Abstract The polarization plane stimulated rotation angle of a probe signal in an intense laser field in plasma is calculated for arbitrary detunings of intense and weak laser waves compared with the ...resonant transition frequency of the medium. Estimates of the residual gas local density in a cesium plasma have been found based on the Faraday, Cotton–Mouton effects and on the effect of stimulated rotation of the polarization plane of the probe signal in an intense laser field. It is shown that the rotation in the medium has a complex structure consisting of the sum of only the influence of the magnetic field, only the influence of the intense laser field and the interfering part of the magnetic and intense laser fields.
The plasma column in a metal inert gas welding process is investigated by optical emission spectroscopy and high-speed imaging. The concentration and repartition of iron vapours are measured and ...correlated with the plasma and electrode geometric configuration. Plasma temperatures and electron densities are also measured for each studied position in the plasma. The temperatures are calculated using two different methods, allowing validation of the local thermodynamic equilibrium state of the plasma. The results show a maximum temperature of 12 500 K in the upper part of the arc, away from the arc axis. The iron concentration reaches a maximum of 0.3% close to the anode and strongly decreases along both the vertical and radial directions.
The plasma thermophysical properties, calculated from this plasma composition, are then discussed regarding the metal transfer mode.
Metal Active Gas (MAG) welding in presence of Argon and CO
2
mixture as shielding gas is a largely developed process allowing the transfer of the liquid metal from the consumable wire anode to the ...workpiece according to various modes (short-arc, globular, spray-arc). The CO
2
presence in the shielding gas leads to the formation of an oxide layer, or gangue, wrapping the droplet, limiting the access to the spray-mode transfer, taking into account the low conductivity and the high viscosity of this layer. Several electrodes of various compositions have been tested thanks to Flux Cored Arc Welding, to limit the gangue formation or its negative contribution, based on Ti, La, Zr and alkali metals addition or reduction in silicon content. The results are interpreted considering the metal transfer mode for a given current intensity (330 and 410 A), with various CO
2
concentrations in the shielding gas. Finally, the role of the gangue, compared to the other factors governing the droplet detachment, is discussed. A decrease in silicon content limits significantly the gangue formation and gives access to spray arc transfer up to 30 vol.% of CO
2
at 330 A. Titanium addition leads to the same results. The tests in presence of zirconium proved the conductivity improvement of the gangue. The addition of alkali allows to stabilize the spray arc up to the noteworthy value of 60 vol.% of CO
2
at 330 A, supporting the hypothesis of a strong influence of viscosity on droplets detachment in the process.
The gas metal arc welding (GMAW) process is strongly influenced by the composition of the shielding gas. In particular, addition of CO2 increases the threshold current for the transition from ...unstable globular to more stable spray transfer mode. We report on the diagnostics-using optical emission spectroscopy-of a GMAW plasma in pure argon and in mixtures of argon, CO2 and N2 while operated in spray and globular transfer modes. The spatially resolved plasma parameters are obtained by applying the Abel transformation to laterally integrated emission data. The Stark widths of some iron lines are used to determine both electron density and temperature, and line intensities yield relative contents of neutral and ionized iron to argon. Our experimental results indicate a temperature drop on the arc axis in the case of spray arc transfer. This drop reduces with addition of N2 and disappears in globular transfer mode when CO2 is added. Despite the temperature increase, the electron density decreases with CO2 concentration. The highest concentration of iron is observed in the plasma column upper part (close to the anode) and for GMAW with CO2. Our results are compared with recently published works where the effect of non-homogeneous metal vapour concentration has been taken into account.
The particular character of the welding arc working in pure argon, whose emission spectrum consists of many spectral lines strongly broadened by the Stark effect, has allowed measurement, sometimes ...for the first time, of the Stark parameters of 15 Mn I and 10 Fe I atomic spectral lines, and determination of the dependence on temperature of normalized Stark broadening in
N
e
= 10
23
m
−3
of the 542.4 nm atomic iron line. These results show that special properties of the MIG plasma may be useful in this domain because composition of the wire-electrode may be easily adapted to the needs of an experiment.
