In that recent campaign of Experimental Advanced Superconducting Tokamak (EAST), a joint fusion team from Institute of Plasma Physics, Hefei Institutes of Physical Science (HFIPS), Chinese Academy of Sciences (CAS) and its partners managed to prove experimentally, for the first time, the current driven by turbulence, via observed data during the experiment and simulations after. The turbulence-driven current, according to the team, was also an important factor to sustain the 100 million degrees of electron temperature stationary long-pulse plasma on EAST.
The team published their work in the journal of Physical Review Letter.
How to confine the high temperature ball of plasma at the core of Tokamak facility is an essential and key problem fusion scientists have to tackle. And the plasma current is considered key to high performance confinement. Turbulence is theoretically predicted to generate a drive force to modify the current, However, which has never been observed in tokamak experiments.
During the past campaign of EAST that has realized long pulse plasmas operation with a super high electron temperature over 100 minions Celsius, the team 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 threshold.
Moreover, as the turbulence increased, the team observed that turbulence-driven current flew into the opposite direction of bootstrap current, which enabled the identification of these two types of current.
In addition to direct observation during the experiment, scientists also bring in gyro-kinetic simulation for further help.
The calculation told scientists the turbulence observed during the experiment was actually electron temperature gradient mode (ETG), by which a residual stress was generated to drive the turbulent current. Then kink mode was triggered leading to the formation of self-regulation among the turbulence, the turbulence-driven current and the kink mode, which worked to keep gradient of electronic temperature at the core stationary.
This physical process was considered by the team as the key to hold stable the long-pulse operation at super-high electron temperature.
The team also pointed out effects of the turbulent current on the plasma macroscopic instability and even disruption.
This work was supported by the Youth Innovation Promotion Association Chinese Academy of Sciences under Grant No.2018483 and partly by National Key R&D Program of China (Grant No. 2019YFE03010002).
Figure. Evolution of Turbulence-driven current and Bootstrap current (Image by EAST Team)
