| In tokamak devices,divertor and limiter are key plasma-facing components(PFCs),which are mainly used to absorb the heat flux coming from the core plasma.As plasma parameters continue to rise,the risks of thermal damage to internal components increase due to the escalating heat flux deposited on their surfaces.Such damage not only diminishes component performance and lifespan but also poses safety concerns for device.Therefore,understanding the physical mechanism of such damages is very important for the plasma operation and engineering design of future fusion reactors.In this thesis,based on the analysis of diverters and limiters in EAST,focuses on the characteristics of damage and investigates the vertical heat flux distribution on the PFCs.It reveals the mechanisms for the local heat flux responsible for the thermal damage to diverters and limiters and assesses the impact of this damage on plasma operation.Firstly,the investigation of the damages on W/Cu monoblocks for upper divertor has been carried out.From 2017,a large number of melting and cracking phenomena have been found on the both sides of cassette,primarily related to steady-state heat flux.Based on a 3D model of the divertor and magnetic field data from the EFIT database,the study about magnetic shadow areas simulated by PFC-Flux code has been carried out.The impact of misalignment between cassettes and the helical magnetic field lines on the toroidal distribution of vertical heat flux has been analyzed.It confirms that divertor damage is primarily related to leading edge due to misalignment.When the misalignment reaches 3 mm,50%to 82%of the heat flux near the strike points on the adjacent cassette is moved to a 3 mm-wide leading edge.Combined with the extreme ultraviolet spectra and radiation power,the study about the first reason for disruption has been carried out,and obtain the disruptiveties due to different impurity burst.Considering the diversity of materials,the study clarifies the impact of damage to different PFCs on plasma operation.About 30%of disruptions are associated with damage to the upper divertor,which is an important factor in constraining the achievement of high-power discharges.Secondly,the investigation of the damage on graphite and molybdenum tiles on the main limiter has been performed.Melting of the copper bolts in the molybdenum tiles located at the ion side of the main limiter indicates that the damage is primarily related to localized heat flux during steady-state discharges.Statistical analysis of hot spot temperatures concerning discharge parameters,combined with observations from JET and C-Mod experiments,it suggests that the crescent-shaped hot spots may be as a result of trapped ions.The impact of trapped ions on the vertical heat flux at the limiter’s surface has been studied by the simulations of trapped ion orbits.It is confirmed that the crescent-shaped hot spots are mainly caused by the trapped ions produced by ICRF.For the parallel heat flux profile during high-power discharges,a new method which uses PFC-Flux and ANSYS code to calculate it based on the temperature on the surface of main limiter has been presented.In this method,two impacts have been considered,which effectively improves the accuracy of the calculation.The first one is the influence of the length of magnetic field line on the parallel heat flux profile.The second one is the influence of the optical transfer function of the infrared system on the comparison between the simulated and experimental temperatures.The simulation results indicate that the parallel heat flux increases by 30 MW/m2 when the power of ICRF increases by 1 MW,and it decreases by 10 MW/m2 when the core plasma density increases by 1×1019 m-3.The maximum vertical heat flux deposited on the graphite tiles reaches 10.7 MW/m2.This study reveals that the local heat flux due to trapped ions produced by ICRF is the primary cause of limiter damage and a significant factor constraining the achievement of the high-confinement discharges under auxiliary heating power more than 10 MW.Finally,the investigation of the damage on tungsten/copper flat-type components on the guard limiter has been carried out.There are two types of damages.The first type contains shallow melting and grid-like toroidal cracks,which is mainly related to transient heat flux.The transient heat flux is mainly due to the disruptions during limiter configuration in low-power cleaning discharges.The second type includes macroscopic poloidal cracks,which is mainly induced by steady-state heat flux.Due to the poloidal cracks,some damaged tiles had been removed from guard limiter.It is found that the radial position of guard limiter had been shifted,significantly altering its function to resemble that of the main limiter.The excessive steady-state heat flux is produced during high-parameter discharges.Such analysis of damage mechanism of divertor and limiter components yields a crucial scientific foundation for optimizing the physical parameters and engineering design of the EAST tokamak.Moreover,it can be extending as a valuable reference for both the experimental operation of ITER and the design of future fusion reactors. |
