Nav: Home

Clarifying the mechanism for suppressing turbulence through ion mass

April 24, 2017

Seeking to further improve plasma performance, from March 7, 2017, plasma experiments utilizing deuterium ions, which have twice the mass of hydrogen, were initiated in the Large Helical Device (LHD) at the National Institute for Fusion Science (NIFS). In numerous plasma experiments being conducted in countries around the world, the use of deuterium is improving the confinement of heat and particles. That is, the phenomenon called "ion mass effect," in which plasma performance is improved, is observed. However, we do not yet understand the detailed physical mechanism of how the increase in ion mass is linked to performance improvement. This has been one of the most important unsolved problems in plasma physics and fusion research from its beginning.

In the plasmas confined in the magnetic field there are various types of waves. In particular conditions those waves grow as time passes, and the so-called "instability" occurs and the plasma becomes turbulent. According to research to date, there has been found to occur a unique flow structure called "zonal flow" that is formed spontaneously in a turbulent plasma. Zonal flows take the stripe structure that flows in the opposite direction to each other, and these flows are known to perform an important role in the suppression of the turbulence. However, there remain many unclarified aspects regarding the conditions by which turbulence and zonal flows are formed. If influences brought about by differences in ion mass can be clarified theoretically, we can accurately predict confinement improvements that are observed in experiments. And because we can link confinement improvement to further enhancement of plasma performance, new developments in research are anticipated.

The research group of Professor Motoki Nakata, through collaborative research with Professor Tomohiko Watanabe of Nagoya University, conducted five-dimensional plasma turbulence simulations utilizing the "Plasma Simulator" at NIFS and the cutting-edge supercomputer "K" at the RIKEN Advanced Institute for Computational Science in order to analyze instabilities (trapped electron modes) caused by electrons that move back and forth along the magnetic field lines and to analyze in detail the turbulence generated from the instability. As a result, we clarified that the influence of the ion mass appeared remarkably in a high-density plasma and that the detailed physical mechanism in which turbulence is suppressed through an effect caused by electron-ion collisions. Further, we discovered that those phenomena exist in both helical and tokamak plasmas. Thus, we were able to clarify the "ion mass effect" broadly observed and one of the important mechanisms to improve plasma performance.

The detailed mechanism that suppresses turbulence is explained below. Turbulence caused due to trapped electron instability weakens the confinement of plasma heat and particles. The collisions among trapped electrons and ions suppress instabilities (suppressing the growth of waves). At a fixed temperature, collisions occur frequently at higher plasma densities. Here, the impacts of collisions in deuterium plasma are remarkable in comparison to hydrogen. As a result, turbulence can be suppressed (Figure 1). Further, we clarified that in the condition in which the instability has weakened, the "zonal flow" becomes stronger and further suppresses the turbulence by grinding large eddies and waves, and eventually improves the confinement of heat and particles (Figure 2).

As has been clarified above, a complete image of turbulence suppression in a plasma with large ion mass may be expressed schematically as in Figure 3. These research results provide fundamental knowledge regarding the complete clarification of the "ion mass effect" which was an unsolved issue for many years in plasma physics and fusion research. Further, the results are anticipated to be beneficial in improving plasma not only in helical devices such as LHD, but also in tokamaks as represented by the International Thermonuclear Experimental Reactor (ITER), which is currently under construction.
-end-
Vocabulary:

Ion Mass Effect


This is called the hydrogen isotope mass effect. This is a general term for physical influences upon stability and confinement brought about by ion mass.

Five-dimensional plasma turbulence simulation

Turbulence behavior in high-temperature plasma confined in the magnetic field is described mathematically through a dynamical equation in five-dimensional space (the three coordinates of space to which two components of particle velocity are added). The flows of water and air as expressed in three-dimensional equations differ significantly from five-dimensional plasma behaviors in complexity and diversity. Utilizing a supercomputer, we solve the five-dimensional equations at high speed to analyze plasma turbulence phenomena. At NIFS, in joint research with Nagoya University we are advancing in developing the "GKV" simulation code.

