489 research outputs found
Solution space and effective model for turbulent transport of helical plasmas
No full textORCID 0000-0002-2459-2392We discuss an effective transport model of magnetized turbulenthelical plasma based on the solution space of first-principle gyrokinetic simulations. If the time evolution of dynamical systems can be regarded as solution trajectories in theoretical phase space, physical phenomena in the saturated stable phase are realized in the solution space formed by these trajectories in the long time limit. Similar properties are found not only in dynamical systems but also in general physical systems with renormalization group flows (Wilson and Kogut 1974 Phys. Rep. C12 75). Therefore, if the solutions effectively form a finite dimensional solution space, the physical system can generally be represented in reduced form. Here, we try to apply this discussion to develop a transport model of turbulent plasma in first-principle gyrokinetic simulations. Based on the solution space due to the trajectory of the simulations with a certain functional form (Fujii and Nunami 2022 Plasma Fusion Res. 17 2403030), we discuss the effective structure of the objective function to represent the transport model. By evaluating the structure with fitting errors of the objective function in the model parameter space, we can determine a plausible functional form. This paper discusses a methodology for constructing such an effective transport model for helical plasmas.journal articl
Relations among Turbulent Fluctuations, Zonal Flows, and Transport Coefficients in Time Series Data of Gyrokinetic Simulations
No full textThe relations among turbulent amplitude, zonal-flow amplitude, and transport level are discussed for the time series data of nonlinear gyrokinetic simulations for magnetized toroidal plasmas. Since it was shown that the transport coefficient can be expressed as a function of the time-averaged turbulent fluctuation level and the zonal flow amplitude [M. Nunami et al., Phys. Plasmas 20, 092307 (2013)], we apply the results to a model function for the turbulent plasma transport coefficient to extend to a functional relation which includes not the time-averaged data but the time-series data of gyrokinetic simulations. We obtain a new functional relation to the turbulent fluctuations, the zonal-flow amplitudes, and the transport coefficients as a function of the input parameters of the gyrokinetic simulations such as plasma temperature gradients. It is also confirmed that the obtained functional relation can reduce relative error which is compared with the original function with time-averages.journal articl
Microinstabilities in hydrogen- and helium-dominated multi-ion-species plasmas in LHD
No full textThe ion scale microinstabilities in the large helical device (LHD) are investigated by the gyrokinetic simulations for the multi-ion-species plasmas including hydrogen, helium, and impurity ions. The observations in the LHD experiments show that the ion temperature increases with the decreases of the ratio of hydrogen density to helium density. It is found from the linear gyrokinetic simulations with the multi-ion-species and real-mass kinetic electrons in the LHD discharges that the growth rates of the ion scale microinstabilities are reduced for the helium-dominated multi-species plasma compared with the hydrogen-dominated one. In addition to the differences of the conditions including the temperature, the density profiles, and the temperature ratio between both plasmas, due to the dependence on the mass number and the electric charge of the mixed ion species, the mixing length estimates obtained from the linear simulations predicts smaller ion thermal diffusivity for the helium-dominated plasma than the hydrogen-dominated one in the hydrogen gyro-Bohm unit, which is consistent with the experimental results.journal articl
Isotope Effects on Trapped-Electron-Mode Driven Turbulence and Zonal Flows in Helical and Tokamak Plasmas
No full textImpacts of isotope ion mass on trapped-electron-mode (TEM)-driven turbulence and zonal flows in magnetically confined fusion plasmas are investigated. Gyrokinetic simulations of TEM-driven turbulence in three-dimensional magnetic configuration of helical plasmas with hydrogen isotope ions and real-mass kinetic electrons are realized for the first time, and the linear and the nonlinear nature of the isotope and collisional effects on the turbulent transport and zonal-flow generation are clarified. It is newly found that combined effects of the collisional TEM stabilization by the isotope ions and the associated increase in the impacts of the steady zonal flows at the near-marginal linear stability lead to the significant transport reduction with the opposite ion mass dependence in comparison to the conventional gyro-Bohm scaling. The universal nature of the isotope effects on the TEM-driven turbulence and zonal flows is verified for a wide variety of toroidal plasmas, e.g., axisymmetric tokamak and non-axisymmetric helical or stellarator systems.journal articl
Linear Gyrokinetic Analyses of ITG Modes and Zonal Flows in LHD with High Ion Temperature
