Plasmoid behavior of pure hydrogen and hydrogen mixed with 5 % neon. In this experiment, a new Thomson Scattering (TS) diagnostic system operating at (an unprecedented rate of) 20 kHz was used to (i) measure the density of the plasmoid at the moment it passed through the observation region, and (ii) identify its position, which verified the theoretical predictions. Credit: National Institute for Fusion Science
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.
More information:
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
Citation:
Cooling 100 million degree plasma with a hydrogen-neon mixture ice pellet (2023, January 6)
retrieved 7 January 2023
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NAIROBI, Kenya (AP) — The body of Ugandan Olympic athlete Rebecca Cheptegei — who died after being set on fire by her partner in Kenya — was received Friday by family and anti-femicide crusaders, ahead of her burial a day later.
Cheptegei’s family met with dozens of activists Friday who had marched to the Moi Teaching and Referral Hospital’s morgue in the western city of Eldoret while chanting anti-femicide slogans.
She is the fourth female athlete to have been killed by her partner in Kenya in yet another case of gender-based violence in recent years.
Viola Cheptoo, the founder of Tirop Angels – an organization that was formed in honor of athlete Agnes Tirop, who was stabbed to death in 2021, said stakeholders need to ensure this is the last death of an athlete due to gender-based violence.
“We are here to say that enough is enough, we are tired of burying our sisters due to GBV,” she said.
It was a somber mood at the morgue as athletes and family members viewed Cheptegei’s body which sustained 80% of burns after she was doused with gasoline by her partner Dickson Ndiema. Ndiema sustained 30% burns on his body and later succumbed.
Ndiema and Cheptegei were said to have quarreled over a piece of land that the athlete bought in Kenya, according to a report filed by the local chief.
Cheptegei competed in the women’s marathon at the Paris Olympics less than a month before the attack. She finished in 44th place.
Cheptegei’s father, Joseph, said that the body will make a brief stop at their home in the Endebess area before proceeding to Bukwo in eastern Uganda for a night vigil and burial on Saturday.
“We are in the final part of giving my daughter the last respect,” a visibly distraught Joseph said.
He told reporters last week that Ndiema was stalking and threatening Cheptegei and the family had informed police.
Four in 10 women or an estimated 41% of dating or married Kenyan women have experienced physical or sexual violence perpetrated by their current or most recent partner, according to the Kenya Demographic and Health Survey 2022.
TEL AVIV, Israel (AP) — A rare Bronze-Era jar accidentally smashed by a 4-year-old visiting a museum was back on display Wednesday after restoration experts were able to carefully piece the artifact back together.
Last month, a family from northern Israel was visiting the museum when their youngest son tipped over the jar, which smashed into pieces.
Alex Geller, the boy’s father, said his son — the youngest of three — is exceptionally curious, and that the moment he heard the crash, “please let that not be my child” was the first thought that raced through his head.
The jar has been on display at the Hecht Museum in Haifa for 35 years. It was one of the only containers of its size and from that period still complete when it was discovered.
The Bronze Age jar is one of many artifacts exhibited out in the open, part of the Hecht Museum’s vision of letting visitors explore history without glass barriers, said Inbal Rivlin, the director of the museum, which is associated with Haifa University in northern Israel.
It was likely used to hold wine or oil, and dates back to between 2200 and 1500 B.C.
Rivlin and the museum decided to turn the moment, which captured international attention, into a teaching moment, inviting the Geller family back for a special visit and hands-on activity to illustrate the restoration process.
Rivlin added that the incident provided a welcome distraction from the ongoing war in Gaza. “Well, he’s just a kid. So I think that somehow it touches the heart of the people in Israel and around the world,“ said Rivlin.
Roee Shafir, a restoration expert at the museum, said the repairs would be fairly simple, as the pieces were from a single, complete jar. Archaeologists often face the more daunting task of sifting through piles of shards from multiple objects and trying to piece them together.
Experts used 3D technology, hi-resolution videos, and special glue to painstakingly reconstruct the large jar.
Less than two weeks after it broke, the jar went back on display at the museum. The gluing process left small hairline cracks, and a few pieces are missing, but the jar’s impressive size remains.
The only noticeable difference in the exhibit was a new sign reading “please don’t touch.”
VICTORIA – The British Columbia government is partnering with a bear welfare group to reduce the number of bears being euthanized in the province.
Nicholas Scapillati, executive director of Grizzly Bear Foundation, said Monday that it comes after months-long discussions with the province on how to protect bears, with the goal to give the animals a “better and second chance at life in the wild.”
Scapillati said what’s exciting about the project is that the government is open to working with outside experts and the public.
“So, they’ll be working through Indigenous knowledge and scientific understanding, bringing in the latest techniques and training expertise from leading experts,” he said in an interview.
B.C. government data show conservation officers destroyed 603 black bears and 23 grizzly bears in 2023, while 154 black bears were killed by officers in the first six months of this year.
Scapillati said the group will publish a report with recommendations by next spring, while an independent oversight committee will be set up to review all bear encounters with conservation officers to provide advice to the government.
Environment Minister George Heyman said in a statement that they are looking for new ways to ensure conservation officers “have the trust of the communities they serve,” and the panel will make recommendations to enhance officer training and improve policies.
Lesley Fox, with the wildlife protection group The Fur-Bearers, said they’ve been calling for such a committee for decades.
“This move demonstrates the government is listening,” said Fox. “I suspect, because of the impending election, their listening skills are potentially a little sharper than they normally are.”
Fox said the partnership came from “a place of long frustration” as provincial conservation officers kill more than 500 black bears every year on average, and the public is “no longer tolerating this kind of approach.”
“I think that the conservation officer service and the B.C. government are aware they need to change, and certainly the public has been asking for it,” said Fox.
Fox said there’s a lot of optimism about the new partnership, but, as with any government, there will likely be a lot of red tape to get through.
“I think speed is going to be important, whether or not the committee has the ability to make change and make change relatively quickly without having to study an issue to death, ” said Fox.
This report by The Canadian Press was first published Sept. 9, 2024.
