## 3 Results
Figure 6 shows that at the moment of \(t\) = 10 us after the pulse, the surface temperature is still about 600 K. The second pulse of the sequence starting at \(t\) = 10 us, interacts with the already heated target. Therefore, it results in higher surface temperature (dashed line in Figure 5). The surface temperature becomes greater than the boiling point and evaporation starts. At the beginning of the third pulse at \(t\) = 20 us, the surface temperature attains 800 K (see Figure 6a), the temperature after  
Figure 3: Dynamics of the temperature field in the target for a single pulse with \(I_{\text{max}}\) = 15 MW/cm\({}^{2}\). \begin{table}
\begin{tabular}{l l} \hline Duration (ns) & 300 \\ \hline Inter-pulse separation (us) & 10 \\ \hline Absorbed intensity at the maximum, \(I_{\text{max}}\) (MW/cm\({}^{2}\)) & 10 \(\ldots\) 15 \\ \hline \end{tabular}
\end{table} 
Table 3: Parameters of the laser pulse. Figure 2: Temporal profiles of the absorbed laser power, the surface temperature, and the evaporation loss for a single pulse with \(I_{\text{max}}\) = 15 MW/cm\({}^{2}\). the third pulse is higher (full line in Figure 5), and evaporation should be more intensive. Figure 6b shows the evolution of the temperature field in the target after the third pulse and Figure 7 presents the gas-dynamic fields. The gas-dynamic fields are qualitatively similar to the fields shown in Figure 4 for a single pulse with \(I_{\rm max}\) = 15 MW/cm\({}^{2}\). The maximum vapor pressure in Figure 4 is about 5 atm, while it is around 2 atm. In Figure 7. Therefore, one can conclude that evaporation is less intensive at \(I_{\rm max}\) = 10 MW/cm\({}^{2}\), even after the third pulse. Figure 4: Evolution of the gas-dynamic fields at single-pulse laser processing with \(I_{\rm max}\) = 15 MW/cm\({}^{2}\). The arrow near each curve points to the corresponding axis. The axis title contains the name of the physical quantity. Flow velocity (dashed curves) and mass densities of vapor (red curves) and gas (blue curves): (**a,c,e**,g**). Temperature (black curves) and pressure (blue curves): (**b,d,f,h**). Time \(t\): 0.1 \(\upmu\)s (**a,b**); 0.2 \(\upmu\)s (**c,d**); 0.3 \(\upmu\)s (**e,f**); 1 \(\upmu\)s (**g,h**). Evaporation/condensation at the target surface is characterized by such parameters of the Knudsen layer as pressure ratio \(p/p_{s}\), temperature ratio \(T/T_{s}\), and Mach number \(M\) related by the boundary conditions of evaporation/condensation, Equations (14)-(16). Figure 8 shows the evolution of these parameters at the studied regimes of laser processing. The effect of laser radiation on the target is characterized by the evolution of the temperature field shown in Figures 3 and 6 along with the melt depth in the target shown in Figure 9.  
Figure 5: Increase of the surface temperature due to heat accumulation in the target at laser processing by the sequence of pulses with \(I_{\text{max}}=10\) MW/cm\({}^{2}\) and the inter-pulse separation 10 \(\upmu\)s.  
Figure 6: Dynamics of the temperature field in the target for a sequence of pulses with \(I_{\text{max}}=10\) MW/cm\({}^{2}\), and inter-pulse separation 10 \(\upmu\)s: (**a**) moment \(t=10\)\(\upmu\)s corresponds to the beginning of the second pulse, the third pulse starts at \(t=20\)\(\upmu\)s, and the fourth pulse starts at \(t=30\)\(\upmu\)s; (**b**) more details for the third pulse. Figure 7: Evolution of the gas-dynamic fields after the third pulse of the sequence with \(I_{\rm max}\) = 10 MW/cm\({}^{2}\) and inter-pulse separation 10 \(\upmu\)s. The arrow near each curve points to the corresponding axis. The axis title contains the name of the physical quantity. Flow velocity (dashed curves) and mass densities of vapor (red curves) and gas (blue curves): (**a**,**c**,**e**,**g**). Temperature (black curves) and pressure (blue curves): (**b**,**d**), (**f**,**h**). Time \(t\): 20.15 \(\upmu\)s (**a**,**b**); 20.2 \(\upmu\)s (**c**,**d**); 20.3 \(\upmu\)s (**e**,**f**); 21 \(\upmu\)s (**g**,**h**).