arXiv subjects

Victor Malka

Publications and source records attributed to Victor Malka.

At least 19 recordsLinked to original sources

arXiv2026

On the generation of astrophysically-relevant intermittent magnetic turbulence in the laboratory

Intermittent magnetic turbulence, namely the presence of non-ordered and clusterized fields, is a ubiquitous phenomenon in space and astrophysical plasmas. It is currently understood that it plays a crucial role in the dynamics of astrophysical systems at all scales, from influencing the evolution of the cosmos as a whole to governing local particle acceleration. While there is direct evidence of turbulence in the solar wind, and despite progress obtained through multi-wavelength observations, most of our knowledge of it outside the solar system derives from indirect evidence, through modeling. Here we show that magnetic turbulence, that quantitatively matches that measured in space, can be reproduced in the laboratory. Starting from a homogeneous magnetized plasma, we randomly perturb it using a speckled laser beam. Using proton radiography, we can follow the development and quantitatively characterize the produced intermittent turbulence from its inception.

physics.plasm-ph↗

arXiv2026

Plasma wakefield dynamics of self-generated electron bunch trains

Laser plasma accelerators can deliver high-energy, quasi-monoenergetic electron beams over centimeter-scale distances. In this work, we report on the generation of narrow, quasi-monoenergetic electron bunch trains with periodic energy spacing issued from downramp injection in a laser driven wakefield accelerator. The periodicity in energy is shaped via relativistic lengthening of the wakefield during the acceleration phase, while the spatial periodicity is obtained via injection into multiple plasma periods. At the end of the accelerator, a rotation in phase-space is performed to compress each bunch in energy, producing narrow periodic spikes in the spectrum. The experimental observations are supported by particle-in-cell simulations, which reproduce the formation and evolution of the periodic bunch trains, providing an insight into the underlying plasma dynamics.

physics.plasm-ph↗

arXiv2026

Electron Acceleration in a Flying-Focus Laser Wakefield Accelerator

Structured light pulses hold significant promise for their ability to overcome dephasing in laser-wakefield accelerators, that should facilitate applications in high-energy physics and XFEL. Numerical studies have shown that sculpting a pulse into a flying focus and using it to drive a wakefield can achieve dephasing-free acceleration of electrons, with gain in excess of 100\,GeV within reachable with existing laser facilities. This work reports on novel experiments using a flying-focus generated laser-wakefield accelerator to accelerate electrons to relativistic energies. The flying-focus pulse is achieved by sculpting the laser-pulse before focusing using spatio-temporal couplings and generating a quasi-Bessel beam with an axiparabola. This combination allows for the tuning of the propagation velocity of the wakefield, which, we demonstrate, has an impact on the maximum achievable electron energy. Optical and particle-in-cell simulations are used to support the data and to provide direct evidence of the partial mitigation of dephasing through this flying-focus scheme. These results are further elucidated in our companion letter [1].

physics.acc-ph↗

arXiv2026

Enhanced dynamic range spatio-spectral metrology of few-cycle laser pulses

Accurate spatio-temporal and spatio-spectral metrology is critical to the characterization and use of ultra-short, high-power lasers. The emergence of few cycle pulses, with bandwidths of tens or hundreds of nanometers, poses a significant challenge to existing metrology techniques. This is due both to large discrepancies in the sensitivities of the measurements at different wavelengths and to variation in the spectral intensity at those wavelengths. In this paper, the authors propose spectral filtering and stitching of the measurements as a robust, simple solution that enhances the dynamic range of the measurements, allowing accurate few-cycle pulse reconstruction. This enhancement is demonstrated using INSIGHT -- the most commonly used spatio-spectral measurement device -- as well as using IMPALA and spatially resolved Fourier transform spectrometry.

physics.optics↗

arXiv2025

Probing Flying-Focus Wakefields

Flying-focus wakefields, which can propagate with a tunable velocity along the optical axis, are promising solutions to electron dephasing in laser-wakefield accelerators. This is accomplished by a combination of spatio-temporal couplings and focusing with an axiparabola, a specialized optical element which produces a quasi-Bessel beam. If implemented, dephasingless acceleration would allow for a hitherto unachievable mixture of high acceleration gradients and long acceleration lengths. Here, we conduct an in-depth study of the structure and behavior of such a flying-focus wakefield, through a combination of direct imaging and simulations. We show the stability of the wakefield structures, explore how the wakefield evolves with changes of density, study the effects of ionization on the wakefield structure with a variety of gases, and analyze the importance of the focusing position. These insights shed light onto this novel wakefield regime and bring understanding that is important to the realization of dephasingless acceleration.

