Plasma

Vladimir Zhdankin earns DOE Early Career award

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Vladimir Zhdankin (credit: Flatiron Institute)

Congrats to Vladimir Zhdankin, assistant professor of physics, on earning a Department of Energy Early Career award! The five-year award will fund his research on energy and entropy in collisionless, turbulent plasmas. 

Systems in equilibrium are easy to describe, but often the most interesting questions in nature are complex and dynamic. Most plasmas, including astrophysical ones and manmade ones on earth, are not in equilibrium, so they are more difficult to characterize. Zhdankin’s research is working toward a more universal understanding of non-equilibrium plasmas, in the form of mathematical equations that can then be broadly applied. 

“We think that our understanding of plasmas isn’t finished yet, and there are still some basic ingredients in the statistical mechanics which, once we understand better, we’ll have a more predictive framework for how plasmas should behave,” Zhdankin says. 

Collisionless plasmas have a low enough particle density where the particles largely flow without bumping into each other. Instead, their trajectories are controlled by the electric and magnetic field, which leads to a generally chaotic flow, like the rapids of a river. It is that dynamic turbulence that causes these plasmas to be non-equilibrium, leading to interesting, if not straightforward, properties. 

“In these systems, energy is conserved — it has to be,” Zhdankin says. “But we don’t quite have a handle on what’s happening with the entropy. We have reason to believe it’s increasing, consistent with the second law of thermodynamics, but it doesn’t seem to reach a maximum.” 

Zhdankin’s goal is to better understand the energy and entropy in these complex plasmas through “particle-in-cell” simulations, where tens of billions of plasma particles — electrons and protons — are simulated in a small box, then manipulated in various ways.

“We imagine stirring the plasma to make it more turbulent and putting some energy into it, and then we want to see how it heats up and how the particles achieve higher energies,” Zhdankin says. “What if we increase or decrease the size of the box? Make the magnetic field stronger? Make the particles collide a little bit?”

The simulations can then be compared to real-world data, including measurements of the solar wind or laboratory plasmas. An ideal outcome would be obtaining formulae that better describe these complex, turbulent plasmas and can be applied across a broad range of systems, from laboratory experiments to the accretion flows of black holes. 

“And there’s a chance we’re just not going to be able to get something predictive out of this work, if there’s just too big of a landscape of possibilities,” Zhdankin says. “But this topic, I consider it one of the most fundamental ones that could be studied in plasma physics.”

Highlights from APS DPP

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Physics Faculty Attend Major Plasma Physics Conference in Atlanta

By Christopher Woolford, Physics PhD student

Over 200 faculty and students represented the University of Wisconsin–Madison at the annual American Physical Society Division of Plasma Physics (DPP) research conference in Atlanta last October. The APS DPP conference presents an excellent opportunity to learn about the newest and cutting-edge research in plasma physics.

a man stands in front of a poster and give a thumbs-up sign
Prof. Rogerio Jorge presents at the APS DPP conference

Rogerio Jorge, an assistant professor of physics at UW–Madison, presented his work on stellarator optimization alongside several of his students. Stellarators are a type of nuclear fusion reactor that uses twisted magnetic fields to confine fusion reactions.

“The weather was amazing, and the conference is a great opportunity for collaborations,” Jorge says.

This year, the APS DPP conference benefited from attendance by a new generation of plasma physicists that has seemed to have grown in recent years.

“I had not seen so many younger plasma physicists in years at an APS DPP conference,” says Paul Terry, a professor of physics at UW–Madison, attributing the rise in attendance to the influx of private companies interested in nuclear fusion.

Eduardo Neto, a postdoc at UW–Madison working with Jorge, gave a talk on his exciting work on Stellarator optimization. This work was the result of an ongoing collaboration with Proxima Fusion and IST in Lisbon, Portugal.

a man stands to the left of the picture in front of a screen. He is showing a powerpoint presentation to an audience of several dozen people
Eduardo Neto delivers an oral presentation at APS DPP in October.

“The most challenging part of this project was getting a lot of different codes to work together,” Neto says. “Startup companies have a strong interest in preventing tungsten buildup in Stellarators.”

Neto plans to continue his collaborations with Proxima Fusion and explore optimizing other parts of Stellarators such as turbulence. His advice for students interested in Stellarator optimization is to gain broad knowledge of plasma physics and nuclear fusion.

The APS DPP conference will be in November this year in Long Beach, California.

a smiling person stands in front of their scientific poster
Cameron Kuchta presents a poster at APS DPP.
two people stand in front of their scientific poster
Tony Qian and Jonathan Pizzo present a poster at APS DPP.
a man stands in front of a scientific poster
Djin Patch presents a poster at APS DPP.

