arXiv subjects

S. Malko

Publications and source records attributed to S. Malko.

15 recordsLinked to original sources

arXiv2026

Laser ion acceleration from concave targets by subpicosecond pulses

Laser-driven proton acceleration provides a powerful route for generating ultrashort, high-charge proton beams. Many applications, including secondary neutron sources and inertial fusion, benefit from tight proton beam focusing. Concave targets offer a robust solution, yet the scaling of proton focusing with laser and target parameters remains poorly understood. Here, we present a numerical study of laser-driven proton acceleration and focusing from hemispherical targets using the fully kinetic, relativistic Particle-In-Cell code EPOCH. We focus on the sub-picosecond laser-pulse regime (duration $\lesssim 10^2$ fs), centering on the laser parameters of our recent experiment at the CSU ALEPH laser facility. We investigate the proton acceleration mechanisms, characterize proton focusing, and assess how focal spot parameters scale with laser and target parameters. We identify Target Normal Sheath Acceleration as the dominant mechanism, supplemented by a secondary post-acceleration stage near the geometrical center of the hemisphere. We demonstrate that both the proton focal spot size and focal plane position scale approximately linearly with the hemisphere radius, with the focal plane consistently located downstream of the geometrical center. The opening angle of the concave target mainly affects the proton beam waist. Energy-dependent proton focusing is interpreted as a consequence of the evolving curvature of the accelerating structure, which departs from the target curvature. Evidence for self-similar proton focusing is found in the regime of nearly uniform target irradiation.

physics.plasm-ph↗

arXiv2026

Evolution of laser-driven magnetic fields from proton tomography

Self-generated magnetic fields are commonly produced in high-power laser-plasma interactions. These fields can inhibit plasma heat-flow which makes them important in inertial fusion and controlled laboratory astrophysics experiments. In this work, we characterize the time evolution of self-generated magnetic fields using multi-view proton tomography at two timings. Tomographic reconstructions of the magnetic field show a clear transition from fields located close to the target at early time to more extended coronal fields at later time. The tomographic inversion and mesh radiography also enable a direct measurement of the magnetic-flux evolution. Comparisons with extended-MHD simulations show only moderate agreement in field structure, but good agreement in magnetic flux. This suggests that the field generation model is largely correct under these conditions, while the magnetic transport model requires additional development to reproduce the observed field structure.

physics.plasm-ph↗

arXiv2025

Laboratory measurements of energy partitioning and anomalous electron heating in magnetized, perpendicular collisionless shocks

We present laboratory results on energy partitioning from supercritical, magnetized collisionless shock experiments ($\rm{M_A} \sim 8$, $\rm{M_{ms}}\sim 4$). We report the first observation of fully-developed laboratory shocks that evolve for more than seven upstream ion gyration periods and have a downstream region that extends more than four shocked ion gyroperiods. Thomson scattering measurements are used to measure electron and ion temperatures, plasma density, and flow speeds. We directly measure a compression ratio of $3.6\pm0.3$, consistent with shock jump conditions. A foot ahead of the shock exhibits super-adiabatic electron and ion heating. The downstream electron temperature has an $\approx 30\%$ excess above adiabatic and collisional electron-ion heating, implying significant collisionless anomalous electron heating. We find a downstream electron-ion temperature ratio $T_e^{(d)}/T_i^{(d)} = 0.8 \pm 0.3$, consistent with spacecraft observations but outside the range of predictions from theory and numerical simulations.

physics.plasm-ph↗

arXiv2024

Proton Radiography Inversions with Source Extraction and Comparison to Mesh Methods

Proton radiography is a central diagnostic technique for measuring electromagnetic (EM) fields in high-energy-density, laser-produced plasmas. In this technique, protons traverse the plasma where they accumulate small EM deflections which lead to variations in the proton fluence pattern on a detector. Path-integrated EM fields can then be extracted from the fluence image through an inversion process. In this work, experiments of laser-driven foils were conducted on the OMEGA laser and magnetic field reconstructions were performed using both "fluence-based" techniques and high-fidelity "mesh-based" methods. We implement nonzero boundary conditions into the inversion and show their importance by comparing against mesh measurements. Good agreement between the methods is found only when nonzero boundary conditions are used. We also introduce an approach to determine the unperturbed proton source profile, which is a required input in fluence reconstruction algorithms. In this approach, a fluence inversion is embedded inside of a mesh region, which provides overconstrained magnetic boundary conditions. A source profile is then iteratively optimized to satisfy the boundary information. This method substantially enhances the accuracy in recovering EM fields. Lastly, we propose a scheme to quantify uncertainty in the final inversion that is introduced through errors in the source retrieval.

