Researchers at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) played a prominent role in the 64th annual meeting of the American Physical Society-Division of Plasma Physics (APS-DPP). Some 170 PPPL scientists, students and others - the largest group from the Laboratory ever - joined the hybrid October 17-21, 2022, conference in Spokane, Washington, that drew a record 2,250 participants from 29 nations.
Scientists at PPPL gave 15 invited talks on wide-ranging plasma research from fusion energy and magnetic reconnection to low-temperature, high-energy and astrophysical plasmas. Representatives also spoke in mini-conferences on public-private partnerships for fusion energy and work-force development through science education and public engagement.
The Laboratory's Science Education staff was active throughout the week. Staff members helped organize an all-day workshop on plasma science for middle and high school teachers aimed at bringing more students from all backgrounds into the workforce. Staffers also presented hands-on activities for students and teachers at the conference's Student Expo. Shannon Swilley Greco, a Science Education senior program leader, cofounded the new Plasma Network for Engagement and Training (Plasma NET) that kicked off during a two-day mini-conference at the week-long event. And some 30 students who participated last summer in the DOE's Science Undergraduate Laboratory Internship (SULI) program at PPPL that Science Education administers presented posters at the conference.
Among this year's APS-DPP honorees was Amitava Bhattacharjee, PPPL physicist and Princeton University professor of astrophysical sciences, who won the James Clerk Maxwell Prize for Plasma Physics, and Jonathan Squire, a 2015 graduate of the Princeton Program in Plasma Physics and now a senior research fellow at the University of Otago, New Zealand, winner of the Thomas H. Stix early career award. Frances Hellman, president of the APS, gave away both prizes at the DPP Banquet on Wednesday evening.
Here we share examples of key research results presented at the meeting in two categories: (1) Fusion Science and Technology and (2) Discovery Plasma Science. Provided are links to results included in convention press releases.
1. Fusion Science and Technology:
Unexpected connection: method used to prevent plasma bursts linked to seemingly contradictory disturbances
PPPL physicists reveal how two seemingly opposing conditions within experimental fusion facilities are strongly linked. Understanding these connections could improve next-generation fusion energy devices built to counter climate change by reproducing on Earth the carbon-free fusion energy that powers the sun and stars.
The surprising connection brings together the use of magnetic fields to confine plasma fusion fuel in doughnut-shaped fusion devices called tokamaks. The plasma, however, may erupt in bursts that can strike and damage the walls of the tokamak, leading researchers to use additional magnetic fields to prevent the bursts. But such fields can make heat flow out of the plasma unevenly and can also damage the machine.
Computer simulations now suggest that the magnets' capability to prevent plasma eruptions cannot be separated from their tendency to disturb the flow of heat and particles in the tokamak's exhaust region, known as the divertor. The simulations showed that when magnets are used to prevent ELMs, they will inevitably disturb the flow of heat and subatomic particles in the divertor exhaust region of the tokamak, either causing that heat to strike unintended parts of the divertor or causing it to overflow into other parts of the tokamak.
"The physics governing the two phenomena is the same," said PPPL physicist Stefano Munaretto, lead author of the paper that reported this research. "You cannot use the magnets to solve the problem of ELMs without having the disturbance in the divertor. The two effects are connected."
These findings are important because understanding how magnetic fields can both help and interfere with fusion plasma is crucial to designing and operating fusion devices. For full release, click here.
A new tool for predicting plasma behavior in stellarators
A new simulation tool - mainly realized by graduate student Daniel Dudt who studies at Princeton and PPPL - will give scientists a clear picture of how to design the next generation of stellarators, fusion devices with twisty coils that have the potential to create a stable and uninterrupted flow of fusion energy. The twisty coils produce magnetic fields that trap hot plasma - a collection of ions and electrons - in a way that prevents it from leaking out. Longer trapping times are key to continuously producing fusion energy by combining lighter ions in the plasma into heavier ions.
The tool, a code called DESC - pronounced "desk" - that Dudt wrote from scratch, creates a fast and accurate method to predict how plasma would behave in a given stellarator design. Such accurate models predicting plasma behavior are much needed before a stellarator is built, as they help physicists and engineers reduce design errors and thus save money.
"Historically the reason that people haven't pursued stellarators as much is because they couldn't solve these complicated optimization problems," Dudt said. "Now that we have the computation tools to solve those problems, stellarators look very attractive because we can make them perform as well if not better [than other approaches]. We're taking these machine learning techniques and continuing to push the envelope on how fast and accurate we can get these things done."
