Study of magnetic presheath stability to parallel drifts

本课题主要研究刮擦层（SOL）等离子体与壁面结构界面处，磁场与表面夹角下的鞘层动力学。由于鞘层e场（垂直于壁，随距离变化强烈）和平行于壁的b场分量，在磁预鞘层（MPS）中存在较大的ExB漂移。这些ExB流将被剪切，可能容易出现类似开尔文-亥姆霍兹的不稳定性。这已经在有限数量的无碰撞PIC模拟中观察到[Theilhaber和Birdsall (1989)， Parker等人（1992）]，这些模拟对现代托卡马克的转流条件的适用性有限，而且解决得也很差。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上，这可能不利于面向等离子体的组件在托卡马克中排气的生存能力。因此，了解这种不稳定性，以及如何减轻这种不稳定性是非常重要的。这个想法是开发一个二维光谱（x, ky），混合代码，具有流体电子和连续的动力学离子表示，以有效地研究MPS中平面的剪切流，电子排斥鞘，包括热离子效应（需要在分流器附近发现Ti > Te）和离子碰撞。代码将侧重于解决整个鞘层的离子VFP方程，包括碰撞预鞘层（厚达几mfp），磁性预鞘层和Debye鞘层，以及绝热电子（即ne与电势omm之间的玻尔兹曼关系）。碰撞-将简化为适当的研究集中在鞘，但优于通常使用的BGK近似。离子-离子库仑和电荷交换离子中性碰撞将包括在内。中性物质是一种规定的、被动的离子碰撞物质。将使用速度空间中的球调和展开，就像在SOL-KiT中使用的电子一样。此代码将用于执行剪切流的MPS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（用“y”表示，即沿壁）。在傅里叶模式较少的情况下，将研究线性增长率和阈值。不稳定性增长的背景将从动力学上自洽地获得。碰撞对不同SOL机制（鞘层、传导受限和分离）阈值的影响可以进行定性探讨。随着傅里叶模式范围的扩大，不稳定饱和可以解决。

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.