Effect of the laser transverse profile in a capillary for laser-plasma acceleration with external injection

Guiding of a high intensity laser beam in a hollow dielectric capillary is investigated with application for a laser plasma wakefield accelerator stage. For such guiding, the transverse profile of the laser intensity at the capillary entrance has a strong influence on the fluctuations of the transverse plasma wakefield during the laser propagation. The effects of these fluctuations on the acceleration of an electron bunch injected at 100 MeV is analyzed in detail. Our results show that, in the best match conditions, the usually used Gaussian transverse profile leads to strong fluctuations generated by the diffraction of the edges of the laser at the capillary entrance. We demonstrate that by reducing this diffraction through the use of a flattened Gaussian laser profile, an efficient acceleration of the electron bunch can be obtained over several Rayleigh lengths