Rogerio Jorge receives first grant as a professor

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Rogerio Jorge

Congrats to Prof. Rogerio Jorge who was awarded his first grant as a professor! The three-year, $500,000 National Science Foundation grant, titled “Moment Approach to Multiscale Plasmas,” will be used to fund a graduate student and postdoc on the project.

“Astrophysical plasmas appear in more than 90% of the universe — for example, on the surface of the sun or in the intergalactic medium — and there’s still a lot of things that we don’t understand about them,” Jorge says. “We need to study phenomena in astrophysical plasmas and try to replicate them numerically to better understand them.”

Jorge’s work will focus on the so-called collisionless regime of these plasmas, where particles travel for a long time before experiencing any collision. He says this regime is difficult to model, both experimentally and numerically.

“We’ve proposed a new method that has two parts. The first one is to try to simplify the equations using a reduced model, called a moment model,” Jorge says. “Second, it’s using machine learning to reduce it even more.”

Jorge and his team have the moment model theory ready to be applied. For the machine learning step, they will use JAX, an open-source machine learning framework developed by the DeepMind team at Google that many physicists are starting to use in their research.

Jorge plans to investigate one intriguing phenomenon in collisionless plasmas: how the acceleration of super-thermal particles occurs versus thermodynamic heating. This will help scientists understand how charged particles in a plasma become energized, a phenomenon applicable to both laboratory and astrophysical plasmas. He will also apply this new approach to the problem of magnetic reconnection in collisionless plasmas, a problem he says is difficult to model due to the topology changes that occur in short time scales.

“We need new models to try to handle these complex scenarios without spending months and months on a single simulation,” Jorge says.

NSF grants require investigators to address the broader impacts of their research, defined as “the potential to benefit society and contribute to the achievement of specific, desired societal outcomes.” Jorge plans to work with the department’s Wonders of Physics outreach program to create realistic movies that simulate these astrophysical plasma environments. For example, he hopes to show, in detail, what is happening with magnetic reconnection in auroras or around the surface of the sun, with both using the new code developed through his research.

For this research, Jorge is collaborating with experimentalists at UW-Madison’s WiPPL facilities, and computational plasma physicists at UCLA, MIT, and Princeton.

Madison Symmetric Torus operates stable plasma at ten times the Greenwald Limit

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If net-positive fusion energy is to ever be achieved, density is key: the more atomic nuclei crashing into each other the more efficient the reaction will be. Nearly 40 years ago, Martin Greenwald identified a density limit above which tokamak plasmas become unstable, and the so-called Greenwald limit has at best been exceeded by a factor of two in the ensuing decades. 

In a new study published July 29 in Physical Review Letters, physicists at the University of Wisconsin–Madison produced a tokamak plasma that is stable at 10 times the Greenwald limit. The findings may have implications for tokamak fusion reactors, though the researchers caution that their plasma is not directly comparable to that in a fusion reactor. 

MST is shown, it is a donut-shaped metal device that is tens of feet in diameter and has hundreds of wires coming in and out of it
The Madison Symmetric Torus (MST). credit: Noah Hurst

“Tokamak devices are considered a leading contender in the race to build a nuclear fusion reactor that generates power in the same way as the sun,” says Noah Hurst, a scientist with the Wisconsin Plasma Physics Laboratory (WiPPL) and lead author on the study. “Our discovery of this unusual ability to operate far above the Greenwald limit is important for boosting fusion power production and preventing machine damage.”  

Tokamaks are toroidal devices, basically hollow metal donuts that churn ionized plasma through the tube by applying both a magnetic field and an electrical current. This shape has been shown to be particularly adept at confining the plasma, which is required to reach the high temperature and density needed for fusion. But the design can also lead to instabilities in the plasma: as its density increases, the plasma becomes more turbulent, causing the plasma to give up all its energy to the wall and cool off. 

The device that the WiPPL team used in this new study is the Madison Symmetric Torus, or MST. For many years, MST has operated as one of the leading programs studying the reversed field pinch, a toroidal configuration closely related to the tokamak. MST was designed to anticipate operation as a tokamak, allowing direct comparison of the two toroidal configurations in the same device. Unlike other tokamaks, the metal donut that houses the MST plasmas is thick and highly conducting, allowing for more stable operation..  

In 2018, MST scientists received National Science Foundation funding to build power supplies that are programmable, facilitating easier access to a range of toroidal plasma configurations, from tokamak to reversed field pinch. Hurst was hired in 2019 to study MST plasmas in tokamak mode with the new power supply.  

