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 give good results concerning
the emittance degradation.
The emittance and energy spread degradation due to wake fields in the
emittance meter experiment is also discussed.