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.

Researchers correlate incidences of rheumatoid arthritis and giant cell arteritis with solar cycles (BMJ Open)

Posted on by Catherine Zandonella
Solar storm
Coronal mass ejection hurling plasma from the sun. (Image credit: NASA)

By John Greenwald, Princeton Plasma Physics Laboratory

What began as a chat between husband and wife has evolved into an intriguing scientific discovery. The results, published in May in BMJ (formerly British Medical Journal) Open, show a “highly significant” correlation between periodic solar storms and incidences of rheumatoid arthritis (RA) and giant cell arteritis (GCA), two potentially debilitating autoimmune diseases. The findings by a rare collaboration of physicists and medical researchers suggest a relationship between the solar outbursts and the incidence of these diseases that could lead to preventive measures if a causal link can be established.

RA and GCA are autoimmune conditions in which the body mistakenly attacks its own organs and tissues. RA inflames and swells joints and can cause crippling damage if left untreated. In GCA, the autoimmune disease results in inflammation of the wall of arteries, leading to headaches, jaw pain, vision problems and even blindness in severe cases.

Inspiring this study were conversations between Simon Wing, a Johns Hopkins University physicist and first author of the paper, and his wife, Lisa Rider, deputy unit chief of the Environmental Autoimmunity Group at the National Institute of Environmental Health Sciences in the National Institutes of Health, and a coauthor. Rider spotted data from the Mayo Clinic in Rochester, Minnesota, showing that cases of RA and GCA followed close to 10-year cycles. “That got me curious,” Wing recalled. “Only a few things in nature have a periodicity of about 10-11 years and the solar cycle is one of them.”

Wing teamed with physicist Jay Johnson of the U.S. Department of Energy’s Princeton Plasma Physics Laboratory, a long-time collaborator, to investigate further. When the physicists tracked the incidence of RA and GCA cases compiled by Mayo Clinic researchers, the results suggested “more than a coincidental connection,” said Eric Matteson, chair of the division of rheumatology at the Mayo Clinic, and a coauthor. This work drew upon previous space physics research supported by the DOE Office of Science.

The findings found increased incidents of RA and GCA to be in periodic concert with the cycle of magnetic activity of the sun. During the solar cycle, dramatic changes that can affect space weather near Earth take place in the sun. At the solar maximum, for example, an increased number of outbursts called coronal mass ejections hurl millions of tons of magnetic and electrically charged plasma gas against the Earth’s magnetosphere, the magnetic field that surrounds the planet. This contact whips up geomagnetic disturbances that can disrupt cell phone service, damage satellites and knock out power grids. More importantly, during the declining phase of the solar maximum high-speed streams develop in the solar wind that is made up of plasma that flows from the sun. These streams continuously buffet Earth’s magnetosphere, producing enhanced geomagnetic activity at high Earth latitudes.

The research, which tracked correlations of the diseases with both geomagnetic activity and extreme ultraviolet (EUV) solar radiation, focused on cases recorded in Olmsted County, Minnesota, the home of the Mayo Clinic, over more than five decades. The physicists compared the data with indices of EUV radiation for the years 1950 through 2007 and indices of geomagnetic activity from 1966 through 2007. Included were all 207 cases of GCA and all 1,179 cases of RA occurring in Olmsted County during the periods and collected in a long-term study led by Sherine Gabriel, then of the Mayo Clinic and now dean of the Rutgers Robert Wood Johnson Medical School.

Correlations proved to be strongest between the diseases and geomagnetic activity. GCA incidence — defined as the number of new cases per capita per year in the county — regularly peaked within one year of the most intense geomagnetic activity, while RA incidence fell to a minimum within one year of the least intense activity. Correlations with the EUV indices were seen to be less robust and showed a significantly longer response time.

The findings were consistent with previous studies of the geographic distribution of RA cases in the United States. Such research found a greater incidence of the disease in sections of the country that are more likely to be affected by geomagnetic activity. For example, the heaviest incidence lay along geographic latitudes on the East Coast that were below those on the West Coast. This asymmetry may reflect the fact that high geomagnetic latitudes — areas most subject to geomagnetic activity — swing lower on the East Coast than on the opposite side of the country. While Washington, D.C., lies just 1 degree farther north than San Francisco geographically, for example, the U.S. capital is 7 degrees farther north in terms of geomagnetic latitude.

Although the authors make no claim to a causal explanation for their findings, they identify five characteristics of the disease occurrence that are not obviously explained by any of the currently leading hypotheses. These include the east-west asymmetries of the RA and GCA outbreaks and the periodicities of the incidences in concert with the solar cycle. Among the possible causal pathways the authors consider are reduced production of the hormone melatonin, an anti-inflammatory mediator with immune-enhancing effects, and increased formation of free radicals in susceptible individuals. A study of 142 electrical power workers found that excretion of melatonin — a proxy used to estimate production of the hormone — was reduced by 21 percent on days with increased geomagnetic activity.

