Wakefield acceleration in a layered plasma waveguide

Plasma wakefield accelerators (PWFA) represent one of the promising new accelerator concepts that are now being developed intensively for future applications in high-energy physics and industry. Among the unresolved problems of practical implementation of PWFA there are maintaining the required quality (emittance, size, energy spread of bunches) and stable transportation of drive and accelerated (witness) bunches over long acceleration distances. For improving the bunch transport, we propose in this work that the plasma channel method should be modified through filling the bunch transport channel with the background plasma, the density of which is lower than the main plasma density building up an accelerating wake wave. We call this waveguide structure with a non-uniform transverse plasma profile a layered plasma waveguide (LPW). The wakefield excitation by a single electron bunch and a regular sequence of electron bunches in the LPW of cylindrical configuration has been explored both analytically and numerically. The layered plasma has been modeled as a combination of a tubular plasma and a plasma column of significantly different densities. The plasma column has a lower density. In the linear approximation of plasma dynamics, analytical expressions for the excited longitudinal and transverse wakefields were obtained. The dispersion dependencies of the TM-modes of the LPW were obtained and analyzed, and it was found that there was a single TM wave resonant with the electron bunch. Based on the obtained analytical expressions, the structures of the axial and radial wakefield amplitudes have been numerically investigated for the cases of a single drive bunch and a regular train of bunches. It is shown that for certain density ratios of the outer and inner plasmas, it is possible both to accelerate and focus simultaneously the drive and witness bunches. A spectral amplitude–frequency analysis of the excited wakefields has been carried out. The 2.5D particle-in-cell code was used to simulate the witness acceleration of an electron bunch by a wakefield created by drive electron bunches in a two-layer PWFA. The simulation showed good agreement with the results of analytical calculations. Analytical studies of wakefield excitation, as well as PIC numerical simulations of the dynamics of accelerated bunches in short layered plasma wake accelerator (LPWA) sections, have shown that LPWA has the potential to simultaneously accelerate and focus both electron and positron witness bunches. Further simulations of significantly longer LPWA accelerator modules are required to determine the final qualitative characteristics of the accelerated bunches.