Search

No abstract available.

International audience; We demonstrate the coupling of microwave radiation into a plasma channel formed by laser filamentation in air, leading to the amplification by two orders of magnitude of longitudinal oscillations of the plasma. Transfer of this longitudinal excitation towards unexcited region of the plasma column occurs over more than 10 cm, in good agreement with a theoretical model describing the propagation of a TM wave guided along the surface between air and plasma. We foresee that high power low frequency electromagnetic waves injected into a multi-filament plasma could initiate and sustain a long-lived plasma over several meters distance.

International audience; A two-dimensional axisymmetric model of the propagation of intense femtosecond laser pulses through dispersion-free transparent media is described. The effects of diffraction, nonlinear Kerr effect (instantaneous and retarded) and multiphoton ionisation are included. Numerical results concerning air and other gases are discussed. In particular, time self-compression of femtosecond pulses is predicted. Stable self-guided pulses are simulated, in agreement with recent experimental observations.

Summary form only given. Optical phase conjugation is well known for its time reversal properties [1]. A phase conjugate mirror is formed when two laser beams of same frequency w meet from opposite directions in a transparent nonlinear medium. When a probe beam is retro-reflected by an optical phase conjugate mirror, wavefront distortions induced by a phase object are removed after re-crossing this object [2, 3]. In this presentation, we use two counter-propagating filaments to create a conjugate mirror with air as a non linear medium [4]. We demonstrate that it operates differently from a usual conjugate mirror. With a filament conjugate mirror, the reflected signal has essentially a perfect spatial mode independent of the initial probe beam profile.The principle of the experiment is represented on figure 1: two separate ultra-short pump laser pulses 1, 2 of equal peak power > 10 GW undergo filamentation in air and then meet from opposite directions to form a middle of the common two signal beams 4, 4', filament phase conjugate mirror (FCM). A weaker fs probe pulse 3 is focused onto the region ionized by the filaments and one co-propagating with the probe beam and amplifying it, the other counter-propagating. Figure 2(a) shows the fluence profile of the incident probe 3. After reflection on the FCM, the pulse 4 displays a high quality beam profile as shown in figure 2(b). Temporal characterizations of the conjugate beam have also been performed. The FCM acts as a plasma mirror and increases the temporal contrast of the pulse (ratio between the peak intensity and any precursor light). is time coincident with the pump pulses, generating In conclusion, filament conjugate mirrors display unusual properties. They should find applications in situations where femtosecond laser pulses with high quality beam are required, for instance in the stage amplifier femtosecond laser system. early stages of a multi-stage amplifier femtosecond laser system.

Two counter propagating filaments can create a phase conjugate mirror in air. This mirror can efficiently clean a femtosecond laser pulse.

This chapter examines operational issues in gas turbine systems, which are caused by, or have an impact on, the performance of the combustion system. Based on a typical gas turbine operation schedule, and with a focus on natural gas based fuels, the chapter assesses the impact of various thermodynamic boundary conditions on the physics of the combustion processes. The chapter begins by summarising the fundamental requirements of a gas turbine combustion system, followed by more detailed discussions on flame stabilisation mechanisms, and the impact of operational conditions of a gas turbine on exhaust gas emissions (especially NOx and CO). The concluding sections review the fundamentals of thermo-acoustics and consider potential future technologies, such as exhaust gas recirculation (EGR), which will have impact on gas turbine combustion.

We classify and discuss all optical guided modes able to propagate in ideal metallic-dielectric structures having the symmetry of a step index fiber. The cases of a metallic core with a dielectric cladding and a dielectric core with a metallic cladding are considered. In each case, the intensity profile of the allowed guided modes is shown. The geometric and frequency dispersion of the effective index of the modes is discussed as a function of the optogeometrical parameters of the fibers. >

In order to describe flame stability of turbulent, non premixed flames, the flow-, mixing- and reaction properties and the corresponding turbulence characteristics have been investigated in a concentric burner with partially blocked air exit and variable geometry. As fuel, natural gas with about 95% CH 4 and several mixtures of natural gas with N 2 or H 2 were used, to get certain variations in the reaction kinetics and the laminar burning velocity S l . The first part of the paper tests a previously suggested model, which describes flame stability in accordance with premixed flames based on Peclet numbers formed with the flow velocity on the one hand and with the laminar burning velocity on the other hand. Therefore the influence of systematic variations of geometry and S l on the stability limits has been investigated and the assumption of premixed flame areas in the stabilization area has been confirmed: that means stoichiometric conditions, at least timely averaged, can be found in this area. The herein presented modification of this model overcomes the disadvantage, that the applicability of the used original model was restricted to a single fuel/air—ratio for each geometry, by taking the influence of the different flow conditions into consideration. In particular the depending turbulence properties in the stabilization zone, which control heat and mass transfer and therefore increase the effective flame speed, are used to replace the laminar flame speed S l by a correlation between the turbulent flame velocity S t and S l . As a result of this modification the shape of the blow off limit for different geometries and fuel compositions can be described by a single correlation.

