The exact solution of a problem on electromagnetic field excitation by a thin annular charged bunch in a semi-infinite round cylindrical waveguide with metal sidewalls and solid homogeneous dielectric filling is obtained. Expressions for all components of electromagnetic field are derived. These formulas describe the excited field at any point and any moment of time. In contrast to previous works, where asymptotic methods (saddle-point technique) were used, we applied a number of successive conformal transformations of integration area in order to carry out the inverse Fourier transformation. Integration along the initial infinite straight-line contour was substituted by integration along the closed circular contour. This allowed us to separate out the integral presentation of the cylindrical Bessel function of first kind and obtain the final solution in the form of infinite converging series. The process of integration is presented in detail. Both cases, when the Cherenkov resonance condition is satisfied and when this condition is not satisfied, are considered. Spatial pictures of field excited by a finite-size electron bunch are calculated numerically and discussed. In the case of the Cherenkov resonance the drift of excited wake field after the bunch with group velocity is demonstrated, and in the nonresonance case the appearance of impulse of transition radiation and the presence of precursor of the signal are shown.
A new scheme for a dielectric wakefield accelerator is proposed that employs a cylindrical multizone dielectric structure configured as two concentric dielectric tubes with outer and inner vacuum channels for drive and accelerated bunches. Analytical and numerical studies have been carried out for such coaxial dielectric-loaded structures (CDS) for high-gradient acceleration. An analytical theory of wakefield excitation by particle bunches in a multizone CDS has been formulated. Numerical calculations are presented for an example of a CDS using dielectric tubes with dielectric permittivity 5.7, having external diameters of 2.121 and 0.179 mm with inner diameters of 2.095 and 0.1 mm. An annular 5 GeV, 6 nC electron bunch with rms length of 0.035 mm energizes a wakefield on the structure axis having an accelerating gradient of $600 MeV=m with a transformer ratio $8:1. The period of the accelerating field is $0:33 mm. If the width of the drive bunch channel is decreased, it is possible to obtain an accelerating gradient of >1 GeV=m while keeping the transformer ratio approximately the same. Full numerical simulations using a particle-in-cell code have confirmed results of the linear theory and furthermore have shown the important influence of the quenching wave that restricts the region of the wakefield to within several periods following the drive bunch. Numerical simulations for another example have shown nearly stable transport of drive and accelerated bunches through the CDS, using a short train of drive bunches.
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