Investigation of two-dimensional radio-frequency sheath properties using a microscale fluid model

In previous work (Kohno H. and Myra J.R. 2023 Comput. Phys. Commun. 291 108841), we developed a numerical scheme based on a two-dimensional microscale radio-frequency (RF) sheath model with periodically curved wall boundaries. Here, we expand the capability of this scheme through modification of the boundary conditions (BCs) on the conducting walls, which allows the ion flow to turn back to the plasma at locations on the walls where the electromagnetic force on the ions is reversed from its usual direction. Numerical simulations are carried out to investigate the dependences of the surface-integrated admittances on the wall bump height, ion magnetization, ion mobility, and the magnetic field angle, and to visualize the sheath structures in several cases. One of the main results is the ion cyclotron admittance resonance observed under the condition of low ion mobility (high normalized frequency). It is shown that the amplitude of the resonance peak depends on the wall bump height and the ion velocity is reversed on the sides of the bump in an RF cycle for the resonance cases. Furthermore, the differences in the admittances between the one- and two-dimensional microscale models are assessed for the purpose of understanding non-locality of the sheath near the wall surface for the parameters considered in this study. This information will be essential for improving the sheath BC for macroscale calculations in the future.

既往研究（Kohno H. 与 Myra J.R. 2023 Comput. Phys. Commun. 291 108841）中，我们基于带有周期性弯曲壁边界的二维微尺度射频（radio-frequency, RF）鞘层模型（sheath model）开发了一套数值方法。本文通过修改导电壁上的边界条件（boundary conditions, BCs）拓展了该方法的功能，使得当壁面上离子所受电磁力方向与常规方向相反时，离子流能够折返至等离子体中。我们开展了数值模拟，以探究表面积分导纳随壁面凸起高度、离子磁化（ion magnetization）、离子迁移率（ion mobility）以及磁场角（magnetic field angle）的变化关系，并对多种场景下的鞘层结构进行可视化。主要研究结果之一是在低离子迁移率（高归一化频率）条件下观测到的离子回旋导纳共振（ion cyclotron admittance resonance）。研究表明，共振峰的振幅随壁面凸起高度变化，且在共振场景下，离子速度在射频周期内于凸起两侧发生反转。此外，为理解本研究中所涉参数下壁面附近鞘层的非局域性，我们评估了一维与二维微尺度模型的导纳差异。该信息将为未来优化宏观尺度计算所用的鞘层边界条件提供必要支撑。