Wakefield spectra in the plasma-dielectric accelerator when changing the plasma density

The analysis of the changes in the spectra of a wake field excited in a cylindrical plasma-dielectric waveguide by relativistic electron bunch is carried out. Three variants of structures are considered: the parameters of the dielectric structure and bunches are fixed; inner or outer radius of the dielectric tube is changed, so that the frequency of the first radial mode coincides with the frequency of the plasma wave. The latter two options are necessary in the case of a regular sequence of bunches to increase the amplitude of the wakefield. It is shown that in case of changes in the outer radius an increase in amplitude of the dielectric wave is due to the transformation of the multimode dielectric wave in a monochromatic wave, with the first harmonic, synchronous with the sequence of bunches.