal can be rotated or otherwise moved without altering its basic character.

In a paper published in Physical Review X this week to coincide with the Nature paper, the team detailed the theory behind how the crystal structure leads to the existence of the hourglass fermion.

An illustration of the complicated dispersion of the surface fermion arising from a background of mercury and bismuth atoms (blue and red). (Image credit: Mingyee Tsang and Aris Alexandradinata)
An illustration of the complicated dispersion of the surface fermion arising from a background of mercury and bismuth atoms (blue and red). (Image credit: Mingyee Tsang and Aris Alexandradinata)

“Our work demonstrates how this basic geometric property gives rise to a new topology in band insulators,” Alexandradinata said. The hourglass is a robust consequence of spatial symmetries that translate the origin by a fraction of the lattice period, he explained. “Surface bands connect one hourglass to the next in an unbreakable zigzag pattern,” he said.

The team found esoteric connections between their system and high-level mathematics. Origin-translating symmetries, also called non-symmorphic symmetries, are described by a field of mathematics called cohomology, which classifies all the possible crystal symmetries in nature. For example, cohomology gives the answer to how many crystal types exist in three spatial dimensions: 230.

In the cohomological perspective, there are 230 ways to combine origin-preserving symmetries with real-space translations, known as the “space groups.” The theoretical framework to understand the crystals in the current study requires a cohomological description with momentum-space translations.

“The hourglass theory is the first of its kind that describes time-reversal-symmetric crystals, and moreover, the crystals in our study are the first topological material class which relies on origin-translating symmetries,” added Wang.

Out of the 230 space groups in which materials can exist in nature, 157 are non-symmorphic, meaning they can potentially host interesting electronic behavior such as the hourglass fermion.

“The exploration of the behavior of these interesting fermions, their mathematical description, and the materials where they can be observed, is poised to create an onslaught of activity in quantum, solid state and material physics,” Cava said. “We are just at the beginning.”

The study was funded by the National Science Foundation, the Office of Naval Research, the David and Lucile Packard Foundation, the W. M. Keck Foundation, and the Eric and Wendy Schmidt Transformative Technology Fund at Princeton University.

The paper, “Hourglass fermions” by Zhijun Wang, A. Alexandradinata, R. J. Cava and B. Andrei Bernevig, was published in the April 14, 2016 issue of Nature, 532,189–194, doi:10.1038/nature17410. Read the preprint.

The paper, “Topological insulators from group cohomology” by A. Alexandradinata, Zhijun Wang, and B. Andrei Bernevig, was published in the April 15, 2016 issue of Phys. Rev. X 6, 021008.

Compressing turbulence to improve inertial confinement fusion experiments (PRL)

Posted on by Catherine Zandonella
Compression of a turbulent plasma. Credit: Seth Davidovits
Compression of a turbulent plasma. Credit: Seth Davidovits

By John Greenwald, Princeton Plasma Physics Laboratory

Physicists have long regarded plasma turbulence as unruly behavior that can limit the performance of fusion experiments. But new findings by researchers associated with the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and the Department of Astrophysical Sciences at Princeton University indicate that turbulent swirls of plasma could benefit one of the two major branches of such research.   The editors of Physical Review Letters highlighted these findings — a distinction given to one of every six papers per issue — when they published the results last week on March 11.

Lead author Seth Davidovits, a Princeton University graduate student, and Professor of Astrophysical Sciences Nathaniel Fisch, his thesis adviser and Associate Director for Academic Affairs at PPPL, produced the findings. They modeled the compression of fluid turbulence, showing effects that suggested a surprising positive impact of turbulence on inertial confinement fusion (ICF) experiments.

Stimulating this work were experiments conducted by Professor Yitzhak Maron at the Weizmann Institute of Science in Israel. Those experiments, on a Z-pinch inertial confinement machine, showed turbulence that contained a surprising amount of energy, which caught Fisch’s attention during a recent sabbatical at Weizmann.

In a Z-pinch and other inertial confinement (ICF) machines, plasma is compressed to create fusion energy. The method contrasts with the research done at PPPL and other laboratories, which controls plasma with magnetic fields and heats it to fusion temperatures in doughnut-shaped devices called tokamaks. The largest Z-pinch device in the United States is at the DOE’s Sandia National Laboratory. Other inertial confinement approaches are pursued at, among other places, the DOE’s Lawrence Livermore National Laboratory.

Present ICF approaches use compression to steadily heat the plasma. Methods range from squeezing plasma with magnetic fields at Sandia to firing lasers at capsules filled with plasma at Livermore’s National Ignition Facility. The presence of turbulence in the plasma is widely thought to increase the difficulty of achieving fusion.

But there could be advantages to turbulence if handled properly, the authors point out, since energy contained in turbulence does not radiate away. This compares with hotter plasmas in which heat radiates away quickly, making fusion harder to achieve. By storing the energy of the compression in turbulence rather than temperature, the authors suppress the energy lost to radiation during the compression.

The turbulent energy also does not immediately lead to fusion, which requires high temperature. This means a mechanism is needed to change the turbulence into the temperature required for fusion once the plasma has been compressed.

Davidovits used a software code called Dedalus to show that turbulent energy is increased during the compression, but then suddenly transformed into heat. As external forces in his simulation compress the turbulence to increase the energy stored within it, they also gradually raise the temperature and viscosity of the plasma. The viscosity, which describes how “thick” or resistant to flow a fluid is, acts to slow the turbulence and convert its energy to temperature. The viscosity started small so that the turbulence was initially unhindered. The rapid compression then kept the viscosity growing until it suddenly catalyzed the transfer of energy from the turbulence to the temperature.

