Multiple currents in charged beams
  It is argued that continuum realisations of distributions of collisionless charged particles should accommodate a dynamically evolving number of electric currents even if the continuum is composed of only one species of particle, such as electrons. A model is proposed that self-consistently describes the interaction of such a continuum and its electromagnetic field. It is formulated using a Lagrangian approach and employs a "folded" flow map to describe the bulk particle motion. An asymptotic perturbation scheme is developed to analyse ultra-relativistic multi-component current configurations. The model is fully relativistic and is formulated over Minkowski spacetime using intrinsic tensor field theory. 
D.A. Burton, J. Gratus, R.W. Tucker