Scientific discoveries, educational opportunities and wide-ranging events highlighted the 62nd American Physical Society-Division of Plasma Physics annual meeting, which attracted participants from around the world. The session this year, held virtually November 9 to 13, drew more than 150 physicists, engineers and students from the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL). They joined more than 2,000 registrants for presentations, poster sessions and seminars on topics ranging from astrophysical plasmas to nanotechnology to fusion development.
PPPL presentations included 11 invited talks on fundamental plasma physics and recent experimental and theoretical findings on the magnetic confinement of plasma that fuels fusion reactions in laboratory facilities. Also speaking was Yuan Shi, a 2018 graduate of the Princeton Program in Plasma Physics and winner of the 2020 Marshall N. Rosenbluth Outstanding Doctoral Thesis Award for his theoretical dissertation on three-wave coupling. He now is a Lawrence fellow at the Lawrence Livermore National Laboratory in California.
Women in Plasma Physics Luncheon
Among special events at the week-long conference was a PPPL-organized presentation at a Women in Plasma Physics Luncheon. Members of the PPPL Science Education Department took part in a Science Teacher's Day on November 7 and November 14 for middle school and high school teachers. Included among the events was announcement of newly elected APS Fellows, which includes Elena Belova, a principal research physicist at PPPL whose theoretical work has advanced key areas of fusion research.
The conference publicized discoveries of general interest through a virtual press room that included three PPPL presentations shown here in shortened form:
Novel Platform Advances Understanding of How Reconnection Creates Solar Flares
Scientists at PPPL and the Department of Astrophysical Sciences at Princeton University have developed a unique robust platform to approximate the flare-like acceleration of high-energy particles across vast distances in space. The recent experiments are powered by high-energy lasers that produce magnetic reconnection similar to that observed in the solar atmosphere.
"Our goal has been to create and measure the acceleration of flare-like particles on an effective new platform," said PPPL physicist Lan Gao. "The laser facilities and the diagnostics we've used in our experiments enable us to characterize the reconnecting plasma and the resulting spectrum of accelerated particles."
The result improves plasma confinement and produces highly accelerated flare-like particles that spectrometers have successfully measured. Researchers used the Omega Laser Facility at the University of Rochester to create a circular current sheet between two semicircular coils and drive reconnection.
Understanding how solar flares generate incredibly hot particles remains an important open question in heliophysics. In the new platform, the lasers both create plasma and drive magnetic reconnection to generate particles at energies similar to those in solar flares. This process contrasts with the lower-energy plasma discharges used in previous experimental facilities. Such experiments lacked sufficient resolution in space, time, and energy to truly observe the full range of these energies.
Going forward, PPPL researchers plan to sharply increase the generation of current in the semicircular coils. "Our next step will be to use the spectrum of measured particles together with theories and simulations to understand the acceleration mechanism," Gao said. "The findings should improve our understanding of these incredibly powerful solar flares and contribute to our ability to predict the occurrence of flares that could impact life on Earth."
The following releases combine PPPL findings with those of other laboratories.
Radiation Therapy for Fusion Plasmas and a Major Puzzle Solved
Recent discoveries at PPPL and the MIT Plasma Science and Fusion Center (MIT PSFC) have advanced the use of radio frequency (RF) waves similar to those used in microwave ovens as a kind of radiation therapy for fusion reactions. These findings help answer some of the biggest open questions for scientists seeking to develop a fusion reactor as a power source to generate electricity: How do you heat the plasma to sufficient temperature? And how do you confine and stabilize this plasma once it's created?
At PPPL, researchers Allan Reiman and Nat Fisch teamed up to predict the RF condensation effect, which suggests that above a power threshold, the RF-driven current becomes concentrated exactly where it stabilizes best: in the center of a magnetic bubble or island. When left unchecked, these islands can cause a disruption in the plasma. But when the key threshold of power is exceeded, the current condenses spectacularly, thereby preventing islands from growing large enough to create disruptions.
At the MIT PSFC, researchers have solved a major puzzle in experimental observations of theoretical simulations that predict how RF waves travel through the plasma. The "missing physics" suggested by the MIT PSFC team lies in the chaotic outside layer of the plasma. This layer may broadly scatter RF waves traveling through the region.