physics.plasm-ph↗

arXiv2025

First Electron Acceleration in a Tunable-Velocity Laser Wakefield

We present the first experimental confirmation that a laser-wakefield accelerator produced by a flying focus pulse is able to maintain the coherent structures necessary to accelerate electrons to relativistic energies. Through a combination of spatio-temporal near-field shaping of the beam and focusing with an axiparabola - a long-focal-depth mirror that produces a quasi-Bessel beam - the propagation velocity of the wakefield is tuned to control the maximum electron energy achievable. The experimental data are supported by advanced optical and particle-in-cell simulations and are aligned with a simplified analytical model. Together, the results significantly strengthen the case for the flying-focus wakefield as a strategy for mitigating dephasing in laser-wakefield acceleration.

physics.acc-ph↗

arXiv2025

Start-to-end modelling of laser-plasma acceleration, beam transport and dose deposition of very high-energy electrons for radiotherapy

Radiotherapy using very high-energy electron (VHEE) beams generated by a laser-plasma accelerator has garnered significant interest due to its dose distribution capabilities and potential to address limitations of traditional photon-based radiotherapy. To explore the feasibility of such approach, the presented study uses the parameters of OONA, the commercial <1.3\,J, <25\,fs pulse duration laser recently installed at the Weizmann Institute of Science. Through particle-in-cell simulations of laser-plasma interaction, realistic electron beams were obtained. After filtering and collimation with a beamline and arranging them into an array of pencil beams, their radiotherapeutic potential was investigated. GEANT4 simulations were used to calculate the dose deposition in water phantoms and heterogeneous phantoms with bone inserts, considering realistic beam parameters for laser-plasma accelerators. Multifield irradiation setup and the dose distribution at the isocenter through different incidence angles were studied, simulating an intensity-modulated delivery. Our findings demonstrate that polychromatic VHEE beams generated from laser-plasma accelerators, when collimated using a compact beamline of quadrupoles and dipoles, can deliver an on-axis dose with enhanced precision and uniformity. This study highlights the transformative potential of laser-plasma accelerators in advancing radiotherapy modalities, paving the way for further research and clinical implementation.

physics.med-ph↗

arXiv2025

First direct observation of a wakefield generated with structured light

The use of structured light to control the phase velocity of the wake in laser-wakefield accelerators has generated significant interest for its ability to mitigate electron dephasing. Combining the diffraction-free properties of Bessel beams with spatio-temporal shaping of the pulse promises to enable acceleration with an unprecedented combination of long acceleration lengths and high gradients. This would facilitate the acceleration of electrons to energies above 100 GeV in existing laser facilities. In-depth understanding of the physical mechanisms involved is critical to achieving dephasing-free electron acceleration. Here we present the first experimental observation of wakefields generated by beams that were spatio-temporally sculpted and then focused with a long-focal-depth mirror, known as an axiparabola, which generates a quasi-Bessel beam. The resulting wakefield was imaged using femtosecond relativistic electron microscopy. Novel insights into this minimally explored regime include mapping the wakefield development over the focal depth and studying the effects of spatio-temporal manipulations of the beam on the structure and phase velocity of the wakefield. Such insights pave the way towards realizing the potential of structured-light based solutions to dephasing in laser-wakefield acceleration.

physics.acc-ph↗

arXiv2024

Compton photons at the GeV scale from self-aligned collisions with a plasma mirror

