Phenomenon Of Filamentation Research Articles

Filamentation effect in a gas attenuator for high-repetition-rate X-ray FELs.

A sustained filamentation or density depression phenomenon in an argon gas attenuator servicing a high-repetition femtosecond X-ray free-electron laser has been studied using a finite-difference method applied to the thermal diffusion equation for an ideal gas. A steady-state solution was obtained by assuming continuous-wave input of an equivalent time-averaged beam power and that the pressure of the entire gas volume has reached equilibrium. Both radial and axial temperature/density gradients were found and describable as filamentation or density depression previously reported for a femtosecond optical laser of similar attributes. The effect exhibits complex dependence on the input power, the desired attenuation, and the geometries of the beam and the attenuator. Time-dependent simulations were carried out to further elucidate the evolution of the temperature/density gradients in between pulses, from which the actual attenuation received by any given pulse can be properly calculated.

Modeling of electrical transport in Zinc Oxide varistors

Abstract. The electrical transport in zinc oxide (ZnO) varistors is analyzed using microstructural material modeling. The fully three dimensional current distribution is computed by means of a nonlinear equivalent circuit model representing the assembly of current carrying grains and grain boundaries of the material. The investigation focuses on the phenomenon of current filamentation due to inhomogeneities of the varistor microstructure. Numerical results highlight the importance of 3-D percolation effects in the modeling of varistor currents as well as that of the grain bulk resistivity which so far has been neglected in previous studies.

Intensity Clamping During Femtosecond Laser Filamentation

The femtosecond laser filamentation is commonly understood as the result of the dynamic balance between optical Kerr effect induced self-focusing and plasma defocusing effect. When the balance of these two processes is achieved, the laser intensity inside filament is not only stabilized along the propagation distance, but also almost invariable with the increase of the input laser energy. This special phenomenon is named intensity clamping. The intensity clamping represents one of the fundamental characteristics of the filamentation phenomenon. It governs the major evolution dynamics of the laser pulse propagation during the filamentation and impacts extensively the applications of the filamentation. This paper summarizes the main research results about the intensity clamping, including current understanding about the underlying mechanism of the intensity clamping, experimental evidences proving the intensity clamping and some recommended approaches to measure the clamped intensity. The paper also introduced some impressive effects brought by the intensity clamping in various applications, such as high stability of the fluorescence signal in remote sensing, beam-cleaning in short pulse production, etc. The effects are in fact in contrary to the common sense about the outcome of nonlinear interaction, which would be unstable under perturbation. Furthermore, the paper presents some methods to increase the clamped intensity inside the filament. At the end, some other effects that may arrest the beam collapse during the filamentation are briefly discussed. These effects mainly include the diffraction, the dispersion and high order Kerr effect.

Filamentation-free self-written waveguides in a photopolymerizable medium initiated by multimode optical fibers

The inscription of light-induced self-written waveguides (LISW) with multimode optical fibers cannot be achieved by using coherent light because of the filamentation phenomena coming from the interferences among the different propagating modes. To suppress this filamentation, we have used two distinct strategies, an incoherent light writing technique and an "on the flow" writing process, to demonstrate the buildup of 1.5 mm long multimode LISWs.

Experimental confirmation of high-stability of fluorescence in a femtosecond laser filament in air

Femtosecond laser filamentation is particularly interesting for remote sensing pollutant in the atmosphere. In this work, we investigate the local shot-to-shot stability of the filament induced fluorescence of nitrogen in air. It is found that the root-mean square fluctuation of the fluorescence signal is at least one order of magnitude lower than that of the linear propagation case. In practice, it would contribute to improve the robustness of long distance spectroscopic analysis of the fluorescence of pollutant molecules inside the filament. We further point out that this unique property of filament induced fluorescence spectroscopy is because of the intensity clamping, a profound phenomenon of filamentation.

Pulsed microwave discharges sustained at atmospheric pressure: study of the contraction and filamentation phenomena

Contraction and filamentation phenomena are studied in a neon atmospheric-pressure microwave discharge sustained in a pulsed-power regime. The time evolution of the discharge is observed by means of an intensified charge coupled device (ICCD) and analyzed in terms of nonuniform gas heating and electromagnetic skin effect.

Long-range detection and length estimation of light filaments using extra-attenuation of terawatt femtosecond laser pulses propagating in air

High-energy femtosecond laser pulses propagating in the atmosphere undergo self-focusing resulting in the appearance of the phenomenon of filamentation. We observed an extra-attenuation of such (terawatt) femtosecond laser pulses propagating in the atmosphere when compared with long pulses (200 ps) with the same energy. This is because, in contrast to the linear propagation of the long pulse, the input femtosecond laser pulse is attenuated owing to either absorption through multiphoton ionization/tunnel ionization or to scattering on the laser-induced plasma; self-phase-modulation and self-steepening further convert partially the energy initially contained in the fundamental bandwidth into the broad side bands of the laser, becoming eventually a white-light laser pulse (supercontinuum). The experimental data allow us to extract an effective extra-attenuation coefficient for an exponential decay of the input pulse energy with the propagation distance. Such a coefficient allows us to estimate an upper bound of the filament length under the experimental conditions used. More generally, our observation leads to a new technique to remotely detect light filaments in the atmosphere.

Long-Alfvén-wave trains in collisionless plasmas. I. Kinetic theory

A generalized kinetic derivative nonlinear Schrödinger equation for the multidimensional dynamics of Alfvén wave trains propagating along an ambient magnetic field is derived from the Vlasov–Maxwell equations by a reductive perturbative expansion. It retains in addition to the Landau damping, the coupling to longitudinally averaged fields driven by both transverse gradients and kinetic effects. These mean fields that modulate the propagation speed of the wave play a main role in transverse instabilities of extended Alfvén wave packets and in the filamentation phenomenon. This long-wave model also provides a benchmark for Landau-fluid descriptions of collisionless plasmas.

Analytical solution for uv filaments

In this paper we derive an analytical solution for long pulse (order of nanoseconds) filaments at a wavelength of 248.6 nm. This is achieved by solving Maxwell's equations in the presence of a quasi-steady-state nonlinear polarization. The resulting solution reproduces the experimental data seen in the case of short pulse (picosecond) uv filaments and offers a quick and intuitive understanding of the complex phenomenon of filamentation.

Contour Dynamics Methods

Contour Dynamics Methods