THEORY AND SIMULATION OF THE INTERACTION OF ULTRAINTENSE LASER-PULSESWITH ELECTRONS IN VACUUM

In order to investigate ponderomotive force in the relativistic regime , the interaction of ultraintense laser pulses with free electrons in vacuum is studied both theoretically and numerically. Various expressi ons for the electromagnetic field of the laser in the case of a Gaussi an transverse profile are given, which take into account corrections t o the monochromatic paraxial approximation, and the effects of finite pulse duration. A detailed demonstration of relativistic ponderomotive force (RPF) is established which makes apparent the domain of validit y of this concept. Computer simulations are carried out using a three- dimensional test-particle code. They show the importance of the correc t description of the fields, and confirm the domain of validity of the RPF which is 1-v(z)/c much greater than 1/kw(0), where v(z) is the co mponent of the electron velocity parallel to the laser propagation dir ection, c is the velocity of light, k is the laser wave vector, and w( 0) the beam waist at focus. Outside of this domain, the electron motio n is more complicated, with a high sensitivity on the initial distance from the laser propagation axis and a relatively low energy gain.