Ballistic cross-field ion beam propagation in a magnetoplasma

Abstract

Long range cross-field ion beam propagation in a magnetoplasma has been studied in the high kinetic beta regime by using computer simulations and analytic techniques. A new regime of long range ballistic beam propagation has been discovered for narrow high-density beams. Ion beams with transverse size Δ≪Rb, where Rb is the ion beam gyroradius and mass density nbMb≫npMp, where nb(Mb),np(Mp) are the particle density (mass) of the beam and the ambient ions can propagate ballistically across the ambient magnetic field over distances varying from a minimum of (nb/np)Rb to over an order of magnitude longer depending on the extent of initial interpenetration of the beam and the background plasma. The propagation mode is characterized by the formation of a front at the head of the beam, which forces the plasma to move sideways and prevents beam-plasma interpenetration. The system momentum is locally balanced by a corresponding displacement of the head of the beam in the opposite direction to the plasma, which is equivalent to erosion of the beam front. The physics of the interaction is distinct from the conventional magnetohydrodynamic picture and requires kinetic treatment for beam ions. Scaling laws that can be tested by laboratory and space experiments are presented.