arXiv subjects

U. Chaulagain

Publications and source records attributed to U. Chaulagain.

8 recordsLinked to original sources

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

Ultrarelativistic electron beams accelerated by terawatt scalable kHz laser

We show the laser-driven acceleration of unprecedented, collimated ($ 2 \ \mathrm{mrad} $ divergence), and quasi-monoenergetic ($ 25 \ \% $ energy spread) electron beams with energy up to $ 50 \ \mathrm{MeV} $ at $ 1 \ \mathrm{kHz} $ repetition rate. The laser driver is a multi-cycle ($ 15 \ \mathrm{fs} $) $ 1 \ \mathrm{kHz} $ optical parametric chirped pulse amplification (OPCPA) system, operating at $ 26 \ \mathrm{mJ} $ ($ 1.7 \ \mathrm{TW} $). The scalability of the driver laser technology and the electron beams reported in this work pave the way towards developing high-brilliance x-ray sources for medical imaging, innovative devices for brain cancer treatment, and represent a step towards the realization of a kHz GeV electron beamline.

physics.plasm-ph↗

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↗

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↗

arXiv2017

Counter-propagating radiative shock experiments on the Orion laser

We present new experiments to study the formation of radiative shocks and the interaction between two counter-propagating radiative shocks. The experiments were performed at the Orion laser facility which was used to drive shocks in xenon inside large aspect ratio gas-cells. The collision between the two shocks and their respective radiative precursors, combined with the formation of inherently 3-dimensional shocks, provides a novel platform particularly suited for benchmarking of numerical codes. The dynamics of the shocks before and after the collision were investigated using point-projection X-ray backlighting while, simultaneously, the electron density in the radiative precursor was measured via optical laser interferometry. Modelling of the experiments using the 2-D radiation hydrodynamic codes NYM/PETRA show a very good agreement with the experimental results.

physics.plasm-ph↗

arXiv2017

Counter-propagating radiative shock experiments on the Orion laser and the formation of radiative precursors

We present results from new experiments to study the dynamics of radiative shocks, reverse shocks and radiative precursors. Laser ablation of a solid piston by the Orion high-power laser at AWE Aldermaston UK was used to drive radiative shocks into a gas cell initially pressurised between $0.1$ and $1.0 \ bar$ with different noble gases. Shocks propagated at {$80 \pm 10 \ km/s$} and experienced strong radiative cooling resulting in post-shock compressions of { $\times 25 \pm 2$}. A combination of X-ray backlighting, optical self-emission streak imaging and interferometry (multi-frame and streak imaging) were used to simultaneously study both the shock front and the radiative precursor. These experiments present a new configuration to produce counter-propagating radiative shocks, allowing for the study of reverse shocks and providing a unique platform for numerical validation. In addition, the radiative shocks were able to expand freely into a large gas volume without being confined by the walls of the gas cell. This allows for 3-D effects of the shocks to be studied which, in principle, could lead to a more direct comparison to astrophysical phenomena. By maintaining a constant mass density between different gas fills the shocks evolved with similar hydrodynamics but the radiative precursor was found to extend significantly further in higher atomic number gases ($\sim$$4$ times further in xenon than neon). Finally, 1-D and 2-D radiative-hydrodynamic simulations are presented showing good agreement with the experimental data.

physics.plasm-ph↗