Study On High-Z Impurity Transport In EAST And WEST Core Plasma

The plasma can be contaminated by impurities caused by the inevitable interaction between the plasma and the PFCs(Plasma Facing Components).The Radiation power and fuel dilution caused by these impurities limits the performance of fusion plasma.As high-Z metallic materials,like Mo and W,can have good properties as PFCs,such as high melting point,low sputtering yield and high thermal conductivity,they are foreseen as plasma facing material for ITER and future fusion reactor.However,compared to low Z impurities,high Z impurity is more easily accumulated in the core plasma,which can cause enormous energy loss because of their high cooling rate.Therefore,it is important to understand the transport of high Z impurities in core plasma for future optimized tokamak operation.In this thesis,the background and mechanism of nuclear fusion are briefly overviewed,as well as the impurity source and the influence of impurity on plasma,followed by atomic processes and the equilibrium of different ionization sates of impurity.Then two kinds of transports,neoclassical transport and turbulent transport,are specifically explained,including the modeling methods and tools.In addition,diagnostics used to monitor impurity behaviors are outlined and the absolute calibration of the spectrum intensity of one EUV spectrometer are introduced in detail.Moreover,two methods of studying impurity transport from the experimental perspective are introduced and preliminary results are presented.One major part of this thesis is quantitative analysis of concentrations of high Z impurities in EAST core plasma,which is essential for study on power or particle balance,as well as for study on impurity transport.Based on absolute measurement of impurity line emissions by EUV spectrometer and combined with Te measured by TS,ne measured by POINT and total power radiation measured by AXUV,as well as 1-D impurity transport model and ADAS atomic database,efficient and reliable methods to calculate concentrations of high Z impurities are successfully developed.With the developed methods,monitoring impurity concentration in quasi real-time and quantitatively statistical analysis of impurity concentration become possible.Some statistical analysis work are carried out,e.g.high Z impurity concentration versus total power injection for both L-and H-mode plasma,variation of the W concentration after the update of guard limiter of LHW2 from graphite to W,contributions to power loss and effective charge by high Z impurities and W concentration versus ELM frequency in both low and high ?p cases jointly heated by RF and NBI.Another important part of this thesis is studying core W transport in WEST long pulse L-mode plasmas.These plasmas are operated with the strike point on the actively cooled upper tungsten divertor.The pulses are mostly heated by lower hybrid waves.It is experimentally found that tungsten does not centrally accumulate throughout these 30s reproducible discharges despite large normalized electron density gradients R/Lne.To explain these observations,turbulent and neoclassical transport of electrons and tungsten ions are computed with GKW and NEO respectively.Additionally,interpretative integrated modelling simulations are also performed to keep data coherency despite the lack of measurements of some quantities such as the Ti profiles.Modelled R/Lne are found consistent with interferometry inversions and the tungsten peaking factor R/LnW remains comparable to R/Lne due to dominant turbulent diffusivities inside r/a=0.3-0.8.In the central region r/a<0.3 neoclassical W transport dominates but the convective velocities are several orders of magnitudes lower compared to plasmas with toroidal rotation velocities induced by a neutral beam injection(NBI)torque.Finally,nitrogen is seeded in these pulses leading to an enhanced energy content which is consistent with stabilized ion temperature gradient modes from dilution.