## 4 Discussion  
The modeling indicates that at inter-pulse separation 10 \(\upmu\)s, the heat transferred to the target by a laser pulse is not completely dissipated when the next pulse of the sequence starts. Thus, thermal energy is accumulated in a near-surface layer of the target. It is illustrated by Figure 5 where the temperature curve of each pulse is above the temperature curve of the previous pulse. The intensity of evaporation by the pulses of a sequence increases because of increasing surface temperature. In the modeled regimes of single and multipulse laser processing, the vapor velocity near the target surface becomes negative even before the termination of the 300 ns laser pulse (see Figures 4 and 7). Thus, back vapor condensation on the target surface starts. According to the model, all the evaporated material condenses back. The period of time necessary for full back condensation depends on the parameters of laser processing. The drop of the absolute value of Mach number \(|M|\) down to zero in Figure 8 indicates the instant of complete back condensation. For single pulse with \(I_{\text{max}}=15\) MW/cm\({}^{2}\) (see Figure 8b), it is \(t=1900\) ns. At \(I_{\text{max}}=10\) MW/cm\({}^{2}\), the duration of back condensation is considerably less. For the second pulse (see Figure 8c), \(t=315\) ns. For the third pulse (see Figure 8d), \(t=415\) ns. In any  
Figure 8: Parameters of the Knudsen layer at evaporation/condensation at different regimes of laser processing: single pulse processing (**a,b**); multipulse processing: (**c,d**). The arrow near each curve points to the corresponding axis: temperature ratio (black curves); Mach number (red curves); inverse pressure ratio (dashed blue curves). Figure 9: Melt depth in the target at single pulse laser processing with \(I_{\text{max}}=15\) MW/cm\({}^{2}\) and multipulse processing with \(I_{\text{max}}=10\) MW/cm\({}^{2}\) and inter-pulse separation 10 \(\upmu\)s.  
case, back condensation finishes well before the beginning of the next laser pulse at \(t=10\) us. This is why the laser radiation of the next pulse cannot interact with the vapor formed by the previous pulse. Figure 8 shows that Mach number \(M\) approaches its theoretical limit for evaporation \(M=1\)[20; 21; 22] but does not attain it. This is because of a relatively low laser intensity. Calculations by the similar model at higher intensities [15] indicated that \(M=1\) can be attained. At evaporation, temperature ratio \(T/T_{s}\) and pressure ratio \(p/p_{s}\) are functions of \(M\) given by Equations (14) and (15). Note that the temperature ratio attains its minimum when evaporation is the most intensive (see Figure 8). At the stage of condensation, \(M\) tends to \(-1\) (see Figure 8). Note that supersonic condensation with \(|M|>1\) is theoretically possible [24; 25]. However, it is not observed in the present work as well as in the previous one [15]. For the calculated examples, temperature ratio at condensation is in the interval between 0.4 and 1 (see Figure 8). This means that the vapor is colder than the target. Such an unusual ratio is explained by adiabatic expansion of vapor where its temperature rapidly decreases as a power function of the density. The temperature of the target surface is controlled by heat conduction and decreases more slowly. Even if the vapor is colder, its pressure is still greater than the saturated vapor pressure at the temperature of the target surface. This is why condensation continues. Once the Mach number of the condensing vapor flow attains \(-1\), the value of \(M\) is around \(-1\) till the complete condensation of the vapor (see Figure 8). Similar behavior of the condensing vapor flow was observed at modeling for much shorter nanosecond pulses [15]. Figure 9 shows the calculated dynamics of the melting/solidification front. The maximum melt depth estimates the thickness of the surface layer with the modified structure.