Advancing Plasma Science
Members of the plasma nanosynthesis team. Front row from left: Alexandros Gerakis, Vladimir Vekselman, Shurik Yatom. Back row from left: Yevgeny Raitses, Bruce Koel, Igor Kaganovich, Alexander Khrabry, Brent Stratton, Rachel Selinsky, Andrei Khodak
“a big step forward in understanding how carbon nanoparticles grow in plasma”
Big steps toward control of production of tiny building blocks
Nanoparticles, superstrong and flexible structures such as carbon nanotubes that are measured in billionths of a meter — a diameter thousands of times thinner than a human hair — are used in everything from microchips to sporting goods to pharmaceutical products. But large-scale production of high-quality particles faces challenges ranging from improving the selectivity of the synthesis that creates them and the quality of the synthesized material to the development of economical and reliable synthesis processes.
PPPL scientists have developed diagnostic tools to advance an improved and integrated understanding of plasma-based synthesis — a widely used but poorly understood tool for creating nanostructures. Researchers and collaborators at the Laboratory have outlined findings in several published papers that could pave the way toward manufacturing advances in a variety of industries.
The papers report unique observations of the synthesis in carbon plasma generated by an electric arc in situ, or as the process unfolds. Researchers create the plasma arc between two carbon electrodes, producing a hot carbon vapor composed of atomic nuclei and molecules that cool and synthesize — or condense — into particles that grow into nanostructures by bunching together.
Examples of three findings:
- Spotting precursors that become nanotubes. Missing from today’s knowledge is a detailed understanding of the precursors of nanotubes that are formed from the vapor during synthesis. PPPL research has now shown that molecular precursors that include “dimers” — molecules formed by two carbon atoms —govern the synthesis of carbon nanotubes in a purely carbon electric arc, opening the door to improved predictive modeling of nanosynthesis in carbon arcs.
- Detecting nanoparticle growth. Researchers have built and demonstrated a unique table-top laser technique for in situ detection of nanoparticle growth. Measuring nanoparticles during large-volume synthesis could advance understanding of the mechanisms behind the particle’s growth.
- Why some synthesis goes wrong. Among the most promising types of nanomaterials are single-wall carbon nanotubes that carbon arc discharges can produce on an industrial scale. A key drawback to this method is the impurity of much of the synthesized nanomaterial, which includes a mix of nanotubes, carbon soot and random carbon particles. Research now shows that a chief source of these drawbacks is the unstable behavior of carbon arcs, highlighting the need for stabilizing the arc for the continuous production of single-wall carbon nanotubes.
Direct observation has produced “a big step forward in understanding how carbon nanoparticles grow in plasma generated by arc,” said physicist Yevgeny Raitses, head of the Laboratory for Plasma Nanosynthesis at PPPL. “The idea now is to combine experimental results with computer modeling for improved control of the process and to apply what we learn to other types of nanomaterials and nanomaterial synthesis.” ☀︎
No longer whistling in the dark: Scientists uncover a little-understood source of waves generated throughout the universe
Magnetic reconnection, the snapping apart and violent reconnection of magnetic field lines in plasma occurs throughout the universe and can whip up space storms that disrupt cell phone service and knock out power grids. Scientists at PPPL and other laboratories have now used data from a NASA four-satellite mission studying reconnection to develop a method for identifying the source of waves in plasma — the state of matter composed of free electrons and atomic nuclei — that help satellites determine their location in space.
The researchers, led by PPPL physicist Jongsoo Yoo, have correlated magnetic field measurements taken by the Magnetospheric Multiscale (MMS) mission that is orbiting at the edge of the magnetic field that surrounds the Earth. The findings identified the source of the propagation of “whistler waves” — waves with whistle-like sounds that drop from high to low and stem from reconnection — whose detection orients the satellites relative to reconnection activity that can affect the Earth.
