PPPL speakers and discoveries prominent at 2021 global plasma physics gathering

Nov. 24, 2021

Path-setting discoveries, special student programs, and mini-conferences on Equity, Diversity and Inclusion highlighted the 63rd American Physical Society-Division of Plasma Physics (APS-DPP) annual meeting in Pittsburgh, Pennsylvania. 

More than 120 scientists, students and engineers from the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) joined a record 2,200 worldwide participants at the hybrid virtual and in-person gathering held Nov. 8-through-12. 

PPPL staffers gave talks, joined panels and presented more than 75 posters covering the full range of fusion and plasma science research at the Laboratory. Among speakers was Elizabeth Paul, a Presidential Postdoctoral Research Fellow at Princeton University who won the highly competitive 2021 Marshall N. Rosenbluth Outstanding Doctoral Thesis Award for her University of Maryland dissertation on advancing the development of twisty magnetic stellarator fusion devices.

Gatekeeping in Admission and Hiring
Barbara Harrison, PPPL’s equity, diversity, and inclusion business partner, spoke on “Gatekeeping in Admission and Hiring” as a panelist in the first of three mini-conferences on diversity, equity, and inclusion.  Harrison touched on PPPL’s efforts in that key area and discussed the importance of creating an atmosphere in which diverse employees have a sense of safety and belonging. The panel discussion focused on ways for organizations to be more inclusive by removing barriers for students and potential employees. 

Arturo Dominguez and Shannon Swilley Greco, both Science Education senior program leaders at PPPL, gave virtual poster presentations, as did Deedee rtiz, science education program manager at the Laboratory.  Greco presented a poster introducing and recruiting members for the Plasma Network for Outreach and Workforce, a multi-institution project intended to engage students, volunteers, and educators in pre-college science, technology, engineering, and mathematics fields. 

Dominguez participated in several activities, including helping to organize the town hall on “Ways to Reduce Equity Gaps in DOE Supported Fusion Energy Sciences Research and Education.” He also authored or co-authored several posters, including one on the new Plasma and Fusion Undergraduate Research Opportunities (PFURO) program, which provides internships to diverse students around the country. Ortiz presented a poster that detailed PPPL’s continuation of science education programs during the COVID-19 pandemic titled, “A Closer Look:  Remote Internships During COVID-19 Times.”

Student posters

Virtual graduate student and recent undergraduate student posters from PPPL ranged from new types of twisty fusion stellarator devices to gravitational waves produced when massive objects like black holes collide and disturb the fabric of space. “I was really excited to learn about updates from fusion start-up companies,” said Tony Qian, a third-year graduate student who presented a poster on “Stellarator fields without stellarator coils: MUSE, a table-top PM stellarator.”

Amelia Chambliss, a two-time former Student Undergraduate Laboratory Internship (SULI) participant and current researcher at PPPL, presented a poster on methods to compute the sensitivities of magnetic islands to parameter perturbations in permanent magnet stellarators. “As someone who just finished an undergraduate program and went straight into a specific project, it’s so nice to be at a conference where so many people are presenting on different topics, though I’m bummed that I didn’t get to meet everyone in person,” she said.

PPPL physicists gave eight invited talks during the week-long event, three of which were also detailed as news releases in the conference’s virtual press room. Here are summaries of the releases:
Upgraded Code Reveals a Source of Damaging Fusion Disruptions
Work led by Min-Gu Yoo of PPPL has discovered a key process behind a major challenge called thermal quenches — the rapid heat loss in hot plasmas that can occur in doughnut-shaped tokamak fusion devices. Such quenches are sudden drops of electron heat in the plasma that fuels fusion reactions, drops that can create damaging disruptions inside the tokamak. Understanding the physics behind these quenches, caused by powerful perturbations in the magnetic fields that confine the plasma in tokamaks, could lead to methods to mitigate or prevent them.

Researchers traced a comprehensive mechanism for thermal quenches to turbulent particle transport. Using the PPPL Gyrokinetic Tokamak Simulation (GTS) code, the physicists explored how the hot plasma, which is composed of free electrons and atomic nuclei, or ions, generates the electric field and the turbulent particle transport at the outset of quenches. The code laid bare the physics behind the mechanism.

The findings showed that the self-generated field mixes up the plasma, causing high-energy electrons to escape from the core and fly toward the wall. This enhanced heat transport produces a rapid and continuous drop in electron temperature, leading to the thermal quench. 

From the simulation results and comparison to experimental observations, researchers found that this novel mechanism could be a major contributor to the abrupt quenches. These breakthrough discoveries could lead to new steps to battle damaging disruptions.

Harnessing Hot Helium Ash to Drive Rotation in Fusion Reactors
In controlled nuclear fusion, heavy isotopes of hydrogen fuse into helium, releasing a huge amount of energy in the process. A large portion of the energy released by a laboratory fusion reaction goes into hot helium ash, an impurity in the plasma that bears no resemblance to ash from a fire. This ash is around 30 billion degrees Celsius, far hotter than the 200 million degree Celsius for the bulk plasma.

Researchers can capture high energy from the ash to help power fusion reactions through a process called alpha channeling, which arises through the interaction of plasma waves and particles. But physicists have not previously known whether the extraction of helium ash by alpha channeling also extracts a net charge from the plasma. Such extraction drives plasma rotation that can stabilize instabilities in the plasma and suppress plasma turbulence.

