Pedestal properties of negative triangularity discharges in ASDEX Upgrade

In this work, we explore the pedestal properties of negative triangularity discharges with upper triangularity of delta ≈ −0.35 and with both ion gradB 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 gradB 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.

本研究针对上三角性δ≈−0.35且涵盖两类离子梯度B漂移方向的负三角形状放电，探究其等离子体台基特性。所有放电均发生向高约束模（H-mode）的转变，伴随出现的边缘局域模（edge-localized modes, ELMs）无法仅通过剥离气球模稳定性分析得到合理解释。观测到离子梯度B漂移方向存在显著影响：在有利漂移工况下，实验测得更低的压强梯度、更浅的径向电场阱以及更强的温度涨落，同时伴随不规则边缘局域模。相较于中性束注入与电子回旋共振加热（electron cyclotron resonance heating, ECRH）联合加热的等离子体，该差异在纯电子回旋共振加热等离子体中更为显著。对两组等离子体参数相近但位形整形不同的放电进行对比分析后发现，磁剪切、辅助加热时序以及位形整形等其他参数间的相互作用，可能在低约束模（L-mode）向高约束模的转变过程中发挥关键作用。值得注意的是，低整形放电维持在低约束模，而高整形放电却反常地进入了带有边缘局域模的高约束模，与预期结果相悖。