Formation Of Multiple Filaments Research Articles

Spatial evolution leading to multiple filament formation of higher order super Gaussian beam in bulk medium at input power P >>Pcr

Multiple filamentation in higher order super Gaussian laser beam propagating in chalcogenide glass is investigated by numerically solving nonlinear Schrödinger equation with split step beam propagation method at input power range of 40 – 160 Pcr and compared to the process in super Gaussian beam. The effect of raising the beam order on the deterministic nature of multiple filaments, threshold power of filamentation, length of filamentation and the number of filaments formed are analysed. The results indicate that within a range of beam order and input power, the process of multiple filamentation in super Gaussian beam may be controlled through beam order. The study further exemplifies the importance of deterministic filaments and the ‘role of beam order as a tool’ in control of self-guided laser beam properties in filamentation process.

Resistive Random Access Memory Filament Visualization and Characterization Using Photon Emission Microscopy

Near InfraRed (NIR) photon emission is observed from filaments in HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Resistive Random Access Memories (ReRAMs). This technique is non-destructive and offers rapid localization of filaments, enabling statistical analysis of their spatial distribution. We show that the emission is electric-field driven. We also report direct experimental evidence of i) completely random spatial distribution of filaments across multiple devices and ii) formation of multiple filaments inside a single large device.

Numerical investigation on the heat deposition characteristics of femtosecond laser pulses undergoing multiple filaments

In this study, the heat deposition characteristics of laser filaments formed by femtosecond laser pulses with different initial energy under influences of external focusing are numerically investigated by means of (3D + 1)-dimensional numerical simulations. The results indicate that the numbers and intensity of filaments increase with growth of the input energy. The formation of multiple filaments limits the increase of filament length. When fixing the input energy, radius of the filament becomes narrow while its numbers and intensity are both enhanced under strong focusing. Under stronger focusing, the filaments can also deposit higher laser energy and result in larger energy deposition rate for laser beams with a given input energy, which further leads to dramatic increase in local temperature and pressure. These are likely to affect the filament induced airflow velocity and eventually the mass of formed snow in the cold supersaturated cloud chamber.

Triggering of the breakdown in a discharge tube by visible-spectrum light pulses

The influence of the pulses of visible-spectrum light on the breakdown in a discharge tube was studied experimentally. The tube of 80 cm length and 1.5 cm inner diameter contained neon at a pressure of 0.6 Torr. High-voltage rectangular or ramp pulses were applied to the anode. The light pulses of 0.1–100 µs duration were produced by the light-emitting diode or laser diode at a wavelength of 460 and 407 nm, respectively. A small area of the tube wall near the anode was illuminated at different moments after or before the voltage pulse onset.It has been found that the light pulse exerts the strongest influence on the breakdown. In the case of rectangular pulse, it drastically decreases the statistical delay time of the breakdown. As a result, all the breakdowns, which did not happen before the light pulse, occur during several microseconds after its onset. The light pulse influences the breakdown delay time even if it terminates before the voltage pulse onset. This can be explained by the finite lifetime of the secondary electrons produced by the light. In the case of the ramp voltage pulse, an essential decrease in the breakdown voltage scatter is observed under illumination. The effect is especially pronounced if the light pulse is applied at the moment when the anode voltage is equal to or somewhat lower than the minimum breakdown voltage. For later moments, the breakdown voltages are strictly bounded by both sides. The average breakdown voltage decreases noticeably.As for the mechanism responsible for the observed effect, electron photodesorption from the tube wall is suggested. This process presumably occurs in dielectric barrier discharge at atmospheric pressure where it can influence the breakdown voltage and provide synchronous breakdown of micro discharges, thus causing the formation of multiple filaments.

Particle simulation of filamentary structure formation in dielectric barrier discharge

Filamentary discharges in dielectric barrier discharge operating in a glow regime are studied by use of two-dimensional particle-in-cell simulation with Monte Carlo collisions included. The formation of multiple filaments and the involved electric fields, electric potentials, plasma densities, and particle temperatures are presented. It is found that the filaments are not ignited simultaneously, and they expire with the shift of the plasma sheaths. “Dark discharges”, which produce ion densities lower by one order of magnitude than a typical filamentary discharge, are observed while the filaments from earlier discharges are decaying.

Cavity ignition and flameholding of ethylene–air and hydrogen–air flows by a repetitively pulsed nanosecond discharge

