Observation of Superposition of Wake Fields Generated by Electron Bunches in A Dielectric-Lined Waveguide

We report results from an experiment, done at the Accelerator Test Facility, Brookhaven National Laboratory, which demonstrates the successful superposition of wake fields excited by 50MeV bunches which travel ∼ 50cm along the axis of a cylindrical waveguide which is lined with alumina. Wake fields from two short (5-6psec) 0.15− 0.35nC bunches are superimposed and the energy losses of each bunch are measured as the separation between the bunches is varied so as to encompass approximately one wake field period (∼ 21cm). A spectrum of 40 TM0meigenmodes is excited by the bunch. A substantial retarding wake field (2.65MV/m·nC for just the first bunch) is developed because of the short bunches and the narrow vacuum channel diameter (3mm) through which they move. The energy loss of the second bunch exhibits a narrow resonance with a 4mm (13.5psec) footprint. This experiment may be compared with a related experiment reported by a group at the Argonne National Laboratory where a ∼ 30 times weaker wake field (∼ 0.1MV/m·nC for the first bunch) having ∼ 10 eigenmodes was excited by a train of much longer bunches [1], and the bunch spacing was not varied.