Laser initiation and early stages of electrical breakdown in charged glasses

A single pulse from a neodymium laser is used to initial electrical breakdown in glasses which accumulate and preserve a space charge induced by a beam of rapid monoenergetic electrons. Laser initiation permits use of high-speed recording apparatus and study of the initial stage of electrical breakdown in the charged glasses. A time delay in electrical breakdown relative to the initiating optical radiation was observed and measured, corresponding to the time of formation of a conductive channel between the region of space-charge localization and the specimen surface. Growth rates of the conductive channel are determined for surface and volume initiation, and possible mechanisms of laser initiation are considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 72–77, July, 1988.     

On the mechanism of fibril rotation at the early stages of amorphous-crystalline polymer reorientation

The microdeformation behavior of polyamide-6 (PA6) oriented films is studied by X-ray diffraction at the early stages of reorientation at an angle of 45° to the direction of primary orientation. In the elastic strain range (up to 22%), the shear of crystallites and the rotation of fibrils take place simultaneously. The rotation is provided by the mutual slip of neighboring lamellas inside the fibrils.     

KrF laser-induced breakdown of gases

Measurements have been made of the threshold intensities for gas breakdown at the KrF 248 nm laser wavelength. The threshold intensities were measured as a function of pressure for air, H₂, CH₄ and the rare gases.     

Criterion for microwave breakdown of gases

An analytical criterion is obtained for microwave breakdown of gases that takes into account the different diffusion coefficients of electrons along and perpendicular to the direction of the electric field. Discharge ignition without electron attachment to the gas molecules and in electronegative gases is examined, along with microwave discharge ignition when a weak dc electric field is applied. Zh. Tekh. Fiz. 69, 25–29 (November 1999)     

Self phase-modulation in the breakdown of gases by ultra-short pulses

In a gas breakdown experiment with ultra-short optical pulses, the transmitted laser light shows spectral alterations attributed to a self phase-modulation. This effect is due to the refractive index change accompanying the ionization growth. A numerical treatment indicates that the predominant mechanism is cascade collisionnal ionization.     

On the early stages of oxidation of iron borides

Pure Fe₂B and FeB powders were compacted and oxidized for 1 h in flowing oxygen in the 300°–450°C temperature range. The reaction products were analyzed by X-ray diffraction, surface Mössbauer spectroscopy and scanning electron microscopy. Thermogravimetric analysis was carried out on powders treated under the same conditions. Iron borides undergo oxidation at low temperatures, with formation of both oxides and oxygen-free products.     

On the acoustoelectronic mechanism of crystallographic orientation of an incomplete electrical breakdown

The contribution of an acoustoelectronic interaction to the streamer discharge is analyzed for a model medium (cadmium sulfide crystals). The specific features of the anisotropy of the nonequilibrium phonon focusing are revealed from analyzing the results of investigating the sixth-order refracting surfaces. These features are responsible for the determinancy of individual phonon modes and permit their assignment to particular orientations of streamers of different types. It is demonstrated that, in this case, the nonlinear effects must be taken into account, primarily, the effect of a strong electric field, the accuracy in measuring the data used in the calculations, and the influence of external factors on these data. It is also necessary to analyze more informative ray velocity surfaces.     

Integrability breakdown in longitudinaly trapped, one-dimensional bosonic gases

A system of identical bosons with short-range (contact) interactions is studied. Their motion is confined to one dimension by a tight lateral trapping potential and, additionally, subject to a weak harmonic confinement in the longitudinal direction. Finite delay time associated with penetration of quantum particles through each other in the course of a pairwise one-dimensional collision in the presence of the longitudinal potential makes the system non-integrable and, hence, provides a mechanism for relaxation to thermal equilibrium. To analyse this effect quantitatively in the limit of a non-degenerate gas, we develop a system of kinetic equations and solve it for small-amplitude monopole oscillations of the gas. The obtained damping rate is long enough to be neglected in a realistic cold-atom experiment, and therefore longitudinal trapping does not hinder integrable dynamics of atomic gases in the 1D regime.     