physics.plasm-ph↗

arXiv2024

X-ray imaging and electron temperature evolution in laser-driven magnetic reconnection experiments at the National Ignition Facility

We present results from X-ray imaging of high-aspect-ratio magnetic reconnection experiments driven at the National Ignition Facility. Two parallel, self-magnetized, elongated laser-driven plumes are produced by tiling 40 laser beams. A magnetic reconnection layer is formed by the collision of the plumes. A gated X-ray framing pinhole camera with micro-channel plate (MCP) detector produces multiple images through various filters of the formation and evolution of both the plumes and current sheet. As the diagnostic integrates plasma self-emission along the line of sight, 2-dimensional electron temperature maps $\langle T_e \rangle_Y$ are constructed by taking the ratio of intensity of these images obtained with different filters. The plumes have a characteristic temperature $\langle T_e \rangle_Y = 240 \pm 20$ eV at 2 ns after the initial laser irradiation and exhibit a slow cooling up to 4 ns. The reconnection layer forms at 3 ns with a temperature $\langle T_e \rangle_Y = 280 \pm 50$ eV as the result of the collision of the plumes. The error bars of the plumes and current sheet temperatures separate at $4$ ns, showing the heating of the current sheet from colder inflows. Using a semi-analytical model, we find that the observed heating of the current sheet is consistent with being produced by electron-ion drag, rather than the conversion of magnetic to kinetic energy.

physics.plasm-ph↗

arXiv2023

Laser-driven ion and electron acceleration from near-critical density gas targets: towards high-repetition rate operation in the 1 PW, sub-100 fs laser interaction regime

Ion acceleration from gaseous targets driven by relativistic-intensity lasers was demonstrated as early as the late 90s, yet most of the experiments conducted to date have involved picosecond-duration, Nd:glass lasers operating at low repetition rate. Here, we present measurements on the interaction of ultraintense ($\sim 10^{20}\,\rm W\,cm^{-2}$, 1 PW), ultrashort ($\sim 70\,\rm fs$) Ti:Sa laser pulses with near-critical ($\sim 10^{20}\,\rm cm^{-3}$) helium gas jets, a debris-free targetry compatible with high ($\sim 1\,\rm Hz$) repetition rate operation. We provide evidence of $\alpha$ particles being forward accelerated up to $\sim 2.7\,\rm MeV$ energy with a total flux of $\sim 10^{11}\,\rm sr^{-1}$ as integrated over $>0.1 \,\rm MeV$ energies and detected within a $0.5\,\rm mrad$ solid angle. We also report on on-axis emission of relativistic electrons with an exponentially decaying spectrum characterized by a $\sim 10\,\rm MeV$ slope, i.e., five times larger than the standard ponderomotive scaling. The total charge of these electrons with energy above 2 MeV is estimated to be of $\sim 1 \,\rm nC$, corresponding to $\sim 0.1\,\%$ of the laser drive energy. In addition, we observe the formation of a plasma channel, extending longitudinally across the gas density maximum and expanding radially with time. These results are well captured by large-scale particle-in-cell simulations, which reveal that the detected fast ions most likely originate from reflection off the rapidly expanding channel walls. The latter process is predicted to yield ion energies in the MeV range, which compare well with the measurements. Finally, direct laser acceleration is shown to be the dominant mechanism behind the observed electron energization.