With today's multi-petawatt lasers, testing quantum electrodynamics (QED) in the strong field regime, where the electric field exceeds the Schwinger critical field in the rest frame of an electron, becomes within reach. Inverse Compton scattering of an intense laser pulse off a high-energy electron beam is the mainstream approach, resulting in the emission of high-energy photons that can decay into Breit-Wheeler electron-positron pairs. Here, we demonstrate experimentally that very high energy photons can be generated in a self-aligned single-laser Compton scattering setup, combining a laser-plasma accelerator and a plasma mirror. Reaching up to the GeV scale, photon emission via nonlinear Compton scattering exhibits a nonclassical scaling in the experiment that is consistent with electric fields reaching up to a fraction $\chi\simeq0.3$ of the Schwinger field in the electron rest frame. These foolproof collisions guaranteed by automatic laser-electron overlap provide a new approach for precise investigations of strong-field QED processes.

physics.plasm-ph↗

arXiv2023

Simple few-shot method for spectrally resolving the wavefront of an ultrashort laser pulse

We present a novel and straightforward approach for the spatio-spectral characterization of ultrashort pulses. This minimally intrusive method relies on placing a mask with specially arranged pinholes in the beam path before the focusing optic and retrieving the spectrally-resolved laser wavefront from the speckle pattern produced at focus. We test the efficacy of this new method by accurately retrieving chromatic aberrations, such as pulse front tilt, pulse front curvature, and higher-order aberrations introduced by a spherical lens. The simplicity and scalability of this method, combined with its compatibility with single-shot operation, make it a promising candidate to become a new standard diagnostic tool in high-intensity laser facilities.

physics.optics↗

arXiv2023

Beam current from downramp injection in electron-driven plasma wakefields

We study the stability of plasma wake wave and the properties of density-downramp injection in an electron-driven plasma accelerator. In this accelerator type, a short high-current electron bunch (generated by a conventional accelerator or a laser-wakefield acceleration stage) drives a strongly nonlinear plasma wake wave (blowout), and accelerated electrons are injected into it using a sharp density transition which leads to the elongation of the wake. The accelerating structure remains highly stable until the moment some electrons of the driver reach almost zero energy, which corresponds to the best interaction length for optimal driver-to-plasma energy transfer efficiency. For a particular driver, this efficiency can be optimized by choosing appropriate plasma density. Studying the dependence of the current of the injected bunch on driver and plasma parameters, we show that it does not depend on the density downramp length as long as the condition for trapping is satisfied. Most importantly, we find that the current of the injected bunch primarily depends on just one parameter which combines both the properties of the driver (its current and duration) and the plasma density.

physics.plasm-ph↗

arXiv2023

Use of spatiotemporal couplings and an axiparabola to control axial energy deposition velocity

This paper presents the first experimental realization of a scheme that allows for the tuning of the axial energy deposition velocity of a focal spot with relativistic intensity. By combining a tunable pulse-front curvature with the axial energy deposition characteristics of the axiparabola, an aspheric optical element, this system allows for controlling the dynamics of laser-wakefield accelerators. We demonstrate the ability to modify the axial energy deposition velocity of 100 TW laser pulses to be superluminal or subluminal. The experimental results are supported by theoretical calculations and simulations, strengthening the case for the axiparabola as a pertinent strategy to achieve dephasingless acceleration.

physics.optics↗

arXiv2023

Proton acceleration with intense twisted laser light

An efficient approach that considers a high-intensity twisted laser of moderate energy (few J) is proposed to generate collimated proton bunches with multi-10-MeV energies from a double-layer hydrogen target. Three-dimensional particle-in-cell simulations demonstrate the formation of a highly collimated and energetic ($\sim 40$ MeV) proton bunch, whose divergence is $\sim 6.5$ times smaller compared to the proton bunch driven by a Gaussian laser containing the same energy. Supported by theoretical modeling of relativistic self-focusing in near-critical plasma, we establish a regime that allows for consistent acceleration of high-energetic proton bunches with low divergence under experimentally feasible conditions for twisted drivers.

physics.plasm-ph↗

arXiv2023

Simultaneous generation and detection of energetic particle and radiation beams from relativistic plasma mirrors driven at kHz repetition rate

We report on the first simultaneous measurement of high-order harmonics, relativistic electrons and low divergence proton beams generated from plasma mirrors driven at kHz repetition rate by relativistic-intensity milliJoule-energy femtosecond laser pulses. This setup enables detailed parametric studies of the particle and radiation spatio-spectral beam properties for a wide range of controlled interaction conditions, such as pulse duration and plasma density scale length. This versatile setup should aid in further understanding the collective laser absorption mechanisms at play during the laser-plasma interaction and in optimizing the secondary beam properties for potential applications.

