Computer Simulation of Secondary Electron Emission From Plasma Facing Materials in Controlled Fusion Devices

磁共聚热核聚变装置中面向等离子体材料(PFM)的粒子诱导二次电子发射是等离子体-表面相互作用过程中的一个重要过程，对于杂质的产生具有重要意义。在本研究中，我们建立了PFM二次电子发射的蒙特卡罗模拟(MCS)模型，计算了发射的二次电子的能量和角分布以及产生的二次电子。MCS模型应用于碗结构和波纹结构，研究了表面粗糙度对二次电子能量和角分布的影响，以及电子轰击下PFM的二次电子产额。表面粗糙度越大，二次电子产额越大，而粗糙度越大，二次电子产额越小；前者是由于能量分布中的低能成分，后者导致斜角发射的电子数减少。MCS模型还与电子输运模型相结合，得到了界面处的电子输运模型。除了垂直于表面的电场的加速外，在斜磁场的作用下，鞘内发射的电子还有回转运动。一些二次电子在它们的第一次回转运动中返回表面，导致相当低的有效二次电子产额。产量的降低伴随着能量分布的低能漂移。

Particle-induced secondary electron emission from plasma-facing materials (PFM) in magnetic cofinement thermonuclear fusion devices is a plasma-surface interaction process of considerable importance for impurity production due to sputtering of the PFM.In this study, We have developed a Monte Carlo simulation (MCS) model of secondary electron emission from PFMs, which calculates the energy and angular distributions of emitted secondary electrons as well as the secondary electron yield.The MCS model is applied to bowl-and ripple-structures for the study of surface roughness effects on the energy and angular distributions of secondary electrons, as well as the secondary electron yield of the PFM under electron bombardment. With increasing roughness, the secondary electron yield becomes greater than that for a flat surface, whereas for large roughness the yield is smaller ; the former is due to the low-energy component in the energy distribution, and the latter results in a decrease the number of electrons emitted with oblique angles.The MCS model is also combined with an electron transport model in a sheath which is found at the interface between the surface and an edge plasma. In addition to acceleration of an electric field normal to the surface, there are gyromotion of emitted electrons in the sheath subjected to an oblique magnetic field. Some of the secondary electrons return to the surface within their first gyromotion, resulting in a considerably low effective secondary electron yield. The lowering of the yield is accompanied with a low-energy shift of the energy distribution.