Discovering unsuspected hurdle for stellarator fusion facilities

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Physicist Roscoe White with figures from his paper. Credit: Elle Starkman/PPPL Office of Communications; Collage by Kiran Sidarsanan

Investigation of a possibly critical issue with twisty magnetic stellarators, promising candidates to serve as models for a U.S. fusion pilot plant, has clarified the potential impact of a largely overlooked concern.

The finding at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) demonstrates how periodic changes in the strength and shape of magnetic fields can, under certain theoretical conditions, facilitate the rapid loss of confinement of plasma particles that fuel .

High energy

“If you want to do you must have high energy,” said PPPL senior physicist Roscoe White, lead author of a Physics of Plasmas paper that editors have selected as a “scilight,” or science highlight.

His paper identifies a new type of energetic particle loss, said Felix Parra Diaz, head of the Theory Department at PPPL. “Studies have so far focused on controlling other types of energetic losses that are dominant, and we are now trying to reduce energetic particle losses even more,” Parra Diaz said. “The paper on which these findings are based identifies a mechanism that we need to include when designing the optimal shape of stellarator magnet fields.

“While this mechanism is included in our more detailed analyses of stellarator configurations among many other effects, it had not been singled out as a problem that needed to be addressed. We cannot use detailed analysis for stellarator optimization due its computational cost. This is why Roscoe’s paper is important: It identifies the problem and proposes an efficient way to evaluate and optimize the stellarator shape to avoid it. This gives us the opportunity to develop stellarator configurations that are even better than existing ones.”

The mechanisms creating this issue are what are called “resonances,” which describe the paths that particles follow as they orbit the magnetic fields that run around the machine. When particles are resonant they repeatedly return to the point they started from. Such returns allow instabilities, or modes, in the hot, charged plasma gas to create what are called islands in the path of orbits, allowing the particles and their energy to escape the confinement.

White used a high-speed software code to search for instabilities called “Alfven modes” that can create islands in doughnut-shaped tokamaks, which are more widely used experimental fusion facilities. “So I thought, ‘Okay,’ I’ll go look at stellarators too,” he said. And in stellarators, “something very different is happening,” he found.

Modes not needed

“Turns out that in a stellarator you don’t need modes,” White said. “In stellarators, when the number of periodic changes in the orbit of resonant high-energy particles matches the number of periodic changes in the , particle losses can occur,” he said. “It’s like pushing a child on a swing. When you want the child to swing higher and higher, every time the swing comes back to you, you push it again, and that’s a push in resonance,” he said.

For White, “The problem up until now is that people have been focusing on the form of the magnetic field. But high energy orbiting particles drift across the field, so you must also consider the particle orbits.”

Going forward, he said, “seeing whether particle resonances in stellarators match the magnetic field period has got to enter into design conditions for finding a good reactor.”

More information:
Roscoe White et al, Poor confinement in stellarators at high energy, Physics of Plasmas (2022). DOI: 10.1063/5.0094458

Discovering unsuspected hurdle for stellarator fusion facilities (2022, July 12)
retrieved 13 July 2022

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Hexbyte Glen Cove NASA's Mars helicopter's third flight goes farther, faster than before thumbnail

Hexbyte Glen Cove NASA’s Mars helicopter’s third flight goes farther, faster than before

Hexbyte Glen Cove

This NASA photo shows the Ingenuity Mars Helicopter(C) hovering during its third flight on April 25, 2021, as seen by the left Navigation Camera aboard NASA’s Perseverance Mars Rover

NASA’s mini helicopter Ingenuity on Sunday successfully completed its third flight on Mars, moving farther and faster than ever before, with a peak speed of 6.6 feet per second.

After two initial flights during which the craft hovered above the Red Planet’s surface, the helicopter on this third covered 64 feet (50 meters) of distance, reaching the speed of 6.6 feet per second (two meters per second), or four miles per hour in this latest flight.

“Today’s flight was what we planned for, and yet it was nothing short of amazing,” said Dave Lavery, the Ingenuity project’s program executive.

