At ITER—the world’s largest experimental fusion reactor, currently under construction in France through international cooperation—the abrupt termination of magnetic confinement of a high temperature plasma through a so-called “disruption” poses a major open issue. As a countermeasure, disruption mitigation techniques, which allow to forcibly cool the plasma when signs of plasma instabilities are detected, are a subject of intensive research worldwide.
Now, a team of Japanese researchers from National Institutes for Quantum Science and Technology (QST) and National Institute for Fusion Science (NIFS) of National Institute of National Sciences (NINS) found that by adding approximately 5% neon to a hydrogen ice pellet, it is possible to cool the plasma more deeply below its surface and hence more effectively than when pure hydrogen ice pellets are injected.
Using theoretical models and experimental measurements with advanced diagnostics at Large Helical Device owned by NIFS, the researchers clarified the dynamics of the dense plasmoid that forms around the ice pellet and identified the physical mechanisms responsible for the successful enhancement of the performance of the forced cooling system, which is indispensable for carrying out the experiments at ITER. These results will contribute to the establishment of plasma control technologies for future fusion reactors. The team’s report was made available online in Physical Review Letters.
The construction of the world’s largest experimental fusion reactor, ITER, is underway in France through international cooperation. At ITER, experiments will be conducted to generate 500 MW fusion energy by maintaining the “burning state” of the hydrogen isotope plasma at more than 100 million degrees. One of the major obstacles to the success of those experiments is a phenomenon called “disruption” during which the magnetic field configuration used to confine the plasma collapses due to magnetohydrodynamic instabilities.
Disruption causes the high-temperature plasma to flow into the inner surface of the containing vessel, resulting in structural damage that, in turn, may cause delays in the experimental schedule and higher cost. Although the machine and the operating conditions of ITER have been carefully designed to avoid disruption, uncertainties remain and for a number of experiments so that a dedicated machine protection strategy is required as a safeguard.
A promising solution to this problem is a technique called “disruption mitigation,” which forcibly cools the plasma at the stage where first signs of instabilities that may cause a disruption are detected, thereby preventing damage to plasma-facing material components. As a baseline strategy, researchers are developing a method using ice pellets of hydrogen frozen at temperatures below 10 Kelvin and injecting it into a high-temperature plasma.
The injected ice melts from the surface and evaporates and ionizes owing to heating by the ambient high-temperature plasma, forming a layer of low-temperature, high-density plasma (hereafter referred to as a “plasmoid”) around the ice. Such a low-temperature, high-density plasmoid mixes with the main plasma, whose temperature is reduced in the process. However, in recent experiments, it has become clear that when pure hydrogen ice is used, the plasmoid is ejected before it can mix with the target plasma, making it ineffective for cooling the high-temperature plasma deeper below the surface.
This ejection was attributed to the high pressure of the plasmoid. Qualitatively, a plasma confined in a donut-shaped magnetic field tends to expand outward in proportion to the pressure. Plasmoids, which are formed by the melting and the ionization of hydrogen ice, are cold but very dense. Because temperature equilibration is much faster than density equilibration, the plasmoid pressure rises above that of the hot target plasma. The consequence is that the plasmoid becomes polarized and experiences drift motion across the magnetic field, so that it propagates outward before being able to fully mix with the hot target plasma.
A solution to this problem was proposed from theoretical analysis: model calculations predicted that by mixing a small amount of neon into hydrogen, the pressure of the plasmoid could be reduced. Neon freezes at a temperature of approximately 20 Kelvin and produces strong line radiation in the plasmoid. Therefore, if the neon is mixed with hydrogen ice before injection, part of the heating energy can be emitted as photon energy.
To demonstrate such a beneficial effect of using a hydrogen-neon mixture, a series of experiments was conducted in the Large Helical Device (LHD) located in Toki, Japan. For many years, the LHD has operated a device called the “solid hydrogen pellet injector” with high reliability, which injects ice pellets with a diameter of approximately 3 mm at the speed of 1100 m/s. Due to the system’s high reliability, it is possible to inject hydrogen ice into the plasma with a temporal precision of 1 ms, which allows measurement of the plasma temperature and density just after the injected ice melts.
Recently, the world’s highest time resolution for Thomson Scattering (TS) of 20 kHz was achieved in the LHD system using new laser technology. Using this system, the research team has captured the evolution of plasmoids. They found that, as predicted by theoretical calculations, plasmoid ejection was suppressed when hydrogen ice was doped with approximately 5 % neon, in stark contrast to the case where pure hydrogen ice was injected. In addition, the experiments confirmed that the neon plays a useful role in the effective cooling of the plasma.
