Ronan Lahaye

Abstract:

Laser-plasma accelerators offer a compact means of producing high-energy electron beams, but their performance is fundamentally limited by dephasing between the accelerated electrons and the plasma wave. To overcome this limitation, we investigate the combination of plasma density tapering and optical guiding to extend the effective acceleration length. Using a Joule-class femtosecond laser coupled into an optical-field-ionized plasma waveguide with a controlled density gradient, we experimentally achieve electron beam energies exceeding 1.6 GeV, a 40% increase compared to the constant-density case. Particle-in-cell simulations reproduce the main experimental features and reveal the central roles of delayed injection, nonlinear laser evolution, and self-focusing in enhancing energy gain.

Abstract:

Laser-plasma accelerators present a promising alternative to conventional accelerators. To fully exploit the extreme amplitudes of the plasma fields and produce high-quality beams, precise control over electron injection into the accelerating structure is required, along with effective laser pulse guiding to extend the acceleration length. Recent studies have demonstrated efficient guiding and acceleration using hydrodynamic optically field-ionized plasma channels. This guiding technique has also been combined with controlled electron injection to produce high-quality electron beams at the GeV level using a 50 TW laser. The present work extends these results to higher laser power, demonstrating the generation of quasimonoenergetic electron beams with peak energies exceeding 2 GeV, for a PW-class laser.

Abstract:

Betatron radiation is produced in laser plasma accelerators when the electrons are accelerated and simultaneously wiggle across the propagation axis [Rousse et al., Phys. Rev. Lett. 93, 135005 (2004)]. The mechanisms of electron acceleration and x-ray radiation production follow different scaling laws [Corde et al., Rev. Mod. Phys. 85, 1–48 (2013)], and the brightest x-ray radiation is often produced for an electron beam with a lower quality in terms of energy and divergence. Here, we report a laser-driven betatron x-ray source where the plasma density profile is tailored in order to separate the acceleration and wiggler stages, which allows for the independent optimizations of acceleration and x-ray production. We demonstrate this concept experimentally and show that the betatron photon energy can be controlled by adjusting the length of the plasma wiggler. This scheme offers a path to overcome the limitations of conventional betatron sources, enabling the production of bright, stable, energetic, and collimated x-ray beams.

Abstract:

This paper presents the first experimental realization of a scheme that allows for the tuning of the velocity of peak intensity of a focal spot with relativistic intensity. By combining a tunable pulse-front curvature with the axial intensity deposition characteristics of an axiparabola, an aspheric optical element, this system provides control over the dynamics of laser-wakefield accelerators. We demonstrate the ability to modify the velocity of peak intensity of ultrashort 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 more efficient acceleration.