The research, reported in Geophysical Research Letters, marks development of “a new methodology for measuring how the wave propagates in reconnection,” said Yoo, lead author of the paper. The source, he said, is what are called “tail electrons” — particles with energy that is far greater than that of the bulk electrons in reconnecting field lines. The temperature of tail electrons differs when measured in different directions.
“What we prove is that you couldn’t have whistler waves without the active X-line” — the central reconnection region — “so whistler waves indicate that reconnection is near,” said Yoo, whose research was conducted on the PPPL Magnetic Reconnection Experiment (MRX).
Going forward, the team plans to investigate the development of whistler waves near the electron diffusion region, the narrow region in the magnetosphere and laboratory experiments where electrons separate from field lines before reconnection takes place. Results could prove relevant to the MMS mission, whose goals include uncovering the role that electrons play in facilitating reconnection. ☀︎
PPPL physicist Jongsoo Yoo at the MRX facility
A new methodology for measuring how the wave propagates in reconnection
PPPL physicist Yi-Min Huang
A gamma-ray burst in our Milky Way galaxy, if pointing towards Earth, could potentially cause a mass extinction event
Plasma bubbles help trigger massive magnetic events in outer space
PPPL scientists have discovered conditions that give rise to fast magnetic reconnection, the process that triggers solar flares, auroras, and geomagnetic storms that can disrupt cell phone service and knock out power grids. Reconnection takes place when the magnetic field lines in plasma, the hot, charged state of matter composed of free electrons and atomic nuclei, break apart and violently reconnect, releasing energy. This happens in thin sheets of plasma, called current sheets, in which electric current is strongly concentrated.
The impact of reconnection can be felt throughout the universe. The process may cause enormous bursts of gamma-ray radiation thought to be associated with supernova explosions and the formation of ultra-dense neutron stars and black holes. “A gamma-ray burst in our Milky Way galaxy, if pointing towards Earth, could potentially cause a mass extinction event,” said PPPL physicist Yi-Min Huang, lead author of a paper reporting the findings in Astrophysical Journal. “Clearly, it is important to know when, how, and why magnetic reconnection takes place.”
Researchers used computer simulations and theoretical analysis to demonstrate that a phenomenon called the “plasmoid instability” creates bubbles within plasma that can lead to fast reconnection when certain conditions are met:
- The plasma must have a high Lundquist number that characterizes how well it conducts electricity.
- Random fluctuations in the magnetic field of the plasma provide “seeds” from which the plasma instability grows.
These two conditions allow plasmoid instabilities to give rise to fast reconnection in current sheets. The instabilities break up sheets of electric current within plasma into bubbles, or plasmoids, and many smaller sheets. The growing number of sheets increases opportunity for reconnection, which also occurs in more than one place, causing the rate of reconnection to increase for an entire system.
Huang and fellow physicists now aim to test their new model using experimental machines with additional capability. One such machine, the Facility for Laboratory Reconnection Experiments (FLARE) has recently been completed and will be housed at PPPL under the supervision of physicist Hantao Ji. ☀︎
Protecting the power grid: Advanced plasma switch can make the grid more efficient for long-distance transmission
Inside your home and office, low-voltage alternating current (AC) powers the lights, computers and electronic devices for everyday use. But when the electricity comes from remote long-distance sources such as hydro-power or solar generating plants, transporting it as direct current (DC) is more efficient — and converting it back to AC current requires bulky and expensive switches. PPPL now is assisting General Electric to develop an advanced switch that will convert high-voltage DC current from the plant to high-voltage AC current more efficiently, enabling reduced-cost transmission of long-distance power. Substations along the route then reduce the high-voltage AC current to the low-voltage AC current that homes and offices use.
GE is testing a tube filled with plasma — the state of matter composed of free electrons and ions that PPPL studies — that the company aims to develop as the conversion device. The switch must be able to operate for years with voltage as high as 300 kilovolts to enable a single unit to cost-effectively replace the assemblies of semiconductor switches that now are required.
