A submillimeter Raman free-electron laser in a dense plasma background

Growth characteristics of a Raman free-electron laser in a dense plasma background are investigated. The background plasma is beneficial in the sense that it allows a beam current over the vacuum space-charge limit. It is found, however, that an excessive plasma density reduces the linear growth rate significantly if the radiation mode is not confined well inside the electron beam. The growth rate reduction accompanies a large positive frequency shift of the radiation field. This indicates that the background plasma causes the optical beam to expand outward farther into the plasma.