Merging Of Filaments Research Articles

Hydrodynamic resistance in wet forming of chemical fibers

A moving model fiber of diameter 0.2-1.0 mm is used to study boundary layer formation as a function of distance, velocity of motion, and diameter of the fiber. An equation is obtained that describes the dependence of the boundary layer thickness on the viscosity, distance, radius, and velocity of motion of the fiber. In forming a complex filament, the boundary layers of adjacent filaments merge at a distance of ∼0.15 mm from the spinneret, so that the coagulation bath is immobilized (trapped) by the moving fiber and moves along with the filament at a velocity equal to 85% of the velocity of the filament. The amount of the coagulation bath drawn off from the spinneret in the form of boundary layers is equal to the product of the area of the perforated part of the spinneret times the velocity (0.85%) of the filament. This amount of the bath approaches the perforated surface of the spinneret in the form of a normal flow at a distance of ∼5 mm from the spinneret. It causes significant stress in the individual filament (jet of spin dope) and may cause the filament to break.

Self-induced magnetic focusing of proton beams by Weibel-like instability in the laser foil-plasma interactions

In the laser foil-plasma interaction the effects of Weibel-like instability have been explored. The self-induced magnetic fields result in the merging of filaments formed at the earlier stage of the instability and subsequent formation of a plasma clump close to the laser propagation axis. A photon cavity is formed in the laser plasma interactions, which can accelerate and focus the proton bunch efficiently, as identified by multidimensional particle-in-cell simulations. These processes are helpful to realize the stable acceleration of hundreds of MeV proton beams with a very low energy spread with circularly polarized intense laser pulses.

Characteristics of multiple filaments generated by femtosecond laser pulses in air: Prefocused versus free propagation

The characteristics of the multiple filaments formed by prefocused and freely propagating femtosecond laser pulses are investigated and compared. It is shown in our experiments that the diameter, length, stability, and interaction for the two cases can be quite different. The filaments formed by prefocused beam indicate dynamic spatial evolution with higher laser intensity and electron density. They have a typical diameter of 100 microm are of shorter length. In the free propagation case, the filaments exhibit interesting properties such as hundred-meter propagation distance and mm-size diameter. Moreover, only the interaction of the filaments with the energy background affects the evolution of the filaments. Filament-filament interactions such as the filament splitting and merging were not observed in this case.

Micro-fluid dynamics via laser–matter interaction: Vortex filament structures, helical instability, reconnection, merging, and undulation

Abstract The Richtmyer–Meskhow flow environment was generated by a laser pulse on the rippled surface of an indium target; the supersonic oscillatory shock caused baroclinic deposition of vorticity and subsequent self-organization (SO) into vortex filaments and the family of related phenomena. Vortex filaments merging, stratification of merged filaments, and undulations along the stratified sheet, were observed on the indium target. These SO structures were favourably compared with those obtained by visiometric studies for the gaseous SF 6 /air and He/air systems. Instead of the turbulent small scale structures that develop in the very late times on gaseous systems, in the case of a metal target very strong surface tension gradients cause the formation of the Marangony structures.

An efficient control of ultrashort laser filament location in air for the purpose of remote sensing

The unavoidable hot spots in a practical terawatt level laser pulse will self-focus in air at a short distance. The short distance cannot be changed significantly by only controlling the chirp or divergence. We overcome such early self-focusing by using a telescope, which enlarges the diameter of the beam, thus that of the hot spots. The telescope’s effective focal length is much shorter than the self-focusing distance of both the enlarged beam and the hot spots. Then, the resulting filaments merge into the geometrical focus whose position is controllable by the telescope. This technique also minimizes the generation of white light.

Simulation of the nonlinear evolution of large scale relativistic electron flow in dense plasmas

A relativistic electron beam of about 100MA is transported through overdense plasmas in the fast ignition. The nonlinear dynamics of the relativistic electron beam in dense plasmas has been investigated using a two-dimensional fluid-particle hybrid (FPH) code [T. Taguchi et al., Phys. Rev. Lett. 86, 5055 (2001)] that combines a particle-in cell code with an Rational Cubic Interpolated Pseudo-Particle fluid code [F. Xiao et al., Comput. Phys. Commun. 93, 1 (1996)]. These simulations show that the relativistic electron beam breaks up into filaments from the Weibel instability and the filaments merge successively to larger filaments. When the relativistic electron beam diameter is large and the total current is over 100 times Alfvén limit current, many large scale filaments remain after the merging process and are confined in the initial beam diameter. It is found that the number of relativistic electron filaments decreases in proportion to t−0.9 in the fixed ion case and t−0.2 in the mobile ion case until ωpe0τ≈1000, respectively. This asymptotic behavior is caused by the random motion of filaments driven by fluctuating magnetic fields.

