arXiv subjects

J. Nejdl

Publications and source records attributed to J. Nejdl.

9 recordsLinked to original sources

arXiv2024

Demonstration of The Brightest Nano-size Gamma Source

Gamma rays selectively interact with nuclei, induce and mediate nuclear reactions and elementary particle interactions, and exceed x-rays in penetrating power and thus are indispensable for analysis and modification of dense objects. Yet, the available gamma sources lack sufficient power and brightness. The predicted and highly desirable laser-driven gamma flash, from here on termed "Gamma Flash", based on inverse Compton scattering from solid targets at extreme irradiances (>$10^{23}W/cm^2$), would be the highest-power and the brightest terrestrial gamma source with a 30-40% laser-to-gamma energy conversion. However, Gamma Flash remains inaccessible experimentally due to the Bremsstrahlung background. Here we experimentally demonstrate a new interaction regime at the highest effective irradiance where Gamma Flash scaled quickly with the laser power and produced several times the number of Bremsstrahlung photons. Simulations revealed an attosecond, Terawatt Gamma Flash with a nanometre source size achieving a record brightness exceeding $~10^{23}photons/mm^2mrad^2s$ per 0.1% bandwidth at tens of MeV photon energies, surpassing astrophysical Gamma Ray Bursts. These findings could revolutionize inertial fusion energy by enabling unprecedented sub-micrometre/femtosecond resolution radiography of fuel mixing instabilities in extremely-compressed targets. The new gamma source could facilitate significant advances in time-resolved nuclear physics, homeland security, nuclear waste management and non-proliferation, while opening possibilities for spatially-coherent gamma rays.

physics.plasm-ph↗

arXiv2024

Sampling requirements in near-field ptychography

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.

physics.app-ph↗

arXiv2023

Long-lasting XUV activation of helium nanodroplets for avalanche ionization

We study the dynamics of avalanche ionization of pure helium nanodroplets activated by a weak extreme-ultraviolet (XUV) pulse and driven by an intense near-infrared (NIR) pulse. In addition to a transient enhancement of ignition of a nanoplasma at short delay times $\sim200$~fs, long-term activation of the nanodroplets lasting up to a few nanoseconds is observed. Molecular dynamics simulations suggest that the short-term activation is caused by the injection of seed electrons into the droplets by XUV photoemission. Long-term activation appears due to electrons remaining loosely bound to photoions which form stable `snowball' structures in the droplets. Thus, we show that XUV irradiation can induce long-lasting changes of the strong-field optical properties of nanoparticles, potentially opening new routes to controlling avalanche-ionization phenomena in nanostructures and condensed-phase systems.

physics.atm-clus↗

arXiv2022

Development of LPA based hard X-ray sources at ELI Beamlines

We report the laser-plasma accelerator-based X-ray sources development at ELI beamlines. One of the main objectives of ELI Beamlines is to provide beams of ultrashort particle and complex X-ray sources to users from various research fields. Two hard X-ray betatron sources with high photon flux based on laser-plasma acceleration (LPA) are being commissioned. The first source is the Gammatron beamline located in Experimental Hall E2. It provides X-ray pulses of energies from 1-100 keV in betatron and up to a MeV in Compton scheme. A novel X-ray optics has been designed as a focusing optics of these hard X-ray sources for the user application. The second hard X-ray source based on LPA is being developed in the ELI plasma physics platform (P3) that will serve as an active diagnostics HED and Laboratory astrophysics, multi-beam experiments, and fundamental research. This source is now being commissioned, we will present the first experimental results. Besides, we have proposed a novel scheme for enhancing the X-ray flux based on betatron oscillations enhanced from nonlinear resonances due to interaction with a two-color laser field. In addition, we will introduce a novel optical probing technique with high sensitivity to characterize a low-density gas target for a laser-plasma accelerator. It has been achieved by employing multiple passes of the probe beam through the object and relay-imaging of the object between the individual passes.

physics.acc-ph↗

arXiv2022

Experimental study of the interaction of two laser-driven radiative shocks at the PALS laser

Radiative shocks (RS) are complex phenomena which are ubiquitous in astrophysical environments. The study of such hypersonic shocks in the laboratory, under controlled conditions, is of primary interest to understand the physics at play and also to check the ability of numerical simulations to reproduce the experimental results. In this context, we conducted, at the Prague Asterix Laser System facility (PALS), the first experiments dedicated to the study of two counter-propagating radiative shocks propagating at non-equal speeds up to 25-50 km/s in noble gases at pressures ranging between 0.1 and 0.6 bar. These experiments highlighted the interaction between the two radiative precursors. This interaction is qualitatively but not quantitatively described by 1D simulations. Preliminary results obtained with XUV spectroscopy leading to the estimation of shock temperature and ion charge of the plasma are also presented.

