Multi-stage scheme for nonlinear Breit-Wheeler pair-production utilising ultra-intense laser-solid interactions

Multi-petawatt (PW) lasers enable intensities exceeding 1023 W cm-2, at which point quantum electrodynamics (QED) processes, such as electron-positron pair-production via the nonlinear Breit-Wheeler process, will play a significant role in laser-plasma interactions. Using 2D QED-particle-in-cell simulations, we present a two-stage scheme in which nonlinear pair-production is induced via an ultra-intense laser-solid interaction. The first stage is the generation of a γ-ray beam, through the interaction of an ultra-intense laser pulse with a thick target, whose features are found to be strongly dependent on collective plasma effects. This compact, high energy γ-ray beam (characterised by a divergence half-angle ∼10° and average photon energy ∼10 MeV) then interacts with two counter-propagating laser pulses. By varying the laser polarisation and angle of incidence, we show that in the case of two circularly polarised laser pulses propagating at an angle equal to the divergence half-angle of the γ-ray beam, the produced positron distribution is highly anisotropic compared to the case of a standard head-on collision

Toothed gearing vibrations analysis : cepstrum, correlation, spectrum

This paper presents the application of cepstral analysis to the vibrations of a toothed gearing . The signal is modeled as an amplitude modulated oscillation and the effect of cepstrum is detailed . Cepstrum and autocorrelation are compared and the resolution of cepstrum is discussed .Cet article présente une application de l'analyse par le cepstre à l'étude des vibrations d'un engrenage. Une modélisation des signaux d'engrenage est utilisée pour détailler l'action du cepstre et la comparer avec celle de l'autocorrélation. Le lien entre la résolution du cepstre et les caractéristiques du signal est mis en évidenc

Intra-pulse transition between ion acceleration mechanisms in intense laser-foil interactions

Multiple ion acceleration mechanisms can occur when an ultrathin foil is irradiated with an intense laser pulse, with the dominant mechanism changing over the course of the interaction. Measurement of the spatial-intensity distribution of the beam of energetic protons is used to investigate the transition from radiation pressure acceleration to transparency-driven processes. It is shown numerically that radiation pressure drives an increased expansion of the target ions within the spatial extent of the laser focal spot, which induces a radial deflection of relatively low energy sheath-accelerated protons to form an annular distribution. Through variation of the target foil thickness, the opening angle of the ring is shown to be correlated to the point in time transparency occurs during the interaction and is maximized when it occurs at the peak of the laser intensity profile. Corresponding experimental measurements of the ring size variation with target thickness exhibit the same trends and provide insight into the intra-pulse laser-plasma evolution

Efficient ion acceleration and dense electron-positron plasma creation in ultra-high intensity laser-solid interactions

The radiation pressure of next generation ultra-high intensity ($>10^{23}$ W/cm$^{2}$) lasers could efficiently accelerate ions to GeV energies. However, nonlinear quantum-electrodynamic effects play an important role in the interaction of these laser pulses with matter. Here we show that these effects may lead to the production of an extremely dense ($\sim10^{24}$ cm$^{-3}$) pair-plasma which absorbs the laser pulse consequently reducing the accelerated ion energy and energy conversion efficiency by up to 30-50\%

Role of magnetic field evolution on filamentary structure formation in intense laser-foil interactions

Filamentary structures can form within the beam of protons accelerated during the interaction of an intense laser pulse with an ultrathin foil target. Such behaviour is shown to be dependent upon the formation time of quasi-static magnetic field structures throughout the target volume and the extent of the rear surface proton expansion over the same period. This is observed via both numerical and experimental investigations. By controlling the intensity profile of the laser drive, via the use of two temporally separated pulses, both the initial rear surface proton expansion and magnetic field formation time can be varied, resulting in modification to the degree of filamentary structure present within the laser-driven proton beam

Modelling the effects of the radiation reaction force on the interaction of thin foils with ultra-intense laser fields

The effects of the radiation reaction (RR) force on thin foils undergoing radiation pressure acceleration (RPA) are investigated. Using QED-particle-in-cell simulations, the influence of the RR force on the collective electron dynamics within the target can be examined. The magnitude of the RR force is found to be strongly dependent on the target thickness, leading to effects which can be observed on a macroscopic scale, such as changes to the distribution of the emitted radiation and the target dynamics. This suggests that such parameters may be controlled in experiments at multi-PW laser facilities. In addition, the effects of the RR force are characterized in terms of an average radiation emission angle. We present an analytical model which, for the first time, describes the effect of the RR force on the collective electron dynamics within the 'light-sail' regime of RPA. The predictions of this model can be tested in future experiments with ultra-high intensity lasers interacting with solid targets

Cohérence et cohérence spectrale: application à la modélisation de la propagation à travers des éléments tournants

International audienc

Analyse des vibrations d'un engrenage: cepstre, corrélation, spectre

International audienc

Estimation of cyclic cumulants by moving average. Application to the diagnosis of rotating machinery.

International audienc

A New Source Extraction Algorithm for Cyclostationary Sources

International audienc