On the electron sheath theory and its applications in plasma–surface interactions

In this work,an improved understanding of electron sheath theory is provided using both fluid and kinetic approaches while elaborating on their implications for plasma–surface interactions.A fluid model is proposed considering the electron presheath structure,avoiding the singularity in electron sheath Child–Langmuir law which overestimates the sheath potential.Subsequently,a kinetic model of electron sheath is established,showing considerably different sheath proflles in respect to the fluid model due to non-Maxwellian electron velocity distribution function and flnite ion temperature.The kinetic model is then further generalized and involves a more realistic truncated ion velocity distribution function.It is demonstrated that such a distribution function yields a super-thermal electron sheath whose entering velocity at the sheath edge is greater than the Bohm criterion prediction.Furthermore,an attempt is made to describe the electron presheath–sheath coupling within the kinetic framework,showing a necessary compromise between a realistic sheath entrance and the inclusion of kinetic effects.Finally,the secondary electron emissions induced by sheath-accelerated plasma electrons in an electron sheath are analysed and the influence of backscattering is discussed.

A Self-Consistent Analysis of Impurity Radiation by SOL-PSI Model

A model of scrape-off layer （SOL）-plasma surface interaction （PSI） coupling is pre- sented for divertor operation. This model can treat the interrelation between impurity production and impurity radiation self-consistently. The model is based on a ＇two-point＇ model and a 0-D steady state impurity particle balance model. The fraction of power radiated in SOL is calcu- lated as a function of the line average density. Compared to the former simple coupling model, this model takes into account the wall-produced impurities that are sputtered by charge-exchange （CX） neutrals. A simple retention factor is also added in this model. A comparison with the experiments is made. The simulation results show the same trend upon the plasma density as shown in experiments. Reasonable qualitative agreement is reached between the results by using the model and those from experiments through the adjustment of the fitted factor.

He bubble-driven growth of W fuzz during the interaction between H_(2)/He plasmas and W materials

The tungsten(W)material as the divertor of fusion reactors is exposed to low-energy and high-flux He/H isotope irradiation,leading to the growth of fuzz layers.The W fuzz growth does not show any dependence on the plasma irradiation devices used;however,it is strongly dependent on He^(+)fluence and energy,irradiation temperature,and impurity level.When the incident He ions collide inside the dense fuzz layers with the extremely high specific area,their mean free path can be up to 690 nm.Up to now,the He bubble-driven W fuzz growth process is not entirely understood;however,it can be closely related to the surface bursting of He bubbles in the W surface layer and W surface erosion by He^(+)implantation.The formation of He bubbles can be attributed to the solute He diffusion into defects or bubbles,which is strongly affected by the temperature and He^(+)fluence.The W fuzz grows over the W surface,where the micro-stress caused by He bubbles in the W surface layer acts as the driving force.The W fuzz layer inhibits He^(+)implantation into W bulk,and provides an entire protection against the He^(+)erosion into W bulk beneath the fuzz layer.In this review article,the current status of experiment and theory are presented,and some of the remaining issues are discussed.

Molecular Dynamics Simulation of Bombardment of Hydrogen Atoms on Graphite Surface

The new potential model of interlayer intermolecular interaction was proposed to represent“ABAB”stacking of graphite.The bombardment of hydrogen atoms on the graphite surface was investigated using molecular dynamics simulation.Before the first graphene fromthe surface side was broken,the hydrogen atoms caused the following processes.In the case of the incident energy of 5 eV,many hydrogen atoms were adsorbed on the front of the first graphite.In the case of the incident energy of 15 eV,almost all hydrogen atoms were reflected by the first graphene.In the case of the incident energy of 30 eV,the hydrogen atoms were adsorbed between the first and second graphenes.The radial distribution function and the animation of the MD simulation demonstrated that the graphenes were peeled off one by one,which is called graphite peeling.One C2H2 was generated in such chemical sputtering.But the other yieldedmolecules often had chain structures terminated by the hydrogen atoms.The erosion yield increased linearly with time.