Modelling Of Erosion Of Divertor Target Plates And Impurity Transport And Deposition

The interaction between the edge plasma and PFCs determines the lifetime of PFCs in fusion devices, which is a crucial parameter for the viability of a fusion reactor. Moreover, the eroded impurities can be transported over long distances and be co-deposited together with fuel species. The resulting build-up of tritium-rich layers could become the main limiting factor for reactor availability due to safety restrictions. Therefore, studies of plasma surface interaction (PSI), impurity transport and re-deposition are crucial to get a better understanding of underlying mechanisms involved and to be able to adopt effective measures to minimize the net erosion and tritium retention.The studies of impurity transport and deposition, tritium retention, the influence of rough surface on impurity deposition and erosion of divertor target due to edge localized modes (ELMs) have been performed in this thesis. A Monte-Carlo code ITCD (Impurity Transport Collision and Deposition) has been developed to study impurity transport, collision, deposition and tritium retention. Furthermore, the ITCD code can be coupled with other kinetic codes, such as EPPIC(Edge Plasma Particle In Cell), to calculate density distribution of the impurity, deposition rate of the impurity and tritium inventory. Moreover, a Monte-Carlo code called SURO (SUrface ROughness) has been developed to investigate dynamic change of rough surface under plasma bombardment and the effect of surface roughness on the impurity deposition characteristic. Finally, a kinetic code SDPIC (SOL and Divertor Particle-In-Cell) has been developed to study the erosion of divertor target during ELMs. The detailed content of the thesis is organized as follows:In chapter2, the KEPIT (Kinetic Edge Plasma&Impurity Transport) code package has been used to study the transport and density distribution of eroded carbon species above divertor target plate. KEPIT comprises three dimensional (3D) Monte-Carlo impurity transport code ITCD and one-dimension-in-space and three dimension-in-velocity (Id3v) Particle-In-Cell Monte-Carlo collision (PIC-MCC) code EPPIC. ID EPPIC is used to study the characteristics of divertor edge plasma and provides physical information such as particle flux and ion energy flux density as input parameters for the PSI model to evaluate the erosion rate of carbon-based material; the resulting erosion rate can then be used as input parameters for the ITCD code to simulate impurity transport, collision, deposition and resulting density distribution of eroded carbon species. To further study the characteristics of density distribution of eroded carbon species, we examine the variations of eroded carbon species density by varying plasma temperature, the incident angle and strength of the magnetic field. It is found that a "density hill" forms at a distance of3-4mm from divertor target plate under certain circumstances:high plasma temperature, small incident angle and strength of the magnetic field.In chapter3, the KEPIT code package has been applied to investigate impurity transport, deposition and tritium retention between the tiles of divertor target plates, which consists of the ITCD code and2d3v EPPIC code.2D EPPIC has been employed to study potential distribution around the tile gap and provides the detailed information such as particle flux and ion energy flux density as input parameters for the PSI model to evaluate erosion rate of carbon-based material; then, the resulting erosion rate can be used as input parameters for the ITCD code to simulate impurity transport, collision and deposition between the tiles of divertor target plates. Moreover, the studies of the influence of carbon substrate and plasma temperatures on the fuel inventory have been carried out. Additionally, the origins of deposited carbon species in gaps have been investigated using KEPIT. Further, more detailed analyses have been made to gain insight into the effects of physical sputtering and chemical erosion on carbon source. Moreover, the impact of erosion, plasma temperature, potential hill and reflection coefficients on deposition of neutral and charged carbon species has been studied by KEPIT. Based on the simulation results, we make a simple estimation that the number of the discharges would be of the order of magnitude of104-105before tritium retention in gaps reaches the safety limit of700g.In chapter4, the MC code SURO has been developed and combined with ERO to investigate the influence of surface roughness on the13C deposition characteristic in13CH4injection experiments in TEXTOR. The simulation domain is divided into two regions: Scrape-Off Layer (SOL) plasma region, in which the transport of particles is treated by ERO, and a near surface region where the particles interact with the smooth or rough surfaces addressed by SURO. The necessary input information for SURO (flux, angle and energy of impinging particles) is then provided by ERO. Modelling of the local deposition of13C has been carried out for various surface roughnesses and varying C and C fluxes. In addition, dynamic change of surface roughness has been studied. For TEXTOR conditions, the exposure time for smoothening of the rough surface was around7000s, which is two orders of magnitude larger than that for an ITER case.In chapter5, Id3v PIC-MCC code SDPIC has been developed to study the erosion of divertor target plate during ELMs. The energy flux at the target during ELMs has been calculated for EAST. The detailed information such as energy, incident angle and flux has been employed to calculate erosion rate of target plate. Moreover, the studies of the impact of ELMs duration on energy flux at the target and corresponding erosion rate has been performed for different substrate materials. For ELMs durations of50μs,100μs and200μs, the peak values of energy fluxes at the target are3.2MWm-2,6.0MWm-2and15.0MWm-2, respectively. Accordingly, the maxima of erosion rates are3.1×10-8ms-1,5.7×10-8ms-1and1.0×10-7ms-1for carbon; while for tungsten, the ones are1.5x10-8ms-1,2.4×10-8ms-1and5.0x10-8ms-1, respectively. The peak vales of substrate temperature are520K and510K for carbon and tungsten, respectively. Therefore, it will not lead to surface melt damage for current plasma parameters of EAST.