Abstract
Laser-driven proton acceleration is a growing field of interest in the high-power laser community. One of the big challenges related to the most routinely used laser-driven ion acceleration mechanism, Target-Normal Sheath Acceleration (TNSA), is to enhance the laser-to-proton energy transfer such as to maximize the proton kinetic energy and number. A way to achieve this is using nanostructured target surfaces in the laser-matter interaction. In this paper, we show that nanowire structures can increase the maximum proton energy by a factor of two, triple the proton temperature and boost the proton numbers, in a campaign performed on the ultra-high contrast 10 TW laser at the Lund Laser Center (LLC). The optimal nanowire length, generating maximum proton energies around 6 MeV, is around 1–2 upmum. This nanowire length is sufficient to form well-defined highly-absorptive NW forests and short enough to minimize the energy loss of hot electrons going through the target bulk. Results are further supported by Particle-In-Cell simulations. Systematically analyzing nanowire length, diameter and gap size, we examine the underlying physical mechanisms that are provoking the enhancement of the longitudinal accelerating electric field. The parameter scan analysis shows that optimizing the spatial gap between the nanowires leads to larger enhancement than by the nanowire diameter and length, through increased electron heating.
Highlights
Laser-driven proton acceleration is a growing field of interest in the high-power laser community
Hydrogen containing contaminants are accelerated at the rear surface of a thin solid foil, typically made of gold (Au), aluminum (Al) or copper (Cu), that is irradiated by a high-intensity ( I0 > 1018 W/cm2 ), short pulse (< 1 ps) laser operating in the near-infrared spectral range
The high aspect ratio between length and diameter of NWs favors an increased laser absorption due to a greater effective interaction surface area with the incoming electromagnetic (EM) wave, occurring within a few laser cycles such as to maximize the interaction with the intact NW forest. This increased interaction ejects electrons from the NW boundaries mainly through Brunel-type and J × B absorption processes, which are further accelerated in the gaps between the NWs by Direct Laser Acceleration (DLA) before re-collision with the target bulk[49], where the electrons seed a cascade of impact ionization events
Summary
We performed PIC simulations using the 2D3V PICLS c ode[53] to determine the optimal NW geometry for proton acceleration and to orient the NW production process. We expect to observe lower enhancement ratios in the experiment since 2D PIC simulations are known to overestimate the electrons energies by a factor in the range of 1.5-258,59 through greater J × B electron heating, this effect can be compensated by the shorter travel time of hot electrons in the NW interspace in 2 D60 This proton energy overestimation is further amplified for NWs due to an overestimated electron confinement, leading to stronger TNSA electric field[60]. The experimentally used gap of g = 200 nmalready provides a substantially increased laser-to-proton conversion efficiency by a factor of 5 with respect to the flat target case, being near-optimal for the laser energy absorption as shown in Fig. 1d (blue diamonds)
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