physics.plasm-ph↗

arXiv2023

Measurements of Extended Magnetic Fields in Laser-Solid Interaction

Magnetic fields generated from a laser-foil interaction are measured with high fidelity using a proton radiography scheme with in situ x-ray fiducials. In contrast to prior findings under similar experimental conditions, this technique reveals the self-generated, Biermann-battery fields extend beyond the edge of the expanding plasma plume to a radius of over 3.5 mm by t=+1.4 ns, a result not captured in state-of-the-art magneto-hydrodynamics simulations. An analysis of two mono-energetic proton populations confirms that proton deflection is dominated by magnetic fields far from the interaction (>2 mm) and electric fields are insignificant. Comparisons to prior work suggest a new physics mechanism for the magnetic field generation and transport in laser-solid interactions.

physics.plasm-ph↗

arXiv2022

Guided Electromagnetic Discharge Pulses Driven by Short Intense Laser Pulses: Characterisation and Modelling

Strong electromagnetic pulses (EMP) are generated from intense laser interactions with solid-density targets, and can be guided by the target geometry, specifically through conductive connections to the ground. We present an experimental characterization, by time- and spatial-resolved proton deflectometry, of guided electromagnetic discharge pulses along wires including a coil, driven by 0.5 ps, 50 J, 1e19 W/cm2 laser pulses. Proton-deflectometry data allows to time-resolve first the EMP due to the laser-driven target charging and then the return EMP from the ground through the conductive target stalk. Both EMPs have a typical duration of tens of ps and correspond to currents in the kA-range with electric-field amplitudes of multiple GV/m. The sub-mm coil in the target rod creates lensing effects on probing protons, due to both magnetic- and electric-field contributions. This way, protons of 10 MeV-energy range are focused over cm-scale distances. Experimental results are supported by analytical modelling and high-resolution numerical particle-in-cell simulations, unraveling the likely presence of a surface plasma, which parameters define the discharge pulse dispersion in the non-linear propagation regime.

physics.plasm-ph↗

arXiv2021

Time-Of-Flight methodologies with large-area diamond detectors for the effectively characterization of tens of MeV protons

A novel detector based on a polycrystalline diamond sensor is here employed in an advanced Time-Of-Flight scheme for the characterization of energetic ions accelerated during laser-matter interactions. The optimization of the detector and of the advanced TOF methodology allow to obtain signals characterized by high signal-to-noise ratio and high dynamic range even in the most challenging experimental environments, where the interaction of high-intensity laser pulses with matter leads to effective ion acceleration, but also to the generation of strong Electromagnetic Pulses (EMPs) with intensities up to the MV/m order. These are known to be a serious threat for the fielded diagnostic systems. In this paper we report on the measurement performed with the PW-class laser system Vega 3 at CLPU (30 J energy, 10^21 W/cm2 intensity, 30 fs pulses) irradiating solid targets, where both tens of MeV ions and intense EMP fields were generated. The data were analyzed to retrieve a calibrated proton spectrum and in particular we focus on the analysis of the most energetic portion (E > 5.8 MeV) of the spectrum showing a procedure to deal with the intrinsic lower sensitivity of the detector in the mentioned spectral-range.

physics.ins-det↗

arXiv2021

Design of proton deflectometry with in situ X-ray fiducial for magnetized HED systems

We report a design and implementation of proton radiography with an in situ reference X-ray image of a mesh to precisely measure non-uniform magnetic fields in expanding plasmas at the OMEGA and OMEGA EP laser facilities. The technique has been developed with proton and x-ray sources generated from both directly-driven capsule implosions and short pulse laser-solid interactions. The accuracy of the measurement depends on the contrast of both the proton and x-ray images. We present numerical and analytic studies to optimize the image contrast using a variety of mesh materials and grid spacing. Our results show a clear enhancement of the image contrast by a factor of 4-6 using a high Z mesh with large grid spacing. This would lead to a further factor of two improvement in accuracy of the magnetic field measurement.

physics.plasm-ph↗

arXiv2021

Proton deflectometry with in situ x-ray reference for absolute measurement of electromagnetic fields in high-energy-density plasmas

