This PDF file contains the front matter associated with SPIE Proceedings Volume 12579, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

X-ray free electron lasers (XFELs) are capable of producing x-ray beams with intense peak brightness, full transverse coherence, and femtosecond-scale pulse duration through Self-Amplified Spontaneous Emission (SASE). However, the SASE FELs suffer from noisy spikes in time and spectrum due to radiation originating from electron beam shot noise. To overcome these limitations and realize bright, fully coherent FEL sources, self-seeding is a promising solution. In this study, we utilized the forward Bragg-diffraction (FBD) monochromator at PAL-XFEL to generate almost fully coherent hard X-ray self-seeded (HXRSS) free-electron laser (FEL) pulses with an unprecedented peak-brightness and a narrow spectrum. Our HXRSS FEL demonstrated outstanding performance across a photon energy range spanning from 3.5keV to 14.6keV. These findings provide valuable insights for the development of advanced X-ray sources and their applications. In order to meet the demands of experimental applications such as resonant inelastic X-ray scattering, nuclear resonance scattering, and X-ray Raman spectroscopy, we have developed x-ray energy scanning method utilizing a double crystal monochromator (DCM). This approach offers improved spectral purity and a fully calibrated energy scale. In this study, we will present recent experimental findings on the characteristics of hard X-ray self-seeded FEL at PAL-XFEL. These results have important implications for the advancement of X-ray spectroscopy and related research fields.

Vacuum laser acceleration (VLA) of electrons has been an intense field of research for a long time due to the extremely high (>1 TV/m) accelerating fields. However, it is very challenging to realize and only a few promising experiments have been performed which have demonstrated the principle. Here, we report on the interaction of relativistic intensity (10²⁰ Wcm^-2) sub-two optical cycle (<5 fs) laser pulses with nanotips to realize and optimize VLA. Various properties of accelerated electrons (angular distribution, charge, and electron spectrum) are measured with different intensities and carrier envelope phases of the laser pulse. Among others, waveform dependence of the electron propagation direction is observed. Furthermore, comparable or even higher electron energies beyond 10 MeV are detected with lower laser intensity, i.e., longer focusing, than with high intensity. These surprising results are reproduced using particle-in-cell simulations, which indicate a nanophotonics electron emission from the nanotip followed by VLA. In fact, the unexpected observations are a direct proof of the VLA process and provide a way to optimize it towards higher energy, isolated, attosecond electron bunch generation.

Experiments have been undertaken using the VEGA-3 petawatt laser system at the Centro de Láseres Pulsados (CLPU) facility in Salamanca to investigate electron and ion acceleration in under-dense plasma. The respective longitudinal and transverse fields of the ‘bubble’ structure of a laser wakefield accelerator (LWFA) simultaneously accelerates electrons to GeV energies, and ions to 100s keV/u to MeV/u energies. The laser is configured to produce two ultra-intense laser pulses, each with a minimum pulse duration of 30 fs and a variable inter-pulse delay up to 300 fs. The double pulses can superpose or resonantly excite the LWFA bubble to increase the accelerating fields. By focusing the laser beam into a 2.74 mm diameter supersonic jet of He gas, using an F/10.4 parabola, an initial intensity of up to ≈10¹⁹ Wcm⁻² can be realized at focus. This ionises the gas to produce plasma and the imposes a ponderomotive force that creates the LWFA accelerating structures. For backing pressures of 30 – 60 bar, corresponding to plasma densities of 1–4×10¹⁹ cm⁻³, the fields of the LWFA can exceed 200 MV/m, which is sufficient to accelerate electrons to GeV energies, and ions to 100s keV/u. This study focuses on ion acceleration in the transverse direction. He⁺¹ and He⁺² ion spectra have been measured using a Thompson parabola spectrometer and a multi-channel plate detector. He ions with energies up to a few hundred keV/u are observed for both single pulses (5.0 J) and double pulses (5.0 J and 3.6 J, respectively), where the inter-pulse delay is varied between 0 fs and ± 300 fs. The measured spectra are consistent with numerical simulations. Ions are observed to undergo electron exchange in the neutral surrounding gas, which produces different charge states ions and neutral atoms.

Electron acceleration by laser pulses with high repetition rate can be used for technical applications. To reach conditions for the wake-field laser acceleration, it was demonstrated recently in experiments that it is beneficial to use near single cycle laser drive pulses with sub-4 fs duration, with narrow waists. To explore possible electron density ramp-up injection as an alternative to ramp-down and ionization injections, we performed numerical simulations of electron bunches generation in the ramp-up region. The PIC code Epoch2D and input parameters near to experiments were used. We assumed thin plasma slabs with super Gaussian density profiles of order 4-80, FWHM about 30 µm. We found that density ramp-up injected bunches can have charges several times higher than those obtained by ionization injection. There can be created a group of up to ten bunches in a sequence of bubbles, with not too mutually different maximum energy and charges. At oblique incidence of drive pulses on steep ramp up profiles, we find significant enhancement of the first bunch charge. For large slant angles -45 or 45 degrees, the bunch charge enhancement is about twenty times. We conclude that the ramp-up injection can be a useful alternative injection on steep enough density profiles.

The dynamics of electron injection from a shock front under a laser condition of a₀ > 3 and tight focusing (FWHM < 10 µm) laser condition has been studied by numerical simulations. Compared to a regular longitudinal shockfront injection, the transverse injection starts near the edge of the bubble with a narrow energy spread of < 13 MeV. The trajectories of the transverse injected electrons are more coherent than the longitudinal injection. By applying the tilted shock front, the betatron oscillation amplitude is significantly larger than the un-tilted shock front. The enhancement of the betatron radiation brightness has been observed.

Shadowgraphic imaging technique is used to study the dynamics of cavitation bubbles produced during the pulsed laser ablation of a silver target in water. Ablation of a solid target immersed in liquid with a pulsed laser beam is a popular technique for the synthesis of colloidal nanoparticles. To understand the highly complex mechanism involved in nanoparticle formation, estimation of the thermodynamical parameters within the bubble, where the nucleation of the nanoparticles occur, is extremely important. The dynamics of the bubble studied using a fast gated CCD camera reveal that the bubble expands initially and then reaches its maximum size after which it starts compressing. The range of temperature and pressure values within the bubble is analytically assessed using two known models, the Rayleigh-Plesset and the van der Waals model.

We have developed a compact liquid target setup that produces a continuous ø50 µm cylindrical water jet, capable of operating at high vacuum. It has been tested with a commercial ultrashort-pulse laser in a series of proof-of-principle laser-driven ion acceleration and x-ray generation experiments at repetition rates up to 1 kHz. In optimized conditions, measurements by the time-of-flight (TOF) method have demonstrated a proton signal cut-off energy of 179±9 keV. The laser-generated x-ray emission was characterized in the range 2-36 keV and used as excitation for x-ray fluorescence spectroscopy (XRF) measurements.

Laser wakefield acceleration is a remarkably efficient method for relativistic electron acceleration that ensures high electric field gradients generated by plasma waves. In this approach, an ultra-short, high-intensity laser pulse propagates through a plasma medium. It has already proven its potential by reaching gradients up to hundreds of GV/m. To further stabilize and control the process of acceleration, a separate source of electrons is widely considered. In order to address this problem, we have performed 3D particle-in-cell simulations using the Smilei code. Several plasma density profiles with different vacuum-plasma transition region and their effect on the external injection were analysed.