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Physicists create stable, strongly magnetized plasma jet in laboratory
Physicist Lan Gao
When you peer into the night sky, much of what you see is plasma, a soupy amalgam of ultra-hot atomic particles. Studying plasma in the stars and outer space requires a telescope, but scientists can recreate the amalgam in the laboratory to examine it more closely.
Now, a team of scientists led by physicists Lan Gao of PPPL and Edison Liang of Rice University, has for the first time created a form of coherent and magnetized plasma jet that could deepen the understanding of the workings of much larger jets that stream from newborn stars and possibly black holes; such holes are stellar objects so massive that they trap light and warp both space and time.
“We are now creating stable, supersonic, and strongly magnetized plasma jets in a laboratory that might allow us to study astrophysical objects light years away,” said astrophysicist Liang, co-author of the paper — with Gao as lead author — reporting the results in The Astrophysical Journal Letters.
The team created the jets using the OMEGA Laser Facility at the University of Rochester’s Laboratory for Laser Energetics (LLE). The researchers aimed 20 of OMEGA’s individual laser beams at a ring-shaped area on a plastic target. Each laser created a tiny puff of plasma that put pressure on the inner region of the ring. That pressure squeezed out a plasma jet reaching over four millimeters in length and created a magnetic field that had a strength of over 100 tesla, more than 50 times stronger than most Magnetic Resonance Imaging (MRI) machines.
“This is the first step in studying plasma jets in a laboratory,” said Gao. “I’m excited because we not only created a jet. We also successfully used advanced diagnostics on OMEGA to confirm the jet’s formation and characterize its properties.”
The experiment reveals new facts about jets. “This is groundbreaking research because no other team has successfully launched a supersonic, narrowly beamed jet that carries such a strong magnetic field, extending to significant distances,” said Liang. “This is the first time that scientists have demonstrated that the magnetic field does not just wrap around the jet, but also extends parallel to the jet’s axis.”
Nuclear warheads? This robot can find them
Detector robot in PPPL hallway. Behind robot from left, student intern Harry Fetsch and Rob Goldston, co-principal investigator of the project
Trust, but verify.
— Ronald Reagan
Picture a swarm of autonomous, three-foot rolling robots armed with smart detectors to support nuclear safeguards and verify arms-control agreements. The prototype of such robots, being developed by PPPL and Princeton University, recently demonstrated the ability to identify the source of nuclear radiation and whether it has been shielded to avoid detection.
The remotely controlled prototype sets the stage for further development of a mobile and fully autonomous swarm. “The demonstration gave total confirmation of the ability of the robot to detect the source of neutrons and provided beautiful data,” said PPPL physicist Rob Goldston, a Princeton University professor of astrophysical sciences and co-principal investigator in the project.
“Everything we saw looks excellent and very promising,” said co-principal investigator Alex Glaser, a Princeton professor of engineering and co-director of the Princeton Program in Science and Global Security.
Three detectors on the “inspector bot” provide both high sensitivity to the energy of detected neutrons and the direction from which neutrons are coming. Low energy could indicate shielding. During the summer Harry Fetsch, a physics student at Harvey Mudd College in the Science Undergraduate Laboratory Internship (SULI) at PPPL, ran thousands of computer hours to simulate the detection system. “These simulations informed the design of the experiments we conducted,” Glaser said.
When fully developed the robot could become part of a swarm of devices that carry out inspection tasks in different types of facilities. Goldston next plans to visit the U.S. Department of Energy’s Savannah River Site nuclear fuel fabrication plant to explore the possibility of testing the inspector bot in a facility where the output from enrichment plants goes.
“We want to see if we can measure the neutrons coming out of the autoclaves,” he said of devices used to heat uranium to send it into the fabrication plant. Doing so could provide a further step toward demonstrating that the simple and robust autonomous neutron detectors can provide a cost-effective means to provide effective and efficient verification.
Fetsch and Goldston examine the detectors
Novel experiment validates a widely speculated mechanism behind the growth of heavenly bodies
Simulated accretion disk swirling around a celestial body (Simulation by Michael Owen and John Blondin, North Carolina State University)
How have stars and planets developed from the clouds of dust and gas that once filled the cosmos? A novel PPPL experiment has demonstrated the validity of a widespread theory known as “magnetorotational instability,” or MRI, that seeks to explain the formation of heavenly bodies.
The theory holds that MRI allows accretion disks, clouds of dust, gas, and plasma that swirl around growing stars and planets and black holes, to collapse into them. According to the theory, this collapse happens because turbulent swirling plasma gradually grows unstable within a disk. The instability causes the process called “angular momentum” that keeps orbiting planets from being drawn into the sun to decrease in inner sections of the disk and fall into celestial bodies.
At PPPL, physicists have simulated the MRI theory in the laboratory’s unique MRI experiment, which consists of two concentric cylinders that rotate at different speeds. Results are reported in Communications Physics. Researchers filled both cylinders with water and attached a water-filled plastic ball tethered by a stretching and bending spring to a post in the center of the device. The spring mimicked the magnetic forces in accretion disk plasmas.
Researchers then rotated the cylinders and compared the motions of the spring-tethered ball when rotating at different speeds. Direct measurement of the results found that when the spring-tethering was weak — analogous to the condition of the magnetic fields in accretion disks —behavior of the angular momentum of the ball was consistent with MRI predictions of developments in a real accretion disk. The weakly tethered rotating ball gained angular momentum and shifted outward during the experiment. Since the angular momentum of a rotating body must be conserved, any gains in momentum must be matched by a loss of momentum in the inner section, allowing gravity to draw the disk into the object it has been orbiting.
