Experimental Study of the Edge Radial Electric Field in Different Drift Configurations and its Role in the Access to H-mode at ASDEX Upgrade

The formation of the equilibrium radial electric field (Er) has been studied experimentally at ASDEX Upgrade (AUG) in L-modes of ’favourable’ (ion ∇B-drift towards primary X-point) and ’unfavourable’ (ion ∇B-drift away from primary X-point) drift configuration, in view of its impact on H-mode access, which changes with drift configuration. Edge electron and ion kinetic profiles, impurity velocity and mean-field Er profiles across the separatrix are investigated, employing new and improved measurement techniques. The experimental results are compared to local neoclassical theory as well as to a simple 1D scrape-off layer (SOL) model. It is found that in L-modes of matched heating power and plasma density the upstream SOL Er and the main ion pressure gradient in the plasma edge are the same for either drift configuration, whereas the Er well in the confined plasma is shallower in unfavourable compared to favourable drift configuration. The contributions of toroidal and poloidal main ion flows to Er, which are inferred from local neoclassical theory and the experiment, cannot account for these observed differences. Furthermore, it is found that in L-mode the intrinsic toroidal edge rotation decreases with increasing collisionality and it is co-current in the bananaplateau regime for all different drift configurations at AUG. This gives rise to a possible interaction of parallel Pfirsch-Schlüter flows in the SOL with the confined plasma. Thus, the different H-mode power threshold for the two drift configurations can not be explained in the same way at AUG as suggested by LaBombard et al. for Alcator C-Mod1. Finally, comparisons of Er profiles in favourable and unfavourable drift configuration at the respective confinement transitions show that also there the Er gradients are all different, which indirectly indicates a different type or strength of the characteristic edge turbulence in the two drift configurations.

本研究针对平衡径向电场（E_r）的形成机制开展实验探究，实验依托ASDEX升级装置（ASDEX Upgrade, AUG）完成，选取了两类漂移构型的L模等离子体：“有利”构型（离子∇B漂移朝向主X点）与“不利”构型（离子∇B漂移背离主X点），重点考察其对随漂移构型变化的H模准入的影响。研究团队采用新型改进的测量技术，对分界面两侧的边缘电子与离子动力学剖面、杂质速度及平均场E_r剖面展开系统分析。将实验结果与局域新经典理论以及简化的一维刮削层（Scrape-off Layer, SOL）模型进行对比后发现：在加热功率与等离子体密度匹配的L模等离子体中，两类漂移构型下上游SOL的E_r以及等离子体边缘的主离子压强梯度均保持一致，但约束等离子体中的E_r阱深度在不利漂移构型中较有利构型更浅。通过局域新经典理论与实验推导得到的环向与极向主离子流对E_r的贡献，无法解释上述观测到的构型差异。进一步研究表明，在L模等离子体中，固有环向边缘旋转随碰撞性升高而降低，且在AUG装置所有不同漂移构型的香蕉高原区中，该旋转均为同向共转。该现象暗示刮削层内的平行皮尔施-许吕特尔流（Pfirsch-Schlüter flows）与约束等离子体之间可能存在相互作用。因此，AUG装置中两类漂移构型对应的H模功率阈值差异，无法通过LaBombard等人针对Alcator C-Mod1提出的解释路径予以说明。最后，对比两类漂移构型在各自约束转变点处的E_r剖面，研究发现二者的E_r梯度均存在显著差异，这间接表明两种漂移构型下特征边缘湍流的类型或强度存在显著不同。