arXiv subjects

Aline Vernier

Publications and source records attributed to Aline Vernier.

16 recordsLinked to original sources

arXiv2024

Differential pumping for kHz operation of a Laser Wakefield accelerator based on a continuously flowing Hydrogen gas jet

Laser-Wakefield Accelerators (LWFA) running at kHz repetition rates hold great potential for applications. They typically operate with low-energy, highly compressed laser pulses focused in high-pressure gas targets. Experiments have shown that the best-quality electron beams are achieved using Hydrogen gas targets. However, continuous operation with Hydrogen requires a dedicated pumping system. In this work, we present a method for designing a differential pumping system, which we successfully implemented in our experiments. This enabled the first demonstration of continuous operation of a kHz LWFA using a high-pressure Hydrogen gas jet. The system effectively maintained a pressure below 3e-4 mbar, even with a free-flowing gas jet operating at 140 bar backing pressure. Numerical fluid dynamics and optical simulations were used to guide and validate the system's design.

physics.plasm-ph↗

arXiv2024

Optical ionization effects in kHz laser wakefield acceleration with few-cycle pulses

We present significant advances in Laser Wakefield Acceleration (LWFA) operating at a 1 kHz repetition rate, employing a sub-TW, few-femtosecond laser and a continuously flowing hydrogen gas target. We conducted the first comprehensive study assessing how the nature of the gas within the target influences accelerator performance. This work confirms and elucidates the superior performance of hydrogen in kHz LWFA. Our system generates quasi-monoenergetic electron bunches with energies up to 10 MeV, bunch charges of 2 pC, and angular divergences of 15 mrad. Notably, our novel scheme relying on differential pumping enables continuous operation at kHz repetition rates, contrasting with previous systems that operated in burst mode to achieve similar beam properties. Particle-in-cell simulations explain hydrogen's superior performances: the ionization effects in nitrogen and helium distort the laser pulse, negatively impacting accelerator performance. These effects are strongly mitigated in hydrogen plasma, thereby enhancing beam quality. This analysis represents a significant step forward in optimizing and understanding kHz LWFA. It underscores the critical role of hydrogen and the imperative need to develop hydrogen-compatible target systems capable of managing high repetition rates, as exemplified by our differential pumping system. These advances lay the groundwork for further developments in high-repetition-rate LWFA technology.

physics.plasm-ph↗

arXiv2024

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↗

arXiv2022

Carrier-envelope phase controlled dynamics of relativistic electron beams in a laser-wakefield accelerator

In laser-wakefield acceleration, an ultra-intense laser pulse is focused into an underdense plasma in order to accelerate electrons to relativistic velocities. In most cases, the pulses consist of multiple optical cycles and the interaction is well described in the framework of the ponderomotive force where only the envelope of the laser has to be considered. But when using single-cycle pulses, the ponderomotive approximation breaks down, and the actual waveform of the laser has to be taken into account. In this paper, we use near-single cycle laser pulses to drive a laser-wakefield accelerator. We observe variations of the electron beam pointing on the order of 10 mrad in the polarisation direction, as well as 30% variations of the beam charge, locked to the value of the controlled laser carrier-envelope phase, in both nitrogen and helium plasma. Those findings are explained through particle-in-cell simulations indicating that low-emittance, ultra-short electron bunches are periodically injected off-axis by the transversally oscillating bubble associated with the slipping carrier-envelope phase.

physics.plasm-ph↗

arXiv2021

Waveform control of relativistic electron dynamics in an underdense plasma

The interaction of ultra-intense laser pulses with an underdense plasma is used in laser-plasma acceleration to create compact sources of ultrashort pulses of relativistic electrons and X-rays. The accelerating structure is a plasma wave, or wakefield, that is excited by the laser ponderomotive force, a force that is usually assumed to depend solely on the laser envelope and not on its exact waveform. Here, we use near-single-cycle laser pulses with a controlled carrier-envelope-phase (CEP) to show that the actual waveform of the laser field has a clear impact on the plasma response. We measure relativistic electron beams that are found to be strongly CEP dependent, implying that we achieve waveform control of electron dynamics in underdense laser-plasma interaction. Our results pave the way to high precision, sub-cycle control of electron injection in plasma accelerators, enabling the production of attosecond relativistic electron bunches and X-rays.

