Icy hot plasmas
Fluffy, electrically charged ice grains reveal new plasma dynamics
California Institute of Technology
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A team of researchers in the Bellan Plasma Lab at Caltech created a plasma in which electrons and positively charged ions exist between ultracold electrodes within a mostly neutral gas environment, injected water vapor, and then watched as tiny ice grains spontaneously formed. They studied the behavior of the grains using a camera with a long-distance microscope lens. The team was surprised to find that extremely "fluffy" grains developed under these conditions and grew into fractal shapes—branching, irregular structures that are self-similar at various scales.
view moreCredit: Bellan Plasma Group/Caltech
When a gas is highly energized, its electrons get torn from the parent atoms, resulting in a plasma—the oft-forgotten fourth state of matter (along with solid, liquid, and gas). When we think of plasmas, we normally think of extremely hot phenomena such as the Sun, lightning, or maybe arc welding, but there are situations in which icy cold particles are associated with plasmas. Images of distant molecular clouds from the James Webb Space Telescope feature such hot–cold interactions, with frozen dust illuminated by pockets of shocked gas and newborn stars.
Now a team of Caltech researchers has managed to recreate such an icy plasma system in the lab. They created a plasma in which electrons and positively charged ions exist between ultracold electrodes within a mostly neutral gas environment, injected water vapor, and then watched as tiny ice grains spontaneously formed. They studied the behavior of the grains using a camera with a long-distance microscope lens. The team was surprised to find that extremely "fluffy" grains developed under these conditions and grew into fractal shapes—branching, irregular structures that are self-similar at various scales. And that structure leads to some unexpected physics.
The scientists describe their work in a paper in the journal Physical Review Letters. The lead author of the paper is Caltech graduate student André Nicolov (MS '22).
"It turns out that the grains' fluffiness has important consequences," says Paul Bellan, professor of applied physics at Caltech. Once such consequence is that the irregular grains, even as they grow, contain much less mass than, say, a solid spherical grain. And, indeed, when other scientists study "dusty plasma" systems they typically inject tiny solid spherical plastic grains into the plasma.
Nicolov and Bellan observed that their fluffy ice grains quickly became negatively charged because the electrons in the plasma move much faster than their positively charged ion counterparts. "They are so fluffy that their charge-to-mass ratio is very high, so the electrical forces are much more important than gravitational forces," Bellan explains. As a result, gravity—which dominates in other experiments, causing solid grains to settle to the bottom of test chambers—is no longer the primary driver of motion.
Instead, the fluffy ice grains dispersed throughout the plasma in the chamber and underwent what Nicolov describes as a "complicated motion that seems to defy gravity." The ice grains bobbed up and down, spun, and whirled in vortices throughout the plasma in ways that were difficult to predict. That remained true even of ice grains that grew to relatively large sizes, hundreds of times larger than the solid plastic spheres previously used. In fact, the researchers say, the fluffiness increases as the grains grow larger.
Nicolov specifies "the microscopic fluffy structure of the grains impacts the motion of the whole cloud of grains and the plasma." The grains are highly confined within the plasma by an inward-directed electric field, and because they are all negatively charged, they repel each other and tend to space out evenly and do not collide. Their fluffiness causes them to interact with the surrounding neutral gas like a feather in the wind.
Bellan says this behavior might help explain how similarly charged fluffy grains interact in astrophysical environments, such as the rings of Saturn and molecular clouds. He adds that because the grains have large surface areas and high charge-to-mass ratios, they may act as intermediaries capable of transferring momentum from electric fields to the neutral gas around them. "You could make a wind where the electric field pushes the dust grains, which then push the neutral gas," he says. The tiny fluffy grains, therefore, might even be responsible for gas and dust streaming across the galaxy.
The findings might also be useful in semiconductor manufacturing, where dust spontaneously formed inside industrial plasmas can deposit on tiny features of the electronic chips being fabricated and so render the chips useless. Understanding the fractal growth and motion of grains within plasma systems could improve strategies for controlling or removing them. "If you want to control the grains, you have to take into account this fractal nature," Nicolov says.
Along with Bellan and Nicolov, former Caltech postdoctoral scholar Seth Pree is also an author of the paper, "Dynamics of Fractal Ice Grains in Cryogenic Plasmas." The work was supported by the National Science Foundation (NSF) and the NSF/Department of Energy Partnership in Plasma Science and Engineering.
