Topology and field strength in spherical, anelastic dynamo simulations

Numerical modelling of convection driven dynamos in the Boussinesq approximation revealed fundamental characteristics of the dynamo-generated magnetic fields and the fluid flow. Because these results were obtained for an incompressible fluid, their validity for gas planets and stars remains to be assessed. A common approach is to take some density stratification into account with the so-called anelastic approximation. The validity of previous results obtained in the Boussinesq approximation is tested for anelastic models. We point out and explain specific differences between both types of models, in particular with respect to the field geometry and the field strength, but we also compare scaling laws for the velocity amplitude, the magnetic dissipation time, and the convective heat flux. Our investigation is based on a systematic parameter study of spherical dynamo models in the anelastic approximation. We make use of a recently developed numerical solver and provide results for the test cases of the anelastic dynamo benchmark. The dichotomy of dipolar and multipolar dynamos identified in Boussinesq simulations is also present in our sample of anelastic models. Dipolar models require that the typical length scale of convection is an order of magnitude larger than the Rossby radius. However, the distinction between both classes of models is somewhat less explicit than in previous studies. This is mainly due to two reasons: we found a number of models with a considerable equatorial dipole contribution and an intermediate overall dipole field strength. Furthermore, a large density stratification may hamper the generation of dipole dominated magnetic fields. Previously proposed scaling laws, such as those for the field strength, are similarly applicable to anelastic models. It is not clear, however, if this consistency necessarily implies similar dynamo processes in both settings.Comment: 14 pages, 11 figure

Anomalous Multiphoton Photoelectric Effect in Ultrashort Time Scales

International audienceIn a multiphoton photoelectric process, an electron needs to absorb a given number of photons to escape the surface of a metal. It is shown for the first time that this number is not a constant depending only on the characteristics of the metal and light, but varies with the interaction duration in ultrashort time scales. The phenomenon occurs when electromagnetic energy is transferred, via ultrafast excitation of electron collective modes, to conduction electrons in a duration less than the electron energy damping time. It manifests itself through a dramatic increase of electron production. A basic hypothesis of the photoelectric process is that the photoemissive properties of matter remain unaltered during the interaction with light. Light-metal coupling is tacitly assumed as a perturbation of the electron population that remains in equilibrium during the interaction. Now, it has recently been shown that transient nonequilibrium electron states can exist in ultrashort time scales, in particular , when electromagnetic energy is transferred from a laser pulse to conduction electrons in a lapse of time shorter than the electron-phonon energy transfer duration [1– 4]. In this Letter, we address the basic question of whether the photoemissive properties of a metal can be modified through ultrafast energy transfer and nonequilib-rium electron heating. In a metallic electron gas, transient density disturbances can result in electron collective oscillation modes in the volume and near the surface. Under certain conditions, these so-called surface plasmon (polariton) modes can be excited by light [5,6]. In the case of thin metal films, the surface plasmon modes on the two surfaces can be coupled [7–9] and energy can be transferred from one surface plasmon mode to the other [10]. Collective electron oscillations can exist as well at the interface [11] between two perfect metals due to symmetry breaking at the metal-metal interface. Furthermore, interface and surface plas-mon modes can be coupled [12] in a bilayer metal system made of a metal M 1 (of electron density n 1) covered by a thin metallic layer M 2 (of electron density n 2 < n 1). If the overlayer metal M 2 is thin enough, the field of the surface plasmon can tunnel through the M 2 bulk and excite electron density fluctuations at the interface between the two metals (see Fig. 1). If the metal overlayer is too thick, the field of the surface plasmon must tunnel through too large a distance to excite the density fluctuations between the two metals. Conversely, if it is too thin, the surface plasmon amplitude is damped because of increasing coupling between the two opposite faces of the overlayer. There exists therefore an optimum thickness of the overlayer for which the amplitude of the induced interface plasmon is maximum. An interesting consequence of the interface or surface plasmon coupling effect is that the electron population in the metal overlayer can be in transient nonequilibrium energy states through ultrafast energy transfer from the coupled interface and surface plasmons. Actually, the conduction electrons near the surface and the metal-metal interface experience an effective nonlinear low-frequency force, the so-called ponderomotive force [13,14], resulting from the strongly inhomogeneous high-frequency field of the plasmons, and are accelerated toward regions of decreasing field amplitude. The ponderomotive force plays the role of an applied electrostatic force that transfers electromagnetic energy in a coherent way to an electron population, in contrast with stochastic energy transfer via thermal heating. The maximum energy that can be transferred to a free electron with initial energy E 0 through ponderomotive acceleration in a strong oscillating electri

On Nori's Fundamental Group Scheme

We determine the quotient category which is the representation category of the kernel of the homomorphism from Nori's fundamental group scheme to its \'etale and local parts. Pierre Deligne pointed out an error in the first version of this article. We profoundly thank him, in particular for sending us his enlightning example reproduced in Remark 2.4 2).Comment: 29 page

Ponderomotive Acceleration of Photoelectrons in Surface-Plasmon-Assisted Multiphoton Photoelectric Emission