DESC, which uses a modern computational method called "automatic differentiation" borrowed from the machine learning community, has a growing user base at institutions around the world. "That allows the code to be much more flexible," said Egeman Kolemen, Dudt's adviser, who holds appointments at PPPL and Princeton. "You can change things around faster, you can add new capabilities on top, and every time you add something you don't have to change the whole code." For full release, click here.
Preventing the Achilles heel of fusion experiments
The super-fast runaway electrons released in disrupted fusion experiments are the "Achilles heel" of fusion experiments in doughnut-shaped tokamaks, according to Luis Delgado-Aparicio. The PPPL physicist has led the development and deployment of a unique diagnostic device - a multi-energy pinhole camera - to detect the birth of runaway electrons on the Madison Symmetric Torus at the University of Wisconsin-Madison. By studying the birth of this process in low-density areas at the edge of the fusion plasma, scientists can understand how the electrons accelerate from low to high energies and move from the edge to the core. Further research with this singular camera could thus produce new understanding of ways to prevent Achilles heels from developing and punching holes in the inner walls of large tokamaks like ITER, the international fusion experiment under construction in Cadarache, France, to demonstrate the practicality of fusion energy.
Fooling fusion fuel to discipline unruly plasma
Maintaining the stability of plasma is crucial to tokamak fusion facilities like ITER that is going up in France. However, while researchers have found that using magnetic coils to stabilize the plasma can go a long way toward avoiding disruptions, the smallest misalignment of the magnetic coils, which is inevitable, can cause tiny field ripples that reduce the stability of the plasma. PPPL research led by physicist Jong-Kyu Park has now developed a novel method to counter the effect by carefully producing quasi-symmetric fields, such as twisty stellarator fusion facilities use, that fool the plasma into behaving as if it was not affected by destabilizing ripples. Researchers have successfully tested this promising method on the Korea Superconducting Tokamak Advanced Research (KSTAR) and the DIII-D National Fusion Facility at General Atomics in San Diego.
Discoveries about the flow of electricity could improve fusion devices
Cored-apple-shaped spherical tokamaks confine plasma, the fuel for fusion reactions, in powerful magnetic fields. But sometimes wiggles in the plasma allow particles and energy to escape in bursts called edge-localized modes (ELMs) that can damage the interior of the machines. PPPL researchers led by physicist Andreas Kleiner have found that incorporating resistivity, the property of any substance that inhibits the flow of electricity, can improve mathematical models that predict such bursts and lead to their mitigation and avoidance.
2. Discovery Plasma Science:
Get energized: How gamma ray bursts could power cosmic rays
The following summarizes a conference press release that combines PPPL and Stanford University presentations into one document.
For decades, scientists have observed powerful explosions known as gamma ray bursts erupting from the cores of distant galaxies thought to host supermassive black holes, which could cause the bursts through an unknown mechanism. The most vivid example is the center of the M87 galaxy, which shoots out luminous gamma-ray flares many times a year. Not only do these gamma ray bursts involve flashes of light, but they are also associated with the acceleration of charged particles called cosmic rays.
Scientists at the Princeton Plasma Physics Laboratory recently completed a simulation of the plasma surrounding the M87 galaxy. Because the plasma is so close to the black hole, the simulation needed to take into account how the electrons and ions move when space-time is warped by the massive object. Including all the necessary physics and scales is an incredibly difficult undertaking given current computational limits. This was also one of the first simulations of magnetic reconnection - when magnetic field lines break apart and snap together in new configurations, releasing tremendous amounts of energy - to consider many of these factors in this environment.
"We built on earlier findings that established the existence of current sheets - layers of flowing electricity - around black holes and examined how much energy they could produce if they reconnected," said Hayk Hakobyan, a postdoctoral researcher associated with Princeton University, PPPL, and Columbia University. "We found that the energy produced by the reconnection process could be enormous and therefore that the model could potentially explain the presence of the flares in M87."
Meanwhile, scientists in the Stanford group headed by Frederico Fiuza found a new mechanism that can lead to gamma ray bursts and cosmic ray energization associated with shocks. This new mechanism, called the plasma cavitation instability, affects the size of the magnetic fluctuations that occur ahead of the gamma-ray burst shock. Particles bounce off these magnetic fields, gaining energy each time. The larger magnetic fields from this instability mean that the particles can be energized more than previously thought possible by the magnetic fluctuations. This would allow the shock to accelerate cosmic rays more efficiently.
"We showed that this plasma cavitation instability can be very efficient in the extremely energetic conditions associated with gamma-ray bursts and amplify magnetic fields to more than an order of magnitude beyond what was previously thought possible" said Ryan Peterson, a PhD student at Stanford and lead author of the Stanford work.