International audience; We reveal stress fields induced by femtosecond laser irradiation by investigating the topography of surface relaxation of a cleavage of silica plates in which irradiation was performed, varying intensity, laser polarization and displacement of the writing beam. The stress field appears to depend on the writing parameters differently according to the laser intensity. For pulse intensity larger than 0.1 µJ, a first shear stress developed. Above 0.25 µJ, another shear stress appears that is dependent on the direction of writing and coupling with a phase matching condition between the pump wave and the third harmonic.

Following positive laboratory-scale experiments, we investigated the ability to trigger real-scale lightning using ionized filaments generated by ultrashort laser pulses in the atmosphere. Under thunderstorm conditions, we observed electric events synchronized with the laser pulses.

Terahertz radiations of laser filament biased by an AC terahertz field or a DC electric field are studied. Similar physical properties of the two THz radiations are observed. Efficiency of both methods is compared.

International audience; We report on what is believed to be the first observation of coherent subterahertz (sub-THz) emission from a 1-m string in the atmosphere. The sub-THz pulse emitted by the filamentary structure from an intense IR femtosecond laser pulse is detected perpendicularly to the laser propagation axis by use of two heterodyne detectors at 94±1 and 118±1GHz . We describe the characteristics of this emission and show evidence of constructive interference between two separate strings.

The NOx emissions of heavy duty gas turbines have been significantly reduced by introducing lean premixed combustion. These flames are known to be prone to combustion instabilities. In this paper, investigations of a single model gas turbine burner are presented with focus on thermo-acoustic eigenmodes of the combustor and the resulting interaction between periodic flow field oscillations and flame front fluctuations. A numerical analysis of the eigenmodes of the combustor rig was performed and compared to pressure measurements. By the numerical analysis Helmholtz mode pressure oscillations were predicted to be present in the air plenum upstream the burner and in the combustion chamber. These oscillation modes were confirmed by the analysis of the pressure signals taken at different positions within the combustor rig. Two major achievements have been proved by experimental evidence: First, the presence of an acoustic eigenmode between plenum and combustor may trigger the combustion instabilities. Due to a phase lag between the pressure oscillations in the air plenum and those in the combustion chamber, fluctuations of air flow through the burner are induced, which cause fluctuations of the equivalence ratio at the burner exit plane. As result, fluctuations of the heat release rate within the flame are established, which interfere with the acoustics of the rig and, thus, unstable combustion results. Second, the amplitude of the combustion oscillations does not increase smoothly when increasing the air fuel ratio. Two different flow field patterns were identified in the combustor depending on the amplitude of the oscillations. At low oscillation levels weak and locally confined velocity fluctuations were observed in the main reaction zone which do not have any significant impact on the flame. By increasing the equivalence ratio high oscillation levels occur and a pumping motion of the flow field in axial direction was identified which causes periodic heat release fluctuations and, hence, a strong disturbance of the flame.

International audience; By studying the conical emission of a blue femtosecond laser filament in air, it is shown that self-improvement of the beams' spatial mode quality occurs for a self-guided laser pulse.

We study the formation of femtosecond filaments in an amplifying medium (Ti:Sa). Subthreshold onset of filamentation as well as flux amplification without fragmentation is observed, in good agreement with simulations.

Summary form only given. We present our results on the characterization and the optimization of the infrared component of the continuum efficiency is studied as a function of several laser parameters, such as pulse energy, duration, emission frequency chirp. For LIDAR application purposes, different beam transmission emitted from light filaments. Infrared configurations are studied. The fundamental beam can be sent in the atmosphere either collimated in a parallel beam or slightly focused. White light can be also re-collimated. Infrared intensities at remote distances are optimized.

International audience; The propagation of femtosecond terawatt laser pulses at reduced pressure (0.7 atm) is investigated experimentally. In such conditions, the non-linear refractive index n 2 is reduced by 30%, resulting in a slightly farther filamentation onset and a reduction of the filament number. However, the filamentation process, especially the filament length, is not qualitatively affected. We also show that drizzle does not prevent the filaments from forming and propagating.

