High-speed mobile fusion plasma turbulence discovered for the first time in the world

Inside Japan’s LHD (Large Helical Device) Stellarator, built to test plasma fusion containment. Credit: Justin Ruckman

New perspectives for understanding turbulence in fusion plasmas.

To achieve fusion in a power plant, it is necessary to stably confine a plasma more than 100 million degrees Celsius in a magnetic field and hold it for a long time.

A research group led by Assistant Professor Naoki Kenmochi, Professor Katsumi Ida and Associate Professor Tokihiko Tokuzawa of the National Institute for Fusion Science (NIFS), National Institutes of Natural Sciences (NINS), Japan, using independently developed measuring instruments and with the cooperation of Professor Daniel J. den Hartog of the University of Wisconsin, USA, discovered for the first time in the world that turbulence moves faster than heat when heat escapes in plasmas of the Large Helical Device (LHD). A feature of this turbulence allows the prediction of plasma temperature changes, and it is expected that the observation of turbulence will lead to the development of a method for real-time control of plasma temperature in the future.

Barrier against turbulence and heat movement

Left: Formation of a barrier in the plasma to confirm the heat inside. Right: Breaking the barrier revealed turbulence that moves faster than heat because heat escapes from within the plasma. Credit: National Institute of Fusion Sciences

In a high temperature plasma confined by the magnetic field, “turbulence”, which is a flow with vortices of different sizes, is generated. This turbulence causes the plasma to be disturbed, and the heat from the confined plasma flows outward, causing the plasma temperature to drop. To solve this problem, it is necessary to understand the characteristics of heat and turbulence in the plasma. However, turbulence in plasmas is so complex that we have not yet fully understood it. In particular, how the generated turbulence moves through the plasma is not well understood, as it requires instruments capable of measuring the time evolution of minute turbulence with high sensitivity and extremely high spatio-temporal resolution. .

A “barrier” can form in the plasma, which acts to block the transport of heat from the center outwards. The barrier creates a strong pressure gradient in the plasma and generates turbulence. Assistant Professor Kenmochi and his research group have developed a method to break this barrier by designing a magnetic field structure. This method allows us to focus on the heat and turbulence that flow vigorously when the barriers break, and to study their relationship in detail. Then, using electromagnetic waves of different wavelengths, we measured the evolution of temperature and heat flow of electrons and fine millimeter turbulences with the highest level of precision. Previously, heat and turbulence were known to move almost simultaneously at a speed of 5,000 kilometers per hour (3,100 miles per hour), about the speed of an airplane, but this experiment led to the world’s first discovery of turbulence moving past heat at a speed of 40,000 kilometers per hour (25,000 miles per hour). The speed of this turbulence is close to that of a rocket.

Assistant Professor Naoki Kenmochi said: “This research has significantly advanced our understanding of turbulence in fusion plasmas. The new feature of turbulence, that it moves much faster than heat in a plasma, indicates that we may be able to predict plasma temperature changes by observing predictive turbulence. In the future, based on this, we plan to develop methods to control plasma temperatures in real time.

Reference: “Previous propagation of turbulence impulses during avalanche events in a magnetically confined plasma” by N. Kenmochi, K. Ida, T. Tokuzawa, R. Yasuhara, H. Funaba, H. Uehara, DJ Den Hartog, I. Yamada, M. Yoshinuma, Y. Takemura, and H. Igami, May 16, 2022, Scientific reports.
DOI: 10.1038/s41598-022-10499-z