Simulation of a high-average power free-electron laser oscillator

In this paper, we compare the 10 kW-Upgrade experiment at the Thomas Jefferson National Accelerator Facility in Newport News, VA, with numerical simulations using the medusa code. medusa is a three-dimensional FEL simulation code that is capable of treating both amplifiers and oscillators in both the steady-state and time-dependent regimes. medusa employs a Gaussian modal expansion, and treats oscillators by decomposing the modal representation at the exit of the wiggler into the vacuum Gaussian modes of the resonator and then analytically determining the propagation of these vacuum resonator modes through the resonator back to the entrance of the wiggler in synchronism with the next electron bunch. The bunch length in the experiment is of the order of 380–420 fsec FWHM. The experiment operates at a wavelength of about 1.6 microns and the wiggler is 30 periods in length; hence, the slippage time is about 160 fsec. Because of this, slippage is important, and must be included in the simulation. The observed single pass gain is 65%–75% and, given the experimental uncertainties, this is in good agreement with the simulation. Multipass simulations including the cavity detuning yield an output power of 12.4 kW, which is also in good agreement with the experiment.