The results of this study show for the first time that the injection of hydrogen ice pellets doped with a small amount of neon into a high-temperature plasma is useful to effectively cool the deep core region of the plasma by suppressing plasmoid ejection. This effect of neon doping is not only interesting as a new experimental phenomenon, but also supports the development of the baseline strategy of disruption mitigation in ITER. The design review of the ITER disruption mitigation system is scheduled for 2023, and the present results will help improve the performance of the system.
A. Matsuyama et al, Enhanced Material Assimilation in a Toroidal Plasma Using Mixed H2+Ne Pellet Injection and Implications to ITER, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.255001
National Institutes of Natural Sciences
Cooling 100 million degree plasma with a hydrogen-neon mixture ice pellet (2023, January 6)
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Rare ‘big fuzzy green ball’ comet visible in B.C. skies, a 50000-year sight
In the night sky, a comet is flying by Earth for the first time in 50,000 years.
Steve Coleopy, of the South Cariboo Astronomy Club, is offering some tips on how to see it before it disappears.
The green-coloured comet, named C/2022 E3 (ZTF), is not readily visible to the naked eye, although someone with good eyesight in really dark skies might be able to see it, he said. The only problem is it’s getting less visible by the day.
“Right now the comet is the closest to earth and is travelling rapidly away,” Coleopy said, noting it is easily seen through binoculars and small telescopes. “I have not been very successful in taking a picture of it yet, because it’s so faint, but will keep trying, weather permitting.”
At the moment, the comet is located between the bowl of the Big Dipper and the North Star but will be moving toward the Planet Mars – a steady orange-coloured point of light- in the night sky over the next couple of weeks, according to Coleopy.
“I have found it best to view the comet after 3:30 in the morning, after the moon sets,” he said. “It is still visible in binoculars even with the moon still up, but the view is more washed out because of the moonlight.”
He noted the comet looks like a “big fuzzy green ball,” as opposed to the bright pinpoint light of the stars.
“There’s not much of a tail, but if you can look through the binoculars for a short period of time, enough for your eyes to acclimatize to the image, it’s quite spectacular.”
To know its more precise location on a particular evening, an internet search will produce drawings and pictures of the comet with dates of where and when the comet will be in each daily location.
Coleopy notes the comet will only be visible for a few more weeks, and then it won’t return for about 50,000 years.
Extreme species deficit of nitrogen-converting microbes in European lakes
Sampling of Lake Constance water from 85 m depth, in which ammonia-oxidizing archaea make up as much as 40% of all microorganisms
An international team of researchers led by microbiologists from the Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures GmbH in Braunschweig, Germany, shows that in the depths of European lakes, the detoxification of ammonium is ensured by an extremely low biodiversity of archaea. The researchers recently published their findings in the prestigious international journal Science Advances. The team led by environmental microbiologists from the Leibniz Institute DSMZ has now shown that the species diversity of these archaea in lakes around the world ranges from 1 to 15 species. This is of particularly concern in the context of global biodiversity loss and the UN Biodiversity Conference held in Montreal, Canada, in December 2022. Lakes play an important role in providing freshwater for drinking, inland fisheries, and recreation. These ecosystem services would be at danger from ammonium enrichment. Ammonium is an essential component of agricultural fertilizers and contributes to its remarkable increase in environmental concentrations and the overall im-balance of the global nitrogen cycle. Nutrient-poor lakes with large water masses (such as Lake Constance and many other pre-alpine lakes) harbor enormously large populations of archaea, a unique class of microorganisms. In sediments and other low-oxygen environments, these archaea convert ammonium to nitrate, which is then converted to inert dinitrogen gas, an essential component of the air. In this way, they contribute to the detoxification of ammonium in the aquatic environment. In fact, the species predominant in European lakes is even clonal and shows low genetic microdiversity between different lakes. This low species diversity contrasts with marine ecosystems where this group of microorganisms predominates with much greater species richness, making the stability of ecosystem function provided by these nitrogen-converting archaea potentially vulnerable to environmental change.
Maintenance of drinking water quality
Although there is a lot of water on our planet, only 2.5% of it is fresh water. Since much of this fresh water is stored in glaciers and polar ice caps, only about 80% of it is even accessible to us humans. About 36% of drinking water in the European Union is obtained from surface waters. It is therefore crucial to understand how environmental processes such as microbial nitrification maintain this ecosystem service. The rate-determining phase of nitrification is the oxidation of ammonia, which prevents the accumulation of ammonium and converts it to nitrate via nitrite. In this way, ammonium is prevented from contaminating water sources and is necessary for its final conversion to the harmless dinitrogen gas. In this study, deep lakes on five different continents were investigated to assess the richness and evolutionary history of ammonia-oxidizing archaea. Organisms from marine habitats have traditionally colonized freshwater ecosystems. However, these archaea have had to make significant changes in their cell composition, possible only a few times during evolution, when they moved from marine habitats to freshwaters with much lower salt concentrations. The researchers identified this selection pressure as the major barrier to greater diversity of ammonia-oxidizing archaea colonizing freshwaters. The researchers were also able to determine when the few freshwater archaea first appeared. Ac-cording to the study, the dominant archaeal species in European lakes emerged only about 13 million years ago, which is quite consistent with the evolutionary history of the European lakes studied.
