Density measurements of laser interaction with ordered structured ‘snow’ targets

Abstract

This paper presents a new method to control the position of a micro-column snow target. This target enables the measurement of the mean electron density of the pre-plasma created by a pre-pulse with different time delays. This research will allow a better understanding of the generation of fast protons from the interaction between a structured pre-plasma and a high intensity laser.Laser driven proton acceleration is an active field of research due to its high potential in reducing the size and cost of conventional accelerators. The acceleration scheme consists of a very high intensity (>10 18 W cm −2 ), high contrast (10 −10 ) laser pulse [1,2] . The laser pulse interacts with a solid target, commonly a thin foil, from which high energy ions and protons are accelerated. Enhancement of the energy of the generated protons using a compact laser source (moderate intensity, 10 17 -10 19 W cm −2 ) is a challenging but rewarding task due to the fact that the proton energy in traditional schemes scales as the electric field employed on the target [3] . One of the promising ways to increase the ion energy is the use of structured targets [4] , mainly nanotargets [5] . In this kind of target the energy conversion efficiency from the laser to the proton beam is increased [6] .Recently, we demonstrated that by using frozen H 2 O micro-column targets, which were grown on a sapphire substrate, significantly improved absorption of the laser energy by the H 2 O micro-column target took place. This study showed that more than 90% of the incident energy was absorbed by the target [7] . Moreover, we have shown that the emitted proton energy scales up by a factor of 10 compared with standard laser driven protons schemes [8][9][10][11] . The maximal proton energy as a function of laser power is presented in Figure 1. Numerical particle in cell (PIC) simulations indicate that the enhancement of proton energy is attributed to the structure of the snow target and the preplasma formed by the pre-pulse and the density gradients formed [10] .

Femtosecond laser-induced damage threshold in snow micro-structured targets

“…The conditions of the snow growth followed the conditions in Refs. [28, 29], in order to achieve the morphology seen by the scanning electron microscope (SEM) in Refs. [28, 29].…”

“…[28, 29], in order to achieve the morphology seen by the scanning electron microscope (SEM) in Refs. [28, 29]. The snow was deposited at a temperature down to and in the pressure range of 0.1–10 Torr.…”

“…More details on the snow targets growth procedure are presented in Refs. [28, 29]. Typical snow target as imaged by an SEM is shown in Figure 1.…”

Proton Acceleration by Ultrashort Intense Laser Interaction with Microstructured Snow Targets

“…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 laser spot size in our experiments was about 4-5 μm, therefore the laser spot may have interacted with one or sequentially with few whiskers, based on the SEM characterization of the snow targets [20,21]. Here, for simplicity, we consider interaction with a single snow whisker.…”