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:

Understanding stars: How tornado-shaped flow in a dynamo strengthens the magnetic field
A new simulation based on the von-Kármán-Sodium (VKS) dynamo experiment takes a closer look at how the liquid vortex created by the device generates a magnetic field.
'Quartz' crystals at the Earth's core power its magnetic field
Scientists at the Earth-Life Science Institute at the Tokyo Institute of Technology report in Nature (Fen.
Brightest neutron star yet has a multipolar magnetic field
Scientists have identified a neutron star that is consuming material so fast it emits more x-rays than any other.
Confirmation of Wendelstein 7-X magnetic field
Physicist Sam Lazerson of the US Department of Energy's Princeton Plasma Physics Laboratory has teamed with German scientists to confirm that the Wendelstein 7-X fusion energy device called a stellarator in Greifswald, Germany, produces high-quality magnetic fields that are consistent with their complex design.
High-precision magnetic field sensing
Scientists have developed a highly sensitive sensor to detect tiny changes in strong magnetic fields.
More Magnetic Field News and Magnetic Field Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Teaching For Better Humans
More than test scores or good grades — what do kids need to prepare them for the future? This hour, guest host Manoush Zomorodi and TED speakers explore how to help children grow into better humans, in and out of the classroom. Guests include educators Olympia Della Flora and Liz Kleinrock, psychologist Thomas Curran, and writer Jacqueline Woodson.
Now Playing: Science for the People

#535 Superior
Apologies for the delay getting this week's episode out! A technical glitch slowed us down, but all is once again well. This week, we look at the often troubling intertwining of science and race: its long history, its ability to persist even during periods of disrepute, and the current forms it takes as it resurfaces, leveraging the internet and nationalism to buoy itself. We speak with Angela Saini, independent journalist and author of the new book "Superior: The Return of Race Science", about where race science went and how it's coming back.