M. Nunami

Effects of equilibrium-scale radial electric fields on zonal flows and turbulence in helical configurations

Effects of equilibrium-scale radial electric fields (Er0) on the zonal flow response and the ion temperature gradient (ITG) instability in magnetically confined plasma with helical configurations are investigated by gyrokinetic simulations. Poloidally global simulations of the collisionless zonal flow damping manifest enhancement of the residual amplitude by the uniform and constant Er0, and show agreement with the zonal flow response function in the long wavelength limit analytically derived from the theory for helical plasmas with multiple-helicity confinement field components. A higher zonal flow response is found with heavier ion mass for the same ion temperature and Er0, that is, the isotope effect, while the ITG mode frequency is Doppler shifted. Accordingly, the isotope dependence of the zonal flow response through Er0 possibly leads to the ion mass (or the poloidal Mach number) dependence of the turbulent transport and fluctuations.

Gyrokinetic studies of the effect of β on drift-wave stability in the National Compact Stellarator Experiment

X iv :1 21 0. 60 84 v1 [ ph ys ic s. pl as m -p h] 2 2 O ct 2 01 2 Gyrokinetic studies of the effect of β on drift-wave stability in NCSX J. A. Baumgaertel, G. W. Hammett, D. R. Mikkelsen, M. Nunami, and P. Xanthopoulos Los Alamos National Laboratory, Los Alamos, New Mexico 87544 Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543 National Institute for Fusion Sciences, Japan Max-Planck-Institut für Plasmaphysik, EURATOM Association, Teilinstitut Greifswald, Wendelsteinstr. 1, 17491 Greifswald, GermanyThe gyrokinetic turbulence code GS2 was used to investigate the effects of plasma β on linear, collisionless ion temperature gradient (ITG) modes and trapped electron modes (TEM) in National Compact Stellarator Experiment (NCSX) geometry. Plasma β affects stability in two ways: through the equilibrium and through magnetic fluctuations. The first was studied here by comparing ITG and TEM stability in two NCSX equilibria of differing β values, revealing that the high β equilibrium was marginally more stable than the low β equilibrium in the adiabatic-electron ITG mode case. However, the high β case had a lower kinetic-electron ITG mode critical gradient. Electrostatic and electromagnetic ITG and TEM mode growth rate dependencies on temperature gradient and density gradient were qualitatively similar. The second β effect is demonstrated via electromagnetic ITG growth rates dependency on GS2s β input parameter. A linear benchmark with gyrokinetic codes GENE and GKV-X is also presented.

Quasisymmetric toroidal plasmas with large mean flows

Geometric conditions for quasisymmetric toroidal plasmas with large mean flows on the order of the ion thermal speed are investigated. Equilibrium momentum balance equations including the inertia term due to the large flow velocity are used to show that, for rotating quasisymmetric plasmas with no local currents crossing flux surfaces, all components of the metric tensor should be independent of the toroidal angle in the Boozer coordinates, and consequently these systems need to be rigorously axisymmetric. Unless the local radial currents vanish, the Boozer coordinates do not exist and the toroidal flow velocity cannot take any value other than a very limited class of eigenvalues corresponding to very rapid rotation especially for low beta plasmas.

Conservation of energy and momentum in nonrelativistic plasmas

Conservation laws of energy and momentum for nonrelativistic plasmas are derived from applying Noethers theorem to the action integral for the Vlasov-Poisson-Ampere system [Sugama, Phys. Plasmas 7, 466 (2000)]. The symmetric pressure tensor is obtained from modifying the asymmetric canonical pressure tensor with using the rotational symmetry of the action integral. Differences between the resultant conservation laws and those for the Vlasov-Maxwell system including the Maxwell displacement current are clarified. These results provide a useful basis for gyrokinetic conservation laws because gyrokinetic equations are derived as an approximation of the Vlasov-Poisson-Ampere system.

A reduced model for ion temperature gradient turbulent transport in helical plasmas

A novel reduced model for ion temperature gradient (ITG) turbulent transport in helical plasmas is presented. The model enables one to predict nonlinear gyrokinetic simulation results from linear gyrokinetic analyses. It is shown from nonlinear gyrokinetic simulations of the ITG turbulence in helical plasmas that the transport coefficient can be expressed as a function of the turbulent fluctuation level and the averaged zonal flow amplitude. Then, the reduced model for the turbulent ion heat diffusivity is derived by representing the nonlinear turbulent fluctuations and zonal flow amplitude in terms of the linear growth rate of the ITG instability and the linear response of the zonal flow potentials. It is confirmed that the reduced transport model is in a good agreement with nonlinear gyrokinetic simulation results for high ion temperature plasmas in the large helical device.

Extended gyrokinetic field theory for time-dependent magnetic confinement fields

A gyrokinetic system of equations for turbulent toroidal plasmas in time-dependent axisymmetric background magnetic fields is derived from the variational principle. Besides governing equations for gyrocenter distribution functions and turbulent electromagnetic fields, the conditions which self-consistently determine the background magnetic fields varying on a transport time scale are obtained by using the Lagrangian, which includes the constraint on the background fields. Conservation laws for energy and toroidal angular momentum of the whole system in the time-dependent background magnetic fields are naturally derived by applying Noethers theorem. It is shown that the ensemble-averaged transport equations of particles, energy, and toroidal momentum given in the present work agree with the results from the conventional recursive formulation with the WKB representation except that collisional effects are disregarded here.

Gyrokinetic simulations of entropy transfer in high ion temperature LHD plasmas

Gyrokinetic simulations of the ion temperature gradient (ITG) turbulence in non-axisymmetric configurations modeled on the Large Helical Device (LHD) are performed with the entropy transfer analysis (Nakata et al 2012 Phys. Plasmas 19 022303). It is clarified that a fluctuation spectrum elongated in the radial wavenumber direction is formed in a neoclassically optimized field configuration with the inward-shifted magnetic axis position, where the zonal flows are more effectively generated than in the standard case. The strong interaction between zonal flows and turbulence causes the successive entropy transfer from low to high radial wavenumber space and the spectral broadening of ITG turbulence.