First experimental observation of QH-mode with EHOs on the HL-3 tokamak

Abstract The quiescent H-mode (QH-mode), free of edge localized mode (ELM), is a promising operational regime for future fusion devices due to its low collisionality and high confinement performance. In this study, we report on the first experimental observation of QH-mode with edge harmonic oscillations (EHOs) on the HL-3 tokamak. The equilibrium point was found to be near but below the kink/peeling instability boundary. The core magnetohydrodynamic modes became weak during the EHO phase, indicating a significant change in the plasma stability. The fundamental mode of the EHOs had a toroidal mode number of n = 1 and rotated in the ion diamagnetic drift direction, while its harmonics rotated in the electron diamagnetic drift direction. The appearance and disappearance of EHOs were linked to critical values of the radial electric field ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>E</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">r</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) minimum and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>E</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">r</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> -curvature ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>E</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">r</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>′</mml:mi> <mml:mi>′</mml:mi> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ). The results suggest that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>E</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">r</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>′</mml:mi> <mml:mi>′</mml:mi> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> is the primary driver of EHOs, rather than <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>E</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">r</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> -shear ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>E</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">r</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>′</mml:mi> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ). Furthermore, significant reductions in divertor particle and heat flux were observed during the EHO phase compared to ELMy and ELM-free stages without EHOs, indicating the potential for achieving high confinement alongside mitigated divertor loads on HL-3. This study provides valuable insights into the physical understanding of QH-mode with EHOs and its potential role in achieving sustainable fusion energy.