Transient Beam Loading in Electron Linear Accelerators

An experiment has been devised to measure directly the beam loading in electron linear accelerators operating with very short beam pulses. A well momentum analyzed and phase bunched electron beam is sent through either an unexcited waveguide or a vacuum pipe. Electron energy loss due to beam loading in the waveguide is detected by measuring the resulting energy shift of elastically scattered electrons with the NBS electron scattering spectrometer. Energy shifts of a few parts in 105 can be measured at 60 MeV. Measurements made to date at 61 MeV and for beam pulse lengths of 5, 10, and 20 nanoseconds appear to confirm the existence of enhanced beam loading relative to the usual non-dispersive linac theory. An attempt is made to compare the measurements to theoretical estimates of this enhancement.