Water-filled version of MRI experiment showing transparent outer cylinder and blackened inner cylinder. Red lasers enter at bottom to measure the local speed of the water (Photo courtesy of Eric Edlund and Elle Starkman)
Scientists deepen understanding of the magnetic fields that surround the Earth
Physicist Eun-Hwa Kim
Vast rings of electrically charged particles encircle the Earth and other planets. Now, a team of scientists has completed research into waves that travel through this magnetic charged environment, known as the magnetosphere, deepening understanding of the region and its interaction with Earth and opening up new ways to study other planets across the galaxy.
The scientists, led by physicist Eun-Hwa Kim, examined a type of wave that travels through the magnetosphere. These waves, called electromagnetic ion cyclotron (EMIC) waves, reveal the temperature and the density of the plasma particles within the magnetosphere.
“Waves are a kind of signal from the plasma,” said Kim, lead author of a paper that reported the findings in JGR Space Physics. “Therefore, the EMIC waves can be used as diagnostic tools to reveal some of the plasma’s characteristics.”
Kim and colleagues focused on the way in which some EMIC waves form. During this process, other waves that travel from outer space collide with Earth’s magnetosphere and trigger the formation of EMIC waves that then zoom off at a particular angle.
Researchers performed simulations on PPPL computers showing that the EMIC waves can propagate through the magnetosphere at an angle that is less than 90 degrees in relation to the border of the region with space. Knowing such characteristics enables physicists to identify EMIC waves and gather information about the magnetosphere with limited initial information.
A better understanding of the magnetosphere could provide detailed information about how Earth and other planets interact with their space environment. “We are really eager to understand the magnetosphere and how it mediates the effect that space weather has on our planet,” said Kim. “Being able to use EMIC waves as diagnostics would be very helpful.”
A shock to behold: Earthbound scientists complement space missions by reproducing the dynamics behind astronomical shocks
Physicist Derek Schaeffer
High-energy shock waves driven by solar flares and coronal mass ejections from the sun erupt throughout the solar system, unleashing magnetic space storms that can damage satellites, disrupt cell phone service and blackout power grids on Earth. Also driving high-energy shock waves is the solar wind — plasma that constantly flows from the sun and buffets the Earth’s protective magnetic field.
Now experiments led by PPPL researchers in the Princeton Center for Heliophysics have for the first time reproduced the process behind the source of such shocks. The findings bridge the gap between laboratory and spacecraft observations and advance understanding of how the universe works.
The experiments, reported in Physical Review Letters, show how the interactions within plasma — the state of matter composed of free electrons and atomic nuclei, or ions — can cause sudden jumps in plasma pressure and magnetic field strength that can accelerate particles to near the speed of light. Such shocks are “collisionless” because they are formed by the interaction of waves and plasma particles rather than by collisions between the particles themselves.
The research, led by physicist Derek Schaeffer of PPPL and Princeton University, was conducted on the Omega laser facility at the University of Rochester. The experiment produced a laser-driven plasma — called a “piston” plasma — that expanded at the supersonic rate of more than one million miles per hour through a pre-existing ambient plasma. The expansion accelerated ions in the ambient plasma to speeds of roughly half-a-million miles per hour, simulating the forerunner to collisionless shocks that occur throughout the cosmos.
Researchers tracked these developments with a diagnostic that detects laser light scattered off electrons in the plasma, enabling measurement of the temperature and density of the electrons and the speed of the flowing ions. The results, the authors write, show that laboratory experiments can probe the behavior of plasma particles in the precursor to collisionless astrophysical shocks, “and can complement, and in some cases overcome the limitations of similar measurements undertaken by spacecraft missions.”
The ultimate goal is to measure the shock-accelerated particles themselves. For that step, said Schaeffer, “the same diagnostic can be used once we develop the capability to drive strong enough shocks,” said Schaeffer. “As a bonus,” he adds, “this diagnostic is similar to how spacecraft measure particle motions in space shocks, so future results can be directly compared.”
NASA-recorded solar flare
Confirming a little-understood source of the process behind northern lights and the formation of stars
Physicists Jonathan Jara-Almonte, right, and Hantao Ji, adviser and coauthor of the paper
Fast magnetic reconnection, the rapid convergence, separation and explosive snapping together of magnetic field lines, gives rise to northern lights, solar flares and geomagnetic storms that can disrupt cell phone service and electric power grids. The phenomenon takes place in plasma, the state of matter composed of free electrons and atomic nuclei, or ions, that makes up 99 percent of the visible universe. But whether fast reconnection can occur in partially ionized plasma — plasma that includes atoms as well as free electrons and ions — is not well understood.
Researchers at PPPL have now produced the first fully kinetic model of the behavior of plasma particles and found that fast reconnection can indeed occur in partially ionized systems. Kinetic models simulate the velocity and distribution of billions of particles, compared with fluid models that treat plasma as a continuous medium rather than a collection of particles.
“There is a whole class of partially ionized plasmas whose link to reconnection has not been well studied,” said physicist Jonathan Jara-Almonte, lead author of a Physical Review Letters paper that reports the recent findings. “We have now demonstrated that fast reconnection can occur in partially ionized systems.”
Such reconnection has important implications for the interstellar medium, the vast clouds of gas and dust that fill the cosmos between stars. The cold, dense regions of the interstellar medium where stars form are very poorly ionized, and fast reconnection occurring within these regions can help remove magnetic fields that prevent star formation.
The new simulations, performed on computers at Princeton University, suggest that the transport of plasma and heat is different in partially ionized plasmas and can alter how and when reconnection occurs. Going forward, Jara-Almonte plans to compare findings of the kinetic simulation with those of fluid simulations that have dominated the previous modeling of partially ionized plasmas.
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