Both of these projects will help scientists better understand the physical mechanisms that heat the fastest, most energetic particles in the universe. For full release, click here.
Shifting momentum: Lab confirms key theory of how planets and stars form
A breakthrough experiment led by physicist Yin Wang has performed the first laboratory confirmation of a long-standing but never-before confirmed theory of the puzzling formation of planets, stars and supermassive black holes by swirling surrounding matter. This breakthrough caps more than 20 years of experiments at the national laboratory devoted to the study of plasma science and fusion energy.
The puzzle arises because the swirling clouds of dust and plasma called accretion disks that collapse into celestial bodies do so in defiance of the conservation of angular momentum, the principle that keeps planets and the rings of Saturn from tumbling from their orbits.
The solution to this puzzle, a theory known as the Standard Magnetorotational Instability (SMRI), was first proposed in 1991 by then-University of Virginia theorists Steven Balbus and John Hawley, who theorized that instabilities in the swirling particles shift the angular momentum away from the center of the clouds toward the edges, allowing the center particles to collapse into the heavenly bodies.
Recent PPPL results on the Laboratory's unique Magnetorotational Instability (MRI) device, a facility conceived by physicists Hantao Ji of PPPL and Jeremy Goodman of Princeton, "have successfully detected the signature of SMRI," said Wang, lead author of a pair of papers detailing the results. Noted theory codeveloper Steven Balbus: "This is great news. To now be able to study this in the laboratory is a wonderful development, both for astrophysics and for the field of magnetohydrodynamics more generally." For full release, click here.
New method for facilitating inertial confinement fusion
An international team of scientists led by PPPL physicist Sophia Malko has uncovered a new method for advancing the development of fusion energy through increased understanding of the properties of warm dense matter, an extreme state of matter similar to that found at the heart of giant planets like Jupiter. The findings detail a new technique to measure the "stopping power" of nuclear particles in plasma using high repetition-rate ultra-intense lasers. Understanding this process is particularly important for inertial confinement fusion (ICF), which differs from the magnetic confinement fusion that PPPL studies.
Surprise finding of a broad new reach for fundamental physics research
Firing high-power electron beams into plasma is a long-standing method for generating plasma for various applications from space propulsion to Northern lights to advanced switches for electric grids. The beam generates waves, like wind on the water, that perturb and impact the behavior of the plasma. Now researchers, led by graduate student Haomin Sun in the Princeton Program in Plasma Physics have demonstrated that the wave has a greater impact on the temperature, density and other parameters of the plasma than had been previously thought.
Discoveries about the flow of electricity could improve fusion devices
Cored apple-shaped spherical tokamaks confine plasma, the fuel for fusion reactions, in powerful magnetic fields. But sometimes wiggles in the plasma allow particles and energy to escape in bursts called edge-localized modes (ELMs) that can damage the interior of the machines. PPPL researchers led by physicist Andreas Kleiner have found that incorporating resistivity, the property of any substance that inhibits the flow of electricity, can improve mathematical models that predict such bursts and lead to their mitigation and avoidance.
Bringing Earth's magnetosphere into the laboratory
The interaction of the Sun's solar wind with the Earth's magnetic field creates the magnetosphere, which is critical for life to exist since it blocks lethal cosmic rays and controls space weather like auroras. Scientists from Princeton, UCLA and the Instituto Superior Técnico, Portugal, have put together a unique new way to study millimeter-sized magnetospheres as test beds for planet-sized magnetospheres in the laboratory. "We have developed a new experimental platform to study mini-magnetospheres on the Large Plasma Device (LAPD) at UCLA," said PPPL and Princeton physicist Derek Schaeffer. This unique platform combines the magnetic field of the LAPD with a fast laser-driven plasma and a current-driven magnet to perform high-repetition-rate 3D measurements that can be compared to 3D simulations and spacecraft observations.
Challenges and mitigations of characterizing plasmas with myriad industrial applications
While the interaction of plasma with liquids produces captivating physical phenomena with wide-ranging industrial uses, diagnosing and interpreting these interactions face vexing challenges. Now PPPL research led by physicist Shurik Yatom and supported by the Princeton Collaborative Low Temperature Research Facility (PCRF), a joint venture of the laboratory and Princeton University, provides a close look at methods for characterizing such low-temperature plasma interactions. The research focuses on laser diagnostic methods of Thomson scattering and laser-induced fluorescence, measuring plasma density and temperature and plasma-induced radicals, respectively. Presented work outlines the details of these approaches, the specific challenges encountered in plasma-liquid systems such as oscillations and quenching, correct ways of setting up the system for diagnostics and analysis of the obtained data.
