Collective Effects in the Jefferson Lab FEL Upgrade

(MO-3-13)

Lia Merminga and Byung C. Yunn  

Jefferson Lab

Recirculating linacs with energy recovery employing superconducting cavities have been shown to be effective driver accelerators for high average power FELs. At sufficiently high bunch charge and/or average beam current, a number of collective effects can degrade the beam quality or induce instabilities. These effects include multibunch Beam Breakup (BBU) due to transverse and longitudinal Higher Order Modes (HOMs), single bunch BBU due to component wakefields and resistive wall, and beam energy and laser power instabilities due to the FEL-rf interaction. We present an overview of all impedance-generated effects for the Jefferson Lab IRFEL Upgrade accelerator, designed to run at 135 pC per bunch, 10 mA average current. Simple expressions for the threshold current of the instabilities and emittance growth and energy spread due to single bunch effects are derived and used to evaluate the design. Scaling of the results with beam and accelerator parameters is presented.