CO2-laser-induced breakdown in mono- and diatomic gases

Several monoatomic and homonuclear diatomic gases absorb energy from a focused CO₂-laser photon field. It has been established that the pressure threshold for the energy absorption correlates qualitatively with the known ionization potentials of those gases. The simplified phenomenological theory of the CO₂-laser-induced dielectric breakdown of gases is invoked to explain this phenomenon. In the H₂–D₂ system, the formation of HD is observed under these conditions. The examination of the reaction yields for HD formation demonstrates that the system studied does not reach equilibrium under our experimental conditions. Considerations regarding kinetics of primary processes reveal that ionic species, created originally via an inverse bremsstrahlung mechanism, are converted into atomic transients in fast ionic association processes. The latter species initiate chain reactions with surrounding molecules of substrates leading to the formation of HD. Simple kinetic analysis based on a non-steady-state assumption permitted the derivation of an expression for the yield of HD formation. This equation was fitted to the experimental data assuming that the temperature of the reaction rises with an increase of the amount of D₂ in the mixture. Some other aspects regarding the behavior of this system in a focused CO₂ laser beam are also discussed.U.S. Department of Energy Document No. DE-AS02-76ER03416-37     

Calculation of the minimum breakdown voltage of inert gases in static fields

Using the analytical expression for the maximum ionization coefficient of the inert gases η_max = 0.325 1/V_i (V_i is the ionization potential of the atom) a more precise expression is obtained for the minimum breakdown voltage of inert gases as a function of the γ-processes at the cathode: $$\left( {Us} \right)_{\min } = 3.08 V_i ln\left( {\frac{1}{\gamma } + 1} \right)$$ .     

Time dependence of the optical emission of laser induced breakdown in gases

A detailed study of the decay of the emission intensity of the laser spark in air, Ar, Xe, Cl₂, Br₂, I₂, ICl, NO₂ and NO reveals an exponential component with one or two time constants and a non-exponential long-time component extending to 100 μsec. The emission volume at the beginning of the spark is shown to be considerably greater than the plasma core.     

On the hydrogen etching mechanism in plasma nitriding of metals

Iron alloys and aluminum were nitrogen implanted in a controlled oxygen atmosphere and the role of hydrogen on the surface etching mechanisms studied. The surface composition was analyzed by in situ photoemission electron spectroscopy (XPS). In iron alloys, hydrogen strongly etches oxygen, improving nitrogen retention on the surface. On the other hand, hydrogen removes nitrogen from aluminum surfaces, with a deleterious effect on the nitriding effectiveness. The oxygen removal in iron alloys is associated with the catalytic effect of electrons in d-orbitals and the nitrogen removal in aluminum is associated with a steric effect.     

Electrical breakdown of microwave plasma in water

The electrical breakdown of microwave plasma in water was investigated between 1 and 30 kPa. The dependency of the ignition power for generating plasma on the size of coaxial electrode was measured. The ignition power decreases with a decrease of the diameter of the inner electrode. The behavior of microwave plasma in water was observed using a high-speed camera. The plasma ignites in a bubble generated by microwave heating. The model for calculating the electric field was created on the basis of the captured images of the bubble just before plasma ignition. The method presented can be used to visualize the electrical field distribution in the bubble. The electric field breakdown was calculated using the measured ignition power. The electric field breakdown of plasma in water is of the same order as gas phase plasma.     

The electrical breakdown of gases in the presence of crossed electric and magnetic fields

The breakdown characteristics of a gas in the presence of crossed electric and magnetic fields are discussed in terms of the Townsend ionization coefficients. The “equivalent pressure” concept is used to assess the effect of a transverse magnetic field on the first Townsend coefficient and the objections which have been raised to the application of this approach to breakdown potentials are shown to be removed by a consideration of the dependence of the second Townsend coefficient upon electric and magnetic field strengths.     

High stability breakdown of noble gases with femtosecond laser pulses

In the past, laser-induced breakdown spectroscopy (LIBS) signals have been reported to have a stability independent of the pulse length in solids. In this Letter, we perform the first stability study of femtosecond LIBS in gases (to our best knowledge) and show a significant improvement in signal stability over those achieved with longer pulses. Our study shows that ultrashort-pulse LIBS has an intrinsically higher stability in gas compared to nanosecond-pulse LIBS because of a deterministic ionization process at work in the femtosecond pulse. Relative standard deviations below 1% are demonstrated and are likely only limited by our laser output fluctuations. This enhanced emission stability may open up possibilities for a range of applications, from monitoring rapid gas dynamics to high-quality broadband light sources.