Excitation of wakefields by relativistic electron bunches in the dielectric waveguide filled with radially inhomogeneous plasma

Abstract

One of shortcomings of the dielectric wakefield accelerators (DWA) limiting the effective length of an acceleration is susceptibility of drive bunches to beam breakup (BBU) instability [1]. With the purpose to suppress this instability recently it has been proposed to fill the drift channel of dielectric structure with plasma of certain density [2]. The investigations of plasma DWA (PDWA) [2] were made for dielectric wakefield structures with homogeneous radial plasma density. Such consideration is right if plasma comes to the drift channel from external source.Other, widely way of plasma creation is capillary discharge. For this case on big times from the beginning of discharge (it is more than a half of pulse width of discharge current) steady distribution of plasma density with inhomogeneous distribution of discharge density forms [3]. At short distances from the axis a radial dependence of plasma density can be described with the parabolic dependence [3,4] 2 0 ( )where a is the inner radius of dielectric tube.Investigations on excitation of wakefield in PDWA with and inhomogeneous transverse distribution of plasma density were not carried out so far. For practical application of the proposed accelerating structure it is important to know how the accelerating gradient and focusing forces acting on test bunches will change in case when plasma in the drift channel is created as a result of the capillary discharge.Here we report the results of numerical simulation of wakefield excitation in the dielectric structure filled with plasma created in capillary discharge. Wakefield is built up at injection of electron bunch in dielectric tube with permittivity of 3.8; outside diameter of dielectric tube (is equal to the diameter of metal waveguide) is 1.2 mm, inner diameter is 1.0 mm. Energy of drive bunch electrons is 5 GeV, bunch charge is 3 nC, its length was 0.2 mm, solid bunch has diameter of 0.9 mm. For the testing the excited wakefield we used a witness bunch. Witness bunch has the same energy and sizes as drive bunch, the charge of witness bunch was less by ten times than drive bunch charge.The drift channel (internal area of dielectric tube) is filled with plasma of on-axis density n 0 = 4.41×10 14 cm -3 . Three different models of dependence of plasma density on radius were investigated: a) homogeneous radial plasma density; b) parabolic dependence of plasma density on radius according eq. (1) At numerical simulation by means of the 2.5D PIC code created by us we examined a structure of wakefield and dynamics of electron bunches at their motion in the drift camera. In Fig. 1 axial profiles of the Lorentz wakefield force acting on test electron in dielectric waveguide with plasma filling for the time t = 26.69 ps from the beginning of injection of drive bunch for different dependences of plasma density on radius are shown: a) homogeneous, b) parabolic (1) and c) the BPM dependence [3]. The grey rectangle near output end shows the location of the drive bunch electrons. Vertical arrows have noted the possible pl...

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

“…Note that the case a) corresponds to the absence of plasma in the drift channel, and the case d) -to the complete plasma filling of the said region. From now on the data shown for the case d) have been partially presented in our paper [37]. It can be seen that the cases b) to d) encourage one to choose the position of the accelerated bunch such as to accelerate and focus the bunch simultaneously.…”

“…[35], which can be described by the parabolic relation for a considerable part of the channel section [36]. The PIC simulation of the test electron bunch acceleration with the model plasma-density dependences [35,36] has shown that the capillary discharge plasma improves the focusing of accelerated bunches [37] in comparison with the case of the uniform plasma density in the section.…”

Investigation of Parameters of Electron and Positron Bunches in a Plasma-Dielectric Wakefield Accelerator

“…The parameters of the structure and bunches, used in the simulations are specified in Table . The p nr for two cases: the plasma fills completely the interior of the dielectric tube (dash lines) and the plasma cylinder of internal radius p1 r = 0.2 mm (solid lines). Red line (2) corresponds to the inhomogeneous distribution of plasma density [15], cyan line (1) is for the homogeneous one…”

Acceleration and focusing of positron bunch in a dielectric wakefield accelerator with plasma in transport channel

“…We have analyzed this possibility analytically and numerically for the case of filling the bunch transport channel of DWA with plasma homogeneous in the cross section [26]. If the plasma is created by means of a capillary discharge [27,28], then the transport of witness electron bunches can even be improved [29,30]. To obtain strong focusing fields, a superdense plasma is not required [30].…”