## 4 Discussion  
As follows from the data of review [13], for laser pulses at the wavelength \(\lambda\) = 1064 nm with a duration \(\tau\) = 60 ps and a spot diameter in the beam waist \(d\) = 31 \(\upmu\)m the breakdown threshold intensity \(I_{thr}\) = 0.14 TW/cm\({}^{2}\). Furthermore, it was found in [47] that for the pulses with a duration of \(\tau\) = 30 ps and spot diameter \(d\) = 19.5 \(\upmu\)m the breakdown threshold is \(I_{thr}\) = 0.37 TW/cm\({}^{2}\). The indicated diameters of \(d\) approximately correspond to the spot diameter in our experiments, i.e., \(d_{0}\) = 40 \(\upmu\)m. The value of the threshold intensity measured in [47] agrees in order of magnitude with the value \(I_{thr}\) = 0.85 TW/cm\({}^{2}\), which was measured in our experiments in water saturated with dissolved gas, i.e., consisting of the phase of bubston clusters. It should be noted that the experimental works considered in review [13] do not report on the samples of degassed water. At the same time, as was obtained in our experiments, the experimental breakdown threshold for degassed samples is \(I_{thr}\) = 3 TW/cm\({}^{2}\), which noticeably exceeds the breakdown threshold for the same water saturated with dissolved gas, i.e., having the bubston clusters. Thus, our results essentially refine the data presented in review [13]: in the case of the optical breakdown of water saturated with dissolved gas, the presence of bubston clusters, which serve as local breakdown centers, should be taken into account. The pattern of optical breakdown on bubston clusters near the threshold corresponds to a set of discrete plasma flashes; each flash corresponds to the breakdown inside a separate cluster, see Figure 9a. Assuming that there are four bright flashes (i.e., four clusters) in the volume of the caustic (a cylinder with a radius of 20 \(\upmu\)m and a length of 3.4 cm), we obtain an estimate for the volume number density of clusters \(n_{cl}\approx 10^{6}\) cm\({}^{-3}\), which is consistent  
Figure 10: Dependence of the breakdown flash intensity \(Lum_{BR}\) vs. the pump intensity \(I\) at the focal area (\(F\) = 20 mm) for degassed Milli-Q water. with the estimate in [35], obtained on the basis of polarization scatterometry; in accordance with this work, \(n_{cl}=0.5\times 10^{6}\) cm\({}^{-3}\). It should be noted that, after the completion of the picosecond laser pulse, the glow of the breakdown plasma is controlled by radiative processes, the duration of which can exceed the duration of the laser pulse, see [13; 47]. All this leads to the initial breakdown pattern, consisting of individual plasma flashes (Figure 9a), turning into a \"smeared\" breakdown pattern shown in Figure 9b. The blurring process is due to the plasma expansion and the effect of moving the breakdown site, see Ref. [13]. The model of moving breakdown assumes that breakdown occurs independently at multiple sites along the beam path; i.e., we are dealing with multiple breakdown. The multiple breakdown occurs whenever the threshold is reached at each breakdown site. In our case, it is straightforward to assume that these breakdown sites are the bubston clusters localized along the caustic. Since the optical breakdown, accompanied by a plasma flash, leads to the generation of a shock wave, the breakdown region expands with the speed of the shock wave, and we can estimate this speed from indirect data. The expansion of the breakdown region due to the movement of the plasma front results in the formation of a new plasma pattern, elongated in the direction of the laser beam from the original breakdown site. The resultant plasma pattern along the caustic looks more uniform due to the merging together of the local breakdown sites, see Figure 9b. Following [13], for the maximum length of the plasma area we have an empirical formula \(z_{\rm max}=z_{R}(\beta-1)1/2\), where \(\beta=I/I_{thr}\), \(z_{R}\) is the caustic length, \(\lambda=1064\) nm. In our case \(z_{R}=3.4\) mm.