Transition from ion- to electron-repelling wall in tokamaks

在用于聚变能的磁化等离子体中，等离子体与壁接触区域的设计决定了包含等离子体的腔室的热量和粒子负荷。简单地将当前的机器缩放到反应堆的大小，就会给墙壁带来比任何已知材料所能承受的最大负荷大一百倍以上的热负荷。我们需要巧妙的解决方案，让这堵墙屹立不倒。这一挑战需要考虑的一个关键方面是壁-等离子体相互作用的特征（等离子体“边缘”的物理学）。如果壁带负电荷，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，带正电荷的壁排斥离子并防止溅射，但它们吸引来自热电子的大量热负荷。由于两种物质的动力学不同，以及等离子体沿磁力线和穿过磁力线的集体动力学不同，壁和磁场之间的角度决定了壁是排斥电子还是排斥离子。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在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利用新的分析技术对电子和离子排斥机制之间的转变进行全面的理论描述。这个想法是通过考虑到一个离子排斥壁会导致离子和电子粒子之间完全不同的平衡，以及能量损失到壁，从而深刻地改变上游（装置内部）的等离子体，来确定1度的角度是否确实有益。该项目属于EPSRC等离子体和激光研究领域。

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.