Kelvin-Helmholtz instability of relativistic beams

A discussion of the linearized Kelvin-Helmholtz instability in the vicinity of a plane infinite vortex sheet separating two fluids in relative motion is presented. The calculation generalizes existing treatments to include relativistic relative motion and relativistic internal sound speeds. The character of the unstable modes is outlined in the two limits (a) when the sound speed ratio is large, and (b) when it is equal to unity. The relevance of these results to beam models of extragalactic radio sources is briefly discussed. (author)