“My job was to try to find ways to make the plasma go unstable,” Hurst says. “I tried, and I found that, well, in many cases, it doesn’t. It was surprising.”

a graph with time [ms] on the x axis and electron density) aka plasma density on the y axis. Several data lines, given in a rainbow of colors, all go up within the first few ms, hold steady for up to 40ms, and then drop down to 0. A dotted line, representing the Greenwald limit, is shown around 0.75 on the y axis; all but one of the data lines goes well above that dotted line, up to 10x the value of the Greenwald limit
WiPPL scientists were able to experimentally create a stable plasma 10x greater than the Greenwald limit (the dashed horizontal line).
Hurst and colleagues looked into plasma density, trying to destabilize the plasma by puffing in more and more gas. They set the power supply to provide whatever voltage was needed to maintain a steady 50000 amps of current in each plasma (as plasma density increases, it becomes more resistive, and more voltage is needed to keep the current steady). They measured the achieved plasma density with interferometers viewing the plasma along 11 different lines of sight. 

The Greenwald limit is just the ratio of the plasma density to the product of the plasma current and plasma size, a simple metric that allows comparison of different devices and operating conditions. Since the limit was defined, only a handful of devices have operated above it, and by at most a factor of two. 

“Here, we were at a factor of ten,” Hurst says. “Future reactor-scale tokamaks will likely need to operate near or above the Greenwald limit, so if we can better understand what’s causing the density limit and understand the physics of how we got to ten times the limit, then maybe we have a shot at doing something about it.” 

Though the researchers feel confident in their results, they are unexpected. The team is actively exploring explanations.  

“The first thing we would ask is, what’s different about our machine relative to other machines?” Hurst says. “MST is very different because it was designed with a thicker wall than most tokamaks. Also, most tokamaks produce lower-resistance plasmas, so they don’t need these large voltages like we did in order to run.”

profile photo of Noah Hurst
Noah Hurst

Hurst also emphasizes that these results are unlikely to be directly applicable to fusion reactors, such as ITER and others that are being built in the hopes of being the first net-positive energy production tokamaks. But he and the team are cautiously optimistic.

“Our results were obtained in a low magnetic field, low temperature plasma, which is not capable of fusion power production. Still, we were the first ones to be able to do this, and you have to start somewhere,” Hurst says. “We’re going to keep studying these plasmas, and we think that what we learn might help higher-performance fusion devices to operate at the higher densities they need to be successful.”  

This study was supported by the U.S. Department of Energy (DE-SC0020245); by the Wisconsin Plasma Physics Laboratory, a research facility supported by the U.S. DOE Office of Fusion Energy Sciences under contract DE-SC0018266; and by a National Science Foundation Major Research Instrumentation grant (PHY 1828159).  

First plasma marks major milestone in UW–Madison fusion energy research

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a cyan blue cloud of light illuminates the majority of the shot

A fusion device at the University of Wisconsin–Madison generated plasma for the first time Monday, opening a door to making the highly anticipated, carbon-free energy source a reality.

Over the past four years, a team of UW–Madison physicists and engineers has been constructing and testing the fusion energy device, known as WHAM (Wisconsin HTS Axisymmetric Mirror) in UW’s Physical Sciences Lab in Stoughton. It transitioned to operations mode this week, marking a major milestone for the yearslong research project that’s received support from the U.S. Department of Energy.

“The outlook for decarbonizing our energy sector is just much higher with fusion than anything else,” says Cary Forest, a UW–Madison physics professor who has helped lead the development of WHAM. “First plasma is a crucial first step for us in that direction.”

WHAM started in 2020 as a partnership between UW–Madison, MIT and the company Commonwealth Fusion Systems. Now, WHAM will operate as a public-private partnership between UW–Madison and spinoff company Realta Fusion Inc., positioning it as major force for fusion research advances at the university.

Read the full story

 

Elliot Claveau, honorary fellow in the Department of Physics and experimental scientist at Realta Fusion, raises his hands in celebration of achieving a plasma from the control room at the Wisconsin HTS Axisymmetric Mirror Project (WHAM) experiment being conducted at the Wisconsin Plasma Physics Laboratory in Stoughton, Wisconsin on July 16, 2024. Part of a public-private partnership between UW–Madison and Realta Fusion Inc, the WHAM achieved the milestone of creating plasma as part of fusion energy research. (Photo by Bryce Richter / UW–Madison)

 

The Wisconsin HTS Axisymmetric Mirror Project (WHAM) experiment being conducted at the Wisconsin Plasma Physics Laboratory in Stoughton, Wisconsin is pictured on July 16, 2024. Part of a public-private partnership between UW–Madison and Realta Fusion Inc, the WHAM achieved the milestone of creating plasma as part of fusion energy research. (Photo by Bryce Richter / UW–Madison)

 

an animated GIF showing fusion at the particle/atomic level, moving from lithium + neutron = tritium + helium waste. Then, tritium + deuterium = neutron + helium waste + lots of energy
The fusion reaction at the atomic level. | Credit: Sarah Perdue, UW–Madison Physics

Cristian Vega awarded Callen Award for Excellence in Theoretical Plasma Physics Research

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Cristian Vega

Congrats to (now) Dr. Cristian Vega who won the Callen Award for Excellence in Theoretical Plasma Physics Research! Vega won the award on April 29, just days before defending his thesis on May 3.