In an experiment, this process would create the conditions for nuclear fusion in a plasma composed of the hydrogen isotopes deuterium and tritium. “This suggests a fundamentally different design for compression-based fusion experiments,” Davidovits said, “and a new paradigm for the inertial technique of producing fusion energy.”

He warns, however, that the simulation includes caveats that could diminish the findings. For example, the model doesn’t consider any possible interaction between the plasma and the containing capsule, and highly energetic turbulence might mix parts of the capsule into the plasma and contaminate the fusion fuel.

Nonetheless, the authors call the rapid transfer of turbulent energy into temperature during ICF experiments a “tantalizing” prospect that could benefit such research. And they note that their findings could lead to new understanding of the evolution of the relationship between the pressure, volume and temperature of a gas that is substantially turbulent. Determining this will be quite challenging, they say, “but the understanding will be important not only for the new fusion approach, but also for many situations involving the behavior of low viscosity compressible fluids and gases.”

This research was initiated through a grant by the Defense Threat Reduction Agency, a unit of the U.S. Department of Defense, and has been supported also by the DOE’s National Nuclear Security Administration through a consortium with Cornell University. Recently, the National Science Foundation and the Israel Binational Science Foundation combined funding opportunities to ensure further experiments at Weizmann on this topic and continued collaboration with the Princeton researchers.

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Read the abstract or paper here.

S. Davidovits, N. Fisch, Sudden Viscous Dissipation of Compressing Turbulence. Phys. Rev. Lett. 116, 105004 – Published 11 March 2016.

This work was supported by DOE through Contracts No. DE-AC02-09CH1-1466 and NNSA 67350-9960 (Prime No. DOE DE-NA0001836), by DTRA HDTRA1-11-1-0037, and by NSF Contract No. PHY-1506122.

 

Down the rabbit hole: how electrons travel through exotic new material (Science)

Posted on by Catherine Zandonella
Electrons on the surface of a Weyl semi-metal
Three-dimensional image using scanning tunneling electron microscopy of electrons on the surface of a Weyl semi-metal, a kind of crystal with unusual conducting and insulating properties. Image credit: Yazdani et al, Princeton University.

By Catherine Zandonella, Office of the Dean for Research

Researchers at Princeton University have observed a bizarre behavior in a strange new crystal that could hold the key for future electronic technologies. Unlike most materials in which electrons travel on the surface, in these new materials the electrons sink into the depths of the crystal through special conductive channels.

“It is like these electrons go down a rabbit hole and show up on the opposite surface,” said Ali Yazdani, the Class of 1909 Professor of Physics. “You don’t find anything else like this in other materials.”

The research was published in the journal Science.

Tantalum arsenide
Crystal structure of tantalum arsenide (TaAs), with Ta in blue and As in green. Image credit: Yazdani et al., Princeton University.

Yazdani and his colleagues discovered the odd behavior while studying electrons in a crystal made of layers of tantalum and arsenic. The material, called a Weyl semi-metal, behaves both like a metal, which conducts electrons, and an insulator, which blocks them. A better understanding of these and other “topological” materials someday could lead to new, faster electronic devices.

The team’s experimental results suggest that the surface electrons plunge into the crystal only when traveling at a certain speed and direction of travel called the Weyl momentum, said Yazdani. “It is as if you have an electron on one surface, and it is cruising along, and when it hits some special value of momentum, it sinks into the crystal and appears on the opposite surface,” he said.

These special values of momentum, also called Weyl points, can be thought of as portals where the electrons can depart from the surface and be conducted to the opposing surface. The theory predicts that the points come in pairs, so that a departing electron will make the return trip through the partner point.

Schematic of the connections between the top and bottom surface of a crystal
New research from Princeton University demonstrates the bizarre movement of electrons through a novel material called a Weyl semi-metal. The image shows a schematic of the connections at special values of electron momentum, which come in pairs and are called Weyl points (red and blue dots). An electron that leaves the surface on a red point can return through its partner blue point, and vice versa. This bizarre behavior is due to the “topological” connections through the bulk of the material. Image credit: Yazdani et al., Princeton University.

The team decided to explore the behavior of these electrons following research, published in Science last year by another Princeton team and separately by two independent groups, revealing that electrons in Weyl semi-metals are quite unusual. For example, their experiments implied that while most surface electrons create a wave pattern that resembles the spreading rings that ripple out when a stone is thrown into a pond, the surface electrons in the new materials should make only a half circle, earning them the name “Fermi arcs.”

To get a more direct look at the patterns of electron flow in Weyl semi-metals, postdoctoral researcher Hiroyuki Inoue and graduate student András Gyenis in Yazdani’s lab, with help from graduate student Seong Woo Oh, used a highly sensitive instrument called a scanning tunneling microscope, one of the few tools that can observe electron waves on a crystal surface.

They obtained the tantalum arsenide crystals from graduate student Shan Jiang and assistant professor Ni Ni at the University of California-Los Angeles.

The results were puzzling. “Some of the interference patterns that we expected to see were missing,” Yazdani said.

Comparison of experiment (left) to theory (right)
Comparison of experiment (left) to theory (right): The image on the left shows a pattern of waves imaged with scanning tunneling microscopy, revealing all the possible ways in which electrons can interfere with each other on the surface. This pattern is dictated by the connection of the surface electrons with the interior of the crystal, which is determined by the Weyl momentum of electrons, the special momentum when the electrons sink easily through the sample. The observed pattern closely matches the pattern predicted by theoretical calculations (right). Image credit: Yazdani et al., Princeton University.

To help explain the phenomenon, Yazdani consulted B. Andrei Bernevig, associate professor of physics at Princeton, who has expertise in the theory of topological materials and whose group was involved in the first predictions of Weyl semi-metals in a 2015 paper published in Physical Review X.

