Analysis of the interaction of thermoacoustic modes with a Green's function approach:

In this paper, we will present a fast prediction tool based on a one-dimensional Green's function approach that can be used to bypass numerically expensive computational fluid dynamics simulations. The Green's function approach has the advantage of providing a clear picture of the physics behind the generation and evolution of combustion instabilities. In addition, the method allows us to perform a modal analysis; single acoustic modes can be treated in isolation or in combination with other modes. In this article, we will investigate the role of higher-order modes in determining the stability of the system. We will initially produce the stability maps for the first and second mode separately. Then the time history of the perturbation will be computed, where both the modes are present. The flame will be modelled by a generic Flame Describing Function, i.e. by an amplitude-dependent Flame Transfer Function. The time-history calculations show the evolution of the two modes resulting from an initial perturbation; both transient and limit-cycle oscillations are revealed. Our study represents a first step towards the modelling of nonlinearity and non-normality in combustion processes

Generazione e rivelazione di strutture elettromagnetiche lentamente variabili in interazione laser-plasma

La propagazione di un impulso laser ad alta intensità in un plasma sottodenso è stata studiata numericamente attraverso simulazioni 'particle-in-cell' (PIC) in due dimensioni (2D), su scale temporali dell'ordine del picosecondo. Sono stati analizzati gli effetti della dinamica degli ioni, che segue il 'self-channeling' dell'impulso. L'accelerazione radiale degli ioni porta alla 'rottura' delle pareti del canale, causando l'inversione del campo elettrico radiale generatosi per la separazione di carica. Per tempi successivi inoltre si osserva le formazione di strutture di campo quasi-periodiche, lentamente variabili, le cui caratteristiche sono simili a quelle di vortici magnetici e 'post-solitoni' allo stesso tempo

Improved ion acceleration via laser surface plasma waves excitation

International audienceThe possibility of enhancing the emission of the ions accelerated in the interaction of a high intensity ultra-short (<100 fs) laser pulse with a thin target (<10 l0), via surface plasma wave excitation is investigated. Two-dimensional particle-in-cell simulations are performed for laser intensities rangingfrom 10^19 to 10^20 W/cm2.µm2. The surface wave is resonantly excited by the laser via the coupling with a modulation at the target surface. In the cases where the surface wave is excited, we find anenhancement of the maximum ion energy of a factor ~2 compared to the cases where the targetsurface is flat

Model and phase-diagram analysis of photothermal instabilities in an optomechanical resonator

A study of the phototermal instabilities in a Fabry-Perot cavity is reported, where one mirror consists of a silicon-nitride membrane coated by the molecular organic semiconductor tris(8-hydroxyquinoline) aluminum and silver layers. We propose a theoretical model to describe the back-action associated with the delayed response of the cavity field to the radiation pressure force and the photothermal force. For the case under investigation, the photothermal force response occurs on a timescale that is comparable to that of mirror oscillations and dominates over the radiation pressure force. A phase diagram analysis has been performed to map the stability of the static solution as a function of the control parameters. The model equations are integrated numerically and the time history is compared to experimental measurements of the transmitted field and displacement of the membrane. In both experimental and theoretical data we observe large amplitude oscillations when the cavity length is scanned at a low speed compared to the growth rate of the instability. The perturbation is found to evolve through three regimes: sinusoidal oscillations, double peaks and single peaks followed by a lethargic regime. When the cavity length is scanned in opposite directions, dynamical hysteresis is observed, whose extension has a power law dependence on the scanning rate

Ion dynamics and coherent structure formation following laser pulse self-channeling

