[en] There is mounting theoretical and experimental evidence that radial electric fields play a pivotal role in the dynamics of turbulence responsible for anomalous transport in tokamaks. In this paper, the effects of these radial electric fields on various electrostatic modes are investigated with a full radius gyrokinetic formulation that unifies the axisymmetric tokamak and helical stellarator perspectives. In particular, it is shown that the shearing rate criterion for stabilization [T .S. Hahm and K.H. Burell, Phys. Plasmas 2, 1648 (1995)] established for tokamaks can be generalized to helical configurations. In addition, another stabilizing mechanism independent of the shearing rate has been found for the cases of toroidal and helical Ion Temperature Gradient (ITG) modes. On the other hand, it is shown that the inclusion of trapped particle (ion and electron) dynamics can have strong consequences: in some cases the presence of radial electric fields is destabilizing rather than stabilizing. (author)

[en] There are a large number of codes treating wave propagation in plasmas. All of these use very different approaches depending on what there are dedicated to study. The LEMan code [1], which was developed in CRPP, is designed to treat electromagnetic waves in the low-frequency range (i.e. Alfven and ion cyclotron domain) in a 3D geometry. With this constraint, the most obvious method was to use a full wave code. The reason for that is the variation of the plasma parameters during a period of the wave because the low frequency domain involves long wavelengths. Other methods like WKB are not fully adapted for this purpose. As a first step, developments were formulated with a cold model. The code solves the following relation derived from the Maxwell's equations. (Author)

[en] Finite beta effects on microinstabilities are investigated with a linear global spectral electromagnetic gyrokinetic formulation. While the toroidal ITG mode is stabilized with increasing beta, another mode of electromagnetic nature becomes unstable below the ideal MHD ballooning limit. Its unique global structure is shown for the first time. The weakly destabilizing effect of trapped electron dynamics on ITG modes is shown for the first time in an axisymmetric bumpy configuration. Applied ExB flows in tokamak and heliac configurations stabilize toroidal and helical ITG modes with a quadratic dependence on the shearing rate. Trapped particle modes can be destabilized by ExB flows. Self-generated zonal flows are studied with a global nonlinear electrostatic formulation that retains parallel nonlinearity and thus allows for a check of the energy conservation property. A quasi steady-state is reached with zonal flow shearing rates fluctuating around a value comparable to the linear growth rate of the most unstable ITG. A semi-Lagrangian approach free of statistical noise is proposed as an alternative to the nonlinear PIC δ_f formulation. (author)

[en] The power deposition profiles during minority ion cyclotron heating are analysed in large tokamaks by using the global, toroidal wave code LION. For tokamaks with large aspect ratio and with circular cross-section, the wave is focused on the magnetic axis and can be absorbed there by cyclotron absorption when the cyclotron resonance passes through the magnetic axis. The power deposition profile is then essentially determined by the Doppler broadening of the ion cyclotron resonance. For equilibria either non-circular or with a small aspect ratio the power deposition profile depends also on the strength of the damping. In this case the power deposition profile can be expressed as a sum of two power deposition profiles. One is related to the power absorbed in a single pass, and its shape is similar to that obtained for large aspect ratio and circular cross-section. The other profile is obtained by calculating the power deposition in the limit of weak damping, in which case the wave electric field is almost constant along the cyclotron resonance layer. A heuristic formula for the power deposition is given. The formula includes a number of calibration curves and functions which has been calculated with the LION code for JET relevant equilibria. The formula enables calculation of the power deposition profile in a simple way when the launched wave spectrum and damping coefficients are known. (author). 7 refs, 11 figs

[en] The confining power of tokamaks is stunning: for instance in ITER, electrons will lose their energy after a 150.000 km trip inside the plasma. Turbulence plays against plasma confining. The computation of turbulence directly from plasma equations is difficult to achieve despite the sharp increase in computer power. The difficulty lies in the fact that the time scale of the system varies from the nanosecond range to a few seconds and that the space scale varies from a few micrometers to several meters and that both scale ranges are different for electrons and ions. Moreover plasmas present a high anisotropy due to the presence of the magnetic field and are the place where numerous non-linear processes happen. The confining time of ITER is not computed but only deduced through extrapolations from experiments in other tokamaks like Jet or Tore-Supra. A right turbulence simulation is necessary because it will give many details on plasma behaviour that cannot be measured. (A.C.)

