Jerome Faure

Laser wakefield acceleration (LWFA) provides a compact pathway to generate high-energy electron beams, but its performance depends on a large set of coupled, nonlinear parameters. Machine-learning approaches, particularly Bayesian optimization, have become valuable for efficiently exploring this space. In this study, we examine how the longitudinal plasma density profile affects the properties of LWFA electron beams using high-fidelity quasi-3D Particle-In-Cell simulations guided by Bayesian optimization. We focus specifically on the influence of the injection gradient and the plasma density down-ramp length. This work establishes a systematic methodology for multi-parameter and multi-objectives optimization of LWFAs and offers design guidance for the upcoming LAPLACE installation. The framework is general and adaptable to other accelerator configurations and optimization goals.Mach. Learn.

The EuPRAXIA project [Walker et al., J. Phys.: Conf. Ser. 874, 012029 (2017)] aims to construct two state-of-the-art accelerator facilities based on plasma accelerator technology. Plasma-based accelerators offer the possibility of a significant reduction in facility size and cost savings over current radio frequency (RF) accelerators. The two facilities—one laser-driven, one a beam-driven—are envisioned to provide electron beams with an energy in the range of 1–5 GeV and beam quality comparable to existing RF machines. This will enable a versatile portfolio of applications from compact free-electron laser drivers to sources for medical and industrial imaging. At the heart of both facilities is the use of plasma-based accelerator components and systems, which encompass not only the accelerating medium itself but also a range of auxiliary systems such as plasma-based electron beam optics and plasma-based mirrors for high-intensity lasers. From a technical standpoint, a high-degree of control over these plasma devices will be essential for EuPRAXIA to achieve its target performance goals. The ability to diagnose and characterize these plasma devices and to simulate their operation will be further essential success factors. Additionally, compatibility with extended operation at high-repetition rates and integration into the accelerator beamline will also prove crucial. In this work, we aim to review the current status of plasma components and related systems for both laser-driven and beam-driven plasma accelerators and to assess challenges to be addressed regarding implementation at future EuPRAXIA facilities.

Laser-Wakefield Accelerators (LWFAs) running at kHz repetition rates hold great potential for applications. They typically operate with low-energy, highly compressed laser pulses focused on 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 3 × 10−4 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.

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 a 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 LWFA driven by few-cycle, low-energy laser pulses. Our system generates quasimonoenergetic electron bunches with energies up to 10 MeV, bunch charges of 2 pC, and angular divergences of 15 mrad. Notably, our 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 the 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 laser plasma accelerator technology. Published by the American Physical Society 2024

We present a laser–plasma electron accelerator module designed to be driven by high-repetition-rate lasers for industrial applications of laser-driven electron beams. It consists of a single vacuum chamber containing all the necessary components for producing, optimizing, and monitoring electron beams generated via laser wakefield acceleration in a gas jet when driven by a suitable laser. The core methods in this paper involve a comprehensive metrological assessment of the driving laser by rigorous temporal laser pulse characterization and contrast measurements, supplemented by detailed spatiotemporal distribution analyses of the laser focus. Results demonstrate the good stability and reproducibility of the laser system, confirming its suitability for advanced scientific and industrial applications. We further demonstrate the functionality of the laser–plasma accelerator module diagnostics, perform target density characterizations, and time-resolved laser–plasma shadowgraphy. Current limitations of the set-up preventing first electron acceleration are analyzed and an outlook for future experiments is given. Our work is a first step towards the wide dissemination of fully integrated laser–plasma accelerator technology.

In this Letter, we investigate coherent phonon dynamics in the incommensurate LaVS3 crystal by femtosecond pump-probe spectroscopy. Two coherent phonon modes are systematically observed in the transient reflectivity, centered at 1.8 and 2.85 THz, respectively, while a third mode centered at 4.5 THz is observed only at high pump fluence. The experimental results obtained at two different polarization configurations as well as a comparison with recent theoretical results allow to assign the two main modes to the interlayer shearing mode and to an intralayer mode, respectively. Two possible assignments are discussed for the third mode, by invoking a possible emergence of nonlinear phonon processes.

We present a laser-plasma electron accelerator module designed to be driven by high repetition rate lasers for industrial applications of laser-driven electron beams. It consists of a single vacuum chamber containing all the necessary components for producing, optimizing, and monitoring electron beams generated via laser wakefield acceleration in a gas jet. The core methods involve comprehensive metrological assessments, including rigorous temporal laser pulse characterization and contrast measurements, supplemented by detailed spatio-temporal distribution analyses of the laser system. Results demonstrate the good stability and reproducibility of the laser system, confirming its suitability for advanced scientific and industrial applications. We demonstrate the full functionality of the laser-plasma accelerator module and perform target density and laser-plasma characterizations. Our work is a first step towards the wide dissemination of fully integrated laser-plasma accelerator technology.

The interaction of ultraintense 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 to show that the actual waveform of the laser field has a clear impact on the plasma response. The beam pointing of our relativistic electron beam oscillates in phase with the carrier-envelope phase of the laser, at an amplitude of 15 mrad, or 30% of the beam divergence. Numerical simulations explain this observation through asymmetries in the injection and acceleration of the electron beam, which are locked to the carrier-envelope phase. These results imply that we achieve waveform control of relativistic electron dynamics. Our results pave the way to high-precision, subcycle control of electron injection in plasma accelerators, enabling the production of attosecond relativistic electron bunches and x rays.

We report on the first observation and control of carrier-envelope phase effects in Laser Wakefield Acceleration. These effects imply the breakdown of the cycle-averaged ponderomotive approximation and are explained through highly localized off-axis electron injection.