Repetitive nanosecond pulse plasma assisted ignition and flameholding of premixed and non-premixed ethylene–air and hydrogen–air flows are studied in a cavity flow at a pressure of 0.2 atm and flow velocities of up to 100 m/s. Ignition occurs via formation of multiple filaments in the fuel–air plasma, although air plasma remains diffuse until the fuel is added. After ignition occurs in the cavity, with ignition delay time of a few milliseconds, the plasma becomes diffuse and the flame couples out to the main flow. The use of a short cavity (length-to-depth ratio L/ D = 1) results in repetitive ignition and flame blow-off, caused by slow mixing between the main flow and the cavity. Increasing the length-to-depth ratio to L/ D = 3, as well as choking inlet air and fuel flows resulted in stable flameholding and nearly complete combustion in both premixed and non-premixed ethylene–air and hydrogen–air flows at u = 35–100 m/s. Air plasma temperature before fuel is added ranges from 70 °C to 200 °C. When the nanosecond pulse discharge is operated in repetitive burst mode, continuous ethylene–air flame is maintained only at a high duty cycle, which increases with the flow velocity. In hydrogen–air, the flame remains stable after the plasma is turned off. Nanosecond pulse discharge ignition of ethylene–air is compared with ignition by DC arc discharge of approximately the same power. DC arc discharge results in sporadic ignition and flame blow-off, much lower burned fuel fraction, and significantly lower flow velocity at which ignition can be achieved. Kinetic modeling is used to identify the reduced mechanism of plasma chemical oxidation and ignition of hydrogen, and to demonstrate the mechanism of energy release low-temperature reactions of radicals generated in the plasma (primarily O and H atoms).

Self and forced periodic arrangement of multiple filaments in glass

The formation of permanent periodic structural changes in fused silica induced by the multifilamentation process was investigated. A cylindrical lens was used to focus 800 nm 50 fs pulses with 0.5 - 3 mJ energy down to a line, resulting in a quasi-periodic linear self-arrangement of multiple filaments (MF). The quasi-period of multiple filaments is shown to be uniquely defined by the critical power of the material and the peak intensity on the sample entrance surface. A novel technique to control this spatial self-arrangement of MF is demonstrated based on the use of a binary phase mask. This technique allowed us to decrease the relative variation of spacing between the adjacent tracks of refractive index modifications by a factor of 4 as compared with the case without the phase mask. 3D + time numerical simulations qualitatively reproduce the main features of multiple filament formation obtained in the experiment.

Nonlinear theory of kinetic Alfvén waves propagation and multiple filament formation

In this paper, the filamentation of kinetic Alfvén waves (KAWs) has been studied by assuming that the incident Gaussian beam of KAWs may not remain Gaussian during the filament formation. The solution of the nonlinear dynamical equation of KAWs has been obtained by developing the semi-analytical approach in solar wind plasma. Magnetic field structures of the KAW have been presented in x-z plane. The analytical theory using eikonal approximation has been used beyond the paraxial domain. The effect on spectral index of the turbulence has also been studied.

Generation of 5 fs, 07 mJ pulses at 1 kHz through cascade filamentation

Two-cycle optical pulses with duration of 5 fs and energy of 0.7 mJ have been generated at 1 kHz by compressing the 38 fs laser pulses from a carrier-envelope phase (CEP) controlled Ti:sapphire laser system through a cascade filamentation compression technique. A simple and effective method is developed to suppress multiple filament formation and stabilize a single filament by inserting a soft aperture with an appropriate diameter into the driving laser beam prior to focusing, resulting in an excellent compressed beam quality. The good beam quality and potentially higher peak power make this ultrashort laser pulse source a significant tool for high-field physics applications.

Optimum small-scale management of random beam perturbations in a femtosecond laser pulse

We have performed statistical simulations of stochastic filament control in a femtosecond laser pulse. Suppression of random multiple filaments is performed by means of a mesh introduced into the transverse beam section at the laser system output. The optimum size of a single square mesh unit contains (3.1-3.2)Pcr for self-focusing in the medium. Controlled filaments are formed in several groups along the laser pulse propagation direction. Almost simultaneous formation of multiple filaments can be attained by means of spatial modulation of the mesh transmission coefficient.

Filamentation of a femtosecond laser pulse with the initial beam ellipticity

The effect of transverse elliptical intensity distribution on multiple filament formation in a femtosecond laser pulse is numerically studied. Two types of filament control are demonstrated. First, the beginning of filamentation can be varied by changing the degree of ellipticity. Second, random multiple filaments can be arranged in a spatial order predetermined by the specific shape of the ellipse. The latter is valid for pulses with a peak power more than ten times higher than the critical power for self-focusing in the medium.

Failure dynamics of the IGBT during turn-off for unclamped inductive loading conditions

The internal failure dynamics of the insulated gate bipolar transistor (IGBT) for unclamped inductive switching (UIS) conditions are studied using simulations and measurements. The UIS measurements are made using a unique, automated nondestructive reverse-bias safe operation area test system. Simulations are performed with an advanced IGBT circuit simulator model for UIS conditions to predict the mechanisms and conditions for failure. It is shown that the conditions for UIS failure and the shape of the anode voltage avalanche-sustaining waveforms during turn-off vary with the IGBT temperature, and turn-off current level. Evidence of single- and multiple-filament formation is presented and supported with both measurements and simulations.

Simulation of multiple filaments in GaAs structures

On the basis of the two-dimensional numerical solution of quasi-hydrodynamic equations for a semiconductor plasma the avalanche injection current filamentation is studied in GaAs n +-i-n +, n +-p-n + and n +-n-n + structures. It is demonstrated that a lengthy n +-contact region in these structures provides the spontaneous formation of multiple current filaments. Effect of structural parameters and dependence upon current are studied. Formation of multiple filaments is explained by a distinction between a small spatial parameter of instability (due to avalanche-injection modulation of the i-region conductivity) and a large spatial parameter of current damping (due to current spreading in the lengthy n +-contact region).