No full textIon temperature (Ti) gradient modes (ITG modes) and zonal flows for high Ti discharges in the Large Helical Device (LHD) are investigated by linear gyrokinetic Vlasov simulation. In recent LHD experiments, high Ti plasmas are generated by neutral beam injection, and spatial profiles of density fluctuations are measured by phase contrast imaging (PCI) [K. Tanaka et al., Plasma Fusion Res. 5, S2053 (2010)]. The observed fluctuations most likely propagate in the direction of the ion diamagnetic rotation in the plasma frame, and their amplitudes increase with the growth of the temperature gradient. The results show the characteristics of ITG turbulence. To investigate the ITG modes and zonal flows in the experiment, linear gyrokinetic simulations were performed in the corresponding equilibria with different Ti profiles by using the GKV-X code [M. Nunami et al., Plasma Fusion Res. 5, 016 (2010)]. The simulation results predict unstable regions for the ITG modes in radial, wavenumber, and phase velocity spaces, in agreement with the PCI measurements. Thus, the fluctuations observed in the experiment are attributed to ITG instability. The responses of the zonal flows show clear contrasts in different field spectra that depend on the Ti profile and the radial position. In addition to the dependence on the field spectra, the zonal flow residual levels are enhanced by increasing the radial wavenumber as theoretically predicted.journal articl
Research of turbulent transport due to dissipative trapped electron mode in tokamak plasmas
No full textORCID 0000-0002-0712-8811The purpose of this article is to study turbulent transport for laboratory plasmas in toroidal devices by gyrokinetic analyses. Linear analysis is performed to clarify the dominant mode for tokamak plasmas. The dissipative trapped electron mode (d-TEM) and the ion temperature gradient (ITG) mode are predicted using the Sugama collision model operator [Sugama et al., Phys. Plasmas 16, 112503 (2009)]. Nonlinear gyrokinetic analysis is used to quantify turbulent transport. The nonlinear simulation results show the levels of particle and energy transport, where the d-TEM and ITG mode are unstable. The effect of zonal flows is studied by the linear and nonlinear simulation results. The results of the analysis are compared when two types of model collision operator, which are the Sugama and Lenard–Bernstein [Phys. Rev. 112, 1456 (1958)] collision model operators, are used. In this study, the simulation results using the Sugama collision operator show a stronger effect of the zonal flows on the turbulent transport than those using the Lenard–Bernstein collision operator, as predicted by the linear simulation result such as the zonal flow decay time.journal articl
マルチスケール粒子シミュレーション手法の開発と宇宙機--宇宙プラズマ相互作用への応用(2.プラズマにおける連結階層シミュレーション,<小特集>連結階層モデルによって見えてきたプラズマシミュレーションの新たな局面)
No full textPolarization 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
Energy exchange between electrons and ions in ion temperature gradient turbulence
No full textORCID 0009-0004-1263-8826Microturbulence in magnetic confined plasmas contributes to energy exchange between particles of different species as well as the particle and heat fluxes. Although the effect of turbulent energy exchange has not been considered significant in previous studies, it is anticipated to have a greater impact than collisional energy exchange in low collisional plasmas such as those in future fusion reactors. In this study, gyrokinetic simulations are performed to evaluate the energy exchange due to ion temperature gradient (ITG) turbulence in a tokamak configuration. The energy exchange due to the ITG turbulence mainly consists of the cooling of ions in the -curvature drift motion and the heating of electrons streaming along a field line. It is found that the ITG turbulence transfers energy from ions to electrons regardless of whether the ions or electrons are hotter, which is in marked contrast to the energy transfer by Coulomb collisions. This implies that the ITG turbulence should be suppressed from the viewpoint of sustaining the high ion temperature required for fusion reactions since it prevents energy transfer from alpha-heated electrons to ions as well as enhancing ion heat transport toward the outside of the reactor. Furthermore, linear and nonlinear simulation analyses confirm the feasibility of quasilinear modeling for predicting the turbulent energy exchange in addition to the particle and heat fluxes.journal articl
Maximal Locality and Predictive Power in Higher-Dimensional, Compactified Field Theories
No full textTo realize maximal locality in a trivial field theory, we maximize the ultraviolet cutoff of the theory by fine tuning the infrared values of the parameters. This optimization proce-dure is applied to the scalar theory in D + 1 dimensions (D ≥ 4) with one extra dimension compactified on a circle of radius R. The optimized, infrared values of the parameters are then compared with the corresponding ones of the uncompactified theory in D dimensions, which is assumed to be the low-energy effective theory. We find that these values approxi-mately agree with each other as long as R−1> ∼ sM is satisfied, where s 10, 50, 50, 100 for D = 4, 5, 6, 7, and M is a typical scale of the D-dimensional theory. This result supports the previously made claim that the maximization of the ultraviolet cutoff in a nonrenormalizable field theory can give the theory more predictive power. Kaluza and Klein’s idea of unifying fundamental forces by introducing extra dimensions1) has been attractive for many decades. Recently, there has been re-newed interest in field theories with extra dimensions.2)–6) Because field theories in more than four dimensions are usually nonrenormalizable, the dependence on th
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