Experimental observation of constructive superposition of wakefields generated by electron bunches in a dielectric-lined waveguide

We report results from an experiment that demonstrates the successful superposition of wakefields excited by 50 MeV bunches which travel ∼50  cm along the axis of a cylindrical waveguide which is lined with alumina. The bunches are prepared by splitting a single laser pulse prior to focusing it onto the cathode of an rf gun into two pulses and inserting an optical delay in the path of one of them. Wakefields from two short (5–6 psec) 0.15–0.35 nC bunches are superimposed and the energy loss of each bunch is measured as the separation between the bunches is varied so as to encompass approximately one wakefield period (∼21   cm). A spectrum of ∼40   TM_0m eigenmodes is excited by the bunch. A substantial retarding wakefield (2.65   MV/m·nC for just the first bunch) is developed because of the short bunch length and the narrow vacuum channel diameter (3 mm) through which they move. The energy loss of the second bunch exhibits a narrow peak when the bunch spacing is varied by only 4 mm (13.5 psec). This experiment is compared with a related experiment reported by a group at the Argonne National Laboratory where the bunch spacing was not varied and a much weaker retarding wakefield (∼0.1   MV/m·nC for the first bunch) comprising only about 10 eigenmodes was excited by a train of long (∼9   mm) bunches.