Wakefield excitation in dielectric wavguides by a sequence of relativistic electron bunches

At the elaboration of the concept of the multi-bunch dielectric wakefield accelerator¹ the investigations of the three issues - multi-bunch, multimode, and resonator schemes, on which the concept is based to enhance the acceleration rate, should be carried out. This presentation gives the results of theoretical and experimental studies of the first scheme, namely, wakefield excitation in dielectric structures by a long sequence of relativistic electron bunches in order to increase the amplitude of the wakefield at summing coherent fields of separate bunches when the bunch repetition frequency coincides with the excited field frequency. It is theoretically shown that in the dielectric waveguide of length L the excited wakefield, consisting of Cerenkov and transition radiation goes out from waveguide with the group velocity νg, so that the summation of fields, leading to an increase the total field amplitude, occurs only for the number of bunches Nmax=(1-νg/ν0)L/d, ν0 - velocity of the bunch, d - distance between the bunches. For experimental verification of this result the facility has been created, in which to excite the wakefield in a tubular dielectric waveguide (inner and outer diameters are 2.1cm 8.5cm, the permittivity is 2.1) with a copper jacket, a sequence 6×10³ of electron bunches (charge 0.32nC, energy 4.5MeV, length 1.7cm, diameter 1.0cm, each), produced by linear resonance accelerator “Almaz”, has been used. In accordance with the theory linearly increasing dependence of the wakefield amplitude upon the dielectric waveguide length is obtained. Stepwise dependence of Cherenkov radiation is smoothed by the inevitable transition radiation occurring at the entrance. For a waveguide with reflections (SWR=2–3) on the increasing dependence the resonant peaks take place corresponding to the waveguide length multiple to the- excited wave length.