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

磁约束热核聚变装置中等离子体表面材料（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，对PFM溅射引起的杂质产生具有重要意义。在本研究中，我们建立了PFMs二次电子发射的蒙特卡罗模拟（MCS）模型，该模型计算了发射的二次电子的能量和角分布以及二次电子的产率。将MCS模型应用于碗状和波纹状结构，研究了表面粗糙度对电子轰击下二次电子能量和角分布的影响，以及二次电子产率的影响。随着粗糙度的增大，二次电子产率大于平坦表面，而粗糙度越大，二次电子产率越小；前者是由于能量分布中的低能量成分，后者是由于斜角度发射的电子数量减少。MCS模型还结合了表面和边缘等离子体界面鞘层中的电子输运模型。除了垂直于表面的电场加速度外，鞘层中发射的电子在倾斜磁场的作用下还存在回旋运动。一些二次电子在它们的第一次回旋运动中返回到表面，导致相当低的有效二次电子产率。产率的降低伴随着能量分布的低能转移。

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.