The Short Range Wakefields in the SLBC Linac

The short-range longitudinal and transverse wakefields of a point charge in the SBLC linac are obtained using a modal summation technique. Simple functional fits to these wakes are given, which can be used as Green functions in beam dynamics simulations of bunches. These results, however, are valid only after the beam has traversed a critical number of cells N{sub crit}. Using time domain computations with Gaussian bunches we have obtained results that are consistent with N{sub crit} varying as {alpha}a{sup 2}/(L{sigma}{sub z}), with a the iris radius and L the period length of the structure, {sigma}{sub z} the bunch length and {alpha} a constant on the order of 1. For the loss per cell to reach to within a few per cent of the asymptotic value we find that {alpha} {approx} 0.5 - 1.0.