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

Abstract

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.

High transformer ratio of multi-channel dielectric wakefield structures

“…The theory (analytical models and numerical simulations) of the single-channel DWAs is quite mature, and has been confirmed and validated by a substantial number of experiments; some examples can be found in [5,6], [10], [14,15].…”

Survey of advanced dielectric wakefield accelerators

“…6. Several experiments have aimed at going a step further, trying to explore more subtle aspects of superposition of fields and waveguide modes [6][7][8].…”

Beam-driven dielectric wakefield accelerating structure as a ThZ radiation source

“…The fundamental theorem of beam loading limits the magnitude of the accelerating gradient seen by the witness bunch to less than or equal to twice the * Work supported by the United States Department of Energy † alancook@physics.ucla.edu decelerating gradient seen by the driving bunch, in the case of a symmetric driving bunch [3]. Experimental and theoretical work done by researchers at Argonne National Laboratory [4,5], and work published more recently [6,7,8,9], has predicted and measured the superposition of beam-driven wakefields in cylindrical dielectric structures. A recent preliminary experimental study by a UCLA/SLAC/LLNL collaboration has used the ultra-short, high-energy beam at the SLAC FFTB facility to explore the GV/m regime of dielectric accelerating structures.…”