  The laser field depletion in laser-electron beam collision is generally
small. However, a properly chosen parameter of the laser and electron, the
laser field depletion can be optimized. To access the laser energy evolution,
simulations of laser-electron beam collision by Particle-in-Cell (PIC)
simulation is performed. In this paper, the laser and electron parameters are
chosen such that the ponderomotive force is compensated by the radiation
reaction force in the head-on collision configuration. Then, the relativistic
electron beam can quiver in the laser pulse for a longer time to increase the
energies conversion. The optimum of laser field energy depletion is observed at
$\gamma_0= a_0 \sim 400$ and limited beyond this point due to the
impenetrability threshold. The total energy conversion from laser and electron
beam into radiation emission is optimum at $\gamma_0= a_0 \sim 250$. This
conversion efficiency can be up to several percent for an electron bunch with
charges of a few nC. This efficient gamma-ray sources may provide some useful
application in photonuclear experiments.