Confirming a causal link between outbreaks of RA and GCA and geomagnetic activity would be an important step towards developing strategies for mitigating the impact of the activity on susceptible individuals. These strategies could include relocating to lower latitudes and developing methods to counteract direct causal agents that may be controlled by geomagnetic activity. For now, say the authors, their findings warrant further investigations covering longer time periods, additional locations and other autoimmune diseases.

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.

This work was funded from NIH grants (NIAMS R01 AR046849, NIA R01 AG034676). This research was supported in part by the Intramural Research Program of the NIH, National Institute of Environmental Health Sciences. This work has also benefited from the works funded by NSF grants (ATM-0802715, AGS-1058456, ATM09002730, AGS1203299), NASA grants (NNX13AE12G, NNH09AM53I, NN09AK63I, NNH11AR07I), and DOE contract (DE-AC02-09CH11466).

Read the abstract

Wing, Simon; Rider, Lisa G.; Johnson, Jay R.; Miller, Frederick W.; Matteson, Eric L.; Crowson, Cynthia S.; Gabriel, Sherine E. “Do solar cycles influence giant cell arteritis and rheumatoid arthritis incidence?” BMJ Open, May 2015

Putting two and two together to make unexplored chemicals (J. American Chemical Society)

Posted on by Catherine Zandonella
Schematic for cobalt-catalyzed [2pi+2pi] reaction.
Schematic for cobalt-catalyzed [2pi+2pi] reaction. Image credit: Chirik group, Princeton University
By Tien Nguyen, Department of Chemistry

Researchers at Princeton have developed a new catalyst that may give unprecedented access to cyclobutanes, four-membered ring-containing molecules that have been relatively unexplored. Held back by the limited scope of previous methods, called [2π+2π] reactions, many cyclobutanes compounds have been out of reach, along with any unique properties that may be of interest to the pharmaceutical or fine chemical industry.

Led by Paul Chirik, the Edwards S. Sanford Professor of Chemistry, the team published the new cobalt-catalyzed [2π+2π] reaction and a thorough investigation of its mechanism in the Journal of the American Chemical Society on June 1.

“Because examples of this reaction are so rare, we wanted to understand why these cobalt complexes were special and how they worked in the reaction,” said Valerie Schmidt, lead author on the article and a postdoctoral researcher in the Chirik lab.

The new cobalt-catalyzed reaction overcame limitations that have plagued other similar methods, such as poor selectivity or requiring very reactive alkenes, which are chemical structures composed of two carbons joined by a double bond, as starting materials. The research team suspected their success came from certain molecules, called bis(imino)pyridine ligands, that are attached to the cobalt center and which are capable of passing electrons to and from the metal.

The Chirik group has used these redox active ligands previously, attached instead to iron to catalyze a [2π+2π] reaction reported in 2006. But the iron catalyst is highly sensitive to air and moisture, an issue that could be mitigated by switching to a less reactive metal like cobalt.

Replacing iron with cobalt presented a unique challenge in analysis because it altered the complex’s overall magnetic state from diamagnetic to paramagnetic. Unlike diamagnetic compounds, paramagnetic compounds can be difficult to identify by nuclear magnetic resonance (NMR) spectroscopy, a technique that uses a strong magnet to pulse atomic nuclei to reveal their environments, and a primary tool for characterizing molecules.

“We really had to be creative in finding ways to confirm our hypotheses about the catalyst,” Schmidt said. One extremely useful tool, analogous to nuclear magnetic resonance but that pulses electrons instead of nuclei, was electron paramagnetic resonance (EPR). This technique allowed the researchers to track the unpaired electrons, called radicals, throughout the reaction.

Additional data gathered from theoretical calculations, kinetic studies and x-ray crystal structure elucidation allowed the research team to sketch out a detailed reaction mechanism. They proposed that the cycle begins with successive coordination of the two tethered alkenes to the metal center.

Coordination of the second alkene was crucial, Schmidt explained, because it changed cobalt’s geometry, from square planar to tetrahedral, and effectively moved the unpaired electron from the ligand to the metal. Only then can the metal based radical promote the carbon-carbon forming event and push the reaction forward.

This action leads to the formation of a metallacycle—a pentagon shaped ring of four carbons and one cobalt atom. Cobalt is then squeezed out of the ring to release the final four-membered cyclobutane product in a process called reductive elimination. After testing a series of catalysts with varying size and electronic properties, the researchers suggested that reductive elimination was the turnover-limiting step, essentially the bottleneck of the reaction.

Armed with a deeper understanding of the cobalt catalyst system, the researchers hope to continually enhance its performance. “We want to make it as easy as possible to access cyclobutane containing molecules, because without this ability, we really have no idea what we are missing out on,” Schmidt said.

