PARTICLE-ACCELERATION IN A RELATIVISTIC WAVE IN THE ADIABATIC REGIME

The adiabatic theory of particle acceleration in a relativistic longitudinal plasma wave of fixed phase velocity is developed. The trapping condition for a low energy particle is found to be more severe than the trapping condition previously obtained for a constant amplitude wave. Intermediate energy particles that are trapped in the wave experience the beam splitting and the beam heating effects, with most of the particles ending in the larger energy beam. The particles that are not trapped experience no acceleration at all in the adiabatic limit. The validity conditions of the adiabatic theory are given for trapped and untrapped particles. Numerical results corresponding to the UCLA (University of California at Los Angeles) beat wave experiment [R. L. Williams, C. E. Clayton, C. Joshi, T. Katsouleas, and W. B. Mori, Laser Part. Beams 8, 427 (1990)] illustrate the theory. They show the important role of the trapping condition.