"With the added physics of turbulence interactions, our confidence in existing RF codes is greatly improved," said doctoral candidate Bodhi Biswas, who led this research. "The scattering of waves by turbulence may, in many cases, be essential to matching the experiment, and to applying RF power effectively in future tokamaks."
Fisch, Director of the Program in Plasma Physics at Princeton University, noted the existence of "an important synergy between the MIT and Princeton discoveries." This synergy, Fisch said, indicates that the scattering of RF rays by turbulence can be counteracted by the discovery by Reiman and his students of the current condensation arising in the center of an island. RF radiation therapy thus "may be an excellent candidate for stabilizing magnetic islands once conditions for the condensation effect set in," he said, "even as turbulent fluctuations at the plasma edge tend to spread RF trajectories as seen and accurately modeled in present-day experiments."
Supercomputers and Wall Coatings: The Road to Better Plasma Confinement
To harness fusion, the energy that powers the sun and stars, scientists have designed many types of magnetic containers to trap ultrahot gas known as plasma and fuel fusion reactions. The stellarator appears to be perhaps the most intricate: a twisty, cruller-shaped bottle that confines plasma using carefully honed magnetic fields. The world's newest, largest, and most advanced stellarator, the $1.2 billion Wendelstein 7-X (W7-X) device in Greifswald, Germany, has adopted this approach with promising results.
In addition to other benefits, the W7-X design appears to reduce turbulence within the plasma. Turbulence - sometimes violent, seemingly random vibrations - can increase the plasma's ability to escape. "Turbulence can be reduced by tailoring the geometry judiciously," said Per Helander, head of the stellarator theory division at the Max Planck Institute for Plasma Physics. "Moreover, these theoretical predictions appear to be borne out in the first experiments of Wendelstein 7-X, which raises the hope that turbulence, which has long plagued magnetic fusion research, could be reduced in this and future devices."
Contributing to that hope is a recently discovered way to recondition the interior walls of stellarators during fusion operations, allowing experiments to continue without interruption.
Scientists have been searching for ways to apply boron - a component of the common household cleaner Borax - to keep stellarator interiors free while the machine is still running. An international team from PPPL and the Max Planck Institute developed a Probe-Mounted Particle Injector (PMPI) to solve this issue. The PMPI can recoat the walls by shooting bits of boron carbide into the plasma.
When the scientists used the PMPI on the W7-X, they found that it not only enabled operations to continue but helped to improve confinement as well. The researchers were surprised to find that the injected boron powder smoothed out the changes in plasma temperature from the hot core to the cooler edge. This smoothing reduces the intensity of unwanted plasma instabilities that transport heat from the core and slow down fusion reactions.
"Reducing the size of these instabilities allows more energy to be stored within the plasma and increases the core ion and electron temperatures," said PPPL physicist Robert Lunsford, leader of the international team. "The controlled introduction of boron carbide could be a way to change the overall plasma profile, allowing for enhanced W7-X performance and greater fusion output."
Also among the virtual press room releases was a morning and afternoon mini-conference on plasma technology for combatting COVID-19 that PPPL physicist Igor Kaganovich chaired in the morning. Here are two references to PPPL research.
Plasma-Based Technology to Fight COVID-19
• Scientists at PPPL and the New Jersey Institute of Technology are using a flexible printed circuit design to develop a cold-atmospheric plasma through the use of a dielectric barrier discharge (DBD). Such discharges are generated between two electrodes separated by an insulating dielectric barrier. They can treat flat or curved surfaces in air with no additional gas flow and reduce the concentration of bacteria on a glass surface by 99.99 percent when in direct contact with the device.
• PPPL scientists and colleagues from Rutgers New Jersey Medical School have also examined the efficacy of commercially available home-use plasma devices for the rapid and inexpensive antiviral treatment of surfaces. These devices include a d'Arsonval high-frequency current source and a plasma globe. Both easy-to-use devices create plasma through the DBD process and are readily available for public purchase through on-line markets.
• A key advantage of plasma-based therapies is their adaptive nature. Such therapies are easily varied based on input parameters - electrical input, voltage, current, and type of gas - unlike the fixed chemical composition of most drugs and disinfectants. This capability can allow scientists to monitor the efficacy of plasma-based treatments and almost instantaneously adjust the plasma composition based on that feedback; scientists are developing rapid detection techniques to realize this feature. All these benefits suggest that cold atmospheric plasmas are a powerful tool in fighting the global pandemic.
