Focusing of Drive and Test Bunches in a Dielectric Waveguide Filled with Inhomogeneous Plasma

The paper presents the results of numerical PIC-simulation of accelerated (test) and drive bunches dynamics in a dielectric waveguide of the THz frequency range filled with radially inhomogeneous plasma. The wakefield was excited by the azimuthally-symmetric on-axis drive electron bunch in a quartz dielectric tube embedded in metal waveguide. A radial displacement of test bunch is absent too. At simulations of plasma, we used different transverse density profiles, viz., the density profile formed in the capillary discharge, and the radially nonuniform density profile with the vacuum channel along the waveguide axis. For all the cases under study the plasma density was low, so that the plasma frequency was lower than the fundamental dielectric mode frequency. The obtained PIC-simulation data have shown that the vacuum channel in the inhomogeneous plasma cylinder improves the accelerated bunch focusing. There is the optimum vacuum-channel size value, at which the focusing turns out to be the strongest. The improvement in the accelerated bunch focusing is accompanied by the decrease in the accelerating gradient as compared with the full plasma filling of the drift channel. The best acceleration takes place in the absence of plasma; however, in that case the accelerated bunch focusing does not occur.