  We discuss radiation reaction effects on charges propagating in ultra-intense
laser fields. Our analysis is based on an analytic solution of the
Landau-Lifshitz equation. We suggest to measure radiation reaction in terms of
a symmetry breaking parameter associated with the violation of null translation
invariance in the direction opposite to the laser beam. As the Landau-Lifshitz
equation is nonlinear the energy transfer within the pulse is rather sensitive
to initial conditions. This is elucidated by comparing colliding and fixed
target modes in electron laser collisions.
