▎ 摘　　要

Electron beams in two-dimensional systems can provide a useful tool to study energy-momentum relaxation of electrons and to generate microwave radiation stemming from plasma-beam instabilities. Naturally, these two applications cannot coexist: if instability exists, it strongly distorts the distribution function of beam electrons; if scattering is strong, it typically suppresses plasma instabilities. Here, we study the role of inevitable electron-electron (e-e) collisions on possible plasma beam instabilities in graphene and show that scattering effects are far less trivial. We find that an unstable plasma mode associated with beam bunching is stabilized already by weak e-e collisions. Quite surprisingly, further enhancement of e-e collisions results in loss compensation and self-excitation of an ordinary graphene plasmon mode. Such instability is interpreted as viscous transfer of momentum from an electron beam to two-dimensional plasmons. Its growth rate reaches its maximum at hydrodynamic-to-ballistic crossover, when plasmon wavelength and electron mean free path are of the same order of magnitude.