Abstract: Accreting black holes emit macroscopic collimated jets, which can reach relativistic bulk flow speeds. Black holes are usually surrounded by a dilute partially ionized gas. Radiation from the approaching jet will ionize most of the latter. This ambient plasma consists of electrons and ions and it interacts electromagnetically with the jet, which is composed of electrons, positrons and a minor fraction of ions. A boundary forms where both plasmas collide. It confines the pair plasma, it separates it from the ambient plasma and it is a source of intense

electromagnetic radiation. I present results of my recent particle-in-cell simulations that model the interaction between a spatially localized microscopic pair cloud and an initially unmagnetized electron-proton plasma for parameters that are representative for the jets of microquasars. I discuss the plasma instabilities at work, which involve carriers of positive charge with different charge-to-mass ratios. These instabilities heat up the plasma and collimate the pair cloud into a magnetized jet.