The propagation of a superintense laser pulse in an underdense, inhomogeneous plasma has been studied numerically by two-dimensional particle-in-cell simulations on a time scale extending up to several picoseconds. The effects of the ion dynamics following the charge-displacement self-channeling of the laser pulse have been addressed. Radial ion acceleration leads to the ``breaking'' of the plasma channel walls, causing an inversion of the radial space-charge field and the filamentation of the laser pulse. At later times a number of long-lived, quasi-periodic field structures are observed and their dynamics is characterized with high resolution. Inside the plasma channel, a pattern of electric and magnetic fields resembling both soliton- and vortex-like structures is observed.Comment: 10 pages, 5 figures (visit http://www.df.unipi.it/~macchi to download a high-resolution version), to appear in Plasma Physics and Controlled Fusion (Dec. 2007), special issue containing invited papers from the 34th EPS Conference on Plasma Physics (Warsaw, July 2007

Observation of the Gamma-Ray Binary HESS J0632+057 with the H.E.S.S., MAGIC, and VERITAS Telescopes

The results of gamma-ray observations of the binary system HESS J0632 + 057 collected during 450 hr over 15 yr, between 2004 and 2019, are presented. Data taken with the atmospheric Cherenkov telescopes H.E.S.S., MAGIC, and VERITAS at energies above 350 GeV were used together with observations at X-ray energies obtained with Swift-XRT, Chandra, XMM-Newton, NuSTAR, and Suzaku. Some of these observations were accompanied by measurements of the Hα emission line. A significant detection of the modulation of the very high-energy gamma-ray fluxes with a period of 316.7 4.4 days is reported, consistent with the period of 317.3 0.7 days obtained with a refined analysis of X-ray data. The analysis of data from four orbital cycles with dense observational coverage reveals short-timescale variability, with flux-decay timescales of less than 20 days at very high energies. Flux variations observed over a timescale of several years indicate orbit-to-orbit variability. The analysis confirms the previously reported correlation of X-ray and gamma-ray emission from the system at very high significance, but cannot find any correlation of optical Hα parameters with fluxes at X-ray or gamma-ray energies in simultaneous observations. The key finding is that the emission of HESS J0632 + 057 in the X-ray and gamma-ray energy bands is highly variable on different timescales. The ratio of gamma-ray to X-ray flux shows the equality or even dominance of the gamma-ray energy range. This wealth of new data is interpreted taking into account the insufficient knowledge of the ephemeris of the system, and discussed in the context of results reported on other gamma-ray binary systems

MAGIC and H.E.S.S. detect VHE gamma rays from the blazar OT081 for the first time: a deep multiwavelength study

https://pos.sissa.it/395/815/pdfPublished versio

Interaction laser-réseau plasma à haute intensité: excitation d'une onde de surface et accélération de particules

