Effect of electric field on plasma control during CO2 laser welding

Experimental results showed that an applied electric field can affect the shielding behavior of the laser-produced plasma during laser welding. It was found that at optimum field strength, the penetration depth can be increased by about 8% and the width of bead can also be reduced. In other words, the electric field can be used for enhancing the efficiency of the laser energy coupling to the workpiece. In addition, the influences of the electric field strength, field direction and laser power on the penetration depth and the bead width are reported.     

Effect of a dc electric field on the high-frequency conductivity of a graphene superlattice

Longitudinal and transverse high-frequency conductivities of a graphene superlattice placed in an additional dc electric field are calculated. It is shown that in a sufficiently strong transverse field, the dependence of the longitudinal high-frequency conductivity of the superlattice on the ac field frequency changes. This effect is explained by the nonadditivity of the electronic spectrum of the investigated structure.     

Influence of the electric field frequency on the performance of a RF excited CO2 waveguide laser

An analysis is presented of the effect of the RF frequency on the active media of CO₂ waveguide lasers. It is found that the characteristics are improved with increasing RF frequency because the space charge sheath width decreases with increasing excitation frequency. We also found that the sheath width decreases with the discharge current; this fact was never discussed before. The higher the exciting frequency the higher is the maximum input power of the discharge in the stable low current mode. It is attractive to extend the input power while keeping the discharge in this mode. Finally, a stabilizing excitation technique is described for the inherent unstable region of the discharge.     

Optical characteristics of the plasma of a nanosecond atmospheric-pressure volume discharge in a nonuniform electric field

Optical characteristics of the plasma of nanosecond volume discharges in air, nitrogen, krypton, argon, neon, and Ar/N₂ and Ar/Xe mixtures at elevated pressures are investigated. The discharges are excited in a gap with a cathode of small curvature radius. The waveforms and spectra of plasma emission from discharges in different gases in the 230-to 600-nm spectral range are measured. Optical generation in an Ar/Xe mixture is achieved at an active length of 1.5 cm. A comparison is performed of the spectral characteristics of the emission from nitrogen, krypton, argon, and neon excited by a volume discharge in a nonuniform electric field, by a nanosecond electron beam, and by a pulsed volume discharge in a uniform electric field at a high initial voltage.     

Dispersion properties and field structures of eigenmodes of nonuniform plasma waveguides in a longitudinal magnetic field

We study the field structure and dispersion properties of a hybrid eigenmode guided by a nonuniform magnetized plasma waveguide. It is shown that the rotational and quasi-potential waves contribute to the formation of such a mode in the whistler frequency range. Depending on the plasma density, the rotational component of the hybrid mode is determined by either waves with complex transverse wave numbers or whistler waves, or by true surface waves. In the presence of an axial nonuniformity of the plasma in a channel, the transverse field structure of the propagating mode changes, which is stipulated by changes in both the values of transverse wave numbers and their dependence on the radial coordinate. It is found that the spectrum of axial wave numbers of eigenmodes of a plasma waveguide undergoes a pronounced condensation when smoothing the waveguide walls. The damping of the hybrid mode of a nonuniform waveguide due to electron collisions is found and it is shown that collisional losses determine the damping of waves trapped in the waveguide in the experiments on ionization self-channeling of whistler waves. We have found the effect of “displacing” the strong field from the inner core to the background outer region of the waveguide with increasing plasma density on its axis and broadening background region. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 7, pp. 607–617, July 2006.     

Effect of an electric field on Ps formation in gaseous CO2

Summary The yield of O-Ps has been studied in moderately dense CO₂ with a static electric field applied across the gas. The results are shown to be consistent with recent models of Ps formation which have considered that Ps is formed in molecular gases by both Ore and spur processes.

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Riassunto La produzione di O-Ps è stata studiata in CO₂ moderatamente denso con un campo elettrico statico applicato attorono al gas. Si mostra che i risultati sono consistenti con recenti modelli di formazione di Ps che considerano che Ps sia formato in gas molecolari per mezzo dei processi di Ore e spur.

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Реэюме Исследуется выход O-Ps в умеренно плотном CO₂ в случае статического электрического поля, приложенного к гаэу. Покаэывается, что реэулътаты согласуются с недавними моделями обраэования Ps которые предполагают, что Ps обраэуется в молекулярных гаэах.

     

X-ray production by a laser plasma in a strong electric field

The X-ray emission of a laser plasma from metal targets in a 26 kV cm^–1 electric field is investigated experimentally. A substructure of the X-ray pulse, corresponding to the laser pulse train period is found. The spectrum of the radiation in the range 2–25 keV is measured.     