physics.plasm-ph↗

arXiv2021

Symmetric and asymmetric shocked gas jets for laser-plasma experiments

Shocks in supersonic flows offer both a high-density and sharp density gradients that can be used, for instance,for gradient injection in laser-plasma accelerators. We report on a parametric study of oblique shocks created by inserting a straight axisymmetric section at the end of a supersonic "de Laval" nozzle. The impact of different parameters such as throat diameter and straight section length is studied through computational fluid dynamics (CFD) simulations. Experimental characterizations of a shocked nozzle are compared to CFD simulations and found to be in good agreement. We then introduce a newly designed asymmetric shocked gas jet, where the straight section is only present on one lateral side of the nozzle, thus providing a gas profile that can be used for density transition injection. In this case, full-3D fluid simulations and experimental measurements are compared and show excellent agreement.

physics.ins-det↗

arXiv2020

Dosimetric characterisation and application to radiation biology of a kHz laser-driven electron beam

Laser-plasma accelerators can produce ultra short electron bunches in the femtosecond to picosecond duration range, resulting in high peak dose rates in comparison with clinical accelerators. This peculiar characteristic motivates their application to radiation biology studies to elucidate the effect of the high peak dose rate on the biological response of living cells, which is still being debated. Electron beams driven by kHz laser systems may represent an attractive option for such applications, since the high repetition rate can boost the mean dose rate and improve the stability of the delivered dose in comparison with J-class laser accelerators running at 10 Hz. In this work, we present the dosimetric characterisation of a kHz, low energy laser-driven electron source and preliminary results on in-vitro irradiation of cancer cells. A shot-to-shot dosimetry protocol enabled to monitor the beam stability and the irradiation conditions for each cell sample. Results of survival assays on HCT116 colorectal cancer cells are in good agreement with previous findings reported in literature and validate the robustness of the dosimetry and irradiation protocol.

physics.acc-ph↗

arXiv2020

Carrier-envelope phase effects in Laser Wakefield Acceleration with near-single-cycle pulses

Driving laser wakefield acceleration with extremely short, near single-cycle laser pulses is crucial to the realisation of an electron source that can operate at kHz-repetition rate while relying on modest laser energy. It is also interesting from a fundamental point of view, as the ponderomotive approximation is no longer valid for such short pulses. Through particle-in-cell simulations, we show how the plasma response becomes asymmetric in the plane of laser polarization, and dependent on the carrier-envelope phase (CEP) of the laser pulse. For the case of self-injection, this in turn strongly affects the initial conditions of injected electrons, causing collective betatron oscillations of the electron beam. As a result, the electron beam pointing, electron energy spectrum and the direction of emitted betatron radiation become CEP-dependent. For injection in a density gradient the effect on beam pointing is reduced and the electron energy spectrum is CEP-independent, as electron injection is mostly longitudinal and mainly determined by the density gradient. Our results highlight the importance of controlling the CEP in this regime for producing stable and reproducible relativistic electron beams and identify how CEP effects may be observed in experiments. In the future, CEP control may become an additional tool to control the energy spectrum or pointing of the accelerated electron beam.

physics.plasm-ph↗

arXiv2020

Demonstration of stable long-term operation of a kilohertz laser-plasma accelerator

We report on the stable and continuous operation of a kilohertz laser-plasma accelerator. Electron bunches with 2.6 pC charge and 2.5 MeV peak energy were generated via injection and trapping in a downward plasma density ramp. This density transition was produced in a newly designed asymmetrically shocked gas nozzle. The reproducibility of the electron source was also assessed over a period of a week and found to be satisfactory with similar values of the beam charge and energy. These results show that the reproducibility and stability of the laser-plasma accelerator are greatly enhanced on the long-term scale when using a robust scheme for density gradient injection.

physics.acc-ph↗

arXiv2020

Generation of XUV spectral continua from relativistic plasma mirrors driven in the near-single-cycle limit

function of the CEP, we observe a transition from a modulated to a continuous SHHG spectrum, indicating the transition from double to high-contrast isolated attosecond pulse emission. Single shot-acquisitions of XUV spectral continua support isolated attosecond pulses with isolation degrees between 10 and 50 for the majority of the driving pulse CEPs, which represents a significant improvement in terms of isolation degree as well as repetition rate over previous results obtained with 2-cycle drivers at 10-Hz. 2D Particle-in-cell simulations corroborate this interpretation and predict percent-level efficiencies for the generation of an isolated attosecond pulse even without any spectral filtering.

