Fundamental radiative instability of accelerated plasmas

Counterstreaming plasmas are well known to be electrostatically unstable and produce charge bunches. A single plasma moving along a curved trajectory is also electromagnetically unstable; i.e., a bunching perturbation is driven by the radiation fields. This relativistic instability is discussed and a physical picture of its origin is given. Calculations are made of when the instability can be quenched by a spread in velocities among the plasma particles. Laboratory relativistic beams rotating in a magnetic field are unstable to this mode, with growth times comparable to their cyclotron frequency, and only relatively large energy spreads Δγ/γ⩾0.2 will stabilize it. Recently observed coherent radiation may result from such bunching of beam electrons.