[en] The theoretical study of plasma turbulent transport is of central importance to fusion research. Experimental evidence indicates that the confinement time is in fact a consequence of the turbulent transport of energy. The magnitude of turbulent transport depends on the turbulent state resulting from nonlinear saturation mechanisms. The ion heat anomalous transport in the plasma core fusion devices seems to be dominated by a class of microinstabilities, the toroidal ion temperature gradient driven modes (ITGs). ITG turbulence is known to self organize to form coherent macroscopic structures extended in the direction perpendicular to the gradient. These structures are essentially axisymmetric flows denominated zonal flows. The amplitude of zonal flows can oscillate: these perturbations are known as Geodesic Acoustic Modes (GAMs). Zonal flows act as a regulating mechanism on plasma microturbulence, the saturated turbulent state being determined by the nonlinear interactions between ITGs, zonal flows and GAMs. We present an analytical study showing the strong impact that plasma geometry has on zonal flow collisionless linear damping. The GAM frequency is shown to scale inversely with the elongation and the aspect ratio. These results are supported by numerical linear analysis, which in addition shows that the GAM damping rate and the undamped zonal flow component are enhanced by elongation and smaller aspect ratio. The same parameters also modify the ITG linear growth rates. Therefore linear analysis suggests that geometry can play a role in the determination of the turbulent transport level. On the other hand, the extent of this action can be quantified only by means of full nonlinear calculations. We present the results of nonlinear gyrokinetic simulations in realistic tokamak magnetohydrodynamic equilibria, focusing on the role of plasma elongation. The effect of the variation of this parameter on the ion heat transport and zonal flow-GAM interactions is investigated in a nonlinear saturated regime. (Author)

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[en] Conventional neoclassical predictions are successfully recovered within a gyrokinetic framework using a minimal Fokker-Planck collision operator. This operator is shown to accurately describe some essential features of neoclassical theory, namely the neoclassical transport, the poloidal rotation and the linear damping of axisymmetric flows while interestingly preserving a high numerical efficiency. Its form makes it especially adapted to Eulerian or Semi-Lagrangian schemes.

No abstract available

[en] This paper presents a class of collision operators, which reproduce neoclassical transport and comply with the constraints of a full-f global gyrokinetic code. The assessment of these operators is based on a variational entropy method, which allows a fast calculation of the neoclassical diffusivity and poloidal velocity.

[en] Linear, fully gyrokinetic, full radius (global), large aspect ratio studies of Alfven-ion temperature gradient mode (AITG) or kinetic ballooning modes or beta-induced Alfven eigenmodes considering only 'passing' species is presented. Effects hitherto completely neglected in a full radius approach such as B_parallel-fluctuations and the ones which have been treated partly [Phys. Plasmas 10, 1424 (2003)] such as Shafranov shifts are included. To this end, an existing code EM-GLOGYSTO has been upgraded to incorporate these effects. Among others, the most interesting results include: (i) For relatively large positive magnetic shear s [1.25s/d ln ρ, where q_s(ρ) is safety factor and ρ minor radius], B_parallel fluctuations have a benign effect on AITG growth rates and for positive but small shear (0.0parallel fluctuations are too weak to play any crucial role. (ii) In the later case, inclusion of Shafranov shift leads to the following: (a) Growth rates without Shafranov shift effects are in general larger than those including Shafranov shift; (b) nonmonotonous dependence of growth rates and frequencies on β=2μ₀NT_i/B₀²; (c) the presence of multiple eigenmodes with competing growth rates for same values of β; (d) no sign of complete stabilization with increasing β. Finally eigenmode structures [φ,A_parallel,A_θ] with and without Shafranov shift are reported. The growth rates and frequencies thus obtained may serve as estimates of transport coefficients and for future bench marking of the (then) global electromagnetic, gyrokinetic, time evolution codes (particle-in-cell or otherwise)

[en] The influence of the initial state on the turbulence and transport is addressed in collisionless, global, and full-f gyrokinetic simulations solving both the equilibrium and the fluctuations. For two strongly differing initial states, it is found that the steady turbulent regime exhibits nearly identical statistical properties. This result is in marked contrast with the claim of different final states. In fact, a long transient with very different properties finally evolves towards the same turbulent regime for long simulation times. When the initial state is a local Maxwellian, i.e., constant on flux surfaces, a large-scale sheared electric potential develops on short time scales to compensate for the vertical curvature and grad-B drifts. We predict analytically (i) the temporal dynamics at short times of this electric potential, (ii) its poloidal structure, and (iii) its saturation time. All agree well with numerical simulations using the GYSELA code. The impact on the transport is twofold, as compared to the canonical initial state, where f only depends on the motion invariants: (i) the turbulence is delayed due to a weaker effective growth rate, (ii) the same transport level is obtained at long times and the turbulence exhibits nearly identical statistical characteristics. In agreement, the electric potential of these two cases has the same magnitude despite very different transients

[en] The energy spectra of the Ion Temperature Gradient driven fluctuations are investigated with the global full-f gyrokinetic code GYSELA. For monotonous q profile, the poloidal spectrum can equally be fitted with two power laws or with a unique exponential. When prescribing an additional sheared radial electric field in view of triggering a transport barrier, the system is found to promptly polarize and screen this field, likely in a transient evolution towards a canonical equilibrium. For a reversed q profile, the negative shear region exhibits larger fluctuations, possibly due to the slab branch of ITG, characterized by a flatter spectrum. No clear transport barrier signature is observed in the vicinity of s = 0 when the radial extent of the gap without resonant modes is smaller than the turbulence correlation length.

No abstract available

[en] It is believed that low frequency microinstabilities such as ion temperature gradient (ITG) driven modes and trapped electron modes (TEMs) are largely responsible for the experimentally observed anomalous transport via the ion and electron channels in a tokamak. In the present work, a comprehensive global linear gyrokinetic model incorporating fully kinetic (trapped and passing) electrons and ions, actual ion to electron mass ratio, radial coupling, and profile variation is used to investigate the ITG driven modes and pure TEMs. These modes are found to exhibit multiscale structures in the presence of nonadiabatic passing electrons. The multiscale structure is related to the large nonadiabaticity of electrons in the vicinity of mode rational magnetic surfaces and leads to reduced mixing length estimates of transport compared to those obtained from adiabatic electron models.