Showing papers on "Filamentation published in 1985"

Formation of filaments by Pseudomonas putida

Abstract: When Pseudomonas putida 40 was grown on a variety of liquid media in which oxygen became a limiting factor during growth, the latter stages of growth involved the elongation of cells without septation, which can result in the complete filamentation of the culture (up to several hundred micrometers long). The filaments appeared to consist of a chain of protoplasts within a common sacculus. Later these filaments were capable of a rapid fragmentation by septation to give a population of ordinary rods with a corresponding increase in the number of viable particles but no appreciable change in total bacterial mass. Filamentation did not occur if slow growth rates were maintained by restriction of oxygen availability from the beginning of growth. In complex media filaments were not formed during growth on 1% peptone alone, but the addition of 0.1 M phosphate or 6.6 × 10−4 M EDTA induced extensive filamentation that was reversed by the addition of 6.6 × 10−4 M Mg2+. In minimal media a much higher Mg2+ concentration than that required for active growth or present in the complex media was usually required for filamentation. A very narrow range of Mg2+ concentration promoted filamentation, and this optimum differed markedly depending on the carbon source used. Other medium variations which influenced the level of filamentation are reported. We found that most strains of P. putida (including the neotype strain) and P. fluorescens gave filaments under the conditions developed with strain 40, whereas several strains of P. aeruginosa failed to give filaments on the same media.

Tilted‐mirror semiconductor lasers

Abstract: Broad-area GaAs heterostructure lasers with a tilted mirror were demonstrated for the first time, with the tilted mirror fabricated by etching. These lasers operate in a smooth and stable single lateral mode with a high degree of spatial coherence. The suppression of filamentation manifests itself in a high degree of reproducibility in the near-field pattern.

Microwave generation from filamentation and vortex formation within magnetically confined electron beams

Abstract: The generation of microwaves from interacting vortices formed in thin magnetized electron beams is investigated experimentally and with three-dimensional electromagnetic particle simulations. Fine-detail photographs of relativistic and nonrelativistic beams show that vortex formation transcends 12 orders of magnitude in beam current. The simulations show a burst of microwave radiation from a rapid magnetic line connection and breaking between vortices when the vortex structure is well defined.

Simultaneous excitation of large-scale geomagnetic field fluctuations and plasma density irregularities by powerful radio waves

Abstract: The physical mechanism of thermal filamentation instability of radio waves whose frequencies can be as low as in the VLF band and as high as in the SHF band are investigated. This instability can excite large-scale magnetic and plasma density fluctuations simultaneously in the ionosphere and magnetosphere. Relevant experiments are reviewed in terms of this instability and other mechanisms.

Anomalous reflection from a laser-produced plasma at ?=0.53 ?m in the nanosecond regime

Abstract: This report describes the behaviour of the reflection from a solid target irradiated at moderate intensity with nanosecond pulses at λ=0.53 μm. The spectral and spatial properties of the scattered light (which amounts to 5–10% at 1014 W/cm2) are consistent with its interpretation in terms of stimulated Brillouin scattering, but a comparison with the theory shows that the instability occurs at much lower intensities than expected. The possibility that the discrepancy may be due to filamentation of the impinging beam is discussed. A recently proposed modified picture of the scattering process which could also account for the low threshold observed is considered.

Extraction and propagation of an intense, rotating electron beam

Abstract: A hollow relativistic electron beam produced in a strong axial magnetic field has been extracted into a neutral gas cell where the field is zero. In the field‐free region, equilibrium is dictated by a balance between the vz×Bθ radial pinch force and the centrifugal outward force. Extraction and propagation of these beams has been studied experimentally with a variety of diagnostics. A strong, low‐frequency filamentation instability is observed after extraction. In general, 80% of the beam energy has been extracted and propagated 75 cm. Beam equilibrium properties are in good agreement with simple theory with the exception of the azimuthal plasma current.

Modulational instabilities of upper-hybrid laser radiation in a strongly magnetized plasma

Abstract: A theoretical investigation is made of the modulational and filamentation instabilities of laser radiation at the upper‐hybrid frequency in the presence of the self‐generated magnetic field of the order of a few megagauss in a laser‐produced plasma. The nonlinear Vlasov equation is solved to obtain the low‐frequency response of magnetized electrons in the plasma. The present theory is valid for short‐wavelength perturbations and arbitrary values of the wavenumber. It is observed that the growth rates of the instabilities are very sensitive to the plasma parameters near the critical density layer. The growth rate of the modulational instability is always larger than that of the filamentation instability for the same power density of the incident laser radiation. It is further noticed that the modulational instability decreases slowly with the magnetic field, while the filamentation instability remains almost insensitive to the magnetic field.

Filamentation instability of the electron plasma wave at the difference frequency of two laser beams in a plasma.

Abstract: A longitudinal electrostatic electron plasma wave excited at the difference frequency of two laser beams is shown to be unstable against filamentation instability in a hot collisionless plasma. The relativistic Vlasov equation has been solved to obtain the nonlinear response of electrons. It is noticed that for a considerable power density of the excited electron plasma wave the growth rate of the filamentation instability is quite high.

Thermal filamentation instability of millimeter waves in laboratory plasmas

Abstract: During the electron cyclotron resonant heating of laboratory plasmas by intense millimeter waves, the thermal filamentation instability can be excited to cause significant fluctuations in plasma density and magnetic field These pump wave-induced perturbations may affect the plasma confinement

Evolution of self-focusing instability in a semiconfined plasma

Abstract: A system of equations obtained in the linear approximation is used to describe the temporal and spatial dependences of the perturbations of the electron density and electromagnetic field pressure as a result of a self-focusing instability. An accurate solution of this system is obtained in a semiconfined plasma neglecting diffraction and the thermal pressure. From this it follows that filamentation occurs far from the plasma boundary, extending to regions increasingly close to the boundary as a function of time.

Surface-Recombination Controlled SNDC and Current Filamentation

Abstract: While the influence of boundary conditions upon current instabilities associated with N-shaped current-voltage characteristics (NNDC) has been convincingly elaborated in the past, see e.g. [1], no similar systematic treatment has been available for the case of S-shaped negative differential conductivity (SNDC). SNDC induced by bulk generation-recombination (g-r) mechanisms occurs in: compensated Ge, CdSe, GaAs and ZnTe at low temperatures [2–5]; amorphous chalcogenide films [6]; pin-diodes [7,8]. Current filamentation resulting from g-r induced SNDC has recently been investigated for infinitely extended systems [9]. The current interest in submicron devices has advanced the need for a theoretical understanding of the influence of finite boundaries upon filamentary SNDC transport. Our analysis shows that surface generation-recombination can be used for efficient control of the SNDC transport properties.

Laser-driven instabilities in long scalelength plasmas

Abstract: In this update lecture we focus on laser-driven instabilities in long scalelength underdense plasmas. Particular attention is given to some recent experiments on Raman scattering of intense laser light. Many important features are in accord with theoretical expectations. These features include a correlation of hot electron generation with Raman scattering, an increase in this scattering as the density scale length increases, and collisional suppression of the instability. Some challenging aspects of the growing data base as well as various deficiencies in the understanding are discussed. The role of the plasmon decay instability 2..omega../sub pe/, Brillouin, and filamentation instabilities is also briefly considered.