Investigation of the absorption coefficient of metal targets irradiated by pulse-periodic CO₂laser radiation in air

Abstract

The emission of positronium negative ions from Cs deposited W(100) surfaces has been studied. A dramatic change in the emission efficiency was observed upon coating a W(100) surface with Cs. The conversion efficiency (the fraction of incident slow positrons yielding the ions) of the W(100) target with a 2.2 × 10 14 atoms cm −2 Cs coverage, measured over a time interval of 3 × 10 3 s immediately after deposition, was found to be 1.25%, which is two orders of magnitude higher than that obtained for the clean, uncoated W(100) surface and 45 times greater than the highest efficiency reported thus far.

Energy Coupling of Laser Radiation on AISI 304 Stainless Steel: Effect of High Temperatures and Surface Oxidation

“…The temperature independent energy coupling efficiency is in good agreement with the results for the absorptivity at temperatures below 600 °C in [24]. As mentioned before, the decrease of the absorptivity below the melting point was attributed to a cleaning effect of the surface [21]. In our experiments we believe that a cleaning effect can be excluded for two reasons.…”

“…The probes were further polished and then cleaned in a ultrasonic ethanol bath. Second, the decrease also continues above the melting point, which contradicts the observations for aluminum [21,22,23]. Therefore, we believe that the measured drop in energy coupling is caused by the temperature increase only.…”

“…Some studies reported higher values of energy coupling on oxidized surfaces [18,19,20]. Contradictorily, others observed an initial decrease of the coupling efficiency while the effect of oxidation, in turn resulting in higher coupling efficiencies, only takes place above a certain temperature range [21,22,23]. The decreasing coupling efficiency was ruled as due to a cleaning effect of the surface due to the laser irradiation.…”

On the theoretical description of the early oxidation stages of copper by cw CO2 laser irradiation

“…However, if in equation 1we use the « laser » values of the thermodiffusive constants which we established in [7], by processing the variation curves of the sample's temperature and its derivative during laser irradiation, i.e. do = 1.4 cm2/s and TD = 15 500 K, a much better agreement with the experimental results is to be pointed out, as one can notice from table I (xl values).…”

“…The variation of the samples' temperature and its derivative were followed with the aid of built-in chromel-alumel thermocouples. The targets absorptivity variation of the 10.6 gm radiation was then inferred according to the procedure described in [7]. If the thermal constants of copper are considered, one can check out that the temperature inside the samples gets uniform in a time interval which is negligible as compared to the irradiation durations of hundreds and thousands of seconds.…”