Longitudinal magnetic field generation during the early stage of relativistic electron beam-plasma interaction

Abstract

A mechanism for longitudinal magnetic field (i.e., the magnetic field parallel to the beam propagation direction) generation during the transport of relativistic electron beams with large edge gradients, through a high density plasma, is explained using two-dimensional (2D) particle-in-cell simulations and analytical theory. A ring structure is first formed due to the unneutralized beam current in the edge. Later, the counter-propagation of plasma return currents in the radial direction is excited due to the induction of a defocusing azimuthal magnetic field in the inner parts of the ring structure, with a characteristic crossing length of the ring width. Finally, the filamentation of plasma radial return currents in the azimuthal direction develops and contributes to the generation of a strong longitudinal magnetic field, which is shown to have strength on the same order of the transverse magnetic fields.