边缘径向电场在有利与不利漂移位形之间不对称性的湍流起源
Turbulent Origin of the Edge Radial Electric Field Asymmetry between Favourable and Unfavourable Drift Configurations
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中文总结 AI 辅助
本研究通过回旋动理学模拟发现，边缘径向电场不对称性源于湍流驱动，有利位形下更强的非线性能量转移导致更深的势阱，并影响L-H功率阈值。
中文摘要 AI 辅助
我们研究了离子磁漂移方向相对于活动X点（有利和不利位形）对ASDEX Upgrade和TCV中边缘湍流、输运以及径向电场$E_r$的影响。利用谱全$f$ GENE-X代码，在偏滤器X点几何中进行了边缘和刮削层的首次原理回旋动理学模拟。模拟了两组匹配的L模放电，并在所有四种情况下与现有测量进行了验证，显示出良好的一致性，特别是对于$E_r$剖面。径向力平衡分析表明，在有利位形中更深的$E_r$势阱是由靠近分界面的较大极向离子流决定的，这种离子流无法由新经典求解器重现，并在轴对称模拟中被识别为湍流驱动的。然后，我们从全$f$回旋动理学涡度方程推导出平均极向流加速度的精确演化方程，该方程将总速度应力的径向梯度确定为其非线性驱动。观察到的不对称性源于有利位形中向平均流的更强非线性能量转移，这与靠近分界面和X点处增强的非线性$E\ imes B$涡度平流有关。通量驱动的TCV模拟进一步表明，在匹配功率超过L模值六倍的情况下，这种不对称性仍然存在。假设在L-H转变处存在临界$E_r$，这得出不利与有利L-H功率阈值之间的估计比率约为1.7，接近实验中的因子二。总体而言，不利位形表现出更大的涨落幅度、更弱的$E_r$势阱、更强的测地声模以及对于相同剖面更短的能量约束时间，这可能解释了在高约束模式进入方面观察到的差异。
英文摘要
We study the influence of the ion magnetic-drift direction relative to the active X-point (favourable and unfavourable configurations) on edge turbulence, transport, and the radial electric field $E_r$ in ASDEX Upgrade and TCV. First-principles gyrokinetic simulations of the edge and scrape-off layer are performed in diverted X-point geometry using the spectral full-$f$ GENE-X code. Two matched pairs of L-mode discharges are simulated and validated against the available measurements in all four cases, showing good agreement, in particular for the $E_r$ profiles. A radial force-balance analysis shows that the deeper $E_r$ well in the favourable configuration is determined by the larger poloidal ion flow near the separatrix, which cannot be reproduced by neoclassical solvers and is identified in axisymmetric simulations as turbulence-driven. We then derive an exact evolution equation for the mean poloidal-flow acceleration from the full-$f$ gyrokinetic vorticity equation, which identifies the radial gradient of the total velocity stress as its nonlinear drive. The observed asymmetry is found to arise from stronger nonlinear energy transfer into the mean flow in the favourable configuration, associated with enhanced nonlinear $E\times B$ advection of vorticity near the separatrix and X-point. Flux-driven TCV simulations further show that the asymmetry persists at matched power more than six times the L-mode value. Assuming a critical $E_r$ at the L-H transition, this yields an estimated ratio of approximately 1.7 between the unfavourable and favourable L-H power thresholds, close to the experimental factor of two. Overall, the unfavourable configuration exhibits larger fluctuation amplitudes, a weaker $E_r$ well, stronger geodesic acoustic modes, and shorter energy confinement times for the same profiles, which may explain the observed difference in access to high-confinement regimes.
发表机构
- Max-Planck-Institut für Plasmaphysik（马克斯·普朗克等离子体物理研究所）
- Laboratoire de Physique des Plasmas, CNRS-École Polytechnique（巴黎综合理工学院等离子体物理实验室）
- École Polytechnique Fédérale de Lausanne (EPFL)（洛桑联邦理工学院）
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