This article describes laser Thomson scattering as applied to investigate laser-induced plasmas originating from gas breakdown or ablation of solid samples. Thomson scattering provides a reliable and ...direct mean of determining plasma electron density and electron temperature with high spatial and temporal resolution. Moreover, unlike e.g. optical emission spectroscopy, no assumptions about axial symmetry, thermodynamic conditions in the plasma or its chemical composition are needed to quantify these fundamental plasma parameters. Because Thomson scattering is inherently accompanied by Rayleigh light scattering, information about concentration of heavy particles and their temperature can be simultaneously derived from the experimental data. The heavy particle temperature and the electron one are the primary indicators of the plasma thermodynamic equilibrium.
The goals of this article are to describe the theory of Thomson scattering relevant for the studies of low-temperature laser-induced plasmas, discuss the instrumental details of Thomson scattering experiments, and review the results of studies in which this technique has been used to characterize laser-induced plasmas.
•Thomson scattering in diagnostics of breakdown in gases and ablative plasmas•Description of the theory of Thomson scattering relevant for low-temperature laser plasmas•Discussion on the instrumental details of Thomson scattering experiments•Review of studies in which TS was used to characterize laser-induced plasmas
Abstract
The equations of motion in the transition layer of a free-electron laser of the relativistic strophotron type are studied. Two types of strophotrons are considered: electric and magnetic. It ...is shown, that the condition of a suddenly switching-on interaction: smallness of the transition layer at the entrance compared to the distance traveled by the electron in longitudinal direction in one period of transverse oscillations in strophotron is realistic and feasible.
A laser Thomson scattering method was applied to investigate the local Saha–Boltzmann equilibrium in aluminum laser-induced plasma. Plasma was created in ambient air using 4.5ns pulses from a Nd:YAG ...laser at 532nm, focused on an Al target. Spatially resolved measurements, performed for the time interval between 600ns and 3μs, show electron density and temperature to decrease from 3.4×1023m−3 to 0.5×1023m−3 and from 61,000K to 13,000K in the plasma core. The existence of local thermodynamic equilibria in the plasma was verified by comparing the rates of the collisional to radiative processes (the McWhirter criterion), as well as relaxation times and diffusion lengths of different plasma species, with the appropriate rate of electron density evolution and its gradients at given, experimentally determined, electron temperatures. We found these criteria to be much easier to satisfy for metallic plasma species than for nitrogen. The criteria are also easier to satisfy in the plasma core of higher electron density.
•Laser Thomson scattering method was applied to investigate aluminum laser-induced plasma.•Spatio-temporal evolution of electron temperature and density was determined.•Three criteria for existence of local thermodynamic equilibrium were verified.•Criteria are much easier to satisfy for metallic plasma species than for nitrogen.•Criteria are easier to satisfy at earlier times and in the plasma core.
Shock wave produced by a laser induced spark in argon at atmospheric pressure was examined using Rayleigh and Thomson scattering. The spark was generated by focusing a laser pulse from the second ...harmonic (λ=532nm) of a nanosecond Nd:YAG laser using an 80mm focal length lens, with a fluence of 2kJ·cm−2. Images of the spark emission were recorded for times between 30ns and 100μs after the laser pulse in order to characterize its spatial evolution. The position of the shock wave at several instants of its evolution and for several plasma regions was determined from the Rayleigh-scattered light of another nanosecond Nd:YAG laser (532nm, 40J·cm−2 fluence). Simultaneously, Thomson scattering technique was applied to determine the electron density and temperature in the hot plasma core.
Attempts were made to describe the temporal evolution of the shock wave within a self-similar model, both by the simple Sedov–Taylor formula as well as its extension deduced by de Izarra. The temporal radial evolution of the shock position is similar to that obtained within theory taking into account the counter pressure of the ambient gas. Density profiles just behind the shock front are in qualitative agreement with those obtained by numerically solving the Euler equations for instantaneous explosion at a point with counter pressure.
► We investigated shock wave evolution by Rayleigh scattering method. ► 2D map of shockwave position for several times after plasma generation is presented. ► Shock wave evolution is not satisfactorily described within self-similar models. ► Evolution of shock position similar to theory taking into account counter pressure. ► Density profile behind the shock similar to numerical solution of Euler equations.
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