Research on stiff transport in electron heating dominated plasmas in EAST

Abstract An experiment was conducted on the Experimental Advanced Superconducting Tokamak (EAST) to investigate the properties of stiff transport in an electron-heated H-mode plasma, using a power deposition scan with electron cyclotron resonance heating (ECRH). During this experiment, a shift in lower hybrid wave heating power and driven current from the plasma core to the outer regions was observed, accompanied by a change in the local magnetic shear. Transport analysis reveals a reduction in the normalized effective electron thermal diffusivity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>χ</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ) at larger minor radii when ECRH was applied off-axis, in alignment with the turbulent instability analysis. A threshold for the stiff transport is identified around <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>R</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:msub> <mml:mi>L</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>Te</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:math> on EAST, above which the electron heat flux ( q e ) increases sharply with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>R</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:msub> <mml:mi>L</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>Te</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> . Moreover, experimental and simulation results indicate that the change in ECRH power deposition influences local micro-instability turbulence, resulting in the transition of TEM to ITG modes, thereby affecting the plasma performance. These findings provide important insights for optimizing plasma performance, improving confinement, and advancing our understanding of plasma transport in future fusion reactors.