In a recent experiment at the Experimental Advanced Superconducting Tokamak (EAST), researchers from the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS) and their partners managed to prove experimentally the existence of current driven by turbulence for the first time, by observed data during the experiment and by following simulations.

Evolution of Turbulence-driven current and Bootstrap current [IMAGE: EAST TEAM]

The turbulence-driven current, according to the researchers, was an important factor in sustaining the 100 million degrees of electron temperature of long-pulse plasma in EAST. Results were published in Physical Review Letters.

How to confine the high temperature ball of plasma at the core of the Tokamak facility is a key problem facing fusion scientists. Plasma current is considered key to high performance confinement. Turbulence is theoretically predicted to generate a drive force to modify the current, but this has never been observed in Tokamak experiments.

During the experiment at EAST that realized long pulse plasmas operation with a super high electron temperature over 100 million ℃, the researchers observed something that excited them.

The real time data from the experiment showed a modulation of plasma current when electron temperature gradient went up over a certain threshold.

Moreover, as the turbulence increased, they found that turbulence-driven current flew in the opposite direction to the bootstrap current, enabling them to identify the two types of current.

In addition to direct observation during the experiment, they also introduced gyro-kinetic simulation for further analysis.

The calculation showed that the turbulence observed during the experiment was actually in electron temperature gradient mode, through which a residual stress was generated to drive the turbulent current. Then a kink mode was triggered to form a self-regulation effect in the turbulence, the turbulence-driven current and the kink mode, keeping the gradient of electronic temperature at the core stationary.

This physical process is considered by the researchers as the key to maintaining stable operation of long pulses at ultra-high electron temperature.

They also point out the effects of the turbulent current on plasma macroscopic instability and disruption.

This work was supported by the Youth Innovation Promotion Association of CAS and the National Key R&D Program of China.

Source: Hefei Institutes of Physical Science (HFIPS),

Chinese Academy of Sciences