An emittance algorithm for a high-intensity low-emittance beam

The Advanced Photon Source (APS) linac together with a photocathode RF gun is the source for a self-amplified spontaneous emission (SASE) free-electron laser (FEL). For optimal performance the APS SASE FEL requires a low-emittance high-intensity electron beam. A three-screen measurement section, located upstream of the Low-Energy Undulator Test Line, where the SASE FEL is located, allows monitoring of the beam emittance. Control and reduction of the emittance growth is achieved experimentally by various methods such as steering or adjusting the RF phases of one or more klystrons along the linac. In particular, emittance growth caused by transverse wakefields can be reduced by strategically placing local trajectory distortions. The factors and methods affecting the beam projection at each of the three screens are systematically investigated using simulations. The results will be used to create an algorithm that allows an effective and systematic means to reduce beam distortions at the end of the linac. Initial experimental results are presented