Instability of a spherical drop in a nonuniform electric field

Analytical calculation in the first order of smallness shows that the equilibrium shape of a drop in the field of a point charge is axisymmetric about the plane passing through the center of mass of the drop normally to the axis connecting the center of mass with the point charge. Whether the equilibrium shape of the drop is stable or not depends on the value of the field parameter, which, in turn, depends on the point charge and the distance to it. There is an asymptotic value of the critical parameter above which all modes become unstable. In the field of the point charge, the mode coupling grows; that is, a mode excited at the zero time generates oscillations of the six nearest modes with amplitudes proportional to that of the initially excited mode. If the initially excited mode loses stability, all the modes coupled with it also become unstable. The surface instability of the drop also develops when the initially excited mode is stable but at least one of the modes coupled with it is unstable.     

Effect of electric field on non-ideal plasma conductivity

An order of magnitude increase in plasma electrical conductivity with increase in electric field up to 10³ V/cm has been experimentally observed for plasma with ζ/Z ≈ 1, where ζ is the number of electrons in Debye sphere, Z = 1–3 is the stage of ionization. The effect vanishes at ζ/Z ? 10.     

High-pressure runaway-electron-preionized diffuse discharges in a nonuniform electric field

High-pressure nanosecond diffuse (volume) discharges in a nonuniform electric field are studied experimentally using a recording system with a ?100-ps time resolution. As the voltage pulse shrinks to a width of ≈100 ps, the initiation of a diffuse discharge without a source of additional ionization is facilitated; specifically, a runaway-electron-preionized diffuse discharge is ignited in atmospheric-pressure air in the case of short interelectrode gaps. It is found that a major energy deposit into the plasma of this discharge is from an abnormal glow discharge following a maximum of the gap voltage.     

Force-extension curve of a polymer in a high-frequency electric field

We study theoretically the conformation and force-extension curve of a semiflexible polymer in a spatially uniform ac electric field. The polymer backbone minimizes its energy by aligning along one of two orientations parallel to the field. In a strong ac field, hairpin kinks develop between regions of opposite alignment. These kinks are mathematically described as sine-Gordon solitons. We calculate the equation of state of the one-dimensional kink gas, which yields the force-extension curve of the polymer. A sufficiently strong ac field causes the polymer to extend spontaneously to almost its full contour length. The theory is applied to recent experiments on dielectrophoretic stretching of DNA.     

Effect of an electric field on the orientation of a liquid crystal in a cell with a nonuniform director distribution

The electric field-induced reorientation of a nematic liquid crystal in cells with a planar helicoidal or a homeoplanar structure of a director field is studied theoretically and experimentally. The dependences of the capacitances of these systems on the voltage in an applied electric field below and above the Fréedericksz threshold are experimentally obtained and numerically calculated. The calculations use the director distribution in volume that is obtained by direct minimization of free energy at various voltages. The inhomogeneity of the electric field inside a cell is taken into account. The calculation results are shown to agree with the experimental data.     

Equilibrium configurations of the conducting liquid surface in a nonuniform electric field

Possible equilibrium configurations of the free surface of a conducting liquid deformed by a nonuniform external electric field are investigated. The liquid rests on an electrode that has the shape of a dihedral angle formed by two intersecting equipotential half-planes (conducting wedge). It is assumed that the problem has plane symmetry: the surface is invariant under shift along the edge of the dihedral angle. A one-parametric family of exact solutions for the shape of the surface is found in which the opening angle of the region above the wedge serves as a parameter. The solutions are valid when the pressure difference between the inside and outside of the liquid is zero. For an arbitrary pressure difference, approximate solutions to the problem are constructed and it is demonstrated the approximation error is small. It is found that, when the potential difference exceeds a certain threshold value, equilibrium solutions are absent. In this case, the region occupied by the liquid disintegrates, the disintegration scenario depending on the opening angle.     

On the discharge in laser plasma in an external electric field

The possibility of creating discharge in laser plasma in an external electric field with quasi-static current generation along the field is discussed. The knowledge of the quasi-static current generation mechanism in laser plasma, associated with the rarefaction shock wave (RSW), allows the determination of the spatial distribution of these currents during laser irradiation of a planar target. Having included the (planar or cylindrical) target into an external contour, we propose a discharge scheme in laser plasma, where the discharge current increases during the laser pulse.