Optical emissivity from a laser-driven shock-heated dense plasma

A plume consisting of vapour and ionized particles of the workpiece is usually formed during various types of laser materials processing. The characteristics of this plume depend on a large number of parameters such as the laser power, spot size, scanning speed, material properties and shielding gas. The height, radius, temperature and absorption coefficient of the plasma are calculated for various values of process parameters. The surface temperature of the melt pool and the vaporization rate are also calculated on the basis of the Stefan condition at the liquid-vapour interface. The absorption coefficient of the plasma given by the Kramers-Unsöld relation and the ionization fraction given by the Saha-Eggert equation are used to model the laser beam propagation through the plasma. A detailed analysis of the plasma stability indicates that absorption of the laser beam by the plasma affects the melt-pool surface temperature nonlinearly.

Section: Introductionmentioning

confidence: 99%

Nonlinear effects of laser-plasma interaction on melt-surface temperature

Sankaranarayanan

Kar

1999

“…We have developed a model [9] used to describe optical emission from a shock heated conductor-insulator interface to describe these results. The model assumes that the shock has a finite width and the temperature and the mass density of the material in the shock front rise linearly from room temperature and solid density to an equilibrium temperature and a compressed density higher than solid density respectively in the distance of the shock width.…”

Section: Resultsmentioning

confidence: 99%

“…In this paper measurement of the time resolved optical reflectivity of a confined surface at the interface of a conductor-insulator, and not the free surface as reported by others [6,7], is presented. The optical emission of aluminium through the transparent material has already been reported in [9] in which a simple model was used to interpret the results. The same conditions of the emissivity model have been used to describe the time resolved reflection of a laser probe pulse from the aluminium.…”

Section: Introductionmentioning

confidence: 99%

Optical reflectivity of dense plasmas produced by laser driven shock waves

Mahdieh

Hall

1997

The time history of the optical reflectivity of a dense plasma at the confined interface of a planar aluminium-plastic target was recorded through the transparent material, during the shock emergence from the aluminium. The shock wave was produced by the irradiation of the aluminium side of the target with a long-pulse (∼2.4 ns FWHM) 1.06 µm laser light at irradiances up to 7 × 10 13 W cm −2 and the interface probed by a low-energy 0.53 µm laser pulse. The reflectivity of the probe laser beam at the interface was recorded across the shocked region. In the central shocked region the reflected probe light decreases in less than 200 ps, as the shock arrives at the rear side, and remains nearly constant at later time. The experimental results are interpreted by employing a simple model. It is shown that the reflection at early times can be explained by Fresnel reflectivity of the aluminium which decreases due to a temperature rise and an absorbing plasma in front of the shock. At later time a WKB approximation is used to calculate the reflectivity from the critical density of the dense plasma at the shock front. This reflectivity is very sensitive to the electron-ion collision frequency which is measured by fitting the model and experimental results.

“…The brightness temperature can be inferred from Planck's law for blackbody radiation with a single wavelength channel but absolute intensity calibration is required to deduce the temperature. Since the calibration of the absolute intensity is usually performed for the low temperature and the temperature of the shocked material is much higher than that, this technique is not very accurate [9].…”

Section: Model and Calculationsmentioning

confidence: 99%

“…Therefore, luminosity technique has been used for both temperature and shock velocity measurements [5][6][7][8]. The optical emission, and reflectivity of a confined conductor-insulator interface, compressed and heated by a shock wave was also reported [9,10]. Using stepped targets, simultaneous measurements of the shock velocity and brightness (or colour) temperature of shock front at the time of breakthrough into the vacuum, was performed [11].…”

Section: Introductionmentioning

confidence: 99%

Real temperature calculation of shock wave driven by sub-nanosecond laser pulses

Mahdieh

Hall

2003

Time history of thermal band emission of a shock front, when breakout from aluminium target into vacuum, has been calculated numerically. It is assumed that the shock is produced by irradiation of high intensity sub-nanosecond pulsed laser on the surface of aluminium planar targets in vacuum. The opacity of dense plasma at the shock front and in the vacuum-aluminium interface, and its effects on thermal emissions was considered in these calculations. Using the results of an experiment that was recently reported and those of our model, the real temperature of the shock front was estimated. In that experiment simultaneous measurements of the colour temperature of dense plasma in a shock front, and the shock velocity at the time of shock breakout from the aluminium targets into the vacuum were reported for the study of the equation of state (EOS). The results of the model show a good agreement with the SESAME library EOS.