Surface waves in solids were first observed by Wood in 1902 as an anomaly in the diffraction of a continuous light source from a metal grating: the diffracted spectrum presented dark lines corresponding to certain wavelengths, which were later explained (Fano, 1941) in terms of the excitation of a surface wave sustained by the grating. Similarly to the metal grating case, a surface plasma wave (SPW) can be resonantly excited by a laser pulse at the surface of a laser-produced over-dense plasma, if the correct matching conditions are provided. SPWs propagate along the plasma-vacuum interface and are characterized by a localized, high frequency, resonant electric field. In the present work we describe numerically the dynamics of the plasma and the field distribution associated to SPW excitation, using two-dimensional particle-in-cell (PIC) simulations, where the plasma surface is initially pre-formed so that the SPW excitation conditions are fulfilled. We examine the surface wave excitation for a large range of laser intensities (Iλ2 =10^15-10^20 μm^2/Wcm^2) in order to study the transition from the non-relativistic to the relativistic regime. The simulations in which the wave is resonantly excited are compared to cases in which the resonant conditions are not provided and the coupling of the laser with the target is analyzed. We have considered the following aspects of the laser-plasma interaction, for different laser and target parameters: i) the laser absorption and the electric field at the surface ii) the generation of a quasi-static magnetic field iii) the electron heating and iiii) the ion acceleration. The possibility to excite a surface plasma wave on a structured target for a large range of laser energies has been demonstrated. In the cases where the surface wave is excited the electric field component normal to the target is amplified at the surface by a factor ranging from 3.2 to 7.2 with respect to the laser field. The absorption is also increased,for example it raises from 27% when the SPW is not excited up to 73% for Iλ^2=10^9 μm^2/Wcm^2. We have defined the optimal conditions for efficient coupling which increase laser absorption, that correspond to the relativistic laser intensities (Iλ^2>10^19 μm^2/Wcm^2). In this regime the main absorption mechanism is vacuum heating, associated to particles oscillating in the field perpendicular to the target, which is enhanced by the stronger, localized field of the SPW. The generation of a quasi-static magnetic field has been studied analytically and compared to the result of PIC simulations. The different field structure in presence of a SPW and for a flat target suggests that the enhanced field strength has caused partial confinement of particles at the target surface when SPW is present. The effects of the surface wave are more pronounced in thin laminar targets where electrons recirculate into the target interacting several times with the wave. Efficient electron heating increases the energy of the ions which are accelerated at both the irradiated and not irradiated target surface by the hot electrons space charge field. For the thinnest target (3.5 μm) the ion cut-off energy is about 14 Mev, approximately twice the value obtained when the SPW is not excited.Les ondes de surface ont été observées pour la première fois par Wood en 1902 qui note des anomalies dans le spectre de diffraction d'une lumière continue sur un réseau métallique. Pour certaines longueurs d'onde, le spectre diffracté présente des lignes noires que Fano interprète quelques années plus tard (1941) comme dues à l'excitation d'ondes de surface. De façon analogue, on peut exciter par laser de façon résonante une onde plasma de surface à la surface d'un plasma sur-dense créé par interaction laser-solide, si les conditions d'excitation de l'onde sont satisfaites. L'onde de surface se propage le long de l'interface plasma-vide et se caractérise par un champ électrique résonant haute-fréquence localisé. Dans ce travail, la dynamique du plasma et les champs associés à l'excitation par laser de l'onde de surface sont décrits numériquement avec des simulations bidimensionnelles Particule-In-Cell dans lesquelles la surface du plasma est initialement pré-structurée de sorte à satisfaire les conditions d'excitation de l'onde de surface. L'intensité laser a été variée entre Iλ2 =10^15 et 10^20 μm^2/Wcm^2 afin d'étudier la transition entre un régime d'excitation non-relativiste et relativiste. Les simulations dans lesquelles l'onde de surface est excitée sont comparées à celles où elle ne l'est pas et le couplage du laser avec la cible est analysé. Pour différents paramètres du laser et de la cible, nous avons considéré les quatre aspects suivants de l'interaction laser plasma : i) l'absorption laser et le champ électrique à la surface du plasma, ii) le champ magnétique quasi-statique généré, iii) le chauffage électronique et iiii) l'accélération des ions. Nous avons démontré la possibilité d'exciter une onde plasma de surface pour une large gamme d'intensité laser. Lorsque l'onde de surface est excitée, la composante perpendiculaire à la surface du plasma du champ électrique est amplifiée par rapport au champ laser sur la surface plasma-vide d'un facteur allant de 3.2 à 7.2 selon les cas. L'absorption augmente également fortement de 27% lorsque l'onde de surface n'est pas excitée à 73% lorsqu'elle l'est pour Iλ2=10^19 μm^2/Wcm^2 par exemple. Cette étude nous a permis de définir les conditions optimales pour lesquelles le couplage entre le laser et l'onde de surface est le plus efficace. Elles correspondent au régime d'intensité laser relativiste dans lequel le mécanisme d'absorption principale est le " vacuum heating " : les particules gagnent de l'énergie en oscillant dans le champ électrique perpendiculaire à la cible. En présence de l'onde de surface, cette oscillation est fortement augmentée par la présence du champ localisé de l'onde de surface plus intense que le celui du laser. La possibilité de créer des champs magnétiques quasi-statiques auto-générés en présence d'une onde de surface a de plus été étudiée analytiquement et les résultats ont été comparés à ceux des simulations. Les structures de champ obtenues suggèrent que l'intensité du champ magnétique généré induit un confinement partiel des particules sur la surface de la cible lorsque l'onde de surface est excitée. Enfin, nous avons observé un effet induit par l'excitation de l'onde de surface encore plus fort dans des cibles minces dans lesquelles les électrons peuvent circuler d'un bord à l'autre de la cible et interagir plusieurs fois avec le champ de l'onde. Le champ de charge d'espace ainsi créé au cours de l'interaction induit une augmentation importante de l'énergie des ions émis sur les deux faces de la cible mince. L'ensemble de ce travail nous a permis de montrer que l'excitation d'une onde de surface par interaction laser-plasma structuré est un mécanisme physique prometteur pour augmenter l'énergie des particules émises. C'est un point particulièrement intéressant pour les applications liées à la production de protons énergétiques telles que la thérapie hadronique ou à celle d'électrons de hautes énergies indispensables dans le processus de fusion inertiel dans lequel le schéma de l'allumeur rapide est utilisée

