Generation of shock trains in free liquid jets with a nanosecond green laser

Shock wave trains in liquid jets were previously generated only by ablation with femtosecond x-ray lasers. Here we show that shock trains in water microjets can be also generated using nanosecond green laser pulses with 1- to 10-mJ energy. The ablation of 15-, 20-, 30-, and 70-$\mu \mathrm{m}$ water microjets opened a gap in the jets and launched an initial shock wave. Fully developed shock trains were observed in the 30- and 70-$\mu \mathrm{m}$ jets up to 250-ns delays, and these trains were also transmitted inside the nozzles. A few tens of nanoseconds after the pulse, the shock dynamics and its pressure became similar to the ones generated by x-ray lasers, with a more rapid pressure decay in thinner jets. At time delays exceeding 100 ns in the 30-$\mu \mathrm{m}$ jets, the leading shock pressure stabilized to an approximately constant pressure of 40 MPa. The energy density deposited in the jets was estimated at 30 MJ/${\mathrm{cm}}^3$ by comparing the jet gaps in the green and x-ray laser experiments, and matched previous estimates for optical ablation in water. The pressure decay in the 30-$\mu \mathrm{m}$ jets was modeled based on the pressure decay observed in x-ray laser experiments.

Figure 1

Experimental setup. Water microjets were ablated with a nanosecond laser and imaged using a femtosecond laser. M1, M2, and M3: silver mirrors; L1 ($f=25$ mm), L2 ($f=100$ mm), L3 ($f=125$ mm), L4 ($f=75.6$ mm), L5 ($f=38.1$ mm), and L6 ($f=125$ mm): lenses; HWP: half waveplate; Pol: polarizing beamsplitter cube.

Figure 2

Water microjets with 15- to 70-$\mu \mathrm{m}$ diameters after ablation by a focused laser pulse in air. The jets flowed downstream and the laser pulse propagated horizontally from right to left. The time delays written in the images represent the time elapsed from the laser pulse. The white or black arrows indicate the location of the leading shock waves. Panels (d1) and (d2) show shock waves transmitted inside the glass nozzles that produced the jets. The dimension bar is 100 $\mu \mathrm{m}$ in all images.

Figure 3

The visibility of shock waves for different time delays, jet diameters, and pulse energies. Shocks were more visible in wider jets and when driven by lower energy pulses.

Figure 4

The pressure of the leading shock wave and its decay in (a) 30-$\mu \mathrm{m}$ jets and (b) 70-$\mu \mathrm{m}$ jets, for the pulse energies shown in the legend. To help distinguish data sets for different pulse energies, the data are plotted shifted by –3, –1, 1, and 3 ns at 1.1, 3.2, 5.7, and 9.7 mJ, respectively.

Figure 5

(a) Comparison of the jet gaps produced by the green laser and an x-ray laser. The similar temporal dynamics allows the estimation of the energy deposited in the jet by the green laser, as listed in the caption. The continuous lines are logarithmic fits to the data. (b) Comparison of the shock pressures with models based on the decay of XFEL-induced shock pressures [see Eqs. (1) and (2)]. The data points are the same as the ones displayed in Fig. 4 and have the same error bars; the error bars in Fig. 5 are shown only for the 3.2-mJ data to avoid excessive clutter in the graph.