37 research outputs found
Impact of Geodesic Curvature on Zonal Flow Generation in Magnetically Confined Plasmas
No full text0000-0003-2693-4859The impact of magnetic geometry on zonal-flow generation in ion temperature gradient driven turbulence is investigated by means of linear and nonlinear gyrokinetic simulations. The modulation of geodesic curvature on various configurations has revealed amplification of the zonal-flow intensity in relatively smaller geodesic curvature. Based on these findings, a nonlinear proxy model for explorations of novel magnetic geometry to activate the zonal-flow dynamics is proposed.journal articl
Polarization and magnetization in collisional and turbulent transport processes
No full textExpressions of polarization and magnetization in magnetically confined
plasmas are derived, which include full expansions in the gyroradius to treat
effects of both equilibrium and microscopic electromagnetic turbulence. Using
the obtained expressions, densities and flows of particles are related to those
of gyrocenters. To the first order in the normalized gyroradius expansion, the
mean part of the particle flow is given by the sum of the gyrocenter flow and
the magnetization flow, which corresponds to the so-called magnetization law in
drift kinetics, while the turbulent part contains the polarization flow as
well. Collisions make an additional contribution to the second-order particle
flow. The mean particle flux across the magnetic surface is of the second-order
and it contains classical, neoclassical, and turbulent transport processes. The
Lagrangian variational principle is used to derive the gyrokinetic Poisson and
Amp\`{e}re equations which properly include mean and turbulent parts so as to
be useful for full- global electromagnetic gyrokinetic simulations. It is
found that the second-order Lagrangian term given by the inner product of the
turbulent vector potential and the drift velocity consisting of the curvature
drift and the drift should be retained in order for the derived
Amp\`{e}re equation to correctly include the diamagnetic current which is
necessary especially for the full- high-beta plasma simulations. The
turbulent parts of these gyrokinetic Poisson and Amp\`{e}re equations are
confirmed to agree with the results derived from the WKB representation in
earlier works.Comment: 16 pages, submitted to Physics of Plasma
Effects of Finite Orbit Width on Neoclassical Transport in High-Temperature Helical Plasmas
No full text総合研究大学院大学博士（理学） 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.application/pdf総研大甲第1412号thesi
Global kinetic simulations of neoclassical toroidal viscosity in low-collisional perturbed tokamak plasmas
No full textNeoclassical transport benchmark of global full-f gyrokinetic simulation in stellarator configurations
No full textNeoclassical electron transport calculation by using delta f Monte Carlo method
No full textHigh electron temperature plasmas with steep temperature gradient in the core are obtained in recent experiments in the Large Helical Device [ A. Komori et al., Fusion Sci. Technol. 58, 1 (2010) ]. Such plasmas are called core electron-root confinement (CERC) and have attracted much attention. In typical CERC plasmas, the radial electric field shows a transition phenomenon from a small negative value (ion root) to a large positive value (electron root) and the radial electric field in helical plasmas are determined dominantly by the ambipolar condition of neoclassical particle flux. To investigate such plasmas' neoclassical transport precisely, the numerical neoclassical transport code, FORTEC-3D [ S. Satake et al., J. Plasma Fusion Res. 1, 002 (2006) ], which solves drift kinetic equation based on δf Monte Carlo method and has been applied for ion species so far, is extended to treat electron neoclassical transport. To check the validity of our new FORTEC-3D code, benchmark calculations are carried out with GSRAKE [ C. D. Beidler et al., Plasma Phys. Controlled Fusion 43, 1131 (2001) ] and DCOM/NNW [ A. Wakasa et al., Jpn. J. Appl. Phys. 46, 1157 (2007) ] codes which calculate neoclassical transport using certain approximations. The benchmark calculation shows a good agreement among FORTEC-3D, GSRAKE and DCOM/NNW codes for a low temperature (Te(0) = 1.0 keV) plasma. It is also confirmed that finite orbit width effect included in FORTEC-3D affects little neoclassical transport even for the low collisionality plasma if the plasma is at the low temperature. However, for a higher temperature (5 keV at the core) plasma, significant difference arises among FORTEC-3D, GSRAKE, and DCOM/NNW. These results show an importance to evaluate electron neoclassical transport by solving the kinetic equation rigorously including effect of finite radial drift for high electron temperature plasmas
