Masanori Nunami

Momentum balance and radial electric fields in axisymmetric and nonaxisymmetric toroidal plasmas

We have investigated the influence of symmetry properties of toroidal magnetic configurations on the mechanisms used for determining the radial electric field such as the momentum balance and the ambipolar particle transport. Both neoclassical and anomalous transport of particles, heat and momentum in axisymmetric and nonaxisymmetric toroidal systems are taken into account. Generally, in nonaxisymmetric systems, the radial electric field is determined by the neoclassical ambipolarity condition. For axisymmetric systems with up–down symmetry and quasisymmetric systems with stellarator symmetry, it is shown using a novel parity transformation that the particle fluxes are automatically ambipolar up to and the determination of the radial electric field Es requires solving the momentum balance equations, where δ denotes the ratio of the thermal gyroradius to the characteristic equilibrium scale length. In axisymmetric systems with large E × B flows on the order of the ion thermal velocity vTi, the radial fluxes of particles, heat and toroidal momentum are dependent on Es and its radial derivative while the time evolution of the Es profile is governed by the toroidal momentum balance equation. In nonaxisymmetric systems, E × B flows of are not generally allowed even in the presence of quasisymmetry because the nonzero radial current is produced by the large flow term in the equilibrium force balance for which the Boozer and Hamada coordinates cannot be constructed.

Linearized model collision operators for multiple ion species plasmas and gyrokinetic entropy balance equations

Linearized model collision operators for multiple ion species plasmas are presented that conserve particles, momentum, and energy and satisfy adjointness relations and Boltzmann’s H-theorem even for collisions between different particle species with unequal temperatures. The model collision operators are also written in the gyrophase-averaged form that can be applied to the gyrokinetic equation. Balance equations for the turbulent entropy density, the energy of electromagnetic fluctuations, the turbulent transport fluxes of particle and heat, and the collisional dissipation are derived from the gyrokinetic equation including the collision term and Maxwell equations. It is shown that, in the steady turbulence, the entropy produced by the turbulent transport fluxes is dissipated in part by collisions in the nonzonal-mode region and in part by those in the zonal-mode region after the nonlinear entropy transfer from nonzonal to zonal modes.

Quasilinear Carbon Transport In An Impurity Hole Plasma In LHD

Comprehensive electrostatic gyrokinetic linear stability calculations for ion-scale microinstabilities in an LHD plasma with an ion-internal transport barrier (ITB) and carbon “impurity hole” are used to make quasilinear estimates of particle flux to explore whether microturbulence can explain the observed outward carbon fluxes that flow “up” the impurity density gradient. The ion temperature is not stationary in the ion-ITB phase of the simulated discharge, during which the core carbon density decreases continuously. To fully sample these varying conditions, the calculations are carried out at three radial locations and four times. The plasma parameter inputs are based on experimentally measured profiles of electron and ion temperature, as well as electron and carbon density. The spectroscopic line-average ratio of hydrogen and helium densities is used to set the density of these species. Three ion species (H,He,C) and the electrons are treated kinetically, including collisions. Electron instability driv...

Physics analyses on the core plasma properties in the helical fusion DEMO reactor FFHR-d1

Physics assessments on magnetohydrodynamics equilibrium, neoclassical transport and alpha particle confinement have been carried out for the helical fusion DEMO reactor FFHR-d1, using radial profiles extrapolated from the Large Helical Device. Large Shafranov shift is foreseen in FFHR-d1 due to its high-beta property. This leads to deterioration in neoclassical transport and alpha particle confinement. Plasma position control using vertical magnetic field has been examined and shown to be effective for Shafranov shift mitigation. In particular, in the high-aspect-ratio configuration, it is possible to keep the magnetic surfaces similar to those in vacuum with high central beta of ~8% by applying a proper vertical magnetic field. As long as the Shafranov shift is mitigated, the neoclassical heat loss can be kept at a level compatible with the alpha heating power. The alpha particle loss can also be kept at a low level if the loss boundary of alpha particles is on the blanket surface and the plasma position control is properly applied. The lost positions of alpha particles are localized around the divertor region that is located behind the blanket in FFHR-d1.