Zonal flow

Flow structure that is spontaneously formed in turbulence. The direction of flow reverses at a certain distance. The term "zonal flow" comes from the striped pattern in which flows continuously reverse. The reversed direction of zonal flow grinds eddies carrying heat and particles, and confinement is improved. Zonal flow is also formed in the striped patterns in Jupiter's atmosphere.

National Institutes of Natural Sciences

Related Magnetic Field Articles:

Origins of Earth's magnetic field remain a mystery
The existence of a magnetic field beyond 3.5 billion years ago is still up for debate.
New research provides evidence of strong early magnetic field around Earth
New research from the University of Rochester provides evidence that the magnetic field that first formed around Earth was even stronger than scientists previously believed.
Massive photons in an artificial magnetic field
An international research collaboration from Poland, the UK and Russia has created a two-dimensional system -- a thin optical cavity filled with liquid crystal -- in which they trapped photons.
Adhesive which debonds in magnetic field could reduce landfill waste
Researchers at the University of Sussex have developed a glue which can unstick when placed in a magnetic field, meaning products otherwise destined for landfill, could now be dismantled and recycled at the end of their life.
Earth's last magnetic field reversal took far longer than once thought
Every several hundred thousand years or so, Earth's magnetic field dramatically shifts and reverses its polarity.
A new rare metals alloy can change shape in the magnetic field
Scientists developed multifunctional metal alloys that emit and absorb heat at the same time and change their size and volume under the influence of a magnetic field.
Physicists studied the influence of magnetic field on thin film structures
A team of scientists from Immanuel Kant Baltic Federal University together with their colleagues from Russia, Japan, and Australia studied the influence of inhomogeneity of magnetic field applied during the fabrication process of thin-film structures made from nickel-iron and iridium-manganese alloys, on their properties.
'Magnetic topological insulator' makes its own magnetic field
A team of U.S. and Korean physicists has found the first evidence of a two-dimensional material that can become a magnetic topological insulator even when it is not placed in a magnetic field.
Scientists develop a new way to remotely measure Earth's magnetic field
By zapping a layer of meteor residue in the atmosphere with ground-based lasers, scientists in the US, Canada and Europe get a new view of Earth's magnetic field.
Magnetic field milestone
Physicists from the Institute for Solid State Physics at the University of Tokyo have generated the strongest controllable magnetic field ever produced.
More Magnetic Field News and Magnetic Field Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Climate Mindset
In the past few months, human beings have come together to fight a global threat. This hour, TED speakers explore how our response can be the catalyst to fight another global crisis: climate change. Guests include political strategist Tom Rivett-Carnac, diplomat Christiana Figueres, climate justice activist Xiye Bastida, and writer, illustrator, and artist Oliver Jeffers.
Now Playing: Science for the People

#562 Superbug to Bedside
By now we're all good and scared about antibiotic resistance, one of the many things coming to get us all. But there's good news, sort of. News antibiotics are coming out! How do they get tested? What does that kind of a trial look like and how does it happen? Host Bethany Brookeshire talks with Matt McCarthy, author of "Superbugs: The Race to Stop an Epidemic", about the ins and outs of testing a new antibiotic in the hospital.
Now Playing: Radiolab

Speedy Beet
There are few musical moments more well-worn than the first four notes of Beethoven's Fifth Symphony. But in this short, we find out that Beethoven might have made a last-ditch effort to keep his music from ever feeling familiar, to keep pushing his listeners to a kind of psychological limit. Big thanks to our Brooklyn Philharmonic musicians: Deborah Buck and Suzy Perelman on violin, Arash Amini on cello, and Ah Ling Neu on viola. And check out The First Four Notes, Matthew Guerrieri's book on Beethoven's Fifth. Support Radiolab today at Radiolab.org/donate.