PPPL is modeling the switch to demonstrate how the high current affects the helium gas that the company is using inside the tube. The simulation modeled the breakdown — or ionization — of the gas into plasma, producing fresh insight into the physics of the process. The results built upon a 2017 PPPL paper published in the journal Physics of Plasmas that modeled the effect of high-voltage breakdown.
Previous research has long studied the lower-voltage breakdown of gases. But “GE is dealing with much higher voltage,” said Igor Kaganovich, deputy head of the PPPL Theory Department and PPPL’s Low Temperature Plasma Laboratory. “The low-pressure and high-voltage breakdown mechanism has been poorly understood because of the need to consider new mechanisms of gas ionization at high voltages, which is what we did.”
The findings have proved useful for GE. “The potential applications of the gas switch depend on its maximum possible voltage,” said GE physicist Timothy Sommerer, who heads the project. “We have already experimentally demonstrated that a gas switch can operate at 100 kilovolts and we are now working to test at 300 kilovolts. The results from the PPPL model are both scientifically interesting and favorable for high-voltage gas switch design.” ☀︎
Plasma glows white in low-pressure helium between magnetized cathode electrode, bottom, and anode electrode, top
The potential applications of the gas switch depend on its maximum possible voltage
PPPL physicist Chuanfei Dong
this work can provide some basic understanding of the scales at which coronal heating occurs
New findings reveal the behavior of turbulence in the exceptionally hot solar corona
The sun defies conventional scientific understanding. Its upper atmosphere, known as the corona, is many millions of degrees hotter than its surface. Astrophysicists are keen to learn why the corona is so hot, and PPPL physicists have completed research that may advance the search.
The scientists found that formation of magnetic bubbles known as plasmoids in an electrically conducting fluid like plasma — the hot, charged state of matter composed of free electrons and atomic nuclei that the sun is made of — can affect the development of turbulence within the fluid. The turbulence, violent or unsteady motion of the plasma, then influences how heat flows through the sun and other astrophysical objects.
The new findings suggest that the formation of plasmoids helps change large turbulent eddies of plasma into smaller whirlpool-like structures. This process creates sheets of intense electric current in the plasma that affect how magnetic energy dissipates as it flows toward the corona.
“Until now, no one had investigated by direct numerical simulation how plasmoids can alter the turbulent energy spectrum in a conducting fluid,” said physicist Chuanfei Dong of PPPL and Princeton University, lead author of the results published in Physical Review Letters. “Our simulations show that in a turbulent conducting fluid the formation of magnetic bubbles causes the turbulent eddies to transition from large scales to small scales more efficiently than previously thought.”
The findings apply not only to the sun, but also to astrophysical objects like accretion disks — clouds of dust and rock that circle dense objects such as black holes and can collapse into stars and planets. “The smallest current sheet size in magnetohydrodynamic turbulence can be smaller than previously predicted,” Dong said. “So the current sheets become more intense before they dissipate. As a result, this work can provide some basic understanding of the scales at which coronal heating occurs.” ☀︎
Surprise finding: Discovering a previously unknown role for a source of magnetic fields
Magnetic forces ripple throughout the universe, from the fields surrounding planets to the gasses filling galaxies, and can be launched by a phenomenon called the Biermann battery effect. PPPL scientists have found that this effect may not only generate magnetic fields, but can sever them to trigger magnetic reconnection, the snapping apart and violent reconnection of magnetic field lines in plasmas.
The Biermann battery effect, a possible seed for the magnetic fields pervading our universe, arises in plasmas —the state of matter composed of free electrons and atomic nuclei — when the plasma temperature and density are misaligned: the tops of such plasmas might be hotter than the bottoms, and the density might be greater on the left side than on the right. This quirky condition gives rise to a force that generates current that leads to magnetic fields.
The researchers reveal through computer simulations a remarkable and surprising new role for the Biermann effect that could improve understanding of reconnection, which gives rise to northern lights, solar flares and geomagnetic space storms that can disrupt cell-phone service and black out electric grids on Earth.
