  We study theoretically the exchange of angular momentum between a photon beam
and a plasma vortex, and demonstrate the possible excitation of photon angular
momentum states in a plasma. This can be relevant to laboratory and space
plasma diagnostics; radio astronomy self-calibration; and generating photon
angular momentum beams. A static plasma perturbation with helical structure,
and a rotating plasma vortex are studied in detail and a comparison between
these two cases, and their relevance to the physical nature of photon OAM, is
established.