International audiencePhotoelectrons emitted from a gold target via a surface-plasmon-assisted multiphoton photoelectric process under a femtosecond laser pulse of moderate intensity are much more energetic than in an ordinary photoeffect without electron collective excitation. The phenomenon is interpreted in terms of time-dependent ponderomotive acceleration of the particles by the resonant field localized at the metal surface. The amplitude of the plasmon resonance may be directly estimated by means of the electron energy spectra. The development of powerful lasers more than three decades ago has allowed the investigation of the generalization of the classical photoelectric emission from metals to processes involving the absorption of several photons [1]. In recent years, the advent of laser pulses of ultra-short duration has favored studies in the femtosecond time regime [2]. These investigations can lead to the creation of new high-current ultrafast electron sources. Experimental studies have revealed that the electron emission rate can be greatly enhanced by the excitation of collective electron modes of the metal, the so-called surface plasmons [3,4]. The increase of the photoelectric signal can be qualitatively explained in terms of an assisted photoelectric effect where the energy of femtosecond light pulses is stored by the surface plasmon, creating a hot-electron population that does not have enough time to transfer its energy to the crystal lattice. While the presence of a surface-plasmon excitation is efficient in increasing the production of photoelectrons, an important open question is how the energy of the emitted electrons in such a " surface-plasmon-assisted " photoelectric process may differ from the energy predicted by the familiar photoelectric equation generalized to multiphoton processes. In this Letter, we show that the photoelectron energy is strongly affected by the surface-plasmon field, the modification from the classical values depending on the characteristics of the plasmon resonance. This fact may be easily understood by considering a simple analysis of the photo-electron behavior in the inhomogeneous high-frequency electric field surrounding the metal surface. The analysis involves simple classical concepts such as the notion of time-dependent ponderomotive effects, which have been successfully used in the context of multiphoton ionization of atoms in high-intensity lasers [5]. Consider an electron released from the metal surface after having absorbed a required number n of photons from the laser beam to overcome the work function W of the metal. While traveling in the vacuum dressed by the high-frequency field E sp of the surface plasmon, the total energy of the electron consists of the sum of its kinetic energy § n (given by the Einstein multiphoton photoelectric equation § n ෇ n ¯ hv 2 W) and its quiver energy U sp ෇ e 2

Electron acceleration by surface plasma waves in the interaction between femtosecond laser pulses and sharp-edged overdense plasmas

International audienceThe relativistic acceleration of electrons by the field of surface plasma waves created in the interaction between ultrashort high-intensity laser pulses with sharp-edged overdense plasmas has been investigated. It is shown that the initial phase of the wave experienced by the electrons play a leading part by yielding a well-defined peaked structure in the energy distribution function. This study suggests that resonant excitation of surface plasma waves could result in quasi-monokinetic energetic electron bunches. When the space charge field becomes too strong, this mechanism can evolve toward a true absorption process of the surface wave energy via an enhanced ''vacuum heating'' mechanism generalized to the case of surface plasma waves

Efficient laser-overdense plasma coupling via surface plasma waves and steady magnetic field generation

International audienceThe efficiency of laser overdense plasma coupling via surface plasma wave excitation is investigated. Two-dimensional particle-in-cell simulations are performed over a wide range of laser pulse intensity from 10 15 to 10 20 W cm À2 lm 2 with electron density ranging from 25 to 100n c to describe the laser interaction with a grating target where a surface plasma wave excitation condition is fulfilled. The numerical studies confirm an efficient coupling with an enhancement of the laser absorption up to 75%. The simulations also show the presence of a localized, quasi-static magnetic field at the plasma surface. Two interaction regimes are identified for low (Ik 2 10 17 W cm À2 lm 2) laser pulse intensities. At " relativistic " laser intensity, steady magnetic fields as high as $580 MG lm/k 0 at 7 Â 10 19 W cm À2 lm 2 are obtained in the simulations

Alginate for cardiac regeneration: From seaweed to clinical trials.

Heart failure is a growing endemic in the aging Western population with a prevalence of over 20 million people worldwide1. Existing heart failure therapies are unable to reverse heart failure and do not address its fundamental cause, the loss of cardiomyocytes2. In order to induce myocardial regeneration for the myocardium and the heart valve, facilitate self-repair, improve tissue salvage, reduce or reverse the adverse-remodeling and ultimately achieve long-term functional stabilization and improvement in the heart function, novel strategies for therapeutic regeneration are being developed which are aiming to compensate for the insufficient and low intrinsic regenerative ability of the adult heart3. Similarly, valve replacement with mechanical or biological substitutes meets numerous hurdles. New approaches using multicellular approaches and new material are extensively studied. Most of those strategies depend on biomaterials that help to achieve functional integrated vasculogenesis and myogenesis in the heart/tissue. Especially for failed heart valve function a number of therapeutic approaches are common from corrective intervention to complete replacement4. However the complexity of the heart valve tissue and its high physical exposure has led to a variety of approaches, however therapeutic regeneration needs to be established. Beside other approaches alginate has been identified as one building block to achieve therapeutic regeneration. Alginate is a versatile and adaptable biomaterial that has found numerous biomedical applications which include wound healing, drug delivery and tissue engineering. Due to its biologically favorable properties including the ease of gelation and its biocompatibility, alginate-based hydrogels have been considered a particularly attractive material for the application in cardiac regeneration and valve replacement techniques. Here, we review current applications of alginate in cardiac regeneration as well as perspectives for the alginate-dependent, cardiac regeneration strategies