Public Release: 

Plasma flow damping observed due to stochastization of the magnetic field

Discovery of a new mechanism to stop flow in plasma

National Institutes of Natural Sciences

This news release is available in Japanese.

Katsumi Ida and colleagues at the National Institute for Fusion Science (NIFS) of the National Institutes of Natural Sciences (NINS) and Shigeru Inagaki of the Research Institute for Applied Mechanics (RIAM) of Kyushu University in Japan have clarified in experiment how the flow of magnetically confined plasmas is damped when the magnetic flux surface confining the plasma is disturbed (stochastization of the magnetic field). This is the first such observation in plasma for nuclear fusion in the world.

In working towards generation of nuclear fusion power, much research to improve the confinement of the high temperature high-density plasma in a magnetic field have been conducted in many countries. As plasma temperature and density rise, turbulence in the plasma deteriorates plasma performance and prevents further increase in plasma temperature. However, when the turbulence develops further, the turbulence drives plasma flow and then turbulence and flow coexist in the plasma. The coexistence of turbulence and flow remains difficult to understand, but it is a universal phenomenon in the plasma, and also is seen in the planet Jupiter and elsewhere. The plasma flow plays an important role in the magnetically confined plasma for nuclear fusion, because the plasma flow grides the vortex in the plasma and suppresses the plasma turbulence, and contributes increasing the temperature or the density. In the universe, the intensity of the plasma flow has been involved substantially in the fate of stars. Thus, plasma flow is an extremely important theme.

In this research, the spatial distribution of the plasma flow is measured precisely in the Large Helical Device (LHD) by developing a spectroscopic technique for measuring plasma flow. When the magnetic shear (magnitude of twist of the magnetic field) is weakened by switching the direction of the heating neutral beam during the discharge, a phenomenon in which the magnetic surface is disturbed (stochastization of the magnetic flux surface) takes place. Associated with the stochastization of the magnetic field, the abrupt stop of plasma flow is observed in the LHD. Although the damping of plasma flow was predicted theoretically 36 years ago (in 1978), the flow damping experimentally observed is much stronger than the prediction. This experimental observation is illustrated in the attached image.

The discovery of a new mechanism to stop the plasma flow due to the stochastization of the magnetic field presents a significant contribution to the future of nuclear fusion research. This newly discovered abnormal brake mechanism of plasma flow is thought to be important in the universe and is expected to have an impact on the wider academic field in the future.

The results of this research have been published in the British science journal Nature Communications on January 8, 2015, 19:00, as an open access electronic version.

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