Particle acceleration and radiation friction effects in the filamentation instability of pair plasmas

The evolution of the filamentation instability produced by two counterstreaming, ultrarelativistic pair plasmas is studied with particle-in-cell simulations. Radiation friction effects are taken into account. Two-dimensional simulations are performed for both cases of the initial momenta being perpendicular (T mode) or parallel (P mode) to the simulation plane. In the initial stage, the instability is purely transverse for both modes and generates small-scale filaments which later merge into larger structures. Particle acceleration leads to a strong broadening of the energy spectrum with the formation of a peak at twice the initial energy for the T mode. In the non-linear stage, significant differences between T- and P modes in the evolution of the fields and in the spectra of accelerated particles are apparent. The presence of radiative losses does not change the dynamics of the instability but strongly affects the structure of the particle spectra in the ultrarelativistic regime (particle energy >100 MeV) and for high plasma densities (>10²¹ cm^-3).