Abstract: Communication Overlap Techniques for Improved Strong Scaling of Gyrokinetic Eulerian Code beyond 100k Cores on the K-Computer

A plasma turbulence research based on 5D gyrokinetic simulations is one of the most critical and demanding issues in fusion science. To pioneer new physics regimes both in problem sizes and in time scales, an improvement of strong scaling is essential. Overlap of computations and communications is a promising approach in improving strong scaling, but it often fails on practical applications with conventional MPI libraries. In this work, this classical issue is revisited, and resolved by communication overlap techniques, which work even on conventional MPI libraries. These techniques dramatically improve the parallel efficiency of a gyrokinetic Eularian code GT5D on the K-computer and the Helios, which are based on dedicated and commodity networks, respectively. On the K-computer, excellent strong scaling is confirmed beyond 100k cores with keeping the peak ratio of ~10% (~307 TFlops using 196,608 cores), and simulations for ITER-size fusion devices are significantly accelerated.

Turbulent transport of heat and particles in a high ion temperature discharge of the Large Helical Device

Turbulent transport in a high ion temperature discharge of the Large Helical Device (LHD) is investigated by means of electromagnetic gyrokinetic simulations, which include kinetic electrons, magnetic perturbations, and full geometrical effects. Including kinetic electrons enables us to firstly evaluate the particle and the electron heat fluxes caused by turbulence in LHD plasmas. It is found that the electron energy transport reproduces the experimental result, and that the particle flux is negative. The contribution of magnetic perturbation to the transport is small because of very low beta. The turbulence is driven by the ion temperature gradient instability, and the effect of kinetic electrons enhances the growth rate larger than that from the adiabatic electron calculation. The ion energy flux is larger than that observed in the experiment, while the flux is close to the experimental observation when the temperature gradient is reduced 20% in the simulation. This significant sensitivity of the energy flux implies that the profile in the experiment is close to the critical temperature gradient. The critical gradient for turbulent energy flux is similar to that for the linear instability, i.e., the Dimits shift is small. This is because the zonal flow in the LHD is weaker than that in tokamaks.

Impact of hydrogen isotope species on microinstabilities in helical plasmas

The impact of isotope ion mass on ion-scale and electron-scale microinstabilities such as ion temperature gradient (ITG) mode, trapped electron mode (TEM), and electron temperature gradient (ETG) mode in helical plasmas are investigated by using gyrokinetic Vlasov simulations with a hydrogen isotope and real-mass kinetic electrons. Comprehensive scans for the equilibrium parameters and magnetic configurations clarify the transition from ITG mode to TEM instability, where a significant TEM enhancement is revealed in the case of inward-shifted plasma compared to that in the standard configuration. It is elucidated that the ion-mass dependence on the ratio of the electron–ion collision frequency to the ion transit one, i.e. , leads to a stabilization of the TEM for heavier isotope ions. The ITG growth rate indicates a gyro-Bohm-like ion-mass dependence, where the mixing-length estimate of diffusivity yields . On the other hand, a weak isotope dependence of the ETG growth rate is identified. A collisionality scan also reveals that the TEM stabilization by the isotope ions becomes more significant for relatively higher collisionality in a banana regime.

Isotope Effects on Trapped-Electron-Mode Driven Turbulence and Zonal Flows in Helical and Tokamak Plasmas

Impacts 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.

Effects of collisions on conservation laws in gyrokinetic field theory

Effects of collisions on conservation laws for toroidal plasmas are investigated based on the gyrokinetic field theory. Associating the collisional system with a corresponding collisionless system at a given time such that the two systems have the same distribution functions and electromagnetic fields instantaneously, it is shown how the collisionless conservation laws derived from Noethers theorem are modified by the collision term. Effects of the external source term added into the gyrokinetic equation can be formulated similarly with the collisional effects. Particle, energy, and toroidal momentum balance equations including collisional and turbulent transport fluxes are systematically derived using a novel gyrokinetic collision operator, by which the collisional change rates of energy and canonical toroidal angular momentum per unit volume in the gyrocenter space can be given in the conservative forms. The ensemble-averaged transport equations of particles, energy, and toroidal momentum given in the present work are shown to include classical, neoclassical, and turbulent transport fluxes which agree with those derived from conventional recursive formulations.

Flux tube train model for local turbulence simulation of toroidal plasmas

A new simulation method for local turbulence in toroidal plasmas is developed by extending the conventional idea of the flux tube model. In the new approach, a train of flux tubes is employed, where flux tube simulation boxes are serially connected at each end along a field line so as to preserve a symmetry of the local gyrokinetic equations for image modes in an axisymmetric torus. Validity of the flux tube train model is confirmed against the toroidal ion temperature gradient turbulence for a case with a long parallel correlation of fluctuations, demonstrating numerical advantages over the conventional method in the time step size and the symmetry-preserving property.