physics.optics↗

arXiv2019

Relativistic near-single-cycle optics at 1 kHz

We present a laser source delivering waveform-controlled 1.5-cycle pulses that can be focused to relativistic intensity at 1 kHz repetition rate. These pulses are generated by nonlinear compression of high-temporal-contrast sub-25\,fs pulses from a kHz Ti:Sapphire double-chirped pulse amplifier in a stretched flexible hollow fiber compressor scaled for high peak power. The unique capabilities of this source are demonstrated by observing carrier-envelope phase effects in laser-wakefield acceleration of relativistic electrons for the first time.

physics.plasm-ph↗

arXiv2018

Anticorrelated emission of high-harmonics and fast electron beams from plasma mirrors

We report for the first time on the anticorrelated emission of high-order harmonics and energetic electron beams from a solid-density plasma with a sharp vacuum interface$-$plasma mirror$-$driven by an intense ultrashort laser pulse. We highlight the key role played by the nanoscale structure of the plasma surface during the interaction by measuring the spatial and spectral properties of harmonics and electron beams emitted by a plasma mirror. We show that the nanoscale behavior of the plasma mirror can be controlled by tuning the scale length of the electronic density gradient, which is measured in-situ using spatial-domain interferometry.

physics.plasm-ph↗

arXiv2017

High-charge relativistic electron bunches from a kHz laser-plasma accelerator

We report on electron wakefield acceleration in the resonant bubble regime with few-millijoule near-single-cycle laser pulses at a kilohertz repetition rate. Using very tight focusing of the laser pulse in conjunction with microscale supersonic gas jets, we demonstrate a stable relativistic electron source with a high charge per pulse up to 24 pC/shot. The corresponding average current is 24 nA, making this kilohertz electron source useful for various applications.

physics.acc-ph↗

arXiv2014

Demonstration of a strong Rydberg blockade in three-atom systems with anisotropic interactions

We study the Rydberg blockade in a system of three atoms arranged in different 2D geometries (linear and triangular configurations). In the strong blockade regime, we observe high-contrast, coherent collective oscillations of the single excitation probability, and an almost perfect van der Waals blockade. Our data is consistent with a total population in doubly and triply excited states below 2%. In the partial blockade regime, we directly observe the anisotropy of the van der Waals interactions between $|nD\rangle$ Rydberg states in the triangular configuration. A simple model, that only uses independently measured two-body van der Waals interactions, fully reproduces the dynamics of the system without any adjustable parameter. These results are extremely promising for scalable quantum information processing and quantum simulation with neutral atoms.

physics.atom-ph↗

arXiv2014

Single-atom trapping in holographic 2D arrays of microtraps with arbitrary geometries

We demonstrate single-atom trapping in two-dimensional arrays of microtraps with arbitrary geometries. We generate the arrays using a Spatial Light Modulator (SLM), with which we imprint an appropriate phase pattern on an optical dipole trap beam prior to focusing. We trap single $^{87}{\rm Rb}$ atoms in the sites of arrays containing up to $\sim100$ microtraps separated by distances as small as $3\;μ$m, with complex structures such as triangular, honeycomb or kagome lattices. Using a closed-loop optimization of the uniformity of the trap depths ensures that all trapping sites are equivalent. This versatile system opens appealing applications in quantum information processing and quantum simulation, e.g. for simulating frustrated quantum magnetism using Rydberg atoms.

physics.atom-ph↗

arXiv2013

Direct measurement of the van der Waals interaction between two Rydberg atoms

We report on the direct measurement of the van der Waals interaction between two isolated, single Rydberg atoms separated by a controlled distance of a few micrometers. By working in a regime where the single-atom Rabi frequency of the laser used for excitation to the Rydberg state is comparable to the interaction energy, we observe a \emph{partial} Rydberg blockade, whereby the time-dependent populations of the various two-atom states exhibit coherent oscillations with several frequencies. A quantitative comparison of the data with a simple model based on the optical Bloch equations allows us to extract the van der Waals energy, and to observe its characteristic $C_6/R^6$ dependence. The magnitude of the measured $C_6$ coefficient agrees well with an \emph{ab-initio} theoretical calculation, and we observe its dramatic increase with the principal quantum number $n$ of the Rydberg state. Our results not only allow to test an important physical law, but also demonstrate a degree of experimental control which opens new perspectives in quantum information processing and quantum simulation using long-range interactions between the atoms.

physics.atom-ph↗