  Beam loading is the phenomenon which limits the charge and the beam quality
in plasma based accelerators. An experimental study conducted with a
laser-plasma accelerator is presented. Beam loading manifests itself through
the decrease of the beam energy, the reduction of dark current and the increase
of the energy spread for large beam charge. 3D PIC simulations are compared to
the experimental results and confirm the effects of beam loading. It is found
that, in our experimental conditions, the trapped electron beams generate
decelerating fields on the order of 1 GV/m/pC and that beam loading effects are
optimized for trapped charges of about 20 pC.
