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Abstract. Transport properties of magnetic fluctuations, in particular the role of Lagrangian Coherent Structures, are investigated from a statistical point of view in a sheared magnetic field. It is shown that field lines escape a tube (jet) over a finite length which is independent of tube size. However this escape length is not uniform in a chaotic sea, and in particular is minimum (indicating maximal transport) in the vicinity of Lagrangian Coherent Structures. Combined with the fact that LCS are not fixed but vary in time and with the velocities of particles, this could reduce their effectiveness as transport barriers in cases where other transport processes exist.

The kinetic origin of the cubic terms found in electromagnetic reduced fluid models of magnetized plasmas is identified and analyzed. The transition from cartesian to guiding center variables in kinetic systems is responsible for the introduction of such cubic terms, and the derivation of a set of such terms is explicitly shown. The role of these terms in fluid simulations, and their importance for energy conservation of cubic terms in both frameworks is discussed

International audience; The nonlinear dynamics of a turbulence driven magnetic island (TDMI) is investigated numerically in a reduced magnetohydrodynamic fluid model. The significance of identifying a characteristic signature of a TDMI for its experimental observation is discussed. The principal focus of our simulations is on the nature of the pressure profile flattening inside a TDMI, and we show that, in agreement with analytical predictions, a partial flattening occurs when the island size exceeds a critical value that is a function of the small scale interchange dynamics. We also present a model and test it numerically, which links explicitly the interchange turbulence and the island pressure flattening. Published by AIP Publishing.

International audience; We investigate, through extensive 3D magneto-hydro-dynamics numerical ă simulations, the nonlinear excitation of a large scale magnetic island ă and its dynamical properties due to the presence of small-scale ă turbulence. Turbulence is induced by a steep pressure gradient in the ă edge region [B. D. Scott, Plasma Phys. Controlled Fusion 49, S25 ă (2007)], close to the separatrix in tokamaks where there is an X-point ă magnetic configuration. We find that quasi-resonant localized ă interchange modes at the plasma edge can beat together and produce ă extended modes that transfer energy to the lowest order resonant surface ă in an inner stable zone and induce a seed magnetic island. The island ă width displays high frequency fluctuations that are associated with the ă fluctuating nature of the energy transfer process from the turbulence, ă while its mean size is controlled by the magnetic energy content of the ă turbulence. (C) 2015 AIP Publishing LLC.

The amplification of a turbulence driven seed magnetic island by the bootstrap current is investigated numerically in a reduced magnetohydrodynamic model as a possible additional channel for the excitation and development of a neoclassical tearing mode (NTM) in a tokamak. The novel mechanism for such an excitation involves a two-step process, namely, the formation of a seed island due to a nonlinear beating of interchange turbulence modes followed by an amplification of the seed island by the bootstrap current leading to a large saturated island. Our study indicates that the level of turbulence controls both the critical seed island size and the saturated island size.