Plasma-loaded free-electron laser with an electromagnetic wave wiggler and axial guide field

A fluid model is used to study the effects of background plasma in an electromagnetically pumped free-electron laser with an axial guide field. The dispersion relation is derived, and the growth rate is formulated for the Raman regime. Numerical calculations are also carried out for both fast electromagnetic wave and whistler wigglers propagating opposite to the relativistic electron beam. In the case of a fast wave wiggler, the background plasma can enhance the growth rate considerably, and a much larger growth rate can be obtained around cyclotron resonance ωi∼Ω0, where ωi is the angular frequency of pump wave, and Ω0 represents the cyclotron frequency of the background plasma electrons. However, in the case of a whistler wiggler, the growth rate decreases rapidly as the plasma density increases, and a considerable decrease also occurs when ωi is near Ω0