We investigate the influence of air pressure on femtosecond filamentation in the laboratory, in the pressure range between 0.2 and 1 bar. Numerical simulations reproduce the main features of the experimental results.

We have studied long distance filamentation in air at high pulse power. Multi-filamentary patterns can be controlled by phase or amplitude masks. Results are in good agreement with numerical simulations from a 3-d propagation code.

Large-eddy simulations (LESs) of an industrial gas turbine burner are carried out for both nonreacting and reacting flow using a compressible unstructured solver. Results are compared with experimental data in terms of axial and azimuthal velocities (mean and RMS), averaged temperature, and existence of natural instabilities such as precessing vortex core (PVC). The LES is performed with a reduced two-step mechanism for methane–air combustion and a thickened flame model. The regime of combustion is partially premixed and the computation includes part of the swirler vanes. For this very complex geometry, results demonstrate the capacity of the LES to predict the mean flow, with and without combustion, as well as its main unstable modes: it is shown, for example, that the PVC mode is very strong for the cold flow but disappears with combustion.

Among the two femtosecond laser damage thresholds exhibited by glasses, we show that the second one is dependent on the chemical composition. The Ge or P doping decreases the power threshold contrary to F.

Summary form only given. Ultrashort laser pulses have proven well-suited for the engraving of modifications in the optical properties of transparent materials, with controllable microscopic dimensions. We report on a new such effect, the permanent inscription of birefringent grooves with a size of a few pm inside thin fused silica platelets. They were obtained by tight focusing of the output of a Ti:sapphire laser delivering ultrashort infrared pulses at 200 kHz repetition rate. Each groove was engraved by displacing the laser spot at constant speed in a direction parallel or perpendicular to its polarization vector. As an example, a periodic structure of 50 grooves, separated by 20 /spl mu/m of unmodified material has been realized.

International audience; Self-guided propagation of femtosecond laser pulses is studied for a converging-beam configuration. Channeling of the pulse energy through various gases is observed over distances well beyond the lens focal point, a fact that cannot be explained by the moving-focus model. The results are in good agreement with three-dimensional numerical simulations.

We discuss techniques allowing to measure the duration of ultrashort laser pulses. In particular, a new method is described, which recovers the phase, amplitude and absolute intensity of femtosecond pulses, allowing to reconstruct the temporal pulse profile with femtosecond resolution. It is based on the spectral analysis, at different time delays, of a probe pulse experiencing cross-phase modulation induced by a pump pulse.

International audience; We present an improved iterative algorithm which allows to reconstruct the temporal pulse profile, phase and absolute intensity of ultra-short laser pulses from a set of cross-phase-modulated spectra. The changes in the centre of gravity of the recorded spectra allow to reconstruct the intensity autocorrelation independently of the phase retrieval algorithm. Ambiguity problems encountered in a previous scheme are eliminated.

International audience; Using cross-phase modulation, the dynamics of the Kerr-like response of air is studied. Spontaneous resurgences of the induced non-linear polarisation are observed, in good agreement with a theoretical model.

The spectrum of intense femtosecond optical pulses after propagation through transparent nonlinear medium contains precious information on the phase and amplitude of the optical pulse, provided the nonlinear response of the medium is known. Alternatively, knowing the phase and amplitude of the optical pulse, it is possible to extract with high accuracy the nonlinear Kerr response of the medium. Examples of pulse characterisation and determination of Kerr coefficients of different solids, liquids, and gases will be presented. A particular interesting phenomenon occurs in air, where spontaneous revivals of the nonlinearity are observed well after the passage of the pulse.

International audience; We present the results of the spectral analysis of intense ultrashort laser pulses propagating through nonlinear transparent media. We discuss the spectral features characteristic of the self-phase-modulation process, which is the most prominent process during nonguided propagation in the weakly nonlinear regime. We describe an application of automatic pulse phase retrieval based on the Kerr-like nonlinearity.

International audience; A fast and accurate method for the measurement of the nonlinear refractive index of transparent materials is presented. The method is applied to various solids and liquids. The effective femtosecond response time of the optical Kerr effect in benzene and methanol is determined.

International audience; In the perspective of the laser lightning rod, the ability of femtosecond filaments to trigger and to guide large scale discharges has been studied for several years. The present paper reports recent experimental results showing for the first time that filaments are able not only to trigger and guide but also to divert an electric discharge from its normal path. Laser filaments are also able to divert the spark without contact between laser and electrodes at large distance from the laser. A comparison between negative and positive discharge polarities also reveals important discrepancies in the guiding mechanism.