Pulsed IR laser ablation of organic polymers in air: shielding effects and plasma pipe formation

Abstract

We report the effect of ‘plasma pipe’ formation on pulsed laser ablation of organic polymers in air under normal conditions. Ablation of polymers (PMMA, polyimide) is carried out in a wide range of CO2 laser fluences with special attention to plasma formation in the ablation products. Evolution of laser ablation plumes in air under different pressures is investigated with simultaneous registration of radiation spectra of the ablation products. An analysis based on thermo-chemical modelling is performed to elucidate the effects of laser light attenuation upon ablation, including plasma and chemical processes in a near-target space. The analysis has shown that the experimental observations of plume development in air can be explained by a combination of processes including formation of a pre-ionized channel along the laser beam propagation, laser-supported detonation wave and effective combustion of the polymer ablation products. A scenario of a streamer-like polymer plasma flow within an air plasma pipe created via laser-induced breakdown is proposed.

Impacts of Ambient and Ablation Plasmas on Short- and Ultrashort-Pulse Laser Processing of Surfaces

“…When the laser beam of relatively high laser intensity is focused on the target surface, air ionization occurs at the beam axis (Figure 2a). In our experiments on polyimide and poly(methyl methacrylate) (PMMA) [14], the ionized air channel in front of the target was detected to be ~5 mm long. Due to the heat release in the ionization zone, ionized air radially expands while the initiated ablation process generates a shock wave with the adjoint LSDW (Figure 2b).…”

“…Here we underline another role of laser-induced air ionization, formation of a low-density channel due to lateral gas expansion from the breakdown zone [13,14]. This rarefied quasi-cylindrical zone termed the "plasma pipe" [14] guides the ablation products off from the ablated surface.…”

“…This rarefied quasi-cylindrical zone termed the "plasma pipe" [14] guides the ablation products off from the ablated surface. Its formation is illustrated in Figure 1.…”

“…Due to the heat release in the ionization zone, ionized air radially expands while the initiated ablation process generates a shock wave with the adjoint LSDW (Figure 2b). The LSDW propagates along the ionized channel with an estimated characteristic velocity of 5-10 km/s [14]. A strong heat release in the LSDW leads to the temperature rise up to several eV along the LSDW path and to further reduction of the density of expanding air within the plasma pipe, up to ten times smaller than the initial level [16].…”

“…Schematics of plasma pipe formation and development under conditions of CO2 laser ablation in air [14]. See explanation in text.…”