Emittance degradation due to wake fields in a high brightness photoinjector

Wake fields effects in addition to space charge forces may have an important impact during the emittance compensation process in a high brightness photo-injector. To study this effect we developed an upgraded version of the Homdyn code including off axis beam dynamics and wake fields. Homdyn describes a bunch as a uniformly charged cylinder, divided in cylindrical slices; in the upgraded version each slice's centroid can be transversally displaced from the nominal axis thus inducing wake fields. When the bunch is short as compared to the beam pipe radius, wake fields for a single cavity are calculated using methods of diffraction theory; instead we use, for a periodic collection of cavities, an asymptotic wake field obtained numerically at SLAC and then fitted to a simple function. As a first application we studied and verified a correction scheme for the SPARC photo-injector to control the bunch trajectory and angle at the entrance of the undulator. The correction scheme consists of a number of steering magnets and beam position monitors placed along the photo-injector. Two different steering approaches are analyzed and the emittance degradation is studied. The code demonstrates the steering positions and number do correct the bunch's orbit and angle and gives good results concerning the emittance degradation. The emittance and energy spread degradation due to wake fields in the emittance meter experiment is also discussed.