Magnetic changes observed in the formation of two filaments in a complex active region: TRACE and MSDP observations

This paper is focused on the formation of two filaments in a complex center of decaying active regions (AR 8329 and AR 8326), located in the northern hemisphere. The observations were obtained in Hα by the Multi-channel Subtractive Double Pass spectrograph (MSDP mounted on the German telescope VTT in Tenerife) and EUV lines with TRACE (Transition Region And Corona Explorer). High Doppler shifts are found to be related to the ends of filament segments where canceling magnetic fields are also located (as seen on magnetograms from Big Bear Solar Observatory). At these locations, velocities along the line of sight, derived by using a cloud model method reach −20 km s−1, the segments of filaments merge and frequently a time-related sub-flare is observed by TRACE. The chirality of the filament segments has been determined by different methods: the segments of dextral chirality join together and form a long dextral filament, and a single filament of sinistral chirality forms end to end with the dextral filament but does not merge with it. Assuming a model of twisted flux tube for filament material, we suggest that the dextral filament has negative helicity and a relationship between its formation and the close by sunspot with the same sign of helicity.

New mode of operating a magnetized coaxial plasma gun for injecting magnetic helicity into a spheromak.

By operating a magnetized coaxial plasma gun continuously with just sufficient current to enable plasma ejection, large gun-voltage spikes (approximately 1 kV) are produced, giving the highest sustained voltage approximately 500 V and highest sustained helicity injection rate observed in the Sustained Spheromak Physics Experiment. The spheromak magnetic field increases monotonically with time, exhibiting the lowest fluctuation levels observed during formation of any spheromak (B/B>/=2%). The results suggest an important mechanism for field generation by helicity injection, namely, the merging of helicity-carrying filaments.

Strong Proton Acceleration during Successive Coalescence of Filament Currents in Relativistic Electron Beam System

We investigate the relaxation process of relativistic electron beam in a plasma, using a two-dimensional fully relativistic electromagnetic particle-in-cell code. The relaxation mechanism of the electron beam with return current is related to the electromagnetic filamentation instability. This instability produces many small current filaments whose size is of order of the electron skin depth. These generated current filaments rapidly merge and the electron pinching occurs in each filament. After that, these filaments merge further. There are two types of coalescence processes. One is the coalescence of almost same size filaments, the other is the coalescence of the different size filaments. We find that during successive coalescence, the strong proton acceleration occurs due to electric field perpendicular to local magnetic field.

Dual-filament initiation of a Coronal Mass Ejection: Observations and Model

We propose a new model for the initiation of solar coronal mass ejections (CMEs) and CME-associated flares. The model is inferred from observations of a quiescent filament eruption in the north-western quadrant of the solar disk on 4 September 2000. The event was observed with the Siberian Solar Radio Telescope (5.7 GHz), the Nobeyama Radioheliograph (17 GHz) and SOHO/EIT and LASCO. Based on the observations, we suggest that the eruption could be caused by the interaction of two dextral filaments. According to our model, these two filaments merge together to form a dual-filament system tending to form a single long filament. This results in a slow upward motion of the dual-filament system. Its upward expansion is prevented by the attachment of the filaments to the photosphere by filament barbs as well as by overlying coronal arcades. The initial upward motion is caused by the backbone magnetic field (first driving factor) which connects the two merging filaments. Its magnetic flux increases slowly due to magnetic reconnection of the cross-interacting legs of these filaments. If a total length of the dual-filament system is large enough, then the filament barbs detach themselves from the solar surface due to magnetic reconnection between the barbs with oppositely directed magnetic fields. The detachment of the filament barbs completes the formation of the eruptive filaments themselves and determines the helicity sign of their magnetic fields. The appearance of a helical magnetic structure creates an additional upward-directed force (second driving factor). A combined action of these two factors causes acceleration of the dual-filament system. If the lifting force of the two factors is sufficient to substantially extend the overlying coronal magnetic arcade, then magnetic reconnection starts below the eruptive filament in accordance with the classical scheme, and the third driving factor comes into play.