physics.plasm-ph↗

arXiv2022

Ultrafast hard X-ray sources based on Relativistic Electrons at ELI Beamlines

We report the LWFA driven X-ray sources development at ELI beamlines. ELI Gammatron beamline provides X-ray pulses of energies from 1-100 keV in betatron scheme, and up to a MeV in Compton scheme. A state-of-the-art Ti:Sa diode-pumped HAPLS laser (10-30 J, 30 fs @10 Hz) is used as a driving laser for these X-ray sources. These sources are characterized with a low divergence (< 10 mrad), small source size (few {\mu}m), short pulse duration (a few fs) and photon flux up to1010 photons per pulse. In addition, a separate Betatron X-ray source is being developed in the ELI Plasma Physics Platform (P3)that will serve as an active diagnostics of various plasma physics experiments. P3 is dedicated to laboratory astrophysics, HED physics, multi-beam experiments, warm dense matter and fundamental research. Besides the presentation of the two X-ray sources, we will also introduce a novel optical probing method with increased interferometric sensitivity for characterization of low-density gas jets. The sensitivity is achieved together with using a novel optical configuration employing multiple passes of the probe beam through the object and relay-imaging of the object between the individual passes, and a short probing beam (405 nm).

physics.plasm-ph↗

arXiv2022

Phase-matched high-order harmonic generation in pre-ionized noble gases

One of the main difficulties to efficiently generating high-order harmonics in long neutral-gas targets is to reach the phase-matching conditions. One issue is that the medium cannot be sufficiently ionized by the driving laser due to plasma defocusing. We propose a method to improve the phase-matching by pre-ionizing the gas using a weak capillary discharge. We have demonstrated this mechanism, for the first time, in absorption-limited XUV generation by an 800 nm femtosecond laser in argon and krypton. The phase-mismatch control ability of our method is confirmed by an analytical model and numerical simulation of the complete generation process. Our method allows increasing the efficiency of the harmonic generation significantly, paving the way towards photon-hungry applications of these short-wavelength compact sources.

physics.optics↗

arXiv2020

Target Design for XUV Probing of Radiative Shock Experiments

Radiative shocks are strong shocks characterized by plasma at a high temperature emitting an important fraction of its energy as radiation. Radiative shocks are commonly found in many astrophysical systems and are templates of radiative hydrodynamic flows, which can be studied experimentally using high-power lasers. This is not only important in the context of laboratory astrophysics but also to benchmark numerical studies. We present details on the design of experiments on radiative shocks in xenon gas performed at the kJ scale PALS laser facility. It includes technical specifications for the tube targets design and numerical studies with the 1-D radiative hydrodynamics code MULTI. Emphasis is given to the technical feasibility of an XUV imaging diagnostic with a 21 nm (~58 eV) probing beam, which allows to probe simultaneously the post-shock and the precursor region ahead of the shock. The novel design of the target together with the improved X-ray optics and XUV source allow to show both the dense post-shock structure and the precursor of the radiative shock.

physics.plasm-ph↗

arXiv2018

Determination of the spectral variation origin in high-order harmonic generation in noble gases

One key parameter in the high-order harmonic generation (HHG) phenomenon is the exact frequency of the generated harmonic field. Its deviation from perfect harmonics of the laser frequency can be explained considering (i) the single-atom laser-matter interaction and (ii) the spectral changes of the driving laser. In this work, we perform an experimental and theoretical study of the causes that generate spectral changes in the HHG radiation. We measured the driving laser spectral shift after high harmonic generation in long medium using a correction factor to take into account the multiple possible HHG initiation distances along the laser path. We separate out the contribution of laser spectral shift from the resultant high harmonic spectral shift in order to elucidate the microscopic effect of spectral shift in HHG. Therefore, in some cases we are able to identify the dominant electron trajectory from the experimental data. Our investigations lead to valuable conclusions about atomic dipole phase contribution to a high harmonic spectral shift. We demonstrate that the significant contribution of a long electron path leads to a high harmonic shift, which differs from that expected from the driving laser. Moreover, we assess the origin of the high-order harmonics spectral broadening and provide an explanation for the narrowest high harmonic spectral width in our experiment.

physics.atom-ph↗