RF stability in energy recovering free electron lasers: theory and experiment

Phenomena that result from the interaction of the beam with the RF fields in superconducting cavities, and can potentially limit the performance of high average power Energy Recovering Free Electron Lasers (FELs), are reviewed. These phenomena include transverse and longitudinal multipass, multibunch beam breakup, longitudinal beam-loading type of instabilities and their interaction with the FEL, Higher Order Mode power dissipation and RF control issues. We present experimental data obtained at the Jefferson Lab IR FEL with average current up to 5 mA, compare with analytic calculations and simulations and extrapolate the performance of Energy Recovering FELs to much higher average currents, up to ∼100 mA.