Particle acceleration via the Kelvin-Helmholtz instability

Collisionless plasma instabilities are a critical ingredient to understand the acceleration of high-energy particles in extreme astrophysical scenarios such as active galactic nuclei and gamma ray bursters. Since these extreme scenarios are usually associated with strain and~rapid variability of the ejecta, it is likely that strong velocity shears are present, triggering the collisionless Kelvin-Helmholtz instability (KHI). We show that particles may accelerate to high energies by scattering in the evolving electric and magnetic fields of the KHI. Moreover,~we present the relativistic two-fluid model of the KHI and perform a detailed comparison with PIC simulations results, namely growth-rates and length-scales of the instability. We analyze the dependence of the KHI on the gradient length of the shear, observing lower~growth-rates for longer gradient-lengths. We further study the particle energy spectra generated by the instability, where we find evidence of non-thermal particle acceleration.