Pedestal properties of negative triangularity discharges in ASDEX Upgrade

Abstract In this work, we explore the pedestal properties of negative triangularity discharges with upper triangularity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>δ</mml:mi> <mml:mi>u</mml:mi> </mml:msub> <mml:mo>≈</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0.35</mml:mn> </mml:mrow> </mml:math> and with both ion <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">∇</mml:mi> <mml:mi>B</mml:mi> </mml:mrow> </mml:math> drift directions. In all cases, the discharges undergo a transition to H-mode with accompanying edge-localized modes (ELMs) that are not explained by the peeling-ballooning stability analysis alone. A variation in the ion <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">∇</mml:mi> <mml:mi>B</mml:mi> </mml:mrow> </mml:math> drift direction is observed. A lower pressure gradient, shallower radial electric field well and increased temperature fluctuations are measured in the favorable case. In addition, irregular ELMs are present. This difference is more pronounced in electron cyclotron resonance heating (ECRH) plasmas compared to plasmas with combined neutral beam injection and ECRH. A comparative analysis between two discharges, featuring similar plasma parameters but varied shaping, suggests that an interplay between other parameters, such as magnetic shear, the timing of the auxiliary heating and shaping, might play a strong role in low- to high-confinement transitions. While the low shaping discharge maintained L-mode, the high shaping discharge entered H-mode with ELMs, contrary to expectations.