Bernevig, with help from postdoctoral researchers Jian Li and Zhijun Wang, realized that the observed pattern made sense if the electrons in these unusual materials were sinking into the bulk of the crystal. “Nobody had predicted that there would be signals of this type of transport from a scanning tunneling microscope, so it came as a bit of a surprise,” said Bernevig.

The next step, said Bernevig, is to look for the behavior in other crystals.

The research at Princeton was supported by the Army Research Office Multidisciplinary University Research Initiative program (W911NF-12-1-0461), the Gordon and Betty Moore Foundation as part of EPiQS initiative (GBMF4530), the National Science Foundation Materials Research Science and Engineering Centers programs through the Princeton Center for Complex Materials (DMR-1420541, NSF-DMR-1104612, NSF CAREER DMR-0952428), the David and Lucile Packard Foundation, and the W. M. Keck Foundation. This project was also made possible through use of the facilities at Princeton Nanoscale Microscopy Laboratory (ARO-W911NF-1-0262, ONR-N00014-14-1-0330, ONR-N00014-13-10661), the U.S. Department of Energy Basic Energy Sciences (DOE-BES) Defense Advanced Research Projects Agency, the U.S. Space and Naval Warfare Systems Command Meso program (N6601-11-1-4110, LPS and ARO-W911NF-1-0606), and the Eric and Wendy Schmidt Transformative Technology Fund at Princeton. Work at University of California–Los Angeles was supported by the DOE-BES (DE-SC0011978).

Read the abstract.

The article, “Quasiparticle interference of the Fermi arcs and surface-bulk connectivity of a Weyl semimetal,” by Hiroyuki Inoue, András Gyenis, Zhijun Wang, Jian Li, Seong Woo Oh, Shan Jiang, Ni Ni, B. Andrei Bernevig,and Ali Yazdani, was published in the March 11, 2016 issue of the journal Science.

Further reading:

M. Weng, C. Fang, Z. Fang, B. A. Bernevig, X. Dai, Phys. Rev. X 5, 011029 (2015)

Q. Lv et al., Phys. Rev. X 5, 1–8 (2015)

X. Yang et al., Nat. Phys. 11, 728–732 (2015)

Y. Xu et al., Science 349, 613–617 (2015)

PPPL physicists simulate innovative method for starting up tokamaks without using a solenoid (Nuclear Fusion)

Posted on by Catherine Zandonella
Francesca Poli
PPPL Scientist Francesca Poli. Photo Credit: Elle Starkman / PPPL Office of Communications. PPPL, located on Princeton University’s Forrestal Campus and managed by the University, is devoted to developing practical solutions for the creation of sustainable energy from fusion and to creating new knowledge about the physics of ultra-hot, charged gases known as plasmas.

By Raphael Rosen, PPPL Office of Communications

Scientists at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have produced self-consistent computer simulations that capture the evolution of an electric current inside fusion plasma without using a central electromagnet, or solenoid.

The computer simulations of the process, known as non-inductive current ramp-up, were performed using TRANSP, the gold-standard code developed at PPPL. The results were published in October 2015 in the journal Nuclear Fusion. The research was supported by the DOE Office of Science.

In traditional donut-shaped tokamaks, a large solenoid runs down the center of the reactor. By varying the electrical current in the solenoid scientists induce a current in the plasma. This current starts up the plasma and creates a second magnetic field that completes the forces that hold the hot, charged gas together.

But spherical tokamaks, a compact variety of fusion reactor that produces high plasma pressure with relatively low magnetic fields, have little room for solenoids. Spherical tokamaks look like cored apples and have a smaller central hole for the solenoid than conventional tokamaks do. Physicists, therefore, have been trying to find alternative methods for producing the current that starts the plasma and completes the magnetic field in spherical tokamaks.

One such method is known as coaxial helicity injection (CHI). During CHI, researchers switch on an electric coil that runs beneath the tokamak. Above this coil is a gap that opens into the tokamak’s vacuum vessel and circles the tokamak’s floor. The switched-on electrical current produces a magnetic field that connects metal plates on either side of the gap.

Researchers next puff gas through the gap and discharge a spark across the two plates. This process causes magnetic reconnection — the process by which the magnetic fields snap apart and reconnect. This reconnection creates a magnetic bubble that fills the tokamak and produces the vital electric current that starts up the plasma and completes the magnetic field.

This current must be nurtured and fed. According to lead author Francesca Poli, the new computer simulations show that the current can best be sustained by injecting high-harmonic radio-frequency waves (HHFWs) and neutral beams into the plasma.

HHFW’s are radio-frequency waves that can heat both electrons and ions. The neutral beams, which consist of streams of hydrogen atoms, become charged when they enter the plasma and interact with the ions. The combination of the HHFWs and neutral beams increases the current from 300 kiloamps to 1 mega amp.

But neither HHFWs nor neutral beams can be used at the start of the process, when the plasma is relatively cool and not very dense. Poli found that HHFWs would be more effective if the plasma were first heated by electron cyclotron waves, which transfer energy to the electrons that circle the magnetic field lines.

“With no electron cyclotron waves you would have to pump in four megawatts of HHFW power to create 400 kiloamps of current,” she said. “With these waves you can get the same amount of current by pumping in only one megawatt of power.

“All of this is important because it’s hard to control the plasma at the start-up,” she added. “So the faster you can control the plasma, the better.”

PPPL is managed by Princeton University for the U.S. Department of Energy’s Office of Science.

Read the abstract.

