RELATIVISTIC PONDEROMOTIVE FORCE, UPHILL ACCELERATION, AND TRANSITIONTO CHAOS

Starting from a covariant cycle-averaged Lagrangian the relativistic o scillation center equation of motion of a point charge is deduced, and analytical formulas for the ponderomotive force in a traveling wave o f arbitrary strength are presented. It is further shown that the ponde rmotive forces for transverse and longitudinal waves are different; in the latter, uphill acceleration can occur. In a standing wave there e xists a threshold intensity above which, owing to transition to chaos, the secular motion can no longer be described by a regular ponderomot ive force.