This PDF file contains the front matter associated with SPIE Proceedings Volume 12582, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

After 30 years since the discovery of the intrinsic orbital angular momentum (OAM) possessed by certain beams, a plethora of applications have been developed in diverse fields such as communications, astrophysics, and biochemistry. Meanwhile, injecting high-order harmonics (HOH) into krypton amplifier plasmas has emerged as a promising alternative to Free Electron Lasers (FEL) for generating table-top, XUV coherent radiation sources. This work brings together these two concepts and asks the following question: what happens when an HOH with OAM is injected into a plasma? Does the amplified beam retain the same OAM, or is this property affected? Understanding the OAM response to this process lays the foundation for new applications. Simulations using the 3D, time-dependent, Maxwell-Bloch code Dagon show that OAM is conserved in low-density plasmas. However, at higher densities, although the OAM is still preserved, the density profiles leave increasing footprints in phase patterns in the form of phase jumps curvature. Finally, a study has been conducted for a plasma with a waveguide, further supporting the potential of OAM for plasma diagnosis.

Empulse is the Swiss table-top near-infrared (λ = 1050 nm) laser system that aims to produce 10-TW class pulses through chirped pulse amplification. The system offers a flexible platform for a number of experiments in materials science, such as advanced spectroscopy and/or materials under extreme conditions. The technical design report (TDR) is presented, highlighting the system architecture and performance. This TDR gives a general overview of the Empulse system and updates on developments towards realization of a joule-class ∼ps-scale short pulse output. In particular, the laser system timing diagram, safety interlock scheme, the front-end pulse jitter characterization, and finally the group delay dispersion (GDD) calculations of the stretcher and compressor optics are presented. The system is complemented by a number of dedicated diagnostics and realtime-monitoring, as reported in the paper by Hemani et al. in this proceedings.

The High-Repetition-rate laser-driven Gas-based High-order Harmonic Generation (HR GHHG) beamlines of the Extreme Light Infrastructure Attosecond Light Pulse Source (ELI ALPS) have started their operation in the recent years. Both beamlines, one designed for gas-phase targets – the HR GHHG Gas beamline –, and one for condensed-phase samples – the HR GHHG Condensed beamline –, now provide high-flux, extreme ultraviolet (XUV) radiation with pump-probe capabilities at 100 kHz repetition rate. The HR GHHG Cond beamline is equipped with a time-compensated XUV monochromator, allowing for tuning the spectral properties while maintaining short, close to Fourier-limited pulse duration in the femtosecond regime. Cutting-edge experimental end stations are also available, for example a Reaction Microscope and a NanoESCA device. Both beamlines are past their first commissioning user experiments. In this presentation the capabilities along with some recent developments and latest experiments will be presented for these two unique attosecond sources.

The process of high-harmonic generation from solid density surfaces results in a train of ultrashort attosecond pulses. Isolating a single attosecond pulse is of interest for many reasons such as atomic and molecular time-scale pump-probe experiments.1 We highlight, via Particle-in-Cell simulations, a viable route to pulse isolation using two colour fields with off integer harmonic frequencies, which allows subtle shaping of the incident laser field which when reflected by the plasma mirror, diminishes some pulses in the train resulting in the potential to isolate a single attosecond pulse.

We present the recent progress done at the Advanced Laser Light Source (ALLS) on the development of a Laser Wakefield Acceleration (LWFA) based X-ray machine. We will describe the pathway we follow to progress towards an industrial solution guided by the concept of Solution Readiness Level (SRL) metric. Two different approaches have been recently studied and assessed to optimize the X-ray beam in the 20keV - 50keV range. This paves the way to the development of a laser-based X-ray machine addressing, with different working points, various strategic challenges. We discuss in the present work the usefulness of the various operational approaches for some aspects of Global Food Security and for the realization of mammography with dose well below the actual clinical standard.

Phase contrast X-ray imaging can be much more sensitive to soft tissue lesions than conventional absorption contrast X-ray imaging, being a potential game changer for medical imaging. A phase contrast method well suited for clinical implementation is the grating interferometry. We show that by using μm period multi-meter long interferometers one can strongly increase the phase sensitivity and lower the dose towards soft tissue imaging applications such mammography. Conventional X-ray tubes do not provide, however, sufficient X-ray flux for clinical imaging with such long interferometers. Instead, 100-TW class lasers could produce highly directional and intense X-ray sources ideal for high sensitivity medical interferometry. We present the X-ray source characteristics required for clinical interferometry, advantages and disadvantages of betatron versus inverse Compton scattering sources for clinical application, and some practical considerations towards laser based interferometric medical imaging.

