Study of magnetic presheath stability to parallel drifts

磁预护套对平行漂移的稳定性研究

• 批准号: 2759197
• 金额: --
• 依托单位: Imperial College London
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2022
• 资助国家: 英国
• 起止时间: 2022 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2759197
• 关键词: Study; magnetic; presheath; stability; parallel

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.

拟议项目的重点是对磁场与表面之间的倾斜角下，刮除层（SOL）中等离子体与壁结构之间界面处的鞘层进行动力学研究。由于鞘层电场（垂直于壁，并随距离强烈变化）和平行于壁的B场分量，磁预鞘（MPS）中存在较大的ExB漂移。这些ExB流将被剪切，并可能倾向于开尔文-亥姆霍兹样不稳定性。在有限数量的无碰撞PIC模拟中观察到了这种情况[Theilhaber和Birdsall（1989），帕克等人（1992）]，这些模拟对现代托卡马克中偏滤器条件的适用性有限，而且分辨率也很差。然而，这种不稳定性有可能改变鞘层特性（例如传输粒子和能量通量），并将通量集中在表面的小区域上，这可能不利于托卡马克中面向等离子体的组件的生存能力。理解这种不稳定性，以及如何减轻它，因此是非常重要的。我们的想法是开发一个二维光谱（x，ky），混合代码，流体电子和连续，动力学离子表示，有效地研究剪切流在MPS平面，电子排斥鞘，包括温暖的离子效应（需要Ti > Te发现偏滤器附近）和离子碰撞。该代码将专注于求解整个鞘层的离子VFP方程，包括碰撞预鞘层（高达几个mfp厚），磁预鞘层和德拜鞘层，以及绝热电子（即ne和电势omm之间的玻尔兹曼关系）。碰撞-将被简化为适当的研究集中在鞘，但上级的BGK近似通常采用。将包括离子-离子库仑和电荷交换离子-中性碰撞。中性粒子将是一种规定的、被动的粒子，供离子碰撞。在速度空间的球谐展开，用于电子在SOL-KiT中，将被使用。这个代码将被用来执行一个基本的物理研究的MPS稳定剪切流。它将描述一个周期系统（以“y”为单位，即，沿着壁），这是由于其光谱特性。在傅立叶模式很少的情况下，将研究线性增长率和阈值。不稳定性增长的背景将是自洽的动力学获得。碰撞的阈值在不同的SOL制度（鞘，传导限制和分离）的效果可以定性探讨。随着傅立叶模式范围的扩大，可以解决不稳定性饱和问题。