F.M. Poli, R.G. Andre, N. Bertelli, S.P. Gerhardt, D. Mueller and G. Taylor. “Simulations towards the achievement of non-inductive current ramp-up and sustainment in the National Spherical Torus Experiment Upgrade.” Nuclear Fusion. Published October 30, 2015. DOI: 10.1088/0029-5515/55/12/123011

Using powerful computers, physicists uncover mechanism that stabilizes plasma (Physical Review Letters)

Posted on by Catherine Zandonella
Virtual plasma
A cross-section of the virtual plasma showing where the magnetic field lines intersect the plane. The central section has field lines that rotate exactly once. Image Credit: Stephen Jardin, PPPL.

By Raphael Rosen, Princeton Plasma Physics Laboratory Communications

A team of physicists led by Stephen Jardin of the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) has discovered a mechanism that prevents the electrical current flowing through fusion plasma from repeatedly peaking and crashing. This behavior is known as a “sawtooth cycle” and can cause instabilities within the plasma’s core. The results have been published online in Physical Review Letters. The research was supported by the DOE Office of Science.

The team, which included scientists from General Atomics and the Max Planck Institute for Plasma Physics, performed calculations on the Edison computer at the National Energy Research Scientific Computing Center, a division of the Lawrence Berkeley National Laboratory. Using M3D-C1, a program they developed that creates three-dimensional simulations of fusion plasmas, the team found that under certain conditions a helix-shaped whirlpool of plasma forms around the center of the tokamak. The swirling plasma acts like a dynamo — a moving fluid that creates electric and magnetic fields. Together these fields prevent the current flowing through plasma from peaking and crashing.

The researchers found two specific conditions under which the plasma behaves like a dynamo. First, the magnetic lines that circle the plasma must rotate exactly once, both the long way and the short way around the doughnut-shaped configuration, so an electron or ion following a magnetic field line would end up exactly where it began. Second, the pressure in the center of the plasma must be significantly greater than at the edge, creating a gradient between the two sections. This gradient combines with the rotating magnetic field lines to create spinning rolls of plasma that swirl around the tokamak and gives rise to the dynamo that maintains equilibrium and produces stability.

This dynamo behavior arises only under certain conditions. Both the electrical current running through the plasma and the pressure that the plasma’s electrons and ions exert on their neighbors must be in a range that is “not too large and not too small,” said Jardin. In addition, the speed at which the conditions for the fusion reaction are established must be “not too fast and not too slow.”

Jardin stressed that once a range of conditions like pressure and current are set, the dynamo phenomenon occurs all by itself. “We don’t have to do anything else from the outside,” he noted. “It’s something like when you drain your bathtub and a whirlpool forms over the drain by itself. But because a plasma is more complicated than water, the whirlpool that forms in the tokamak needs to also generate the voltage to sustain itself.”

During the simulations the scientists were able to virtually add new diagnostics, or probes, to the computer code. “These diagnostics were able to measure the helical velocity fields, electric potential, and magnetic fields to clarify how the dynamo forms and persists,” said Jardin. The persistence produces the “voltage in the center of the discharge that keeps the plasma current from peaking.”

Physicists have indirectly observed what they believe to be the dynamo behavior on the DIII-D National Fusion Facility that General Atomics operates for the Department of Energy in San Diego and on the ASDEX Upgrade in Garching, Germany. They hope to learn to create these conditions on demand, especially in ITER, the huge multinational fusion machine being constructed in France to demonstrate the practicality of fusion power. “Now that we understand it better, we think that computer simulations will show us under what conditions this will occur in ITER,” said Jardin. “That will be the focus of our research in the near future.”

Learning how to create these conditions will be particularly important for ITER, which will produce helium nuclei that could amplify the sawtooth disruptions. If large enough, these disruptions could cause other instabilities that could halt the fusion process. Preventing the cycle from starting would therefore be highly beneficial for the ITER experiment.

Read the abstract.

S.C. Jardin, N. Ferraro, and I. Krebs. “Self-Organized Stationary States of Tokamaks.” Physical Review Letters. Published November 17, 2015. DOI: http://dx.doi.org/10.1103/PhysRevLett.115.215001

This article is courtesy of the Princeton Plasma Physics Laboratory.

PPPL physicists propose new plasma-based method to treat radioactive waste (Journal of Hazardous Materials)

Posted on by Catherine Zandonella
Caption: Securing a shipment of mixed, low-level waste from Hanford for treatment and disposal. Credit: U.S. Department of Energy
Caption: Securing a shipment of mixed, low-level waste from Hanford for treatment and disposal. Credit: U.S. Department of Energy

By Raphael Rosen, Princeton Plasma Physics Laboratory Communications

Physicists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) are proposing a new way to process nuclear waste that uses a plasma-based centrifuge. Known as plasma mass filtering, the new mass separation techniques would supplement chemical techniques. It is hoped that this combined approach would reduce both the cost of nuclear waste disposal and the amount of byproducts produced during the process. This work was supported by PPPL’s Laboratory Directed Research and Development Program.

“The safe disposal of nuclear waste is a colossal problem,” said Renaud Gueroult, staff physicist at PPPL and lead author of the paper that appeared in the Journal of Hazardous Materials in October. “One solution might be to supplement existing chemical separation techniques with plasma separation techniques, which could be economically attractive, ideally leading to a reevaluation of how nuclear waste is processed.”

The immediate motivation for safe disposal is the radioactive waste stored currently at the Hanford Site, a facility in Washington State that produced plutonium for nuclear weapons during the Cold War. The volume of this waste originally totaled 54 million gallons and was stored in 177 underground tanks.

In 2000, Hanford engineers began building machinery that would encase the radioactive waste in glass. The method, known as “vitrification,” had been used at another Cold War-era nuclear production facility since 1996. A multibillion-dollar vitrification plant is currently under construction at the Hanford site.