One of the major objectives of the ELI Beamlines project is to employ the state-of-the-art high power lasers for the generation of ultra-short pulses of particles and X-rays with unprecedented parameters to deliver stable beamline capacity to serve the broad scientific and industrial user community. The development of laser plasma-based X-ray sources at ELI beamlines, especially the ELI Gammatron beamline based on the laser plasma accelerator (LPA) is presented here. The ELI Gammatron beamline is capable of providing X-ray pulses of energies from 1-100 keV in the Betatron scheme and up to few MeV’s in the inverse Compton scheme. A dedicated LPA-based Betatron X-ray source, developed at the ELI plasma physics platform (P3) will serve as an active diagnostic of the high energy density (HED) plasma and laboratory astrophysics multi-beam experiments. In addition to this, a novel scheme for enhancing the X-ray flux of the betratron sources was proposed based on nonlinear resonances in a two-colour laser field interaction. A high-sensitivity optical probing technique for characterizing gas targets for LPA was also developed in-house to support the beamline development.

A compact undulator-based soft X-ray radiation source furnishing the laser wakefield electron acceleration concept is currently being developed at ELI-Beamlines in the Czech Republic. It will bring to the user community a high-repetition-rate (up to 50 Hz) soft X-ray radiation to enable high-temporal-resolution pump-probe experiments, combined with XANES spectroscopy, high-resolution microscopy, investigations of biological molecules and chemical reactions, evaluation of soft X-ray multilayer optics, as well as with coherent radiation applications like ptychography or coherent diffraction imaging, at later development stages. Now the first stage called LUIS is under development, which will result in production of uncoherent radiation in the ‘water window’ spectral range. The next stages will enable coherent extreme ultraviolet and soft X-ray radiation. We present suggestions on the user-oriented program to the community.

Exploring and understanding ultrafast processes at the atomic level is a scientific challenge. Femtosecond X-ray Absorption Near-Edge Spectroscopy (XANES) arises as an essential experimental probing method, as it can simultaneously reveal both electronic and atomic structures, and thus potentially unravel their non-equilibrium dynamic interplay which is at the origin of most of the ultrafast mechanisms. The key point of this investigation is the achievement of a femtosecond X-ray source suitable for routine experiments. This paper will show the progressive development and improvement of such laser-plasma-based X-ray sources, starting from the picosecond down to the femtosecond scale. Time-resolved XANES measurements have been achieved and interpreted using ab initio quantum molecular dynamics simulations. This diagnostic was used to shed new light on the non-equilibrium physics involved in various materials. This paper will focus on results devoted to the non-equilibrium dynamics of a copper foil brought from solid to warm dense matter regime, by the use of a femtosecond laser pulse. Particular emphasis will be given to the recent study of the ultrafast electronic transport, which was revealed by the femtosecond time resolution.

We demonstrate High-Energy X-ray (HEX-ray) generation using 2.8-mJ, 850fs pulses on a liquid metal target in a Laser Plasma X-ray Source (LPXS). The measured HEX-ray spectra reach into the MeV spectral range. The spectra follow Boltzmann energy distributions with a maximum HEX-ray “temperature” of 420 keV. The low laser intensity requirement, orders of magnitude less than previously reported, enables operation with widely available picosecond, millijoule laser systems with hundreds of Watt average laser power. Based on lasers with Yb-doped active media, HEX-ray sources driven with kilowatt average laser power are achievable in the very near future.

Novel hybrid reflection zone plates as dispersive elements will allow time-resolved Near Edge X-ray Absorption Fine Structure (tr-NEXAFS) studies in a wide photon energy range from 100 – 1500 eV. We describe two tr-NEXAFS setups using a laser produced plasma (LPP) as well as a high harmonics generation (HHG) source.

