Abnormal magnetic field topology in the beam-plasma instability driven by ion dynamics

We report on abnormal topological structures in the particles and electromagnetic field distributions induced by current filamentation instability, with emphasis on the effects of plasma ions. In the plasma filament (PF) situation, the transverse magnetic field reaches its maximum at the edge of the plasma filaments, in contrast to the case of beam filament (BF, i.e., the magnetic field reaches the maximum at the edge of the beam filaments). An analytical model is proposed to estimate the response time of the beam electrons and plasma ions, which is essential to determine the criterion of PF occurrence. With increase in beam energy and density, and decrease in plasma ion mass, the transition from BF to PF can be observed. The results are also confirmed by detailed electromagnetic particle-in-cell simulations. Moreover, the influence of PF on the synchrotron photon emission power is also discussed.