Characterisation of real GPRS traffic with analytical tools

With GPRS and UMTS networks lunched, wireless multimedia services are commercially becoming the most attractive applications next to voice. Because of the nature of bursty, packet-switched schemes and multiple data rates, the traditional Erlang approach and Poisson models for characterising voice-centric services traffic are not suitable for studying wireless multimedia services traffic. Therefore, research on the characterisation of wireless multimedia services traffic is very challenging. The typical reference for the study of wireless multimedia services traffic is wired Internet services traffic. However, because of the differences in network protocol, bandwidth, and QoS requirements between wired and wireless services, their traffic characterisations may not be similar. Wired network Internet traffic shows self-similarity, long-range dependence and its file sizes exhibit heavy-tailedness. This paper reports the use of existing tools to analyse real GPRS traffic data to establish whether wireless multimedia services traffic have similar properties as wired Internet services traffic

Nonlinear enhancement of the efficiency of the second harmonic radiation produced by ultrashort laser pulses on a gold surface

Summarization: An unexpected nonlinear enhancement in the efficiency of the second harmonic generation as a function of the fundamental laser intensity is observed, when a gold surface is illuminated by few tens of GW/cm2 subpicosecond pulses of an excimer-pumped dye laser at grazing incidence. The evidence of this effect is tested by time resolved autocorrelation measurements. This enhancement is attributed to the thermal nonequilibrium, between the electron gas and lattice subsystems, produced by the subpicosecond laser pulses on the metallic surface.Παρουσιάστηκε στο: Optics Communication

Observation of the inversion of second and third harmonic generation efficiencies on a gold surface in the femtosecond regime

Summarization: We present measurements of second and third harmonic generation efficiencies of gold, induced by p-polarized 290 fs Ti:sapphire laser pulses at 800 nm, in a broad intensity regime up to the plasma threshold. Inversion of second and third harmonic generation conversion efficiencies is successfully demonstrated for laser intensities below the limit of breakdown of perturbation theory. Non-linear enhancement in the efficiencies of harmonics, caused by the thermal non-equilibrium between the electron gas and lattice, as well as intensity saturation phenomena, are also observed and discussed.Παρουσιάστηκε στο: Optics Communication

Kilohertz extreme-ultraviolet light source based on femtosecond high-order harmonic generation from noble gases

International audienceA compact, versatile table-top kilohertz source of coherent extreme-ultraviolet (XUV) radiation in the wavelength region 18–100 nm, based on high-order harmonic generation from noble gases induced by a 40-femtosecond Ti:sapphire laser system, is presented. The XUV beamline delivers at its output 10^8 photons/s at a wavelength of 23 nm. The monochromatized XUV radiation is directly focused onto a 10^-2-mm^2 spot by a toroidal grating, allowing one to reach intensities higher than 10^6 W/cm^2. Optimization results are presented for a new XUV-generating geometry, utilizing a ‘semi-infinite' quasi-static gas cell and strong focusing. In those conditions, we observe an anomalous inversion between the cutoffs of argon and krypton, with the krypton spectrum extending to much higher orders than expected in an adiabatic limit

Temporal characterization of ultra short laser pulses based on multiple harmonic generation on a gold surface

Μη διαθέσιμη περίληψηNot available summarizationΠαρουσιάστηκε στο: Applied Physics

Enhanced harmonic conversion efficiency in the self-guided propagation of femtosecond ultraviolet laser pulses in argon

International audienceHarmonic generation during the self-guided propagation of femtosecond ultraviolet (UV) laser pulses (248-nm, 450-fs) in argon is investigated. The third (82.7-nm) and fifth (49.6-nm) harmonics are generated in the UV filament. The energy-conversion efficiencies for the harmonics are found to be at least two orders of magnitude higher than those reported in the literature for similar gas pressures. The enhancement is attributed to the quasi-phase matching of the harmonics due to the self-guiding of the driving pulse

A numerical study on laboratory plasma dynamics validated by low current x-pinch experiments

The computational study of x-pinch plasmas driven by pulsed power generators demands the development of advanced numerical models and simulation schemes, able to enlighten the experiments. The capabilities of PLUTO code are here extended to enable the investigation of low current produced x-pinch plasmas. The numerical modules of the code used and modified are presented and discussed. The simulations results are compared to experiments, carried out on a table-top pulsed power plasma generator implemented in a mode of producing a peak current of ~45 kA with a rise time (10%–90%) of 50 ns, loaded with Tungsten wires. The structural evolution of plasma density is studied and its influence on the magnetic field is analyzed with the help of the new simulation data. The simulated areal mass density is compared with the experimentally measured dense opaque region to enlighten the dense plasma evolution. In addition, the measured areal electron density is compared to the simulation results. Moreover, the new simulation data offer valuable insights to the main jet formation mechanisms, which are further analyzed and discussed in relation to the influence of the J × B force and the momentum. © 2020 IOP Publishing Ltd Printed in the U

High performance simulations of a single X-pinch

The dynamics of plasmas produced by low current X-pinch devices are explored. This comprehensive computational study is the first step in the preparation of an experimental campaign aiming to understand the formation of plasma jets in table-top pulsed power X-pinch devices. Two state-of-the-art magneto-hydro-dynamic codes, GORGON and PLUTO, are used to simulate the evolution of the plasma and describe its key dynamic features. GORGON and PLUTO are built on different approximation schemes and the simulation results obtained are discussed and analyzed in relation to the physics adopted by each code. Both codes manage to accurately handle the numerical demands of the X-pinch plasma evolution and provide precise details on the mechanisms of the plasma expansion, the jet-formation, and the pinch generation. Furthermore, the influence of electrical resistivity, radiation transport and optically thin losses on the dynamic behaviour of the simulated X-pinch produced plasma is studied in PLUTO. Our findings highlight the capabilities of the GORGON and PLUTO codes in simulating the wide range of plasma conditions found in X-pinch experiments, enabling a direct comparison to the scheduled experiments. © 2021 IOP Publishing Ltd