Transition from ion- to electron-repelling wall in tokamaks

在用于聚变能量的磁化等离子体中，与壁接触的等离子体区域的设计决定了包含等离子体的腔室的热量和粒子负载。简单地将现有的机器按反应堆的尺寸缩放，会给壁带来比任何已知材料所能承受的最大负荷高出一百倍的热负荷。我们需要聪明的解决方案，使墙生存。这一挑战的一个关键方面是要考虑的壁等离子体相互作用（等离子体的物理“边缘”）的字符。如果壁带负电，它将排斥电子并加速离子。然后加速的离子撞击壁并溅射掉部分材料。相反，带正电的壁排斥离子并防止溅射，但它们从热电子吸引大量的热负荷。由于两种物质的不同动力学以及等离子体沿着和跨磁场线的不同集体动力学，壁和磁场之间的角度决定了壁排斥电子还是离子。目前的机器似乎处于电子排斥状态，但未来聚变反应堆的设计正在考虑电子排斥和离子排斥情况之间过渡的大约一度范围内的角度（这些设计基于其他优化考虑，忽略了这种过渡）。这个博士项目的重点是一个完整的理论表征之间的电子和离子排斥制度使用新的分析技术。这个想法是要确定是否角度的顺序为一度确实是有益的，考虑到离子排斥墙将导致一个完全不同的平衡之间的离子和电子粒子和能量损失的墙壁，深刻地改变等离子体上游（内设备）。这个项目属于EPSRC等离子体和激光器的研究领域福尔斯。

In magnetised plasmas for fusion energy, the design of the region of the plasma in contact with the wall determines the heat and particle load to the chamber that contains the plasma. Simply scaling current machines to reactor size would give heat loads to the wall that are more than one hundred times the maximum load that any known material withstands. We need clever solutions to enable the wall to survive. A key aspect of this challenge to consider is the character of the wall-plasma interaction (the physics of the plasma "edge"). If the wall charges negatively, it will repel electrons and accelerate ions. The accelerated ions then hit the wall and sputter part of the material away. In contrast, positively charged walls repel ions and prevent sputtering, but they attract a large heat load from hot electrons. Due to the different dynamics of the two species and also different collective dynamics of the plasma along and across the magnetic field lines, the angle between the wall and the magnetic field determines whether the wall repels electrons or ions. Current machines seem to be in the electron-repelling regime, but designs of future fusion reactors are considering angles that are within approximately one degree of the transition between the electron-repelling and the ion-repelling cases (these designs are based on other optimisation considerations, which ignore this transition). This DPhil project focuses on a full theoretical characterisation of the transition between the electron- and ion-repelling regimes using novel analytical techniques. The idea is to determine whether angles of the order of one degree are indeed beneficial by considering that an ion-repelling wall would lead to a completely different balance between the ion and electron particle and energy losses to the wall, profoundly changing the plasma upstream (inside the device).This project falls within EPSRC Plasma and Lasers research area.