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Direct numerical simulations of incompressible nonhelical randomly forced MHD turbulence are used to demonstrate for the first time that the fluctuation dynamo exists in the limit of large magnetic Reynolds number Rm ≫ 1 and small magnetic Prandtl number Pm ≪ 1. The dependence of the critical Rmc for dynamo on the hydrodynamic Reynolds number Re is obtained for 1 Re 6700. In the limit Pm ≪ 1, Rmc is about three times larger than for the previously well established dynamo at large and moderate Prandtl numbers: Rmc 200 for Re 6000 compared to Rmc ∼ 60 for Pm ≥ 1. Is is not as yet possible to determine numerically whether the growth rate of the magnetic energy is ∝ Rm 1/2 in the limit Rm → ∞, as it should if the dynamo is driven by the inertial-range motions at the resistive scale.PACS numbers: 91.25. Cw, 95.30.Qd, 96.60.Hv Introduction. The amplification of magnetic field by turbulent fluid motion, or dynamo, is believed to be the cause of cosmic magnetism [1,2,3]. Two types of turbulent dynamo should be distinguished. The first is the mean-field dynamo defined as the growth of magnetic field at scales larger than the outer (energy-containing) scale L of the turbulent fluid motion. The second, which is the focus of this Letter, is the fluctuation dynamo (or small-scale dynamo) defined as the growth of magneticfluctuation energy at or below the outer scale [25].

The feasibility of a mean-field dynamo in nonhelical turbulence with a superimposed linear shear is studied numerically in elongated shearing boxes. Exponential growth of the magnetic field at scales much larger than the outer scale of the turbulence is found. The characteristic scale of the field is lB proportional S(-1/2) and the growth rate is gamma proportional S, where S is the shearing rate. This newly discovered shear dynamo effect potentially represents a very generic mechanism for generating large-scale magnetic fields in a broad class of astrophysical systems with spatially coherent mean flows.

Abstract. -We study the effect of different boundary conditions on the kinematic dynamo threshold of von Kármán type swirling flows in a cylindrical geometry. Using an analytical test flow, we model different boundary conditions: insulating walls all over the flow, effect of sodium at rest on the cylinder side boundary, effect of sodium behind the impellers, effect of impellers or side wall made of a high magnetic permeability material. We find that using high magnetic permeability boundary conditions decreases the dynamo threshold, the minimum being achieved when they are implemented all over the flow.Dynamo action, i.e., self-generation of magnetic field by the flow of an electrically conducting fluid, is at the origin of planetary, stellar and galactic fields [1]. Fluid dynamos have been observed only recently in laboratory experiments in Karlsruhe [2] and Riga [3] by geometrically constraining the flow lines in order to mimic laminar flows that were known analytically for their dynamo efficiency [4]. More recently, the VKS experiment displayed self-generation in a less constrained geometry, e.g. a von Kármán swirling flow generated between two counterrotating impellers in a cylinder [5]. However, until now, dynamo action in the VKS geometry has been found only when the impellers are made of soft iron. It is thus of primary importance to understand how the dynamo problem is modified by the presence of magnetic material at the flow boundaries. We address this problem here using a kinematic dynamo code in a cylindrical geometry. Two important approximations are made to simplify the study. First, an analytic test flow that mimics the geometry of the mean flow of the VKS experiment is considered. Second, the magnetic boundary conditions are taken in the limit of infinite magnetic permeability of the boundaries compared to the one of the fluid. This seems a reasonable approximation for soft iron compared to liquid sodium. Our main result is that the critical magnetic Reynolds number, Rm c , for dynamo generation is significantly decreased with boundaries of high magnetic permeability all over the flow.The VKS experimental set-up is sketched in Figure 1. A turbulent von Kármán flow of liquid sodium is generated by two counter-rotating impellers (rotation frequencies F 1 and F 2 ). The impellers are made of iron disks of radius 154 mm, fitted with 8 iron blades of height 41.2 mm, and are placed 371 mm apart in an inner cylinder of radius 206 mm and length 524 mm. It is surrounded by sodium at rest in another concentric cylindrical vessel, 578 mm in inner diameter. This has been shown to decrease the dynamo threshold in kinematic computations based on the mean flow velocity [6]. When the impellers are operated at equal and opposite rotation rates F , a statistically stationary magnetic field is generated above a magnetic Reynolds number R m ∼ 30 [5]. The large scale field involves an azimuthal component and a poloidal one which is dominated by an axial dipole. This geometry has been understood with a simple α − ω dy...

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We address issues associated with non-local magnetic boundary conditions for non-spectral dynamo simulations. We introduce an integro-differential formulation for a domain bounded by an insulating outer domain. We show how to combine the flexibility of a local discretisation with a rigorous formulation of magnetic boundary conditions in arbitrary geometries. This formulation substantiates from mathematical point of view a new method for numerical solution of magnetohydrodynamic problems with non-local boundary conditions based on coupling finite volumes and boundary elements. Finally, we discuss practical efficiency of this new method.

Femtosecond laser-produced plasmas are bright ultrafast line x-ray sources potentially suitable for different applications including material science and biology. The conversion efficiency of the laser energy incident onto a solid target into the x-ray emission is significantly enhanced when a laser prepulse precedes the main pulse. The details of x-ray line emission from solid targets irradiated by a pair of ultrashort laser pulses are investigated both theoretically and experimentally. Insight into spatial and temporal characteristics of the line x-ray source is provided by numerical simulations and a simplified analytical model. Optimal time separation of the laser pulses is searched for in order to reach the maximum conversion of laser energy into the emission of selected x-ray lines. We deduced how the optimal pulse separation scales with laser and target parameters.

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