The Callen Award is awarded annually to a UW–Madison plasma physics graduate student for achievements in plasma theory. Now-retired Professor Emeritus Jim Callen was a long-time faculty member in the Nuclear Engineering and Engineering Physics department. Callen was also an affiliate faculty member of the Physics department.

Welcome, Professor Vladimir Zhdankin!

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Vladimir Zhdankin (credit: Flatiron Institute)

Theoretical plasma astrophysicist Vladimir Zhdankin ‘11, PhD ’15, returns to UW–Madison as an assistant professor of physics on January 1, 2024. As a student, Zhdankin worked with Prof. Stas Boldyrev on solar wind turbulence and basic magnetohydrodynamic turbulence, which are relevant for near-Earth types of space plasmas. After graduating, Zhdankin began studying plasma astrophysics of more extreme environments. He first completed a postdoc at CU-Boulder, then a NASA Einstein Fellowship at Princeton University. He joins the department from the Flatiron Institute in New York, where he is currently a Flatiron Research Fellow.

Please give an overview of your research. 

These days, most of my interest is in the field of plasma astrophysics — the application of plasma physics to astrophysical problems. Much of the matter in the universe is in a plasma state, such as stars, the matter around black holes, and the interstellar medium in the galaxy. I’m interested in understanding the plasma processes in those types of systems. My focus is particularly on really high energy systems, like plasmas around black holes or neutron stars, which are dense objects where you could get extreme plasmas where relativistic effects are important. The particles are traveling at very close to the speed of light, and there’s natural particle acceleration occurring in these systems. They also radiate intensely, you could see them from halfway across the universe. There’s a need to know the basic plasma physics in these conditions if you want to interpret observations of those systems. A lot of my work involves doing plasma simulations of turbulence in these extreme parameter regimes.

What are one or two research projects you’ll focus on the most first?

One of them is on making reduced models of plasmas by using non-equilibrium statistical mechanical ideas. Statistical mechanics is one of the core subjects of physics, but it doesn’t really seem to apply to plasmas very often. This is because a lot of plasmas are in this regime that’s called collisionless plasma, where they are knocked out of thermal equilibrium, and then they always exist in a non-thermal state. That’s not what standard statistical mechanics is applicable to. This is one of the problems that I’m studying, whether there is some theoretical framework to study these non-equilibrium plasmas, to understand basic things like: what does it mean for entropy to be produced in these types of plasmas? The important application of this work is to explain how are particles accelerated to really high energies in plasmas. The particle acceleration process is important for explaining cosmic rays which are bombarding the Earth, and then also explaining the highest energy radiation which we see from those systems.

Another thing I’m thinking about these days is plasmas near black holes. In the center of the Milky Way, for example, there’s a supermassive black hole called Sagittarius A*, which was recently imaged a year or two ago by the Event Horizon Telescope. It’s a very famous picture. What you see is the shape of the black hole and then all the plasma in the vicinity, which is in the accretion disk. I’m trying to understand the properties of that turbulent plasma and how to model the type of radiation coming out of the system. And then also whether we should expect neutrinos to be coming out, because you would need to get very high energy protons in order to produce neutrinos. And it’s still an open question of whether or not that happens in these systems.

What attracted you to UW–Madison?

It’s just a perfect match in many ways. It really feels like a place where I’m confident that I could succeed and accomplish my goals, be an effective mentor, and build a successful group. It has all the resources I need, it has the community I need as a plasma physicist to interact with. I think it has a lot to offer to me and likewise, I have a lot to offer to the department there. I’m also really looking forward to the farmers’ market and cheese and things like that. You know, just the culture there.

What is your favorite element and/or elementary particle?

I like the muon. It is just a heavy version of the electron, I don’t remember, something like 100 times more massive or so. It’s funny that such particles exist and this is like the simplest example of one of those fundamental particles which we aren’t really familiar with, it’s just…out there. You could imagine situations where you just replace electron with a muon and then you get slightly different physics out of it.

What hobbies and interests do you have?

They change all the time. But some things I’ve always done: I like running, skiing, bouldering indoors, disk golf, racquet sports, and hiking. (Cross country or downhill skiing?) It’s honestly hard to choose which one I prefer more. In Wisconsin, definitely cross country. If I’m in real mountains, the Alps or the Rockies, then downhill is just an amazing experience.