High Intensity Laser-Plasma Grating Interaction (surface wave excitation and particle acceleration)

Les ondes de surface ont été observées pour la première fois par Wood en 1902 qui note des anomalies dans le spectre de diffraction d'une lumière continue sur un réseau métallique. Pour certaines longueurs d'onde, le spectre diffracté présente des lignes noires que Fano interprète quelques années plus tard (1941) comme dues à l'excitation d'ondes de surface. De façon analogue, on peut exciter par laser de façon résonante une onde plasma de surface à la surface d'un plasma sur-dense créé par interaction laser-solide, si les conditions d'excitation de l'onde sont satisfaites. L'onde de surface se propage le long de l'interface plasma-vide et se caractérise par un champ électrique résonant haute-fréquence localisé. Dans ce travail, la dynamique du plasma et les champs associés à l'excitation par laser de l'onde de surface sont décrits numériquement avec des simulations bidimensionnelles Particule-In-Cell dans lesquelles la surface du plasma est initialement pré-structurée de sorte à satisfaire les conditions d'excitation de l'onde de surface. L'intensité laser a été variée entre I 2 =10^15 et 10^20 m^2/Wcm^2 afin d'étudier la transition entre un régime d'excitation non-relativiste et relativiste. Les simulations dans lesquelles l'onde de surface est excitée sont comparées à celles où elle ne l'est pas et le couplage du laser avec la cible est analysé. Pour différents paramètres du laser et de la cible, nous avons considéré les quatre aspects suivants de l'interaction laser plasma : i) l'absorption laser et le champ électrique à la surface du plasma, ii) le champ magnétique quasi-statique généré, iii) le chauffage électronique et iiii) l'accélération des ions. Nous avons démontré la possibilité d'exciter une onde plasma de surface pour une large gamme d'intensité laser. Lorsque l'onde de surface est excitée, la composante perpendiculaire à la surface du plasma du champ électrique est amplifiée par rapport au champ laser sur la surface plasma-vide d'un facteur allant de 3.2 à 7.2 selon les cas. L'absorption augmente également fortement de 27% lorsque l'onde de surface n'est pas excitée à 73% lorsqu'elle l'est pour I 2=10^19 m^2/Wcm^2 par exemple. Cette étude nous a permis de définir les conditions optimales pour lesquelles le couplage entre le laser et l'onde de surface est le plus efficace. Elles correspondent au régime d'intensité laser relativiste dans lequel le mécanisme d'absorption principale est le " vacuum heating " : les particules gagnent de l'énergie en oscillant dans le champ électrique perpendiculaire à la cible. En présence de l'onde de surface, cette oscillation est fortement augmentée par la présence du champ localisé de l'onde de surface plus intense que le celui du laser. La possibilité de créer des champs magnétiques quasi-statiques auto-générés en présence d'une onde de surface a de plus été étudiée analytiquement et les résultats ont été comparés à ceux des simulations. Les structures de champ obtenues suggèrent que l'intensité du champ magnétique généré induit un confinement partiel des particules sur la surface de la cible lorsque l'onde de surface est excitée. Enfin, nous avons observé un effet induit par l'excitation de l'onde de surface encore plus fort dans des cibles minces dans lesquelles les électrons peuvent circuler d'un bord à l'autre de la cible et interagir plusieurs fois avec le champ de l'onde. Le champ de charge d'espace ainsi créé au cours de l'interaction induit une augmentation importante de l'énergie des ions émis sur les deux faces de la cible mince. L'ensemble de ce travail nous a permis de montrer que l'excitation d'une onde de surface