[en] Full text: Microturbulence leading to anomalous transport of particles and heat is studied in magnetic fusion relevant plasmas using gyrokinetic simulations. In the Ion Temperature Gradient (ITG) and the Trapped Electron Mode (TEM) regimes, the frequencies of the underlying instabilities are sufficiently small that an obvious assumption often made is to consider the response of passing electrons to be adiabatic. However, the response from these particles may in fact become non-adiabatic around mode rational surfaces, characterized by a low rational safety factor. In such regions, the parallel wave number k_|| of the corresponding resonant Fourier mode component goes to zero and the parallel phase velocity ω/k_|| thus becomes larger than the electron thermal velocity. The GENE code is used to perform linear and nonlinear simulations addressing these non-adiabatic electron effects. A reduced model considering kinetic trapped and adiabatic passing electrons is also applied. (author)

[en] The study of wave propagation is an important topic in plasma physics as it can play a considerable role in heating or instability processes. The LEMan code is designed to treat low-frequency waves e.g. in the Alfven and ion-cyclotron domain. In this range of frequencies, the main points of interest are heating in the ion-cyclotron domain (ICRH) and the global Alfven modes that can be driven unstable by the fast ions resulting either from fusion reactions or from neutral beam injection (NBI). A warm model has been implemented in the LEMan code. To determine the corresponding dielectric tensor, the linearized Vlasov equation is solved by the same method used in, but retaining only the lowest order terms in the finite Larmor radius expansion. In order to compute the parallel gradient of the perturbed distribution function, the latter is decomposed in term of Fourier harmonics. A linear system is thus obtained and leads to a dielectric tensor whose form corresponds to a convolution connecting together the components of the Fourier series of the electric current density and field. Such a method permits to take into account the poloidal up shift of the parallel wave vector and to avoid using approximations that are quite delicate in stellarator geometries. In the Alfven range, the main kinetic effects that can be modelled with this formulation are the Kinetic Alfven Wave (KAW) and the electron Landau damping. Lack of symmetry in stellarator systems gives rise to a larger variety of global modes (TAE, HEA, MAE, etc...). These modes already exist in the cold model but can also be influenced by kinetic effects. Investigation will be done to see how the different methods act on the results. The computation of a straight helix has been done and points out the presence of a helicity-induced Alfven Eigenmode (HAE). Adding other effects like toroidicity to the previous geometry will influence the spectrum and may modify the characteristics of the HAE. (Author)

No abstract available

[en] The absorption of power is studied with fluid and gyrokinetic plasma models, when two Alfven or ion-ion hybrid resonances provide for a weak damping in a partially standing wave-field. Examples chosen in slab and toroidal geometry show that the the fluid predictions based on resonance absorption are generally very different from the Landau damping of mode-converted slow waves. They in particular suggest that the continuum damping of toroidal Alfven eigenmodes and the power deposition profiles obtained in the ion-cyclotron range of frequencies using fluid plasma models are very misleading. (orig.)

[en] Experimental results from the Tokamak a Configuration Variable (TCV) experiment have shown a heat transport coefficient χ_e two times greater with a triangularity δ= +0.4 than in a case with δ = -0.4 in L-mode plasma. These results were the motivation for a systematic study of shaping effects, and especially triangularity, on turbulent transport using the flux-tube gyrokinetic code GENE. In order to enable simulations of realistic tokamak plasma conditions and geometry, the code is extended from the s-alpha approximation to general axisymmetric geometry using an interface with an ideal MHD equilibrium code, CHEASE. In a second stage the code will be used to compare numerical results with experimental data from Electron Internal Transport Barriers (eITBs) studies conducted at TCV in a fully non-inductive discharge. The relative importance of Trapped Electron Modes and Electron Temperature Gradient modes will be investigated. The current status of this work will be presented. (Author)