Ptychography is a robust lensless form of microscopy routinely used for applications spanning life and physical sciences. The most common ptychography setup consists in using a detector to record diffraction patterns in the far-field. A near-field version has been more recently introduced, and its potential is yet to be fully exploited. In this work, the sampling requirements for near-field ptychography are analysed. Starting from the characterisation available in literature, the formalism of the fractional Fourier transform is used to generalise analytically the sampling conditions. The results harmonise the far- and near-field regimes and widen the applications of the technique with respect to the current knowledge. This study is supported by simulations and provides clear guidelines on how to optimise the setup and acquisition strategies for near-field ptychography experiments. The results are key to drive the translation of the technique towards low brilliance sources.

Burst Intensification by Singularity Emitting Radiation (BISER) in underdense relativistic laser plasma is a bright source of coherent extreme ultraviolet (XUV) and x-ray radiation. In contrast to all harmonic generation mechanisms, high-resolution experimental BISER spectra in the XUV region contain spectral fringes with separation much finer (down to 0.12 eV) than the initial driving laser frequency (~1.5 eV). We show that these fringe separations result from two main factors: laser frequency downshift (redshift) due to the quasi-adiabatic energy loss to the plasma waves, and spectral interference of different harmonic orders from different emission moments, i.e. alloharmonics [Pirozhkova et al., arXiv:2306.01018]

Although the origin of cosmic rays (CRs) remains an open question, collisionless magnetized shock waves are widely regarded as key sites for particle acceleration. Recent theories further suggest that shock-shock collisions in stellar clusters could provide the additional acceleration needed to explain the observed high-energy CR spectrum. Here, we investigate this hypothesis through a laser-based experiment that creates magnetized plasma conditions similar to astrophysical environments. Our results demonstrate that interpenetrating collisionless shocks can significantly boost the energy of ambient protons previously energized by the individual shocks, while also improving the overall acceleration efficiency. Numerical kinetic simulations corroborate these findings, revealing that protons are reaccelerated via their bouncing motion in the convective electric fields of the colliding magnetized flows. By allowing to highly energize ambient protons, our novel colliding-shock platform opens the prospect to test the long-discussed mechanism of diffusive shock acceleration in a controlled laboratory setting.

We report on progress in the understanding of the effects of kilotesla-level applied magnetic fields on relativistic laser-plasma interactions. Ongoing advances in magnetic-field-generation techniques enable new and highly desirable phenomena, including magnetic-field-amplification platforms with reversible sign, focusing ion acceleration, and bulk-relativistic plasma heating. Building on recent advancements in laser-plasma interactions with applied magnetic fields, we introduce simple models for evaluating the effects of applied magnetic fields in magnetic-field amplification, sheath-based ion acceleration, and direct laser acceleration. These models indicate the feasibility of observing beneficial magnetic-field effects under experimentally relevant conditions and offer a starting point for future experimental design.