par interaction laser-plasma structuré est un mécanisme physique prometteur pour augmenter l'énergie des particules émises. C'est un point particulièrement intéressant pour les applications liées à la production de protons énergétiques telles que la thérapie hadronique ou à celle d'électrons de hautes énergies indispensables dans le processus de fusion inertiel dans lequel le schéma de l'allumeur rapide est utilisée.Surface waves in solids were first observed by Wood in 1902 as an anomaly in the diffraction of a continuous light source from a metal grating: the diffracted spectrum presented dark lines corresponding to certain wavelengths, which were later explained (Fano, 1941) in terms of the excitation of a surface wave sustained by the grating. Similarly to the metal grating case, a surface plasma wave (SPW) can be resonantly excited by a laser pulse at the surface of a laser-produced over-dense plasma, if the correct matching conditions are provided. SPWs propagate along the plasma-vacuum interface and are characterized by a localized, high frequency, resonant electric field. In the present work we describe numerically the dynamics of the plasma and the field distribution associated to SPW excitation, using two-dimensional particle-in-cell (PIC) simulations, where the plasma surface is initially pre-formed so that the SPW excitation conditions are fulfilled. We examine the surface wave excitation for a large range of laser intensities (I 2 =10^15-10^20 m^2/Wcm^2) in order to study the transition from the non-relativistic to the relativistic regime. The simulations in which the wave is resonantly excited are compared to cases in which the resonant conditions are not provided and the coupling of the laser with the target is analyzed. We have considered the following aspects of the laser-plasma interaction, for different laser and target parameters: i) the laser absorption and the electric field at the surface ii) the generation of a quasi-static magnetic field iii) the electron heating and iiii) the ion acceleration. The possibility to excite a surface plasma wave on a structured target for a large range of laser energies has been demonstrated. In the cases where the surface wave is excited the electric field component normal to the target is amplified at the surface by a factor ranging from 3.2 to 7.2 with respect to the laser field. The absorption is also increased,for example it raises from 27% when the SPW is not excited up to 73% for I ^2=10^9 m^2/Wcm^2. We have defined the optimal conditions for efficient coupling which increase laser absorption, that correspond to the relativistic laser intensities (I ^2>10^19 m^2/Wcm^2). In this regime the main absorption mechanism is vacuum heating, associated to particles oscillating in the field perpendicular to the target, which is enhanced by the stronger, localized field of the SPW. The generation of a quasi-static magnetic field has been studied analytically and compared to the result of PIC simulations. The different field structure in presence of a SPW and for a flat target suggests that the enhanced field strength has caused partial confinement of particles at the target surface when SPW is present. The effects of the surface wave are more pronounced in thin laminar targets where electrons recirculate into the target interacting several times with the wave. Efficient electron heating increases the energy of the ions which are accelerated at both the irradiated and not irradiated target surface by the hot electrons space charge field. For the thinnest target (3.5 m) the ion cut-off energy is about 14 Mev, approximately twice the value obtained when the SPW is not excited.PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF