  Three dimensional Particle in Cell simulations of Laser Wakefield
Acceleration require a considerable amount of resources but are necessary to
have realistic predictions and to design future experiments. The planned
experiments for the Apollon laser also include two stages of plasma
acceleration, for a total plasma length of the order of tens of millimeters or
centimeters. In this context, where traditional 3D numerical simulations would
be unfeasible, we present the results of the application of a recently proposed
envelope method, to describe the laser pulse ant its interaction with the
plasma without the need to resolve its high frequency oscillations. The
implementation of this model in the code Smilei is described, as well as the
results of benchmark simulations against standard laser simulations and
applications for the design of two stage Apollon experiments.
