Laser-Plasma Accelerators (LPAs) produce electric fields exceeding 100 GV/m, that is 3 orders of magnitude larger than those obtained in metallic-cavity accelerators. They could thus allow for a drastic decrease of the size of accelerators for scientific, medical and industrial applications. A high field-gradient is however not sufficient for reaching high-energies; the electron beam has also to experience the accelerating field on long distances, which is challenging in a LPA because of 3 phenomenons: diffraction, pump depletion and dephasing. Diffraction and pump depletion leads to a decrease of the laser intensity during the acceleration, down to a level from which the laser can no more drive a wakefield. Dephasing corresponds to electrons reaching a decelerating phase of the electric field. It occurs because the phase velocity of the accelerating field is smaller than the velocity of the electron beam. To date, the highest beam energies have been obtained by guiding the laser in a capillary discharge, thus overcoming diffraction.
Here we propose a new acceleration concept, based on the use of high-intensity quasi-Bessel beams and spatio-temporal couplings, which allows to overcome not only diffraction but also pump depletion and dephasing. The velocity of the quasi-Bessel beam is superluminal in vacuum and dephasing is suppressed by using spatio-temporal couplings to phase lock the electron beam on the accelerating field. In this scheme, the electron energy is proportional to the laser energy and inversely proportional to the laser pulse length (the shorter the laser, the higher the beam energy).
We will first present Particle-In-Cell simulations demonstrating this concept. We will then show preliminary experimental results illustrating the generation of high-intensity quasi-Bessel beams as well as the generation of a 1 cm plasma-waveguide.