## 4 Discussion
Thus, at \(\beta\approx 1\), there will be no blurring effect of local breakdown sites (apparently, this situation is illustrated in Figure 9a), while in the case of \(\beta\geq 1\), each plasma point will experience a slight blurring; that is, the breakdown pattern will take the shape exhibited in Figure 9b; in this case \((\beta-1)^{1/2}=(1.2/0.85-1)^{1/2}=0.64\); that is, the value of \(z_{\rm max}\) will be slightly less than the length of the caustic \(z_{R}\). Let us now estimate the velocity \(u\) of the plasma front at which the pattern of the plasma flash becomes uniform along the caustic. Let us assume that the volume number density of clusters is equal to about \(10^{6}\) cm\({}^{-3}\); that is, the average distance between clusters is \(l\approx 100\) um. In addition, we assume that for the spatial homogeneity of the flash, the plasma front must cover the distance \(l/2\approx 50\) um during the plasma flash. Taking \(\tau_{pl}=1.6\) ns for the plasma flash duration (this value was taken from study [47] for plasma glow during the optical breakdown of water excited by an 80 ps pulse at wavelength \(\lambda=1064\) nm), we obtain the estimate \(u=l/(2\tau_{pl})=31\) km/\(s\); i.e., here we are dealing with a shock wave of very high intensity. Therefore, optical breakdown at laser pulse intensities in the range \(I\sim 10^{12}\) W/cm\({}^{2}\) is obviously not applicable in ophthalmology, since the shock wave should inevitably lead to undesirable side effects. At the same time, in the case of breakdown on bubston clusters, the first stage of breakdown is stimulated optical coalescence, which requires quite moderate intensities (of the order of \(10^{7}\) W/cm\({}^{2}\)) to occur. If the intensity of laser radiation is fixed at this level, then a plasma flash will not occur; that is, it will be possible to avoid the appearance of a shock wave. According to the results of this work, the volume number density of bubston clusters in doubly distilled water saturated with dissolved gas \(n_{cl}\approx 10^{6}\) cm\({}^{-3}\). As shown in [32], the volume number density of bubston clusters can be estimated using the empirical formula \(n_{cl}=\kappa/\sigma_{sca}\), where \(\kappa\) is the extinction coefficient of radiation at a wavelength of 532 nm in an aqueous salt solution; \(\sigma_{sca}\) is the scattering cross section of radiation at this wavelength on a separate bubston cluster. Note that at this wavelength the absorption of light in water is absent; that is, extinction is due only to scattering by particles exceeding the wavelength in size. In [32], the extinction coefficient was measured in a dust-free aqueous solution of NaCl in the concentration range \(10^{-3}<C<3\) M. As shown in this work, for ion concentrations \(C<1.5\) M, the values of the dependence of extinction coefficient \(\kappa\), measured in the experiment on laser light scattering vs. \(C\), can be approximated by the empirical formula \(\kappa=\alpha(C-C_{0})^{1/2}\), where \(\alpha\) is a dimensional constant and \(C_{0}\) is the lower limit of ion content at which bubston clusters are formed; this quantity was not determined by us. Assuming that the scattering cross section \(\sigma_{sca}\) does not depend on the content of ions,we obtain that an increase in the ion concentration by a factor of 100 should lead to an increase in the volume number density \(n_{cl}\) of bubston clusters by approximately a factor of 10. In a physiological solution of NaCl (this solution is apparently close by its properties to intraocular fluid), the content of ions is \(C=0.14\) M, while for doubly distilled water we have an estimate of \(C=10^{-6}\) M. Thus, the volume number density of bubston clusters \(n_{cl}\) in physiological saline solution should increase by about 300 times compared to doubly distilled water, i.e., \(n_{cl}\approx 10^{8}\) cm\({}^{-3}\). Therefore, bubston clusters are likely to be centers of optical breakdown in intraocular fluid.