RF System Upgrades to the Advanced Photon Source Linear Accelerator in   Support of the Fel Operation
  The S-band linear accelerator, which was built to be the source of particles and the front end of the Advanced Photon Source injector, is now also being used to support a low-energy undulator test line (LEUTL) and to drive a free-electron laser (FEL). The more severe rf stability requirements of the FEL have resulted in an effort to identify sources of phase and amplitude instability and implement corresponding upgrades to the rf generation chain and the measurement system. Test data and improvements implemented and planned are described 
T.L. Smith, M.H. Cho, A.E. Grelick, G. Pile, A. Nassiri, N. Arnold