Enhanced laser-driven proton acceleration using nanowire targets

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 $$\upmu$$ μ m. 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.

High Efficiency Laser-Driven Proton Sources Using 3D-Printed Micro-Structure

“…Therefore, before doing comparison we rst nd out the optimal foil thickness that gives highest proton energy. This was omitted in previous reports [26][27][28] , where the structure target is compared to the planar target with the same thickness of only the substrate, or the structure (substrate thickness is ignored). We scan over the thickness of planar SiN layers from 0.2 μm to 4.0 μm and summarize the measured results in Fig.…”

“…Employing carbon nanotube foam (CNF) on a planar diamondlike carbon (DLC) foil has realized triple energy gain for protons 18,19 . An alternative method is to introduce periodic/aperiodic nano/micro-structures in front of planar targets [20][21][22][23][24][25][26][27][28] . Experimental data indicate that microstructure can greatly raise the temperature of hot electrons beyond that of the ponderomotive acceleration 29,30 .…”

“…Considerable energy enhancement for protons has been observed by using defocused picosecond lasers with normal incidence (10 17 ~10 18 W/cm 2 ) 25 . The effect of pre-plasma in ll is suppressed when employing ultrashort femtosecond relativistic lasers 28 . Judging from the reported results, when using ne surface structures to improve proton acceleration in TNSA, high contrast femtosecond lasers with an incidence angle as small as possible is more favorable.…”

“…The two-photon polymerization (TPP) method can readily achieve lateral resolution ~100 nm and vertical resolution ~500 nm 34 . Compared to other approaches such as Si semiconductor-based technique 33 and electrochemical deposition method 28 , 3D nano-printing can accurately print various complex structures as desired in experiments at micro or sub-micro scale. It is therefore timely to apply this advanced technique to laser-plasma physics such as to manipulate ultrafast laser-plasma interaction 22 , generation of high bright x/gamma-ray 35 , high power coherent Terahertz 36 and positrons 37 .…”

Structuring Free-Standing Foils for Laser-Driven Particle Acceleration Experiments

“…Experimental studies on laser-driven particle acceleration, including particle-in-cell simulation, have been previously reported on aligned nanowires, with sub-wavelength dimensions [28][29][30][31][32][33], which are similar in structure to the anodic aluminium oxide template (AAO). One of the approaches used to improve the laser absorption in the laser-driven ion acceleration experiments is the nanostructuring of the substrates in a brush-like geometry to obtain nanometre-sized rods standing in an upright position [31]. These structures have the advantage of increasing the hot electron density and temperature that in turn can lead to enhancement ratios of 2-3 for the maximum proton energy.…”