The simulations showed that temperature spiked in the reconnecting field lines and reversed the role of the Biermann effect that originated the magnetic field lines. Because of the spike, the Biermann effect destroyed the very lines it had created, cutting them like a pair of scissors cutting a rubber band. The sliced lines then reconnected downstream, away from the original reconnection point.
“This is the first simulation to show Biermann battery-mediated magnetic reconnection,” said Jackson Matteucci, a graduate student in the Program in Plasma Physics at PPPL and lead author of a description of the process in Physical Review Letters. “This process had never been known before.” Moreover, the unexpected results “provide a new platform for simulating the reconnection observed in astrophysical plasmas,” he said. ☀︎
Physicists Jackson Matteucci and coauthor and advisor Will Fox with poster displaying their research
PPPL graduate student Denis St-Onge
Turbulence in space might solve outstanding astrophysical mystery
Contrary to what many people believe, outer space is not empty. In addition to an electrically charged soup of ions and electrons known as plasma, space is permeated by magnetic fields whose production, sustainment and amplification have long puzzled astrophysicists. PPPL scientists have now shown that turbulence in the plasma might be the cause, providing a possible answer to what has been called one of the most important unsolved problems in plasma astrophysics.
Researchers used high-performance computers to simulate how the turbulence could intensify magnetic fields through what is known as the dynamo effect, in which the magnetic fields become stronger as the magnetic field lines twist and turn. “This work constitutes an important step toward answering for the first time the question of whether turbulence can amplify magnetic fields to dynamical strengths in a hot, dilute plasma, such as that residing within clusters of galaxies,” said Matthew Kunz, an astrophysics professor at Princeton University and a coauthor of the paper, which was published in The Astrophysical Journal Letters.
Past research has focused on dynamos as they might occur in so-called collisional plasmas, in which particles collectively behave as a fluid. But intergalactic plasmas are collisionless, so past experiments are not necessarily relevant. This new research is meant to address that gap. “We wanted to see how the dynamo would behave in the collisionless regime,” said Denis St-Onge, graduate student in the Princeton Program in Plasma Physics at PPPL and lead author of the paper.
In computer simulations, the physicists observed that types of plasma turbulence known as “mirror” and “firehose” instabilities caused plasma particles to scatter. This scattering broke the link between the energy of the particles and the magnetic fields, in which the two tend to increase or decrease together, and allowed the amplitudes of the magnetic fields to grow closer to what is observed in nature.
Going forward, “we would like to investigate the specifics of particle scattering,” St-Onge said. “How exactly do the instabilities cause the particles to scatter, how often does the scattering occur, and can the scattering lead to sudden, dramatic growth of a magnetic field? The last idea is a notion proposed by PPPL Director Steven Cowley years ago. We would like to investigate whether this is true.” ☀︎
Engage engines! New research illuminates complex processes inside plasma propulsion systems for satellites
If you think plasma thrusters are found only in science fiction, think again. Researchers at PPPL have been uncovering the physics behind these high-tech engines, which maneuver satellites in space. The research began with an Air Force Office of Scientific Research award to PPPL to investigate the origin of spoke-like plasma structures that appear in Hall thrusters, propulsion devices used to correct the orbits of satellites circling Earth.
Former PPPL physicist Johan Carlsson, working with Princeton University graduate student Andrew Powis and other colleagues, simulated a Penning discharge device — a machine that is simpler and easier to model than a plasma thruster. The simulations accurately reproduced the spokes observed earlier in experiments conducted by PPPL principal research physicist Yevgeny Raitses with the help of students.
The new simulations were performed on a high-powered computer network in western Canada, and at the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility at Lawrence Berkeley National Laboratory
“We now have confidence that we have the right ingredients built into our model to reproduce the physics of plasma propulsion devices,” said Carlsson, the lead author of a paper reporting the results in Physics of Plasmas.
“This paper shows that three components of fundamental research — experimental study, theoretical calculations, and advanced computation — can help increase the understanding of complex phenomena like spokes.” ☀︎
Physicist Johan Carlsson