To reduce the cost of high-level waste vitrification and disposal, it may be advantageous to reduce the number of high-level glass canisters by packing more waste into each glass canister. To reduce the volume to be vitrified, it would be advantageous to separate the nonradioactive waste, like aluminum and iron, out of the waste, leaving less waste to be vitrified. However, in its 2014 report, the DOE Task Force on Technology Development for Environmental Management argued that, “without the development of new technology, it is not clear that the cleanup can be completed satisfactorily or at any reasonable cost.”

The high-throughput, plasma-based, mass separation techniques advanced at PPPL offer the possibility of reducing the volume of waste that needs to be immobilized in glass. “The interesting thing about our ideas on mass separation is that it is a form of magnetic confinement, so it fits well within the Laboratory’s culture,” said physicist Nat Fisch, co-author of the paper and director of the Princeton University Program in Plasma Physics. “To be more precise, it is ‘differential magnetic confinement’ in that some species are confined while others are lost quickly, which is what makes it a high-throughput mass filter.”

How would a plasma-based mass filter system work? The method begins by atomizing and ionizing the hazardous waste and injecting it into the rotating filter so the individual elements can be influenced by electric and magnetic fields. The filter then separates the lighter elements from the heavier ones by using centrifugal and magnetic forces. The lighter elements are typically less radioactive than the heavier ones and often do not need to be vitrified. Processing of the high-level waste therefore would need fewer high-level glass canisters overall, while the less radioactive material could be immobilized in less costly wasteform (e.g., concrete, bitumen).

The new technique would also be more widely applicable than traditional chemical-based methods since it would depend less on the nuclear waste’s chemical composition. While “the waste’s composition would influence the performance of the plasma mass filter in some ways, the effect would most likely be less than that associated with chemical techniques,” said Gueroult.

Gueroult points out why savings by plasma techniques can be important. “For only about $10 a kilogram in energy cost, solid waste can be ionized. In its ionized form, the waste can then be separated into heavy and light components. Because the waste is atomized, the separation proceeds only on the basis of atomic mass, without regard to the chemistry. Since the total cost of chemical-based techniques can be $2,000 per kilogram of the vitrified waste, as explained in the Journal of Hazardous Materials paper, it stands to reason that even if several plasma-based steps are needed to achieve pure enough separation, there is in principle plenty of room to cut the overall costs. That is the point of our recent paper. It is also why we are excited about our plasma-based methods.”

Fisch notes that “our original ideas grew out of the thesis of Abe Fetterman, who began by considering centrifugal mirror confinement for nuclear fusion, but then realized the potential for mass separation. Now the key role on this project is being played by Renaud, who has developed the concept substantially further.”

According to Fisch, the current developments are a variation and refinement of a plasma-based mass separation system first advanced by a private company called Archimedes Technology Group. That company, started by the late Dr. Tihiro Ohkawa, a fusion pioneer, raised private capital to advance a plasma-based centrifuge concept to clean up the legacy waste at Hanford, but ceased operation in 2006 after failing to receive federal funding.

Now an updated understanding of the complexity of the Hanford problem, combined with an increased appreciation of new ideas, has led to renewed federal interest in waste-treatment solutions. Completion of the main waste processing operations, which was in 2002 projected for 2028, has slipped by 20 years over the last 13 years, and the total cleanup cost is now estimated by the Department of Energy to be greater than 250 billion dollars, according to the DOE Office of Inspector General, Office of Audits and Inspections. DOE, which has the responsibility of cleaning up the legacy nuclear waste at Hanford and other sites, conducted a Basic Research Needs Workshop on nuclear waste cleanup in July that both Fisch and Gueroult attended. The report of that workshop, which is expected to highlight new approaches to the cleanup problem, is due out this fall.

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Read the abstract.

Renaud Gueroult, David T. Hobbs, Nathaniel J. Fisch. “Plasma filtering techniques for nuclear waste remediation.” Journal of Hazardous Materials, published October 2015. doi:10.1016/j.jhazmat.2015.04.058.

Identifying new sources of turbulence in spherical tokamaks (Physics of Plasmas)

Posted on by Catherine Zandonella

By John Greenwald, Princeton Plasma Physics Laboratory Communications

Turbulence 1
Computer simulation of turbulence in a model of the NSTX-U, a spherical tokamak fusion facility at the U.S. Dept. of Energy’s Princeton Plasma Physics Laboratory. Credit: Eliot Feibush

For fusion reactions to take place efficiently, the atomic nuclei that fuse together in plasma must be kept sufficiently hot. But turbulence in the plasma that flows in facilities called tokamaks can cause heat to leak from the core of the plasma to its outer edge, causing reactions to fizzle out.

Researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have for the first time modeled previously unsuspected sources of turbulence in spherical tokamaks, an alternative design for producing fusion energy. The findings, published online in October in Physics of Plasmas, could influence the development of future fusion facilities. This work was supported by the DOE Office of Science.

Spherical tokamaks, like the recently completed National Spherical Torus Experiment-Upgrade (NSTX-U) at PPPL, are shaped like cored apples compared with the mushroom-like design of conventional tokamaks that are more widely used. The cored-apple shape provides some distinct characteristics for the behavior of the plasma inside.

