Showing papers on "Filamentation published in 1981"

Role of sulA and sulB in filamentation by lon mutants of Escherichia coli K-12.

Abstract: Cells containing the pleiotropic Escherichia coli mutation lon filament extensively and die after exposure to ultraviolet light Outside suppressors of the ultraviolet sensitivity, called sul, have previously been described at two loci; these mutations reverse the ultraviolet sensitivity of lon strains but do not affect the mucoidal or degradation defect of these strains An isogenic set of strains carrying combinations of lon, sulA, and sulI was constructed, and their behavior during normal growth and after ultraviolet treatment was studied sulA mutations had no detectable phenotype in lon+ cells; the lon sulA strains filamented transiently after ultraviolet irradiation, as did lon+ sul+ cells We found that the sulB mutation, which alters cell morphology and slows recovery from transient filamentation after ultraviolet treatment, was epistatic to both lon and sulA Whereas sulA mutations were recessive to the wild-type allele, sulB was partially dominant The simplest model to account for our observations is that sulA and lon participate in a pathway of filamentation independent of that which produces transient filamentation in wild-type strains; sulB product may be the target of sulA action and may play a role in normal cell division Images

Nonlinear evolution of the sheet pinch

Abstract: An incompressible, dissipative numerical code of the spectral type is used to follow the nonlinear evolution of a magnetohydrodynamic sheet pinch in two spatial dimensions. The evolution involves considerable turbulent activity in the electric current field, with the excited spatial scales ranging from the size of the containing volume down to the dissipation lengths of the magnetic and velocity fields. Strong current filamentation near magnetic X-points is observed, as is 'jetting', or expulsion of magnetofluid from the vicinity of the X-point parallel to the current sheet.

Growth of laser ripple in a plasma and its effect on plasma wave excitation

Abstract: An investigation of the growth of a radially symmetrical ripple, superimposed on a Gaussian laser beam in a collisionless unmagnetized plasma is presented. The effect of the position and the width of the ripple on its growth has been studied in some detail. In a special case, the growth rate of the ripple is seen to be comparable to the growth rate of filamentation instability. In addition to the growth of the ripple, the effect of the presence of the ripple on the excitation of an electron plasma wave has also been studied.

Stress Distribution during Peeling of Adhesive Tapes

Abstract: The pattern of stress distribution observed during the peeling of pressure sensitive tapes is not adequately described by existing theoretical analyses of peel, which make over-simplifying assumptions. In particular, the consequence of filamentation or “legging” in the peeling zone is neglected by the theories. In the present work an attempt is made to assess the effect of filamentation by analysis of the peeling profile obtained by photography. The deflection of the backing film from its unrestricted “bent beam” configuration is interpreted in terms of a “filamentation force”. The stress distributions obtained show that filamentation makes an important contribution to the peel force and show good correlation with the results obtained from related systems by a different experimental method.

Self-focusing of 10-μm laser pulses in SF 6

Abstract: Self-focusing and self-defocusing of 1.6-nsec CO2 laser pulses were observed in SF6 gas for several selected 10-μm P-branch lines. The lines P(8), P(10), P(14), and P(16) self-focused, P(28) defocused, and either effect occurred at P(20), depending on the incident energy. The self-focusing was found to depend on incident fluence rather than on intensity and weakly on He buffer. Under the appropriate conditions beam filamentation occurred.

Filamentation and Two-Stream Instabilities of Light Ion Beams in Fusion Target Chambers

Abstract: Filamentation and two-stream instabilities for a light ion beam (LIB) fusion system are investigated. Stability conditions for these two types of instabilities are derived for propagation through a background plasma. The results are illustrated by plotting stability boundaries for a proton beam propagating in a plasma.

Theory of lossless suppression of filamentation in high-power laser beams

Abstract: This paper supports an earlier suggestion that saturation of the optical nonlinearity of a medium by a plane wave will suppress filamentation of the wave. A variational approach and exact solutions of the slowly‐varying‐envelope approximation of the linearized wave equation describe the growth of localized filaments on plane waves. The solutions show that a wave will be stable against filamentation if the wave is weak or if it is intense enough to saturate the nonlinearity of the medium. Experiments are proposed to demonstrate suppression of filamentation of CO2 laser beams exceeding 1010 W and Nd laser beams exceeding 1012 W in fully ionized hydrogen plasmas. The beams saturate the nonlinearity of the plasma by ponderomotive forces; absorption is negligible in the rarefied channel produced by the laser. Applications may include lossless filters that remove intensity hot spots from beams or convert mulitmode beams to nearly single mode.

Filamentation instability of a relativistic hollow electron beam

Abstract: The filamentation instability properties of a relativistic hollow electron beam confined in axial flow by a uniform magnetic field in a pipe are investigated via the Vlasov-Maxwell equations. The instability is found to have two side-bands, one with a spectrum of positive wavenumbers k and the other with a spectrum of negative wavenumbers. The spectral point k=0, associated with the diokotron instability, is excluded from the filamentation instability's two unstable sidebands. Only in the limit of zero axial beam flow, the diokotron instability becomes asymptotically part of the filamentation instability spectrum. In this limit the filamentation instability's two sidebands merge asymptotically and symmetrically toward the diokotron instability spectral point, k=0, in agreement with the basic driving physical mechanisms and geometry configurations for these two distinct and different instabilities.

Suppression of optical beam filamentation in plasmas and saturable dielectrics

Abstract: The first use of a variational approach to analyze the growth of a localized filament on an optical plane wave in a nonlinear medium shows that saturation of the nonlinearity of the medium by the wave can suppress filamentation of the wave. Applications may include lossless filters that remove hot spots from beams or convert multimode beams to single mode.

Filamentation instability in heavy ion beams

Abstract: The filamentation instability for a focused heavy ion beam is analyzed with a particle simulation code. Appreciable growth is prevented by high plasma conductivity and by the nonlinear orbit mixing characteristic of the simulation code.

Temporal growth rate of filamentation instability in a strongly ionised magnetoplasma

Abstract: Presents an investigation of the temporal growth rate of filamentation instability of a uniform laser beam propagating along and perpendicular to the static magnetic field in a strongly ionised plasma. The nonlinearity of the dielectric constant arises on account of diffusion of non-uniformly heated carriers (on account of fluctuations in the intensity of the main beam). Both the mechanism of energy loss of electrons, namely the thermal conduction and collisions with heavy particles have been taken into consideration.

Nonlinear Development of the Resistive Filamentation Instability

Abstract: A two-dimensional particle simulation of the resistive filamentation instability on a charge- and current-neutralized beam is performed. The model equations employed depend essentially on only one dimensionless parameter, making feasible a numerical survey of the parametric dependence of the nonlinear behavior of the instability. The simulation results are analyzed with reference to the focusing requirements of heavy-ion beams acting as drivers for pellet fusion.