Breaking and merging of liquid sheets and filaments

The surface of a sheet of liquid which contracts due to surface tension, breaks, and then pulls apart into two pieces, is calculated. Before breaking, the flow is the self-similar one found by Keller, Milewski and Vanden-Broeck. After breaking, it is the self-similar flow found by Keller and Miksis. A general numerical scheme, which includes the previous ones, is presented and new numerical results are discussed. There is an analogous flow of an axially symmetric liquid filament, but it is not calculated.

Diffusion-Generated Motion by Mean Curvature for Filaments

Diffusion-generated motion by mean curvature is a simple algorithm for producing motion by mean curvature of a surface, in which the motion is generated by alternately diffusing and renormalizing a characteristic function. In this paper, we generalize diffusion-generated motion to a procedure that can be applied to the curvature motion of filaments, i.e., curves in R ^3, that may initially consist of a complex configuration of links. The method consists of applying diffusion to a complex-valued function whose values wind around the filament, followed by normalization. We motivate this approach by considering the essential features of the complex Ginzburg-Landau equation, which is a reaction-diffusion PDE that describes the formation and propagation of filamentary structures. The new algorithm naturally captures topological merging and breaking of filaments without fattening curves. We justify the new algorithm with asymptotic analysis and numerical experiments.

Breakup and fusion of self-guided femtosecond light pulses in air.

We report experiments showing the breakup and the merging of filaments formed by the modulational instability of femtosecond optical pulses in air. For input powers as high as 25 times the self-focusing threshold, the beams are shown to split into two spots, which coalesce into a self-guided beam. This effect occurs in an optically Kerr regime and plays an important role in the guiding process. Numerical simulations and theoretical estimates both support the comparison with the experimental data.

Shear-flow driven current filamentation: Two-dimensional magnetohydrodynamic-simulations

The process of current filamentation in permanently externally driven, initially globally ideal plasmas is investigated by means of two-dimensional magnetohydrodynamic-simulations. This situation is typical for astrophysical systems like jets, and the interstellar and intergalactic medium where the dynamics is dominated by external forces. Two different cases are studied. In one case, the system is ideal permanently and dissipative processes are excluded. In the second case, a system with a current density dependent resistivity is considered. This resistivity is switched on self-consistently in current filaments and allows for local dissipation due to magnetic reconnection. Thus, one finds tearing of current filaments and, besides, merging of filaments due to coalescence instabilities. Energy input and dissipation finally balance each other and the system reaches a state of constant magnetic energy in time.

Crystalloid bodies in skeletal muscle of hypothyroid myopathy

Crystalloid bodies in skeletal muscle fibers have been described in myopathic and non-myopathic conditions. They have been interpreted as viral, glycogen, protein-glycogen complex, artifacts and of unknown nature. This report described similar crystalloid bodies in the postmortem muscle samples of two patients with hypothyroid myopathy. The crystalloid bodies were preferentially located in the I band and Z line region and in the subsarcolemmal region closely associated with lipofuscin. Some were present within basophilic bodies. They were formed by parallel filaments of 6-10 nm beaded periodically by electron-dense particles of 10-18 nm in a lattice, hexagonal or parallel-ripple pattern. Merging of filaments of crystalloid bodies into actin filaments of the I band was noted. The electron-dense particles stained strongly with periodic acid-thiocarbohydrazide-silver proteinate method for polysaccharides and were unaffected or partially digested by diastase treatment on the ultrathin sections. The filamentous component was neither stained for polysaccharides nor digested by diastase treatment. It is suggested that crystalloid bodies of muscle fibers are composed of two distinct subunits with particles of glycogen complex attached to filaments of unknown nature.

Numerical studies of motion and decay of vortex filaments

A computational code is developed for the integro-differential equations governing the motion of the centerlines of vortex filaments submerged in a background potential flow. These equations, which are derived from the method of matched asymptotic analysis, include the effect of decaying large-magnitude circumferential and axial velocity components in the vortical cores. Numerical examples are presented to assess the effect of large axial velocity and of nonsimilar initial profiles in vortical cores. The initial configurations of the filaments are chosen so as to fulfill the basic assumption of asymptotic analysis, which is the effective vortical core size is much smaller than all other length scales in the flowfield, e.g., the radius of curvature and interfilament distance. The computations are continued until the basic assumption is no longer valid, that is, when the merging or intersection of filaments have begun. Various types of local or global merging or intersection of filaments are classified and demonstrated by numerical examples.