Direct observation reveals “two-in-one” roles of plasma turbulence

Background

Producing fusion energy requires heating plasma to more than one hundred million degrees and confining it stably with strong magnetic fields. However, plasma naturally develops fluctuations known as turbulence, and they carry heat outward and weaken confinement. Understanding how heat and turbulence spread is therefore essential.

Conventional theory has assumed that heat and turbulence move gradually from the center toward the edge. Yet experiments have sometimes shown heat and turbulence spreading much faster, similar to American football players passing a ball quickly across long distances so that a local change influences the entire field almost at once. Clarifying the cause of this rapid, long-range response has been a long-standing challenge.

Results

A research team from the National Institute for Fusion Science carried out short duration heating of the plasma core in the Large Helical Device and used high-precision diagnostic instruments, based on electromagnetic waves of various wavelengths, to measure temperature, turbulence, and heat propagation with fine spatial and temporal resolution.

The measurements revealed a close relationship between heat spreading and the behavior of turbulence. Immediately after heating, a type of turbulence appeared that connected distant regions of the plasma in less than one ten thousandth of a second. This mediator-type turbulence resembles football players calling out to each other and passing the ball rapidly, allowing separated regions to respond together (Figure 1).

After this rapid response, another type of turbulence spread more slowly. This heat carrying turbulence behaves like a player holding the ball securely and running it forward, shaping the overall temperature profile of the plasma.

The experiments also showed that shorter heating pulses made the mediator turbulence stronger and caused heat to spread more quickly. These observations demonstrate that plasma turbulence plays two roles at the same time. One role is to carry heat outward, and the other is to connect distant regions so that heat can spread suddenly across the entire plasma.

Significance and Outlook

This study provides the first high resolution experimental identification of the mediator type turbulence that links distant parts of a plasma at the same time. It also presents the first direct demonstration that turbulence plays two distinct roles: one that carries heat outward and another that connects distant regions so that heat can spread rapidly across the plasma.

These findings explain how heat introduced at the plasma center can spread rapidly to the edge and form a scientific basis for predicting and controlling heat transport in future fusion reactors. Controlling the mediator turbulence may help create plasma conditions in which heat spreads more slowly, improving confinement.

The property that distant regions respond simultaneously is also seen in other natural systems, including ocean and atmospheric circulation and energy transfer inside materials. Therefore, the present results may be relevant to fields beyond fusion energy research.

Glossary

Turbulence

Fluctuations in plasma density or temperature that grow into waves, flows, or vortices. At high temperatures the structure becomes irregular and chaotic, causing heat and particles to be transported outward.

Large Helical Device (LHD)

One of the world’s largest superconducting helical plasma experimental devices, located at the National Institute for Fusion Science (NIFS) in Toki, Gifu, Japan.

Mediator

A medium within the plasma that links distant regions at the same time and speeds up the transfer of heat and energy. Its behavior resembles American football players rapidly passing the ball between teammates as they move down the field. When the heat is regarded as the ball, the mediator serves as the relay that connects the behavior of the plasma as a whole.

High precision diagnostics

Advanced measurement instruments that use electromagnetic waves of different wavelengths to observe turbulence, electron temperature, and heat propagation. They provide microsecond time resolution and millimeter spatial resolution.