TRANSIT-TIME SCATTERING AND HEATING OF A RELATIVISTIC ELECTRON-BEAM IN STRONG LANGMUIR TURBULENCE

A Fokker-Planck theory is developed to describe the diffusion in momen tum space of a beam of relativistic electrons due to multiple transit- time interactions with an ensemble of coherent Langmuir wave packets. The theory incorporates two ingredients: a perturbed-orbit calculation of the momentum change of a test particle during a single transit-tim e interaction, and an ensemble average of the resulting Fokker-Planck coefficients based on the statistical properties of strong Langmuir tu rbulence. An approximate analytic solution of the Fokker-Planck equati on is obtained for the case of a strongly collimated beam, and is used to interpret measurements of energy and pitch-angle scattering in rel ativistic-electron-beam (REB) experiments. Fokker-Planck coefficients are also calculated for a weakly collimated beam. It is shown that the theory correctly predicts the amount of energy scattering in REB expe riments, but underestimates the pitch-angle scattering regardless of t he distribution of wave packet orientations and the degree of collimat ion of the beam. This discrepancy may be a product of the approximate wave-packet structure assumed in the analysis, or of systematic errors in the experimental data; alternatively, it may imply that a non-tran sit-time process is responsible for part of the pitch-angle scattering observed. (C) 1996 American Institute of Physics.