Slowed evolution of freshwater archaea
The major freshwater species in Europe changed relatively little over the 13 million years and spread almost clonally across Europe and Asia, which puzzled the researchers. Currently, there are not many examples of such an evolutionary break over such long time periods and over large intercontinental ranges. The authors suggest that the main factor slowing the rapid growth rates and associated evolutionary changes is the low temperatures (4 °C) at the bottom of the lakes studied. As a result, these archaea are restricted to a state of low genetic diversity. It is unclear how the extremely species-poor and evolutionarily static freshwater archaea will respond to changes induced by global climate warming and eutrophication of nearby agricultur-al lands, as the effects of climate change are more pronounced in freshwater than in marine habitats, which is associated with a loss of biodiversity.
Publication: Ngugi DK, Salcher MM, Andre A-S, Ghai R., Klotz F, Chiriac M-C, Ionescu D, Büsing P, Grossart H-S, Xing P, Priscu JC, Alymkulov S, Pester M. 2022. Postglacial adaptations enabled coloniza-tion and quasi-clonal dispersal of ammonia oxidizing archaea in modern European large lakes. Science Advances: https://www.science.org/doi/10.1126/sciadv.adc9392
PhDr. Sven-David Müller, Head of Public Relations, Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures GmbH
Phone: ++49 (0)531/2616-300
About the Leibniz Institute DSMZ
The Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures is the world’s most diverse collection of biological resources (bacteria, archaea, protists, yeasts, fungi, bacteriophages, plant viruses, genomic bacterial DNA as well as human and animal cell lines). Microorganisms and cell cultures are collected, investigated and archived at the DSMZ. As an institution of the Leibniz Association, the DSMZ with its extensive scientific services and biological resources has been a global partner for research, science and industry since 1969. The DSMZ was the first registered collection in Europe (Regulation (EU) No. 511/2014) and is certified according to the quality standard ISO 9001:2015. As a patent depository, it offers the only possibility in Germany to deposit biological material in accordance with the requirements of the Budapest Treaty. In addition to scientific services, research is the second pillar of the DSMZ. The institute, located on the Science Campus Braunschweig-Süd, accommodates more than 82,000 cultures and biomaterials and has around 200 employees. www.dsmz.de
Scientists are closing in on why the universe exists
Particle astrophysicist Benjamin Tam hopes his work will help us understand a question. A very big one.
“The big question that we are trying to answer with this research is how the universe was formed,” said Tam, who is finishing his PhD at Queen’s University.
“What is the origin of the universe?”
And to answer that question, he and dozens of fellow scientists and engineers are conducting a multi-million dollar experiment two kilometres below the surface of the Canadian Shield in a repurposed mine near Sudbury, Ontario.
The Sudbury Neutrino Observatory (SNOLAB) is already famous for an earlier experiment that revealed how neutrinos ‘oscillate’ between different versions of themselves as they travel here from the sun.
This finding proved a vital point: the mass of a neutrino cannot be zero. The experiment’s lead scientist, Arthur McDonald, shared the Nobel Prize in 2015 for this discovery.
The neutrino is commonly known as the ‘ghost particle.’ Trillions upon trillions of them emanate from the sun every second. To humans, they are imperceptible except through highly specialized detection technology that alerts us to their presence.
Neutrinos were first hypothesized in the early 20th century to explain why certain important physics equations consistently produced what looked like the wrong answers. In 1956, they were proven to exist.
Tam and his fellow researchers are now homing in on the biggest remaining mystery about these tiny particles.
Nobody knows what happens when two neutrinos collide. If it can be shown that they sometimes zap each other out of existence, scientists could conclude that a neutrino acts as its own ‘antiparticle’.
Such a conclusion would explain how an imbalance arose between matter and anti-matter, thus clarifying the current existence of all the matter in the universe.
It would also offer some relief to those hoping to describe the physical world using a model that does not imply none of us should be here.
Guests in this episode (in order of appearance):
Benjamin Tam is a PhD student in Particle Astrophysics at Queen’s University.
Eve Vavagiakis is a National Science Foundation Astronomy and Astrophysics Postdoctoral Fellow in the Physics Department at Cornell University. She’s the author of a children’s book, I’m A Neutrino: Tiny Particles in a Big Universe.
Erica Caden is a research scientist at SNOLAB. Among her duties she is the detector manager for SNO+, responsible for keeping things running day to day.
*This episode was produced by Nicola Luksic and Tom Howell. It is part of an on-going series, IDEAS from the Trenches, some stories are below.
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