Particle Simulation Of The Divertor Target Erosion And One-Dimensional Scrape-off Layer Physics In Tokamak

Tokamak is one of the most promising experimental devices for nuclear fusion.Plasma surface interaction(PSI)is one of the critical issues for the tokamak,which affects not only the lifetime but also the steady-state operation.PSI procedures mainly occur in the divertor region in tokamak.After a long time exposure to the high heat fluxes,especially during edge local modes(ELMs)and disruption,the wall of the divertor can be damaged,which reduces the lifetime of the device.In addition,the wall material may be eroded,which can introduce impurities into the tokamak.If impurities concentration in the core plasma is too high,the plasma discharge could be shut down.Tokamak experiments are complicated and expensive;therefore,the PSI numerical simulation study is the alternative method to study and understand those physics processes.Due to manufacture constraint and ion bombardment,divertor wall surfaces have different roughnesses,which may have impacts on the physical sputterings.Besides,the divertor target consists of many tiles to reduce heat stress and eddy-current effect,leaving many gaps between tiles.The divertor gaps cause hydrogen isotope fuel retention and leading-edge sputtering issues.To investigate these problems,we should focus on the plasma conditions near the target,which are related to the whole scrape-off layer(SOL)region.Therefore,one-dimensional(1D)and two-dimensional(2D)parallel particle codes are developed to study the divertor target erosion and SOL plasma problems in this thesis.The background of the research is introduced in chapter 1,the numerical methods and simulation codes are given in chapter 2,and the contents of other chapters are described in the following paragraphs.In chapter 3,a 2D particle code is used to study the rough surface problem,and irregular boundaries are handled by step-grids.The electric potential distribution near the rough surface of carbon target is calculated,and the effects of different plasma temperatures,surface topgraphies,and magnetic field strengths on the nominal angle,as well as the physical sputtering yields are considered.The results show that the effect of plasma temperature on physical sputtering is mainly due to the ion impact energy;the physical sputtering yield increases with the valley width and magnetic field strength.In chapter 4,the 2D particle code is applied to study the effects of different tile edge shapes on the heat flux density distribution,and the physical sputtering by some low-Z impurity ions.The simulation results show that the particle flux density,heat flux density and the physical sputtering are very high in the gap near the tile edge of the plasma-facing side.For the poloidal gaps,rounded edge tiles at the plasma-facing side can reduce the peak heat flux density.In chapter 5,a 1D particle code is used to simulate the plasma transport in the SOL and the effects of carbon injection and deuterium recycling on the plasma.The plasma density,temperature,potential distribution and the plasma sheath structure are calculated self-consistently.The velocity distrubiton functions of electron and ion near the target are given.The simulation results show that,with carbon atom injection or deuterium recycling,the electron density increases,while the electron temperature,the sheath potential drop,and the heat flux on the target decrease,which means the carbon injection and the deuterium recycling will mitigate the target erosion.