Investigation Of Impurity Transport In Tokamak Edge Plasma

Ihe impurity transport in the Tokamak boundary plays an important role in the control of the generation of impurities and their content in the plasma core,the mitigation of the interaction between the plasma and the plasma-facing component(PFC)to extend the life of the PFCs and improve the radiation divertor.There are experimental and simulation approaches for the study of the transport of impurities at the boundary.The former has the disadvantages of limited diagnostic access,limited information volume,insufficient resolution,systematic errors,and high diagnostic costs,making it difficult to systematically describe the overall process and characteristics of the boundary plasma.The simulation study can overcome the above shortcomings and is a powerful complement to the experimental research.It can link the scattered experimental information with each other,allowing for interpretation and diagnosis of each diagnostic measurements,and analysis of the overall characteristics of the boundary impurity transport.In addition,the simulation and prediction of impurity transport properties in the boundary plasma provides an important means of evaluating divertor performance and guiding divertor design and optimization.This thesis first briefly introduces the significance of nuclear fusion and impurity transport in fusion research,followed by an overview of the basic physics of boundary transport,including the introduction of divertors and scrape-off layer(SOL),the two-point model of boundary plasma,and the generation and transport of impurities,as well as the present status of experimental research and simulation on impurity transport.Then,the main simulation program used for this thesis is introduced:OEDGE(Osm-Eirene-Divimp)boundary plasma code package,which consists of an onion skin plasma model(OSM),Monte Carlo neutral impurity ion program EIRENE and Monte Carlo impurity transport program,DIVIMP.OEDGE has the advantages of being simple,easily coupled with experiment,fast and flexible to use.The main work of this thesis is based on experiments and simulations on the EAST device.A major part of this thesis is based on experiments and simulations on the EAST device.In the EAST experiment in 2014,the upper divertor was modified to a tungsten divertor.For high power operation with the upper and lower tungsten/carbon divertors,radiative divertor experiments were performed with Ar injection from different locations in the divertors.Impurity gas puffing in the Tokamak divertor region is an important means to assess the impurity screening effect of the divertor.In this work,a fast-response extreme-ultraviolet(EUV)spectrometer is used to monitor the Ar emission lines during the divertor Ar injection experiment in EAST.Based on the NIST(National Institute of Standards and Technology)atomic spectrum database,the emission lines from different ionized Ar ions in 20-500A wavelength range,e.g.Ar IV,Ar ?-? and Ar XIV-XVI,are identified.Ar XVI 353.9A and Ar IV 442.2A with the ionization energy of 918.4eV and 59.6eV,respectively,are monitored during the experiment with Ar puffing to study the behavior of Ar impurities in different regions in plasma simultaneously.The preliminary analysis on divertor impurity screening efficiency is carried out with the time evolution of the intensities of these two Ar emission lines.The results with Ar puffing from the same gas puffing inlet(e.g.from the lower outer target inlet)but with different plasma configurations(e.g.lower single null,upper single null)show that the screening efficiency for Ar injected from the divertor region is better than that from the main plasma region,and the screening efficiency of lower divertor,as well as the particle exhaust by the internal cryopump installed in the lower divertor,is stronger than the upper divertor.In addition,modeling with OEDGE has been carried out to assess the initial and long-term plasma contamination efficiency of Ar puffing from the different divertor locations,i.e.,the inner divertor,the outer divertor and the dome,in EAST for typical Ohmic plasma conditions.It is found that the initial Ar contamination efficiency is dependent on the local plasma conditions at the different gas puff locations.However,it quickly approaches a similar steady state level due to high Ar recycling efficiency,>0.9.OEDGE modeling shows that the final equilibrium Ar contamination efficiency is significantly lower for the more closed lower divertor than that for the upper divertor.The last part of this thesis is the simulation of the tungsten rings experiment on the DIII-D device.The integration of the OEDGE program into the OMFIT platform was introduced.OEDGE simulation is carried out to understand the divertor W transport in the SOL for different divertor configurations during the divertor metal rings campaign(MRC)on DIII-D.It is found that the W leakage(the fraction of the sputtered W that enters the core plasma)from the strike point(SP)region only differs by 5%between partially closed and open divertor configurations.Simulations of the discharge with partially closed divertor configuration indicate that the W leakage from the far SOL is about two times higher than that from the strike point due to higher density near the strike point,which increases W prompt re-deposition,even though the absolute level of contamination of the far SOL W source is lower.