## 1 Introduction  
Pulsed laser processing of materials has some advantages over continuum-wave processing because it can offer thinner processed and heat-affected zones and higher surface temperatures. The serious drawback of laser pulses with durations ranging from nanoseconds to microseconds is the ignition of plasma. The plasma shields the treated surface from the laser beam. Thus, the efficiency of laser treatment decreases with the pulse energy. High repetition-rate sequences of pulses is a well-known solution to increase the efficiency [1; 2; 3; 4]. The computational model developed by [PERSON] et al. [2] to understand the influence of multiple millisecond laser pulses on the surface finish is in agreement with experimentally observed surface roughness [2]. [PERSON] et al. [3] numerically simulated the dynamics of vapor plumes induced by nanosecond multi-pulse laser irradiation in the burst mode. They showed that the multi-pulse mode can considerably decrease the plasma ignition threshold and increase the efficiency of ablation. [PERSON] et al. [4] modeled laser processing by the sequences of sub-millisecond pulses. Their model provided a theoretical guidance for optimizing the process parameters of laser ablation. The complex of physical phenomena related to the interaction of a laser pulse with material includes absorption of laser radiation, heat transfer in the irradiated target followed by melting and evaporation, vapor expansion and interaction with the ambient gas, and laser beam absorption in the vapor, which can result in optical breakdown and plasma ignition. They were studied experimentally [5; 6; 7; 8; 9] and analyzed theoretically [9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. The above works indicate that the evaporation process is very important in laser processing. First, it is responsible for laser ablation. Conventional surface evaporation and phase explosion (volumetric evaporation) are the principal mechanisms for nanosecond pulses [19]. It seems that the phase explosion is more important for higher energies and shorter pulses while the surface evaporation becomes the principal ablation mechanism for longer pulses [2]. Due to high latent heat, evaporation can be the principal mechanism of heat loss. Thus, it influences heat transfer in the target. Finally, low ionization potential and high temperature of metal vapor facilitate optical breakdown [16], which drastically changes the mode of laser/material interaction. The surface temperature is the principal parameter influencing the rate of evaporation. This is why one can decouple gas dynamics and heat transfer in the target [19] under certain conditions. Generally, evaporation from the target surface and back condensation from the gas phase are competing processes [20]. Thus, the net evaporation rate depends also on the gas-phase parameters. Gas dynamics of vapor and ambient gas coupled with heat transfer in the target was calculated by [PERSON]'s direct Monte Carlo simulation (DSMC) [12; 13; 18]. If the mean free path in the gas phase is small, the numerical solution of [PERSON]'s gas-dynamic equations is useful [10; 11]. A strongly non-equilibrium Knudsen layer is formed in the gas phase near the target surface [20; 21; 22; 23; 24; 25; 26; 27]. Rigorous boundary conditions at the Knudsen layer should be taken into account [15]. Gas-dynamic modeling of laser ablation [9; 10; 11; 12; 13; 14; 15; 16; 17; 18] shows formation of a shock wave in the ambient gas and a contact temperature jump between the vapor and the ambient gas. The results of these calculations describe plume expansion and shock wave propagation at nanosecond pulsed laser ablation in ambient gas, which were observed experimentally [5; 7]. Net back condensation of the vapor is expected at later stages of the gas-dynamic flow when the laser pulse is over [15]. The DSMC of laser evaporation by the sequences of nanosecond pulses [3] indicated that the second and the following pulses generate shock waves not only in the ambient gas but also in the vapor formed by the previous pulses.