The paper, with PPPL principal research physicist Weixing Wang as lead author, identifies two important new sources of turbulence based on data from experiments on the National Spherical Torus Experiment prior to its upgrade. The discoveries were made by using state-of-the-art large-scale computer simulations. These sources are:

  • Instabilities caused by plasma that flows faster in the center of the fusion facility than toward the edge when rotating strongly in L-mode — or low confinement — regimes. These instabilities, called “Kelvin-Helmholtz modes” after physicists Baron Kelvin and Hermann von Helmholtz, act like wind that stirs up waves as it blows over water and are for the first time found to be relevant for realistic fusion experiments. Such non-uniform plasma flows have been known to play favorable roles in fusion plasmas in conventional and spherical tokamaks. The new results from this study suggest that we may also need to keep these flows within an optimized level.
  • Trapped electrons that bounce between two points in a section of the tokamak instead of swirling all the way around the facility. These electrons were shown to cause significant leakage of heat in H-mode — or high-confinement — regimes by driving a specific instability when they collide frequently. This type of instability is believed to play little role in conventional tokamaks but can provide a robust source of plasma turbulence in spherical tokamaks.

Most interestingly, the model predicts a range of trapped electron collisions in spherical tokamaks that can be turbulence-free, thus improving the plasma confinement. Such favorable plasmas could possibly be achieved by future advanced spherical tokamaks operating at high temperature.

Findings of the new model can be tested on the NSTX-U and will help guide experiments to identify non-traditional sources of turbulence in the spherical facility. Results of this research can shed light on the physics behind key obstacles to plasma confinement in spherical facilities and on ways to overcome them in future machines.

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by Princeton University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Read the abstract:

Weixing X. Wang, Stephane Ethier, Yang Ren, Stanley Kaye, Jin Chen, Edward Startsev, Zhixin Lu, and Zhengqian Li. “Identification of new turbulence contributions to plasma transport and confinement in spherical tokamak regime.” Physics of Plasmas, published October 2015. doi:10.1063/1.4933216.

‘Material universe’ yields surprising new particle (Nature)

Posted on by Catherine Zandonella

By Staff

tungsten ditelluride
A crystal of tungsten ditelluride is shown. Image courtesy of Wudi Wang and N. Phuan Ong, Princeton University.

An international team of researchers has predicted the existence of a new type of particle called the type-II Weyl fermion in metallic materials. When subjected to a magnetic field, the materials containing the particle act as insulators for current applied in some directions and as conductors for current applied in other directions. This behavior suggests a range of potential applications, from low-energy devices to efficient transistors.

The researchers theorize that the particle exists in a material known as tungsten ditelluride (WTe2), which the researchers liken to a “material universe” because it contains several particles, some of which exist under normal conditions in our universe and others that may exist only in these specialized types of crystals. The research appeared in the journal Nature this week.

The new particle is a cousin of the Weyl fermion, one of the particles in standard quantum field theory. However, the type-II particle exhibits very different responses to electromagnetic fields, being a near perfect conductor in some directions of the field and an insulator in others.

The research was led by Princeton University Associate Professor of Physics B. Andrei Bernevig, as well as Matthias Troyer and Alexey Soluyanov of ETH Zurich, and Xi Dai of the Chinese Academy of Sciences Institute of Physics. The team included Postdoctoral Research Associates Zhijun Wang at Princeton and QuanSheng Wu at ETH Zurich, and graduate student Dominik Gresch at ETH Zurich.

The particle’s existence was missed by physicist Hermann Weyl during the initial development of quantum theory 85 years ago, say the researchers, because it violated a fundamental rule, called Lorentz symmetry, that does not apply in the materials where the new type of fermion arises.

Particles in our universe are described by relativistic quantum field theory, which combines quantum mechanics with Einstein’s theory of relativity. Under this theory, solids are formed of atoms that consist of a nuclei surrounded by electrons. Because of the sheer number of electrons interacting with each other, it is not possible to solve exactly the problem of many-electron motion in solids using quantum mechanical theory.

Instead, our current knowledge of materials is derived from a simplified perspective where electrons in solids are described in terms of special non-interacting particles, called quasiparticles, that move in the effective field created by charged entities called ions and electrons. These quasiparticles, dubbed Bloch electrons, are also fermions.

Just as electrons are elementary particles in our universe, Bloch electrons can be considered the elementary particles of a solid. In other words, the crystal itself becomes a “universe,” with its own elementary particles.

In recent years, researchers have discovered that such a “material universe” can host all other particles of relativistic quantum field theory. Three of these quasiparticles, the Dirac, Majorana, and Weyl fermions, were discovered in such materials, despite the fact that the latter two had long been elusive in experiments, opening the path to simulate certain predictions of quantum field theory in relatively inexpensive and small-scale experiments carried out in these “condensed matter” crystals.

These crystals can be grown in the laboratory, so experiments can be done to look for the newly predicted fermion in WTe2 and another candidate material, molybdenum ditelluride (MoTe2).

“One’s imagination can go further and wonder whether particles that are unknown to relativistic quantum field theory can arise in condensed matter,” said Bernevig. There is reason to believe they can, according to the researchers.

The universe described by quantum field theory is subject to the stringent constraint of a certain rule-set, or symmetry, known as Lorentz symmetry, which is characteristic of high-energy particles. However, Lorentz symmetry does not apply in condensed matter because typical electron velocities in solids are very small compared to the speed of light, making condensed matter physics an inherently low-energy theory.

“One may wonder,” Soluyanov said, “if it is possible that some material universes host non-relativistic ‘elementary’ particles that are not Lorentz-symmetric?”

This question was answered positively by the work of the international collaboration. The work started when Soluyanov and Dai were visiting Bernevig in Princeton in November 2014 and the discussion turned to strange unexpected behavior of certain metals in magnetic fields (Nature 514, 205-208, 2014, doi:10.1038/nature13763). This behavior had already been observed by experimentalists in some materials, but more work is needed to confirm it is linked to the new particle.