But if the channeling does not extract net charge, fuel ions must be pulled into the hot plasma center, which is also an advantageous effect. The key question, for which researchers led by Princeton University graduate student Ian Ochs have developed a model to answer, is: which advantageous effect is more likely to occur?

 The new model shows that while a wave that grows over time creates conditions that cancel all the charge extraction from the ash, steady state waves that grow in space allow for extraction of charge and rotation drive. More complex waves exhibit a mix of these behaviors, in a readily predictable way. Thus, this model self-consistently establishes the conditions under which alpha channeling extracts charge and drives rotation, bringing us closer to finer-tuned plasma control.

The following release combines the findings of PPPL physicist Florian Effenberg and research scientist Theresa Wilks of the Massachusetts Institute of Technology (MIT).  

Integrating Hot Cores and Cool Edges in Fusion Reactors
Future fusion reactors have a conundrum: maintain a plasma core that is hotter than the surface of the sun without melting the walls that contain the plasma. Fusion scientists refer to this challenge as “core-edge integration.” Researchers working at the DIII-D National Fusion Facility at General Atomics have recently tackled this problem in two ways: the first aims to make the fusion core even hotter, while the second focuses on cooling the material that reaches the wall.

A research team from PPPL has experimented with the injection of a powder consisting of boron, boron nitride, and lithium. This use of powder rather than gas allows a larger range of potential impurities that can also be made purer and less likely to chemically react with the plasma. Measurements of the experiment showed only a marginal decrease in fusion performance during the heat production. 

At the same time, experiments led by MIT scientists on DIII-D ran in a special phase called Super H-mode, which raises the pressure at the edge of the plasma beyond what had been thought possible, combatted the decrease in fusion performance by making the fusion core even hotter. The researchers also developed ways to reduce the likelihood of the plasma becoming unstable.

The two sets of experiments developed a balanced approach that achieved significant edge cooling with only modest effects on core performance. Incorporating powder injection or the use of the Super H-mode into future reactor designs may allow them to maintain high levels of fusion performance while increasing the lifetime of divertor surfaces that exhaust waste heat, and could become compatible with future devices like ITER, the international tokamak under construction in France.

Five more PPPL invited talks that were not included in the press room are briefly summarized as follows. 

A cool solution to protecting hot fusion devices 
Researchers believe future fusion power plants will have a flowing liquid lithium wall facing the plasma to protect fusion devices from the superhot heat of the plasma and enhance plasma performance. However, the liquid metal can splash into the plasma and contaminate it. PPPL scientists led by senior engineering analyst Andrei Khodak have proposed a solution. Using computer simulations, the scientists have developed the concept of layering a porous, sponge-like wall with tiny holes on top of the fast-flowing liquid lithium. The porous material would stabilize the lithium surface and allow the liquid metal to serve as a cooling agent to absorb excess heat from the plasma.

A novel tool to maintain plasma heat in future fusion devices
Scientists are seeking to trap plasma, also known as the fourth state of matter, to replicate the process that powers the sun and stars. But sometimes the plasma undergoes disruptions, becoming unstable and losing valuable heat that could otherwise stoke the fusion reactions. Runaway electrons that can damage the walls of fusion facilities can accompany these disruptions. Now, PPPL physicist Chang Liu and Laboratory researchers have helped to develop a new computational tool to improve understanding of this disruptive heat loss and possibly prevent it in the future. The researchers combined two computer codes to model plasma more accurately than previously. This new tool could keep plasma hot and be used to support 
future studies of runaway electron mitigation strategies in ITER, the international experiment being built in France.

Decreasing turbulent wiggles to increase plasma confinement
When scientists seek to confine electrically charged gas known as plasma within a twisty machine known as a stellarator, they need to keep as much heat within the plasma as possible. But plasma, which makes up 99 percent of the visible universe, resists control and often slips its confining magnetic cage, quenching the fusion process. Now, PPPL physicist Federico Nespoli and colleagues have helped to confirm that adding tiny specks of boron, an element found in the common household cleaner Borax, can help tame the plasma. When the scientists dropped the boron into theLarge Helical Device (LHD) in Japan — a stellarator-like facility that the Japanese call a heliotron. The drops reduced plasma turbulence, the unwanted wiggling that can cause heat to escape, and appeared to prevent unwanted material from the facility’s walls from seeping into the plasma and lowering its temperature.

Untwisting twisty stellarator coils
Scientists around the world are exploring the use of twisty magnetic facilities known as stellarators to harness the fusion process that powers the sun and stars as a carbon-free source of energy to generate electricity. But stellarators require electromagnets with extremely complex shapes that are costly to build. Physicist Coaxiang Zhu and fellow PPPL researchers are leading the design of a new type of stellarator that uses permanent magnets far more powerful than the kind used on refrigerator doors. Such magnets could greatly simplify the design and construction of stellarators and could show the way to creating fusion power more easily and economically.

Found: A quick way to optimize heat in fusion devices
A model once thought to be nearly impossible to quickly and accurately design radio frequency (RF) waves that help heat the plasma that fuels fusion reactions has now been produced at PPPL. The model, developed by graduate student Nick Lopez, demonstrates a fast and accurate way to calculate the energy and path of RF waves that are distorted by roadblocks called “caustics,” that make the behavior of the waves highly complex. The technique applies a geometrical technique called a “metaplectic transform” — a kind of math trick that removes the caustic from an equation that describes the wave running into a roadblock and enables the model to produce quick simulations showing the energy and path of the injected waves. The result can be faster optimizations of RF-based schemes for heating fusion plasmas.

PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit energy.gov/science.