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

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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.