The researchers found that while relativistic theory only allows a single species of Weyl fermions to exist, in condensed matter solids two physically distinct Weyl fermions are possible. The standard type-I Weyl fermion has only two possible states in which it can reside at zero energy, similar to the states of an electron which can be either spin-up or spin-down. As such, the density of states at zero energy is zero, and the fermion is immune to many interesting thermodynamic effects. This Weyl fermion exists in relativistic field theory, and is the only one allowed if Lorentz invariance is preserved.

The newly predicted type-2 Weyl fermion has a thermodynamic number of states in which it can reside at zero energy – it has what is called a Fermi surface. Its Fermi surface is exotic, in that it appears along with touching points between electron and hole pockets. This endows the new fermion with a scale, a finite density of states, which breaks Lorentz symmetry.

Left: Allowed states for the standard type-I Weyl fermion. When energy is tuned from below, at zero energy, a pinch in the number of allowed states guarantees the absence of many-body phenomena such as superconductivity or ordering. Right: The newly discovered type-II Weyl fermion. At zero energy, a large number of allowed states are still available. This allows for the presence of superconductivity, magnetism, and pair-density wave phenomena. Credit B. Andrei Bernevig et al.
Left: Allowed states for the standard type-I Weyl fermion. When energy is tuned from below, at zero energy, a pinch in the number of allowed states guarantees the absence of many-body phenomena such as superconductivity or ordering.
Right: The newly discovered type-II Weyl fermion. At zero energy, a large number of allowed states are still available. This allows for the presence of superconductivity, magnetism, and pair-density wave phenomena.
Credit
B. Andrei Bernevig et al.

The discovery opens many new directions. Most normal metals exhibit an increase in resistivity when subject to magnetic fields, a known effect used in many current technologies. The recent prediction and experimental realization of standard type-I Weyl fermions in semimetals by two groups in Princeton and one group in IOP Beijing showed that the resistivity can actually decrease if the electric field is applied in the same direction as the magnetic field, an effect called negative longitudinal magnetoresistance. The new work shows that materials hosting a type-II Weyl fermion have mixed behavior: While for some directions of magnetic fields the resistivity increases just like in normal metals, for other directions of the fields, the resistivity can decrease like in the Weyl semimetals, offering possible technological applications.

“Even more intriguing is the perspective of finding more ‘elementary’ particles in other condensed matter systems,” the researchers say. “What kind of other particles can be hidden in the infinite variety of material universes? The large variety of emergent fermions in these materials has only begun to be unraveled.”

Researchers at Princeton University were supported by the U.S. Department of Defense, the U.S. Office of Naval Research, the U.S. National Science Foundation, the David and Lucile Packard Foundation and the W.M. Keck Foundation. Researchers at ETH Zurich were supported by Microsoft Research, the Swiss National Science Foundation and the European Research Council. Xi Dai was supported by the National Natural Science Foundation of China, the 973 program of China and the Chinese Academy of Sciences.

The article, “Type II Weyl Semimetals,” by Alexey A. Soluyanov, Dominik Gresch, Zhijun Wang, QuanSheng Wu, Matthias Troyer, Xi Dai, and B. Andrei Bernevig was published in the journal Nature on November 26, 2015.

Read the abstract.

Long-sought chiral anomaly detected in crystalline material (Science)

Posted on by Catherine Zandonella

By Catherine Zandonella, Office of the Dean for Research

A study by Princeton researchers presents evidence for a long-sought phenomenon — first theorized in the 1960s and predicted to be found in crystals in 1983 — called the “chiral anomaly” in a metallic compound of sodium and bismuth. The additional finding of an increase in conductivity in the material may suggest ways to improve electrical conductance and minimize energy consumption in future electronic devices.

“Our research fulfills a famous prediction in physics for which confirmation seemed unattainable,” said N. Phuan Ong, Princeton’s Eugene Higgins Professor of Physics, who co-led the research with Robert Cava, Princeton’s Russell Wellman Moore Professor of Chemistry. “The increase in conductivity in the crystal and its dramatic appearance under the right conditions left little doubt that we had observed the long-sought chiral anomaly.”

The study was published online today in the journal Science.

Handedness
This sketch illustrates the concept of handedness, or chirality, which is found throughout nature. Most chemical structures and many elementary particles come in right- and left-handed forms. Source: Princeton University

The chiral anomaly – which describes how elementary particles can switch their orientation in the presence of electric and magnetic fields – stems from the observation that right- and left-handedness (or “chirality” after the Greek word for hand) is ubiquitous in nature. For example, most chemical structures and many elementary particles come in right- and left-handed forms that are mirror images of each other.

Early research leading up to the discovery of the anomaly goes back to the 1940s, when Hermann Weyl at the Institute for Advanced Study in Princeton, New Jersey, and others, discovered that all elementary particles that have zero mass (including neutrinos, despite their having an extremely small mass) strictly segregate into left- and right-handed populations that never intermix.

A few decades later, theorists discovered that the presence of electric and magnetic fields ruins the segregation of these particles, causing the two populations to transform into each other with observable consequences.

This field-induced mixing, which became known as the chiral anomaly, was first encountered in 1969 in work by Stephen Adler of the Institute for Advanced Study, John Bell of the European Organization for Nuclear Research (CERN) and Roman Jackiw of the Massachusetts Institute of Technology, who successfully explained why certain elementary particles, called neutral pions, decay much faster — by a factor of 300 million — than their charged cousins. Over the decades the anomaly has played an important if perplexing role in the grand quest to unify the four fundamental forces of nature.

