Nonlinear Motion in Electromagnetic Fields via Multiscale Expansions
  The consideration of dynamics of relativistic beams/particles is based on variational approach to rational (in dynamical variables) approximation for equations of motions. It allows to control contribution from each scale of underlying multiscales and represent solutions via exact nonlinear eigenmodes expansions. Our approach is based on methods provided possibility to work with well-localized bases in phase space and good convergence properties of the corresponding expansions. 
Antonina N. Fedorova, Michael G. Zeitlin