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EAST Team Uses Its Diagnostic System to Obtain Discovery of Microturbulence in EAST Core Plasma

May 20, 2020

The Experimental Advanced Superconducting Tokamak (EAST) team from Hefei Institutes of Physical Science has reported that they used the diagnostic system which is a subsystem of EAST facility to obtain new discovery of microturbulence in EAST Core Plasma.

Controlling fusion reaction rate in tokamak is a frontier science in fusion research. At present, the main fusion reaction rate control scheme is to adjust the plasma density in tokamak core by fueling core area, such as pellet injection and supersonic molecular beam injection, while the main challenge is that the energy confinement time is uncertain during fueling process due to the nonlinear physical feature of the burning plasma.

Through EAST experimental research utilizing CO2 laser collective scattering diagnostics, EAST scientists concluded with a new idea that the average value of fusion reaction rate can be controlled by modulating the frequency and intensity of the core peaked density in the tokamak.

Actually, as early as 2018 EAST steady-state discharge experiments with neutral beam power injection, the team found that the modulated electron cyclotron heating pulse could regulate the turbulence intensity in the core region leading to the modulation of core turbulence intensity and the regulation of plasma poloidal rotation velocity in the core, thus realized the density peaking factor modulation in the corresponding region.

"Based on our findings," said Prof. LI Yadong, who leads the CO2 diagnosis team, "It may be a feasible method to adjust the fusion reaction rate through modulating the frequency and intensity of the core peaked density."

By analyzing diagnostic data, they also discovered the coupling phenomenon between electronic mode turbulence and the internal kink mode under the condition with low collision rate in EAST.

As indicated in their report, micro-turbulence could couple with low-n MHD in low collision rate and high-parameter plasma, resulting in free energy exchange, which may cause low-n MHD to grow naturally.

Turbulent transport across the lines of magnetic force may influence the stability of low-n MHD or even lead to crash. To tackle this problem, EAST scientists reckoned that fusion researchers must take the plasma density gradient into account when it comes to the operating regime qmin <3 with allowed high-parameter plasma as the most effective driving source for micro-turbulence was plasma density gradient.

The research above is critical to answer how to get burning plasma under control that has been seen as a key problem before developing actical fusion power.

"There is now high readiness for our team to take a closer look on the electron thermal transport and the multi-scale physics involving electronic mode turbulence," said Prof. LI Yadong.

The CO2 laser collective scattering diagnostic system on EAST tokamak was independently developed by Institute of Plasma Physics, Hefei Institutes of Physical Science, based on which, EAST scientists have conducted simultaneous monitoring of the electronic mode turbulence in the core and outer regions of EAST tokamak.

Their research work also involves many other related topics, including the interaction between electronic mode turbulence and Alfvén eigenmode, the influences of turbulence in radial and poloidal on energy confinement, dynamic evolution of electronic mode turbulence in the confinement mode conversion, the characteristic of microturbulence, etc.

Since 2012, the diagnostic system has contributed to more than ten facilitate outputting scientific achievements.

 

CO2 laser collective scattering diagnostics (Image by WU Guojiang) 

 

Core density peaking factor modulation experiment (Image by WU Guojiang ) 

 

Nonlinear coupling between microturbulence and internal kink mode (Image by WU Guojiang) 

Study of turbulence modulation and core density peaking with CO2 laser collective scattering diagnostics in the EAST tokamak

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