Effects of finite orbit width on neoclassical transport in high-temperature helical plasmas 高温ヘリカルプラズマにおける新古典輸送に対する有限軌道幅の効果
Effects of finite orbit width on neoclassical transport in high-temperature helical plasmas
Improving confinement of the particle and energy transport is an important task torealize a nuclear fusion reactor in toroidal magnetic devices and a great effort hasbeen devoted theoretically and experimentally to achieve this aim. Neoclassical (NC) transport theory has been studied in detail, since it describes a diffusive transportphenomenon caused by particle collisional interactions in a torus configuration, andthus, determines a irreducible minimum transport level depending on the magneticgeometry. In helical/stellarator devices which have the three-dimensional magneticstructure, neoclassical transport has a character of increasing with T7=2n low collisionality regime, where Ta is the temperature of species a = e; i. In addition to this,the radial electric field (Er), which in general reduce both neoclassical and anomaloustransport, is determined by the ambipolar condition of neoclassical particle transportin helical devices. The plasmas of Ti > 5 keV are successfully obtained in the recent LHD experiments.Neoclassical transport analyses are performed for such plasmas. We confirm that whenTi increases, the NC transport flux is reduced by two orders of magnitude comparedto that without Er due to the existence of the ambipolar radial electric field. Theparameter survey calculations on Ti and ne are also carried out to consider the NCtransport flux dependence on the plasma parameter. With these calculations, it isshown that NC ion thermal diffusivity is reduced to small level as that of electroneven for plasmas with the fusion reactor relevant parameter if Te ' Ti is numerically retained. It is found that the radial electric field in high Ti plasmas with high Te hasa significant impact on the reduction of the NC transport. This fact provides us the opportunity to reconsider the NC transport more rigorously in high Te plasmas. Neoclassical transport in an asymmetric magnetic field has been estimated and calculated by using numerical simulations based on local assumptions, which neglect theparticle drift, or the deviation from a certain magnetic surface. Although it has been pointed out that the finite orbit width effect for ions plays an important role in neoclasisical transport theory in recent studies, it has been considered that such conventionallocal assumptions have been valid for electrons. This is because the deviation fromthe magnetic surface on which the electron is located initially has been considered tobe small enough. On the other hand, very high Te plasmas exceeding Te ' 20 keVat the plasma core region followed by electron internal transport barrier formation ofthe steep Te gradient have been achieved in recent experiments in LHD. These plasmasare called Core Electron-Root Confinement, CERC, since they are accompanied by the formation of the strong positive radial electric field, or the electron root. The high electron temperature makes helically-trapped electrons drift away from the initialmagnetic surface. As a result, local assumptions of the neoclassical transport may bebroken even for electrons in CERC plasmas. This effect of electron drift, however, has not been considered seriously so far and it is quite unclear whether the local treatment is valid or not. In this thesis, the electronfinite orbit width effect on neoclassical transport is investigated in detail by using nonlocal_f Monte Carlo simulation code, FORTEC-3D, which is newly extended to applyto electrons in this work. It is found that the electron finite drift makes qualitativedifference between the local treatment and the non-local treatment in neoclassical transport calculations.This thesis is organized as follows. First, we performed NC transport analysis basedon the local treatment for high Ti plasmas. With these calculations, the electron-root Eris obtained in high Ti plasmas numerically considering Te is high at the same time. This suggests that the non-local electron drift plays an important role in the Er formation. Second, FORTEC-3D, which solves the drift kinetic equation without the local assumptions is newly extended to apply to electrons including electron-ion collisions. Precise benchmark calculations are carried out with DCOM/NNW and GSRAKE code, which are both widely used local neoclassical transport simulation codes. By numerical calculations, it is found that the electron ∇B and curvature drifts change the particleand energy flux due to the particle poloidal precession and collisionless detrapping process in high Te and the low collisionality regime, while results in low Te and high collisionality regime reproduce the similar transport dependence on Er obtained byDCOM/NNW and GSRAKE. The changes of NC transport in the low collision alityregime appear as the reduction of the peak value and/or shift of peak position in flux dependence on the radial electric field. Non-local effect is confirmed by fully taking the particle drift and its orbit into account in neoclassical transport calculations byFORTEC-3D. Third, the extended FORTEC-3D for electrons is applied to a CERC plasma obtained in the LHD experiment. Such non-local electron transport analysis for the LHD experimental discharge is performed for the first time in this study. The radial electric field is analyzed in two ways: (1) the electron particle flux is calculated by FORTEC-3D with the fixed radial electric field for given plasma profiles and the steady state radial electric field is determined so as to satisfy the ambipolar condition of the electron particle flux obtained by FORTEC-3D and the ion particle flux by DCOM/NNW. (2) Time evolution of the radial electric field is followed as an initial value problem with given plasma profiles using ion-particle-flux-radial-electric-field table made by DCOM/NNW. The ambipolar radial electric field is obtained as its steady state solution. This separate procedure for electron and ion is adopted in order to reduce the calculation alburden since simultaneously calculating the NC transport for both species waste toomuch computational resources. It is shown that the resultant Er differs from that obtainedby DCOM/NNW in the core region, while it agrees with ion-root Er evaluatedby DCOM/NNW in the edge region. In this study, the importance of the electron finite orbit width effect in determining the neoclassical transport flux and its influence on the radial electric formation in high temperature helical plasmas is investigated by directly the the drift kinetic equation including the finite orbit width effect of electrons. With this approach, we provide asufficient and reasonable basis on how the electron drift affects the neoclassical transportand the resultant radial electric field. This enables one to analyze the neoclassical transport property with a desirable accuracy, and thus, leads ones to obtain more detailed physical insight to the plasma physics involving the transport and the radial electric field.