The Perseverance rover, which carried the four-pound (1.8 kilograms) rotorcraft to Mars, filmed the 80-second third flight. NASA said Sunday that would be sent to Earth in the coming days.

The lateral flight was a test for the helicopter’s autonomous navigation system, which completes the route according to information received beforehand.

“If Ingenuity flies too fast, the flight algorithm can’t track surface features,” NASA explained in a statement about the flight.

Ingenuity’s flights are challenging because of conditions vastly different from Earth’s—foremost among them a rarefied atmosphere that has less than one percent the density of our own.

This means that Ingenuity’s rotors, which span four feet, have to spin at 2,400 revolutions per minute to achieve lift—about five times more than a helicopter on Earth.

NASA announced it is now preparing for a fourth flight. Each flight is planned to be of increasing difficulty in order to push Ingenuity to its limits.

This black and white image was taken by NASA’s Ingenuity helicopter during its third flight on April 25, 2021. Credit:  NASA/JPL-Caltech

The Ingenuity experiment will end in one month in order to let Perseverance return to its main task: searching for signs of past microbial life on Mars.

© 2021 AFP

NASA’s Mars helicopter’s third flight goes farther, faster than before (2021, April 25)
retrieved 26 April 2021

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Hexbyte Glen Cove Evolution favors new diseases of 'intermediate' severity thumbnail

Hexbyte Glen Cove Evolution favors new diseases of ‘intermediate’ severity

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Credit: CC0 Public Domain

New epidemic diseases have an evolutionary advantage if they are of “intermediate” severity, research shows.

Scientists tested the theory that (disease-causing organisms) that inflict intermediate levels of harm on their host are the most evolutionarily successful.

The study, by the University of Exeter, Arizona State University and Auburn University, found that natural selection favors pathogens of intermediate (how much harm a pathogen causes) at the point the disease emerges in a new host species.

This occurs because virulence and transmission are linked, with virulence arising because pathogens need to exploit hosts to persist, replicate and transmit.

While too-low virulence will be detrimental for pathogens if they cannot transmit, virulence that is too high will also be a disadvantage if infection kills hosts so fast that the pathogen does not have time to transmit.

Over time, pathogens that show intermediate levels of virulence should therefore have an .

“For a long time, conventional wisdom held that new diseases evolved to become harmless,” said Dr. Camille Bonneaud, of the Centre for Ecology and Conservation on Exeter’s Penryn Campus in Cornwall.

“Although theoretical developments in evolutionary biology in the 1980s showed that this was not necessarily the case, such belief still holds firm, even today.

“Our study focussed on the ‘virulence-transmission trade-off’ hypothesis, which allows us to make predictions about pathogen evolution.

“Experimental evidence for this theory is rare, but we were able to test it by using more than 50 variants of the infectious bacterial pathogen Mycoplasma gallisepticum, which infects house finches.”

In the study, house finches from populations that had never encountered the disease were exposed to one of the different variants, simulating conditions at epidemic outbreak.

“We found that variants that were more virulent transmitted faster, but that variants of intermediate virulence were the most evolutionarily successful,” Dr. Bonneaud said.

“Our results therefore provide support for using the virulence-transmission trade-off hypothesis as a framework for understanding and predicting emerging pathogen evolution.”

Counter to commonly held beliefs, however, variants of the pathogen that replicated faster during infection and achieved higher densities did not transmit better or faster than those that achieved lower densities.

“This tells us that transmission is not always a numbers game and that we cannot use pathogen numbers as a proxy for their success.”

The paper, published in the journal Evolution Letters, is entitled: “Experimental evidence for stabilizing selection on virulence in a bacterial pathogen.”

More information:
Camille Bonneaud et al, Experimental evidence for stabilizing selection on virulence in a bacterial pathogen, Evolution Letters (2020). DOI: 10.1002/evl3.203

Evolution favors new diseases of ‘intermediate’ severity (2020, November 12)
retrieved 12 November 2020

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