## 2 Properties of Microstructured Snow Targets  
The snow structured targets were generated on liquid nitrogen cooled sapphire substrate on which water vapor was deposited. The snow structure is determined by parameters of growth conditions, such as pressure, flow velocity, and temperature, and by the nucleation centers [20,21]. The snow surface can be described as a highly structured surface with three roughness scales: (1) pillars in the scale of 100 \(\upmu\)m; (2) spikes in the length of 10 \(\upmu\)m on top of them; and (3) whiskers in the scale of 1 \(\upmu\)m on top of the spikes. The distance between the structures is of the order of their roughness scales. A key factor determining the interaction of the main pulse with the snow target is how its highly structured surface is affected by prepulse or lower pedestals intensity levels irradiation, in particular, whether each roughness scale prevails and how steep of the plasma gradient generated by the prepulse is. One of the parameters determining the state of the target after the interaction with the prepulse and the plasma density gradient at the interaction of the main pulse with the target is the Optical Damage Threshold (ODT). As it will be shown here, the ODT intensity level is lower than the prepulse intensity; however, it represents a lower limit for controlling the level of possible existing pedestal in the laser system. This feature is identified in the experiments reported here by irreversible change of the surface [22]. The ODT is a consequence of multiphoton followed by impact ionization of the snow [23,24,25,26]. The free electrons generated by ionization escape the target and due to charge separation they pull the ions out of the target. In order to measure the prepulse influence on the microstructure targets with tens of femtoseconds gate resolution and a few microns for space resolution we used an imaging system and gating technique [22]. The measurements of the ODT of snow were performed at the Hebrew University High Intensity Laser Facility with a Ti: Sapphire laser based on Chirped Pulse Amplification (CPA) that can deliver peak power of 1 TW in 50 fsec with 10 Hz repetition rate. In order to determine the snow ODT, the laser energy on target was controlled using filters and reduced to the prepulse intensity level. The diagnostic method was strobe illumination by short pulse white light source and imaged at three times:1. Before the interaction of the laser with the snow target. 2. 7 ns after the interaction (around the delay between the prepulse and the main pulse). 3. At longer times, minutes after the interaction. The target was imaged at these three times with a telescope system to a CCD camera placed outside the vacuum chamber. The diagnostic beam was part of the main beam that was used for generating super-continuum strobe, enabling gives a controllable delayed probing with fast gating at the time scale of the laser pulse duration. The measurements were conducted for various values of the laser fluence on target, in the range (0.1-3) J/cm\({}^{2}\). Three images were recorded for each laser fluence (Figure 2). The first image of the snow target, long before the interaction, was obtained by blocking the main beam and using the strobe alone. The second image, 7 nanoseconds after the interaction, of the order of the time duration between the prepulse and the main pulse of the laser system, was obtained using both beams of the laser, and the third image, long after the interaction, again by blocking the main beam and using the probe beam only. Damage was identified as irreversible change in the target surface and was identified by measuring the intensity as a function of distance along lines on the image. Using this processing procedure, we have seen that at fluence higher than 1 J/cm\({}^{2}\) on target a definite damage has been occurred to the target, as can be seen in Figure 2 (a.1-3). At laser fluence on target less than 0.35 J/cm\({}^{2}\) we could not see any damage with our imaging system and processing procedure (Figure 2 (c.1-3)).