Journal
Physical Review Letters
Article Title
Dynamics of Fractal Ice Grains in Cryogenic Plasmas
Ice grains, illuminated by a green sheet of laser light, are suspended in the plasma discharge (purple). Insets show individual ice grains imaged with 20x magnification.
Credit
Bellan Plasma Group/Caltech
Direct observation reveals “two-in-one” roles of plasma turbulence
High resolution measurements show that turbulence acts both as a heat carrier and a mediator that rapidly spreads heat across a plasma
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(Top) The mediator-type turbulence resembles players calling out and rapidly passing the ball(heat) among teammates. It links distant regions of the plasma almost instantaneously and accelerates the spread of heat.
(Bottom) The heat-carrying turbulence acts like an American football player running forward while firmly holding the ball. It spreads more slowly and shapes the overall temperature profile of the plasma.
view moreCredit: National Institute for Fusion Science
Background
Producing fusion energy requires heating plasma to more than one hundred million degrees and confining it stably with strong magnetic fields. However, plasma naturally develops fluctuations known as turbulence, and they carry heat outward and weaken confinement. Understanding how heat and turbulence spread is therefore essential.
Conventional theory has assumed that heat and turbulence move gradually from the center toward the edge. Yet experiments have sometimes shown heat and turbulence spreading much faster, similar to American football players passing a ball quickly across long distances so that a local change influences the entire field almost at once. Clarifying the cause of this rapid, long-range response has been a long-standing challenge.
Results
A research team from the National Institute for Fusion Science carried out short duration heating of the plasma core in the Large Helical Device and used high-precision diagnostic instruments, based on electromagnetic waves of various wavelengths, to measure temperature, turbulence, and heat propagation with fine spatial and temporal resolution.
The measurements revealed a close relationship between heat spreading and the behavior of turbulence. Immediately after heating, a type of turbulence appeared that connected distant regions of the plasma in less than one ten thousandth of a second. This mediator-type turbulence resembles football players calling out to each other and passing the ball rapidly, allowing separated regions to respond together (Figure 1).
After this rapid response, another type of turbulence spread more slowly. This heat carrying turbulence behaves like a player holding the ball securely and running it forward, shaping the overall temperature profile of the plasma.
The experiments also showed that shorter heating pulses made the mediator turbulence stronger and caused heat to spread more quickly. These observations demonstrate that plasma turbulence plays two roles at the same time. One role is to carry heat outward, and the other is to connect distant regions so that heat can spread suddenly across the entire plasma.
Significance and Outlook
This study provides the first high resolution experimental identification of the mediator type turbulence that links distant parts of a plasma at the same time. It also presents the first direct demonstration that turbulence plays two distinct roles: one that carries heat outward and another that connects distant regions so that heat can spread rapidly across the plasma.
These findings explain how heat introduced at the plasma center can spread rapidly to the edge and form a scientific basis for predicting and controlling heat transport in future fusion reactors. Controlling the mediator turbulence may help create plasma conditions in which heat spreads more slowly, improving confinement.
The property that distant regions respond simultaneously is also seen in other natural systems, including ocean and atmospheric circulation and energy transfer inside materials. Therefore, the present results may be relevant to fields beyond fusion energy research.
Glossary
Turbulence
Fluctuations in plasma density or temperature that grow into waves, flows, or vortices. At high temperatures the structure becomes irregular and chaotic, causing heat and particles to be transported outward.
Large Helical Device (LHD)
One of the world’s largest superconducting helical plasma experimental devices, located at the National Institute for Fusion Science (NIFS) in Toki, Gifu, Japan.
Mediator
A medium within the plasma that links distant regions at the same time and speeds up the transfer of heat and energy. Its behavior resembles American football players rapidly passing the ball between teammates as they move down the field. When the heat is regarded as the ball, the mediator serves as the relay that connects the behavior of the plasma as a whole.
High precision diagnostics
Advanced measurement instruments that use electromagnetic waves of different wavelengths to observe turbulence, electron temperature, and heat propagation. They provide microsecond time resolution and millimeter spatial resolution.
Journal
Communications Physics
Method of Research
Experimental study
Article Title
Direct observation of coexisting local and nonlocal turbulence in a magnetically confined plasma
Article Publication Date
10-Dec-2025
