Thermal analysis of polymethyl methacrylate ablation by pulsed IR lasers

We present theoretical studies of polymethyl methacrylate (PMMA) ablation by single pulses of C0 2 -laser radiation (9.17 μm) and by pulse trains of free-electron laser (FEL) operating in the wavelength range of 100-200 μm. Laser heating and vaporization of the PMMA sample and the mechanisms of ablation have been investigated for the particular experimental conditions. The theoretical description of laser-induced target heating and ablation is based on the widely used thermal model which allows one to describe mass removal over a fairly wide range of laser fluences from ablation threshold up to transition to phase explosion. It has been shown that, in contrast to metal ablation, PMMA ablation continues up to several milliseconds after laser pulse termination. Phase explosion aspects are analyzed on the basis of the target superheating data obtained by modeling. It has been demonstrated that, in the case of PMMA ablation, the pulsed regime of FEL can be modeled by continuous irradiation with the equivalent power. A methodology has been proposed to evaluate the absorption coefficients of PMMA for wavelengths of FEL, which are unknown at present, basing on the vaporization rate.