Anomalous free electron laser interaction

Free electron lasers (FELs) are considered, typically, as fast wave devices. The normal FEL interaction satisfies the tuning condition ω≅(k_z + k_W)V_z , where ω and k_z are the em-wave angular frequency and longitudinal wave number, respectively, V_z is the electron axial speed, and k_W is the wiggler periodicity. This paper presents an anomalous FEL interaction, which may occur in slow-wave FELs (i.e. loaded by dielectric or periodic structures). The anomalous FEL effect presented here satisfies the tuning condition ω≅(k_z - k_W)V_z , and it resembles the anomalous effect in slow-wave cyclotron resonance masers. A necessary condition for the anomalous interaction is ω/k_z<V_z (i.e., the em-wave phase velocity should be slower than the electron beam). The paper presents a preliminary experimental result demonstrating the anomalous FEL effect in a stripline dielectric-loaded FEL experiment. A linear Pierce equation is applied to describe both the anomalous and normal FELs in the same framework. The paper is concluded with a conceptual discussion.