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ASDEX升级装置（ASDEX Upgrade, AUG）中，离子∇B漂移朝向主X点的"有利"漂移位形与离子∇B漂移远离主X点的"不利"漂移位形下的L模（低约束模，L-mode），开展了平衡径向电场（Er）形成机制的实验探究，研究初衷在于该电场对随漂移位形变化的H模（高约束模，H-mode）准入阈值的影响。本研究采用新型改进的测量技术，对穿过分界面（separatrix）的边缘电子与离子动力学分布、杂质速度及平均场Er分布进行了系统测量与分析。实验结果分别与局地新经典理论（neoclassical theory）以及简化的一维刮削层（scrape-off layer, SOL）模型进行了对标对比。研究发现，在加热功率与等离子体密度匹配的L模条件下，两种漂移位形的上游刮削层Er以及等离子体边缘的主离子压强梯度均保持一致；但受限等离子体中的Er势阱在不利漂移位形下较有利漂移位形更为浅缓。通过局地新经典理论与实验推导得到的环向与极向主离子流对Er的贡献，无法解释上述观测到的差异。此外，本研究还观测到，在L模条件下，本征环向边缘旋转随碰撞性（collisionality）增强而减弱；且在AUG装置的所有不同漂移位形中，香蕉高原区（bananaplateau regime）的本征旋转均为同流旋转。这一现象表明刮削层内的平行皮尔施-许特尔流（Pfirsch-Schlüter flows）或可与受限等离子体发生相互作用。因此，AUG装置中两种漂移位形对应的H模功率阈值差异，无法采用LaBombard等人针对Alcator C-Mod1所提出的解释方案进行统一说明。最后，针对两种漂移位形各自约束模转变点处的Er分布进行对比后发现，二者的Er梯度亦存在显著差异，这间接表明两种漂移位形下典型边缘湍流的类型或强度存在明显区别。