TESLA Report 2003-18 NUMERICAL CALCULATION OF SMALL-ANGLE COLLIMATOR WAKEFIELDS FOR SHORT BUNCHES

Collimators are used to eliminate halo particles from the beam. To relax the wakefield effects a gradual transition from a large to a small aperture is used. However, the existing computer codes face severe problems for long tapered transitions. Two main sources of the problems are the grid dispersion and the staircase geometry approximation. Using recently developed time domain numerical approach, which is able to model curved boundaries and does not suffer from dispersion in longitudinal direction, we calculate the short-range geometric wakefields of the TESLA and NLC collimators. Wake fields and corresponding integral parameters are given for bunches of different length. The numerical results are compared to measurements and to analytical estimations. The applicability range for the analytical formulas is highlighted.