Study of magnetic presheath stability to parallel drifts

该项目的重点是研究刮离层(SOL)中等离子体与壁结构界面处的鞘层在磁场与表面之间的斜角下的动力学。由于鞘层电场(垂直于壁面，且随距离强烈变化)和平行于壁面的B场分量的作用，在磁预制程(MPS)中存在较大的ExB漂移。这些exb资金流将被削减，并可能容易出现开尔文-亥姆霍兹式的不稳定。在有限数量的无碰撞PIC模拟[Theilhaber和Birdsall(1989)，Parker等人(1992)]中观察到了这一点，这些模拟对现代托卡马克中的偏滤器条件的适用性有限，而且解决得很差。然而，这种不稳定性有可能改变鞘层特性(例如，传输的粒子和能量通量)，并将通量集中在表面的小区域，这可能不利于面向等离子体的组件在托卡马克中排出的生存能力。因此，了解这种不稳定性以及如何缓解这种不稳定性是很重要的。我们的想法是开发一种2D光谱(x，Ky)，混合代码，包含流体电子和连续统动离子表示，以有效地研究平面电子排斥鞘的MPS中的剪切流动，包括热离子效应(在偏滤器附近发现的钛和Te所需的)和离子碰撞。该程序将集中于求解整个鞘的离子VFP方程，包括碰撞预能层(最多几个MFP厚度)、磁性预能层和德拜鞘，以及绝热电子(即Ne和电势OMM之间的Boltzmann关系)。碰撞--对于以鞘为中心的研究，将适当简化，但优于通常使用的BGK近似。将包括离子-离子库仑和电荷交换离子-中性碰撞。中性体将是离子碰撞的一种规定的被动物种。速度空间中的球谐展开将被用来处理SOL-KIT中的电子。该程序将被用来执行MPS对剪切流动稳定性的基本物理研究。由于它的光谱性质，它将有效地描述一个周期系统(在‘y’中，即沿墙)。在傅里叶模较少的情况下，我们将研究其线性增长率和阈值。不稳定性增长的背景将是自洽的动力学获得的。在不同的溶胶区(鞘层、传导受限和分离)下，碰撞对阈值的影响可以被定性地研究。通过扩大傅立叶模式的范围，可以解决不稳定饱和问题。

The proposed project focuses on the kinetic study of sheaths at the interface between plasma in the scrape-off layer (SOL) andwall structures, for oblique angle between the magnetic field and the surface. There are large ExB drifts in the magnetic presheath (MPS) due to the sheath E-field (normal to the wall, and strongly varying with distance) and the component of B-field parallel to the wall. These ExB flows will be sheared and potentially prone to Kelvin-Helmholtz-like instabilities. This has been observed in a limited number of collisionless, PIC simulations [Theilhaber and Birdsall (1989), which Parker et al (1992)], which were of limited applicability to divertor conditions in modern tokamaks and also poorly resolved. Yet such instabilities have the potential to change the sheath characteristics (e.g.transmitted particle and energy fluxes) and concentrate fluxes on small areas of the surface, which could be detrimental to the survivability of plasma facing components to exhaust in tokamaks. Understanding this instability, and how to mitigate it, is therefore of importance.The idea is to develop a 2D spectral (x, ky), hybrid code, with fluid electrons and a continuum, kinetic ion representation to efficiently study sheared flows in the MPS for planar, electron-repelling sheaths, including warm ion effects (needed for Ti > Te found near the divertor) and ion collisions. The code will focus on solving the ion VFP equation across the entire sheath, encompassing the collisional presheath (up to several mfp thick), magnetic presheath and Debye sheath, together with adiabatic electrons (i.e. Boltzmann relation between ne and electric potential omm). Collisions - Will be simplified as appropriate for a study focussed on the sheath, but superior to the BGK approximation usually employed. Ion-ion Coulomb & charge-exchange ion-neutral collisions will be included. Neutrals will be a prescribed, passive species for ions to collide against. A spherical-harmonic expansion in velocity space, as used for electrons in SOL-KiT, will be used.This code will be used to perform a fundamental physics study of the MPS stability to sheared flows. It will describe a periodic system (in 'y', i.e., along the wall) efficiently, owing its spectral nature. With few Fourier modes, the linear growth rate and thresholds will be investigated. The background on which the instability grows will be self-consistently kinetically obtained. The effect of collisions on the thresholds in different SOL regimes (sheath-, conduction-limited and detached) can be qualitatively explored. With an expanded range of Fourier modes, instability saturation can be tackled.