We report a technique of proton deflectometry which uses a grid and an in situ reference x-ray grid image for precise measurements of magnetic fields in high-energy density plasmas. A D$^3$He fusion implosion provides a bright point-source of both protons and x-rays, which is split into beamlets by a mesh grid. The protons undergo deflections as they propagate through the plasma region of interest, whereas the x-rays travel along straight lines. The x-ray image therefore provides a zero-deflection reference image. The line-integrated magnetic fields are inferred from the shifts of beamlets between the deflected (proton) and reference (x-ray) images. We developed a system for analysis of this data, including automatic algorithms to find beamlet locations and calculate their deflections from the reference image. The technique is verified in an experiment performed at OMEGA to measure a non-uniform magnetic field in vacuum, then applied to observe the interaction of an expanding plasma plume with the magnetic field.

physics.plasm-ph↗

arXiv2020

A quasi-monoenergetic short time duration compact proton source for probing high energy density states of matter

We report on the development of a highly directional, narrow energy band, short time duration proton beam operating at high repetition rate, suitable for measurements of stopping power in high energy density plasmas as well as other applications. The protons are generated with an ultrashort-pulse laser interacting with a solid target and converted to a pencil-like narrow-band beam using a compact magnet-based energy selector. We experimentally demonstrate the production of a proton beam with an energy of 500 keV and energy spread well below 10%, and a pulse duration of 260 ps. The energy loss of this beam is measured in a 2 $μ$m thick solid Mylar target and found to be in within 1% of theoretical predictions. The short time duration of the proton pulse makes it particularly well suited for applications involving the probing of highly transient plasma states produced in laser-matter interaction experiments, in particular measurements of proton stopping power.

physics.plasm-ph↗

arXiv2020

Integrated quantitative PIXE analysis and EDX spectroscopy using a laser-driven particle source

Among the existing elemental characterization techniques, Particle Induced X-ray Emission (PIXE) and Energy Dispersive X-ray (EDX) spectroscopy are two of the most widely used in different scientific and technological fields. Here we present the first quantitative laser-driven PIXE and laser-driven EDX experimental investigation performed at the Centro de Láseres Pulsados in Salamanca. Thanks to their potential for compactness and portability, laser-driven particle sources are very appealing for materials science applications, especially for materials analysis techniques. We demonstrate the possibility to exploit the X-ray signal produced by the co-irradiation with both electrons and protons to identify the elements in the sample. We show that, using the proton beam only, we can successfully obtain quantitative information about the sample structure through laser-driven PIXE analysis. These results pave the way towards the development of a compact and multi-functional apparatus for the elemental analysis of materials based on a laser-driven particle source.

physics.plasm-ph↗

arXiv2019

Enhanced relativistic-electron beam collimation using two consecutive laser pulses

The double laser pulse approach to relativistic electron beam (REB) collimation has been investigated at the LULI-ELFIE facility. In this scheme, the magnetic field generated by the first laser-driven REB is used to guide a second delayed REB. We show how electron beam collimation can be controlled by properly adjusting laser parameters. By changing the ratio of focus size and the delay time between the two pulses we found a maximum of electron beam collimation clearly dependent on the focal spot size ratio of the two laser pulses and related to the magnetic field dynamics. Cu-K alpha and CTR imaging diagnostics were implemented to evaluate the collimation effects on the respectively low energy (< 100 keV) and high energy (> MeV) components of the REB.

physics.plasm-ph↗

arXiv2018

Generation of high energy laser-driven electron and proton sources with the 200 TW system VEGA 2 at the Centro de Laseres Pulsados

The Centro de Laseres Pulsados in Salamanca Spain has recently started operation phase and the first User access period on the 6 J 30 fs 200 TW system (VEGA 2) already started at the beginning of 2018. In this paper we report on two commissioning experiments recently performed on the VEGA 2 system in preparation for the user campaign. VEGA 2 system has been tested in different configurations depending on the focusing optics and targets used. One configuration (long focal length f=130 cm) is for under-dense laser-matter interaction where VEGA 2 is focused onto a low density gas-jet generating electron beams (via laser wake field acceleration mechanism) with maximum energy up to 500 MeV and an X-ray betatron source with a 10 keV critical energy. A second configuration (short focal length f=40 cm) is for over-dense laser-matter interaction where VEGA 2 is focused onto an 5 microns thick Al target generating a proton beam with a maximum energy of 10 MeV and average energy of 7-8 MeV and temperature of 2.5 MeV. In this paper we present preliminary experimental results.

physics.plasm-ph↗