Read the abstract.

Schmidt, V. A.; Hoyt, J. M.; Margulieux, G. W.; Chirik, P. J. “Cobalt-Catalyzed [2π+2π] Cycloadditions of Alkenes: Scope, Mechanism and Elucidation of Electronic Structure of Catalytic Intermediates.” Journal of the American Chemical Society 2015, Just Accepted Manuscript.

This work was supported by the National Institutes of Health Ruth L. Kirschstein National Research Service Award (F32 GM109594) and Princeton University Intellectual Property Accelerator Fund.

 

Giant structures called plasmoids could simplify the design of future tokamaks (Physical Review Letters)

Posted on by Catherine Zandonella
Plasmoid formation in plasma simulation
Left: Plasmoid formation in simulation of NSTX plasma during CHI. Credit: Fatima Ebrahimi, PPPL / Right: Fast-camera image of NSTX plasma shows two discrete plasmoid-like bubble structures. Credit: Nishino-san, Hiroshima University

By Raphael Rosen, Princeton Plasma Physics Laboratory

Researchers at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have for the first time simulated the formation of structures called “plasmoids” during Coaxial Helicity Injection (CHI), a process that could simplify the design of fusion facilities known as tokamaks. The findings, reported in the journal Physical Review Letters, involve the formation of plasmoids in the hot, charged plasma gas that fuels fusion reactions. These round structures carry current that could eliminate the need for solenoids – large magnetic coils that wind down the center of today’s tokamaks – to initiate the plasma and complete the magnetic field that confines the hot gas.

“Understanding this behavior will help us produce plasmas that undergo fusion reactions indefinitely,” said Fatima Ebrahimi, a physicist at both Princeton University and PPPL, and the paper’s lead author.

Ebrahimi ran a computer simulation that modeled the behavior of plasma and the formation of plasmoids in three dimensions thoughout a tokamak’s vacuum vessel. This marked the first time researchers had modeled plasmoids in conditions that closely mimicked those within an actual tokamak. All previous simulations had modeled only a thin slice of the plasma – a simplified picture that could fail to capture the full range of plasma behavior.

Researchers validated their model by comparing it with fast-camera images of plasma behavior inside the National Spherical Torus Experiment (NSTX), PPPL’s major fusion facility. These images also showed plasmoid-like structures, confirming the simulation and giving the research breakthrough significance, since it revealed the existence of plasmoids in an environment in which they had never been seen before. “These findings are in a whole different league from previous ones,” said Roger Raman, leader for the Coaxial Helicity Injection Research program on NSTX and a coauthor of the paper.

The findings may provide theoretical support for the design of a new kind of tokamak with no need for a large solenoid to complete the magnetic field. Solenoids create magnetic fields when electric current courses through them in relatively short pulses. Today’s conventional tokamaks, which are shaped like a donut, and spherical tokamaks, which are shaped like a cored apple, both employ solenoids. But future tokamaks will need to operate in a constant or steady state for weeks or months at a time. Moreover, the space in which the solenoid fits – the hole in the middle of the doughnut-shaped tokamak – is relatively small and limits the size and strength of the solenoid.

A clear understanding of plasmoid formation could thus lead to a more efficient method of creating and maintaining a plasma through transient Coaxial Helicity Injection. This method, originally developed at the University of Washington, could dispense with a solenoid entirely and would work like this:

  • Researchers first inject open magnetic field lines into the vessel from the bottom of the vacuum chamber. As researchers drive electric current along those magnetic lines, the lines snap closed and form the plasmoids, much like soap bubbles being blown out of a sheet of soapy film.
  • The many plasmoids would then merge to form one giant plasmoid that could fill the vacuum chamber.
  • The magnetic field within this giant plasmoid would induce a current in the plasma to keep the gas tightly in place. “In principle, CHI could fundamentally change how tokamaks are built in the future,” says Raman.

Understanding how the magnetic lines in plasmoids snap closed could also help solar physicists decode the workings of the sun. Huge magnetic lines regularly loop off the surface of the star, bringing the sun’s hot plasma with them. These lines sometimes snap together to form a plasmoid-like mass that can interfere with communications satellites when it collides with the magnetic field that surrounds the Earth.

While Ebrahimi’s findings are promising, she stresses that much more is to come. PPPL’s National Spherical Torus Experiment-Upgrade (NSTX-U) will provide a more powerful platform for studying plasmoids when it begins operating this year, making Ebrahimi’s research “only the beginning of even more exciting work that will be done on PPPL equipment,” she said.

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

Ebrahimi and R. Raman. “Plasmoids Formation During Simulations of Coaxial Helicity Injection in the National Spherical Torus Experiment. Physical Review Letters. Published May 20, 2015. DOI: http://dx.doi.org/10.1103/PhysRevLett.114.205003