Study Of Drift Wave Instabilities With Scale Length Of Ion Gyro-radius In Tokamak Plasmas

As a magnetically confined fusion device,tokamak is one of the most promising approaches to enable controlled fusion,which can solve the energy and environment problems ultimately.Actually,the transport coefficients given by the classical and neoclassical transport theories are less than the tokamak experimental results,which have been known as anomalous transport and constitute a major obstacle to the ignition and self-sustaining burning of the reactor.Microinstability-driven turbulence is widely recognized to be responsible for most of the observed cross field particle,momentum,and energy anomalous transports in tokamak plasmas.Understanding turbulent transports in fusion experiments,through both ion and electron channels,is one of the key open subjects for magnetic confinement fusion studies.It is generally believed that the ion temperature gradient(ITG)mode and trapped electron mode(TEM)are two of the most plausible candidates for turbulent transports with characteristic scale length of ion gyro-radius.Therefore,it is of great significance to elucidate the physical properties of these two microinstabilities,to explore the means of controlling them,and to understand the basic behavior of plasma and the research on advanced operation scenarios.After several decades development,gyrokinetic simulation has become a major tool to study the low frequency microinstabilities and turbulent transport in tokamak plasmas.In this thesis,we systematically study the effect of impurities on the ITG mode and TEM,the influence of plasma shaping on the ITG mode,and the properties of ITG mode and TEM with inverted density profile by using the gyrokinetic theory and numerical code.The research of this paper is helpful to understand the physical mechanism of turbulent transport and the control of microinstability,providing an important basis for improving the confinement performance of tokamak plasma.The basic structure and main research contents of the thesis are summarized as follows:In Chapter Ⅰ,the background and significance,basic physical model of this thesis are introduced,the overview of the microturbulence and work progress of the ITG mode and TEM are presented.The gyrokinetic integral equation in toroidal geometry employing s-αlocal equilibrium model is formulated.Also,the local gyrokinetic integral eigenvalue code HD7 is briefly outlined.In Chapter Ⅱ,the properties of the ITG mode and TEM in the presence of impurity ions is numerically investigated using the gyrokinetic integral eigenmode equation employing s-α local equilibrium model.The dependences of the growth rate,real frequency,eigenmode structure,and wave spectrum on charge concentration,charge number,and mass of impurity ions are analyzed in detail.The interesting finding of this work is that the effects of impurity ions on the TEM depend not only on the impurity density profiles but also on the ion temperature gradient,electron temperature gradient,electron density gradient.In particular,for flat and/or moderate electron density gradient,two independent unstable modes,which propagate in electron and ion diamagnetic drift directions,corresponding to TEM and ion mode,respectively,are found to coexist in a certain range.For peaked electron density gradient,however,only one mode is driven unstable by the combination of the driving forces of ions and trapped electrons.In Chapter Ⅲ,the effects of impurity ions on the ITG driven instability in elongated tokamak plasmas are numerically investigated using the upgraded HD7 code to solve the gyrokinetic integral eigenmode equation employing Miller’s local equilibrium model.It is found that the effect of elongation κ on the ITG mode mainly depend on the electron density gradient and poloidal wave number.Moreover,the low Z impurity ions with inwardly(outwardly)peaked density profiles have stabilizing(destabilizing)effects on the ITG mode in elongated plasmas.In particular,the high Z tungsten impurity ions with a weakly outwardly peaked density profile still have a stabilizing effect.Finally,the results indicating that the impurity mode is harder to be excited in elongated plasmas than in circular ones.In Chapter Ⅳ,linear characteristics of the ITG mode and TEM in tokamak plasmas with inverted density profiles are numerically investigated using the local eigenvalue code HD7 and global initial particle in cell code gKPSP.A thorough scan of crucial local parameters has been carried out in order to cover a wide range of physical conditions expected to occur in pellet fueled tokamak plasmas.The conditions for the existence of the ITG mode and TEM,separately or simultaneously,and the transitions between those two modes are discussed.It is found that the(▽T_i,▽T_e)parameter plane may be divided into four regions,viz.,the TEM unstable,the ITG mode unstable,the coexistence unstable,and the stable region.Stability diagrams in terms of(▽n,▽T)are divided into three regions:either TEM or ITG mode is unstable,both are unstable,and stable.Interestingly,the global linear simulations show that,with non-monotonic density profie,both the linear ITG mode and TEM eigenmode exhibit a double humped structure.In particular,in the flat density region,the ITG mode has the unconventional mode structure peaking at the poloidal angle θ ≠ 0.A possible implication of this work on particle transport in pellet fueled tokamak plasmas is also discussed.Conclusions are finally given,and future work is also discussed in Chapter V.