physics.plasm-ph↗

arXiv2022

Control of electron beam current, charge and energy spread using density downramp injection in laser wakefield accelerators

Density dowmramp injection has been demonstrated to be an elegant and efficient approach for generating high quality electron beams in laser wakefield accelerators. Yet, the charge of the produced beam is tens of pC per Joule of laser energy, still limiting its use for a wider range of applications. The possibility of generating high charge beam while keeping a good beam quality, stays to be explored. Moreover, despite previous studies focused on separate physical processes such as beam loading which affects the uniformity of the acceleration field and thus the energy spread of the trapped electrons, repulsive force from the rear spike of the bubble which reduces the transverse momentum $p_\perp$ of the trapped electrons and results in small beam emmittance, and the laser evolution when travelling in plasma. A more general investigation of the plasma density parameters on the final beam properties is required. In this work, we demonstrate that the current profile of the injected electron beam is directly correlated with the density transition parameters, which further affects the beam charge and energy spread. By fine-tuning the plasma density parameters, high-charge (up to several hundreds of pC) and low-energy-spread (around 1\% FWHM) electron beams can be obtained. All these results are supported by large-scale three-dimensional particle-in-cell simulations.

physics.plasm-ph↗

arXiv2022

Low divergence proton beams from a laser-plasma accelerator at kHz repetition rate

Proton beams with up to 100 pC bunch charge, 0.48 MeV cut-off energy and divergence as low as a $3^{\circ}$ were generated from solid targets at kHz repetition rate by a few-mJ femtosecond laser under controlled plasma conditions. The beam spatial profile was measured using a small aperture scanning time-of-flight detector. Detailed parametric studies were performed by varying the surface plasma scale length from 8 to 80 nm and the laser pulse duration from 4 fs to 1.5 ps. Numerical simulations are in good agreement with observations and, together with an in-depth theoretical analysis of the acceleration mechanism, indicate that high repetition rate femtosecond laser technology could be used to produce few-MeV protons beams for applications.

physics.acc-ph↗

arXiv2022

Spatiotemporal pulse characterization with far-field beamlet cross-correlation

We present a novel, straightforward method for spatiotemporal characterization of ultra-short laser pulses. The method employs far-field interferometry and inverse Fourier transform spectroscopy, built on the theoretical basis derived in this paper. It stands out in its simplicity: it requires few non-standard optical elements and simple analysis algorithms. This method was used to measure the space-time intensity of our 100 TW class laser and to test the efficacy of a refractive doublet as a suppressor of pulse front curvature (PFC). The measured low-order spatiotemporal couplings agreed with ray-tracing simulations. In addition, we demonstrate a one-shot measurement technique, derived from our central method, which allows for quick and precise alignment of the compressor by pulse front tilt (PFT) minimization and for optimal refractive doublet positioning for the suppression of PFC.

physics.optics↗

arXiv2021

Conceptual Design Report for the LUXE Experiment

This Conceptual Design Report describes LUXE (Laser Und XFEL Experiment), an experimental campaign that aims to combine the high-quality and high-energy electron beam of the European XFEL with a powerful laser to explore the uncharted terrain of quantum electrodynamics characterised by both high energy and high intensity. We will reach this hitherto inaccessible regime of quantum physics by analysing high-energy electron-photon and photon-photon interactions in the extreme environment provided by an intense laser focus. The physics background and its relevance are presented in the science case which in turn leads to, and justifies, the ensuing plan for all aspects of the experiment: Our choice of experimental parameters allows (i) effective field strengths to be probed at and beyond the Schwinger limit and (ii) a precision to be achieved that permits a detailed comparison of the measured data with calculations. In addition, the high photon flux predicted will enable a sensitive search for new physics beyond the Standard Model. The initial phase of the experiment will employ an existing 40 TW laser, whereas the second phase will utilise an upgraded laser power of 350 TW. All expectations regarding the performance of the experimental set-up as well as the expected physics results are based on detailed numerical simulations throughout.

hep-ex↗