Tungsten Erosion And Transport Processes In The Edge Plasma Of TOKAMAKs

As one of the most important plasma facing material,tungsten's erosion processes under different edge plasma conditions are studied in this paper.The paper includes two parts:1,Tungsten erosion processes in the DIII-D H-mode discharges;2,Evaluation of the tungsten erosion and tungsten core concentration for CFETR.3 discharges with varying type-I ELM sizes(AW/W ranging from 3%to 10%)and ELM frequencies(18 Hz to 70 Hz)are chosen from the DIII-D Metal Rings Campaign to study the relationship between the tungsten gross erosion and the ELM characteristics.The one-dimensional 'free-streaming' model is used to analyze the ELM transport time and electron density at the target plane.The ELM transport time is used to evaluate the charge state of C originating from pedestal,and the divertor electron density is used to accurately calculate the intra-ELM tungsten effective ionizations/photon(S/XB)value.The OEDGE code is employed to calculate the inter-ELM impurity distribution and tungsten gross erosion.Then a semi-analytical model based on the TRIM.SP sputtering code is used to simulate the intra-ELM tungsten gross erosion profiles.For both inter-and intra-ELM simulations,a carbon-tungsten mixed target material is used.The OEDGE results reveal that the inter-ELM tungsten gross erosion rate is not sensitive to the carbon fraction on the tungsten surface layer in a wide range,which is consistent with a simple analytical model.Carbon fractions from 0.3 to 0.6 can give similar tungsten gross erosion profiles and can match the diagnostic data.But a further intra-ELM simulation indicates the carbon fraction should be about 0.3.For both inter-and intra-ELM cases,carbon is predicted to dominate the tungsten erosion.However,unlike the inter-ELM tungsten erosion which is dominated by locally redeposited C2+,the energetic C6+ originating from the pedestal can contribute substantial tungsten erosion in the near-separatrix region during ELMs.To match the experimental data,the C6+ is assumed to have a smaller decay length than deuterium.The ratio of carbon-deuterium decay lengths has a strong relationship with the energy ELM carried.For CFETR,the tungsten erosion and distribution are evaluated for cases of different divertor running regimes,different impurity injections,and different injection locations.The results reveal that,running in the detachment is the only acceptable choice for the divertor protection.When running in the detachment,the impurity injection location will not affect the impurity distribution very much,so as to the tungsten core concentration.It is easier for the tungsten originating from the first wall to transport into the core region,thus for the detached cases,the tungsten erosion from the first wall determines the tungsten core concentration.Comparing with Ne and N injections,taking advantage of the strongest radiation ability,the Ar injection leads to the least impurity concentration in the plasma,and thus smallest first wall tungsten erosion.Therefore,the Ar injection is the best choice for the first wall protection.