Intense attosecond scale pulses of extreme-ultraviolet and soft X-ray light can be generated from plasma surfaces driven relativistically by intense laser pulses. The temporal profile consists of a train of pulses separated by the laser’s optical period and manifests in the spectral domain as harmonics of the laser frequency. Isolating individual attosecond pulses is a key challenge for applications of these sources to time-resolved experiments for attosecond science and plasma-based sources allow the use of ultra-high energies and intensities that can enable fully attosecond scale pump-probe measurements. Results are presented here for numerical Particle-In-Cell simulations of a scheme to angularly sweep the pulses so that one is temporally gated out from the others in the reflected direction. Using two identical laser pulses that are incident noncollinearly on the surface with a time delay causes the instantaneous wavefront to sweep between each of them with the attosecond pulses also being swept in their emission angle accordingly. This method naturally separates out the remaining reflected laser energy due to the angular gap between the incident pulses negating the need for spectral filtering after the interaction. We demonstrate clear gating of a single pulse along the reflected axis in both 2D and 3D simulations and discuss the effect of spectral isolation from the laser frequency. We extend the investigation to further examine techniques to improve the temporal gating by tailoring the laser and target properties.

We review a number of instruments employed in a high-intensity J-KAREN-P laser-solid interaction experiment and discuss the applicability of the diagnostics to the best target position determination with a ~10 μm accuracy, while the focal spot size was ~1 μm and peak intensity was up to 7×10²¹ W/cm². We discuss both front- and back-side diagnostics, some of them operated in the infrared, visible and ultraviolet ranges, while others in the extreme ultraviolet, soft X-ray and gamma-ray ranges. We found that the applicability of some of the instruments to the best at-focus target position determination depends on the thickness of the target.

Compact X-Ray laser is a hot topic in the field of laser research, enabling 24/7 advanced spectroscopy and overcoming the beamline bottleneck. The investigated systems are either scaled-down replicas of accelerators, or tabletop architectures based on high-harmonic generation, plasmas, or wakefield acceleration. Ideally, one would enable a large range of applications if the X-Ray source would be portable. For that, some groups are working on accelerators-on-a-chip. A new class of active materials exploiting distributed feedback was proposed 50 years ago, as a candidate for an X-Ray laser gain medium. A Fabry-Perot analysis of a selection of "röntgen materials", based on their refractive index, Bragg's coupling coefficient, and threshold gain, is presented. The alkaline earth metal oxide showed the highest gain value of all the materials considered in this work. A relationship between the refractive index of the material and the threshold gain value is given. In addition, details on the geometry of the gain medium are discussed. Theoretical analysis revealed that alkaline earth metal oxides are a promising material with a higher gain coefficient of about 77.4 nm-1 for a 0.001 μm3 crystal and the highest of all the materials investigated in this work. Except for alkaline earth metal oxide, all other oxide materials, such as transition and lanthanide metal oxide, have the lowest gain value. While nitrides, carbides, and compound semiconductors outperform oxide materials in terms of gain, they have still one order of magnitude less gain than alkaline earth metal oxide. The details of röntgen material calculations and design parameters are covered in depth.

The development of laser technologies to produce extreme intensities has allowed the generation of high-order harmonics from relativistic laser-plasma mirror interactions to become attainable to observe experimentally. Numerical plasma simulations are invaluable for understanding the dynamic processes underpinning this mechanism. However, accuracy in describing high-frequency electromagnetic waves is challenging. Finite Difference Time Domain methods give rise to numerical dispersion when used to solve Maxwell’s equations, inducing a dispersive change in vacuum refractive index, which causes significant errors in physical properties of the reflected field, such as an angular deviation in the harmonic spatial profiles from the predicted specular reflection. EPOCH Particle-In-Cell (PIC) code is used to perform two-dimensional (2D) simulations to extensively study and control the effects of numerical dispersion on the generated harmonics for several Maxwell solvers. Effects on angular deviation across a range of angles of incidence and strategies to mitigate dispersive effects via controlling interaction geometry are discussed.

We present a four-pass tomography setup for the characterization of the density distribution of a gas jet. The data acquired with a wavefront sensor is processed using a tomography algorithm, providing a 3D reconstruction of the gas density distribution. We applied this technique to characterize a supersonic gas jet produced by a de Laval nozzle and obstructed by a razor blade in ambient pressure, which are among the common targets used in high-repetition rate laser wakefield acceleration (LWFA) experiments. The results revealed the complex density distribution of the jet, including the formation of barrel shocks. The four-pass tomography setup with a wavefront sensor provides a valuable tool for the detailed characterization of gas jets, enabling the investigation of fundamental fluid mechanics phenomena and contributing to the development of advanced propulsion systems and LWFA gas targets.