Astrophysical Neutrinos and Blazars

We review and discuss recent results on the search for correlations between astrophysical neutrinos and γ-ray-detected sources, with many extragalactic studies reporting potential associations with different types of blazars. We investigate possible dependencies on blazar sub-classes by using the largest catalogues and all the multi-frequency data available. Through the study of similarities and differences in these sources we conclude that blazars come in two distinct flavours: LBLs and IHBLs (low-energy-peaked and intermediate-high-energy-peaked objects). These are distinguished by widely different properties such as the overall spectral energy distribution shape, jet speed, cosmological evolution, broad-band spectral variability, and optical polarisation properties. Although blazars of all types have been proposed as neutrino sources, evidence is accumulating in favour of IHBLs being the counterparts of astrophysical neutrinos. If this is indeed the case, we argue that the peculiar observational properties of IHBLs may be indirectly related to proton acceleration to very high energies.

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

We applied 3D-printed microwire-array (MWA) structure to boost the energy conversion efficiency of laser proton acceleration. The advanced nano-printing technique allows precise control on the spacing and geometrical size of 3D structures at 100-500 nm resolution. Under irradiation of high contrast laser pulse (15J, 35fs), the MWA target generates over 1.2×1012 protons (> 1MeV) with cut-off energies extending to 25MeV, corresponding to top-end of 8.7% energy conversion efficiency from femtosecond lasers. When comparing to flat foils the efficiency is enhanced by three times, while the cut-off energy is increased by 30-70% depending on their thicknesses. By precisely controlling the array period via 3D nano-printing, we found the dependence of proton energy/conversion-efficiency on the spacing of the MWA. The experimental trend is well reproduced by hydrodynamic and Particle-In-Cell simulations, which reveal for the first time the modulation of pre-plasma profile induced by laser diffraction within the fine structures. Optimal geometry for laser-proton acceleration is therefore strongly modified. Our work validates the use of 3D-printed micro-structures to produce high efficiency laser-driven particle sources and pointed out the new effect in optimizing the experimental conditions.

Parametric Study of Proton Acceleration from Laser-Thin Foil Interaction

We experimentally investigated the accelerated proton beam characteristics such as maximum energy and number by varying the incident laser parameters. For this purpose, we varied the laser energy, focal spot size, polarization, and pulse duration. The proton spectra were recorded using a single-shot Thomson parabola spectrometer equipped with a microchannel plate and a high-resolution charge-coupled device with a wide detection range from a few tens of keV to several MeV. The outcome of the experimental findings is discussed in detail and compared to other theoretical works.

Enhanced Proton Acceleration by Laser-Driven Collisionless Shock in the Near-Critical Density Target Embedding with Solid Nanolayers

Effects of solid nanolayers embedded in a near-critical density plasma on the laser-driven collisionless shock acceleration are investigated by using two-dimensional particle-in-cell simulations. Due to the interaction of nanolayers and the incident laser, an additional number of hot electrons are generated and an inhomogeneous magnetic field is induced. As a result, the collisionless shock is reinforced within the nanolayer gaps compared to the target without the structured nanolayers. When the laser intensity is 9.8 × 10 19 W / cm 2 , the amplitude of the electrostatic field is increased by 30% and the shock velocity is increased from 0.079c to 0.091c, leading to an enhancement of the peak energy and the cutoff energy of accelerated protons, from 6.9 MeV to 9.1 MeV and 12.2 MeV to 20.0 MeV, respectively. Furthermore, the effects of the width of the nanolayer gaps are studied, by adjusting the gap width of nanolayers, and optimal nanolayer setups for collisionless shock acceleration can be acquired.