Power limitations in the OK-4/Duke storage ring FEL

In this paper, we present results of our experimental and theoretical studies of average power in the OK-4/Duke storage ring FEL. Our theoretical studies are based on the 3D FEL macro-particle model, which includes the local interactions, diffusion, radiation damping and spontaneous radiation. The OK-4/Duke storage ring FEL is operational since 1996 and demonstrated lasing in a wavelength range from 193.7 to 730 nm using electron beam energies from 220 to 800 MeV. It operated in both CW and giant pulse modes. During this period of time we collected substantial amounts of data regarding the FEL power and electron beam dynamics. We compare selected results on CW lasing with our theoretical predictions based on the rigorous numerical model. We also discuss a number of simplified scaling laws for the FEL gain and power as functions of electron beam energy and current, as well as, the cavity losses.