The prediction that the chiral anomaly could also be observed in crystals came in 1983 from physicists Holger Bech Nielsen of the University of Copenhagen and Masao Ninomiya of the Okayama Institute for Quantum Physics. They suggested that it may be possible to detect the anomaly in a laboratory setting, which would enable researchers to apply intense magnetic fields to test predictions under conditions that would be impossible in high-energy particle colliders.

Recent progress in the development of certain kinds of crystals known as “topological” materials has paved the way toward realizing this prediction, Ong said. In the crystal of Na3Bi, which is a topological material known as a Dirac semi-metal, electrons occupy quantum states which mimic massless particles that segregate into left- and right-handed populations.

To see if they could observe the anomaly in Na3Bi, Jun Xiong, a graduate student in physics advised by Ong, cooled a crystal of Na3Bi grown by Satya Kushwaha, a postdoctoral research associate in chemistry who works with Cava, to cryogenic temperatures in the presence of a strong magnetic field that can be rotated relative to the direction of the applied electrical current in the crystal. When the magnetic field was aligned parallel to the current, the two chiral populations intermixed to produce a novel increase in conductivity, which the researchers call the “axial current plume.” The experiment confirmed the existence of the chiral anomaly in a crystal.

“One of the key findings in the experiment is that the intermixing leads to a charge current, or axial current, that resists depletion caused by scattering from impurities,” Ong said. “Understanding how to minimize the scattering of current-carrying electrons by impurities — which causes electronic devices to lose energy as heat — is important for realizing future electronic devices that are more energy-efficient. While these are early days, experiments on the long-lived axial current may help us to develop low-dissipation devices.”

The research was supported by the National Science Foundation, the Army Research Office and the Gordon and Betty Moore Foundation.

Read the abstract or paper.

The paper, “Evidence for the chiral anomaly in the Dirac semimetal Na3Bi,” was published online in the journal Science by Jun Xiong; Satya K. Kushwaha; Tian Liang; Jason W. Krizan; Max Hirschberger; Wudi Wang; Robert J. Cava; and N. Phuan Ong.

X marks the spot: Researchers confirm novel method for controlling plasma rotation to improve fusion performance (Physical Review Letters)

Posted on by Catherine Zandonella
Representative plasma geometries, with the X-point location circled in red. (Reprinted from T. Stoltzfus-Dueck et al., Phys. Rev. Lett. 114, 245001, 2015. Copyright 2015 by the American Physical Society.)
Representative plasma geometries, with the X-point location circled in red. (Reprinted from T. Stoltzfus-Dueck et al., Phys. Rev. Lett. 114, 245001, 2015. Copyright 2015 by the American Physical Society.)

By Raphael Rosen, Princeton Plasma Physics Laboratory

Rotation is key to the performance of salad spinners, toy tops, and centrifuges, but recent research suggests a way to harness rotation for the future of mankind’s energy supply. In papers published in Physics of Plasmas in May and Physical Review Letters this month, Timothy Stoltzfus-Dueck, a physicist at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), demonstrated a novel method that scientists can use to manipulate the intrinsic – or self-generated – rotation of hot, charged plasma gas within fusion facilities called tokamaks. This work was supported by the DOE Office of Science.

Such a method could prove important for future facilities like ITER, the huge international tokamak under construction in France that will demonstrate the feasibility of fusion as a source of energy for generating electricity. ITER’s massive size will make it difficult for the facility to provide sufficient rotation through external means.

Rotation is essential to the performance of all tokamaks. Rotation can stabilize instabilities in plasma, and sheared rotation – the difference in velocities between two bands of rotating plasma – can suppress plasma turbulence, making it possible to maintain the gas’s high temperature with less power and reduced operating costs.

Today’s tokamaks produce rotation mainly by heating the plasma with neutral beams, which cause it to spin. In intrinsic rotation, however, rotating particles that leak from the edge of the plasma accelerate the plasma in the opposite direction, just as the expulsion of propellant drives a rocket forward.

Stoltzfus-Dueck and his team influenced intrinsic rotation by moving the so-called X-point – the dividing point between magnetically confined plasma and plasma that has leaked from confinement – on the Tokamak à Configuration Variable (TCV) in Lausanne, Switzerland. The experiments marked the first time that researchers had moved the X-point horizontally to study plasma rotation. The results confirmed calculations that Stoltzfus-Dueck had published in a 2012 paper showing that moving the X-point would cause the confined plasma to either halt its intrinsic rotation or begin rotating in the opposite direction. “The edge rotation behaved just as the theory predicted,” said Stoltzfus-Dueck.

A surprise also lay in store: Moving the X-point not only altered the edge rotation, but modified rotation within the superhot core of the plasma where fusion reactions occur. The results indicate that scientists can use the X-point as a “control knob” to adjust the inner workings of fusion plasmas, much like changing the settings on iTunes or a stereo lets one explore the behavior of music. This discovery gives fusion researchers a tool to access different intrinsic rotation profiles and learn more about intrinsic rotation itself and its effect on confinement.

The overall findings provided a “perfect example of a success story for theory-experiment collaboration,” said Olivier Sauter, senior scientist at École Polytechnique Fédérale de Lausanne and co-author of the paper.

Along with the practical applications of his research, Stoltzfus-Dueck enjoys the purely intellectual aspect of his work. “It’s just interesting,” he said. “Why do plasmas rotate in the way they do? It’s a puzzle.”

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Read the abstract.

Stoltzfus-Dueck, A. N. Karpushov, O. Sauter, B. P. Duval, B. Labit, H. Reimerdes, W. A. J. Vijvers, the TCV Team, and Y. Camenen. “X-Point-Position-Dependent Intrinsic Toroidal Rotation in the Edge of the TCV Tokamak.” Physical Review Letters 114, 245001 – Published 17 June 2015.