MDPI : Acceleration of an Electron Bunch with a Non–Gaussian Transverse Profile in Proton-Driven Plasma Wakefield

Liang, Linbo; Xia, Guoxing; Farmer, John Patrick; Pukhov, Alexander

doi:10.3390/app122110919

Article
Report number	arXiv:2208.04585
Title	Acceleration of an Electron Bunch with a Non–Gaussian Transverse Profile in Proton-Driven Plasma Wakefield
Related title	Acceleration of an electron bunch with a non-Gaussian transverse profile in a quasilinear plasma wakefield
Author(s)	Liang, Linbo (Manchester U. ; Daresbury) ; Xia, Guoxing (Manchester U. ; Daresbury) ; Pukhov, Alexander (Heinrich Heine U., Dusseldorf) ; Farmer, John Patrick (CERN ; Munich, Max Planck Inst.)
Publication	2022-10-27
Imprint	2022-08-09
Number of pages	14
In:	Appl. Sciences 12 (2022) 10919
DOI	10.3390/app122110919
Subject category	Accelerators and Storage Rings
Abstract	Beam-driven plasma wakefield accelerators typically use the external injection scheme to ensure controllable beam quality at injection. However, the externally injected witness bunch may exhibit a non-Gaussian transverse density distribution. Using particle-in-cell simulations, we show that the common beam quality factors, such as the normalized RMS emittance and beam radius, do not strongly depend on the initial transverse shapes of the witness beam. Nonetheless, a beam with a highly-peaked transverse spatial profile can achieve a higher fraction of the total beam charge in the core. The same effect can be seen when the witness beam's transverse momentum profile has a peaked non-Gaussian distribution. In addition, we find that an initially non-axisymmetric beam becomes symmetric due to the interaction with the plasma wakefield, and so it does not cause a detrimental effect for the beam acceleration.
Copyright/License	preprint: (License: CC BY-NC-SA 4.0) publication: © 2022-2025 The authors (License: CC-BY-4.0)

$The simplified scheme of AWAKE Run 2 acceleration stage. The densities of plasma electrons ($n_{e}$, green), the proton driver bunch ($n_{pb}$, pink) and the electron witness bunch ($n_{eb}$, black) are shown in colourmaps, with values normalized by the unperturbed plasma electron density is $n_0=7\times10^{14}$ $\mathrm{cm^{-3}}$. Particle beams propagate from left to right. $\xi=z-ct$ is the longitudinal coordinate in the co-moving frame. The blue solid line represents the loaded longitudinal wakefield $E_z$, and the red dashed line is the transverse wakefield $W_x=E_x-cB_y$ at one $\sigma_{re}$ off the longitudinal axis. $\sigma_{re}$ is the RMS beam radius of the electron witness bunch.$ $Plasma wakefields for the low-charge (120 pC) case in colourmaps. (a) The longitudinal wakefield $E_z$. The 1D lineout shows the on-axis value of $E_z$, whose value range is the same as the right colorbar. (b) The transverse wakefield $W_x=E_x-cB_y$. The dashed line and the dotted line show the value at $r=\sigma_{re}$ and $r=2\sigma_{re}$, respectively. The initial loading position $\xi_{0e}$ (or the longitudinal centroid) of the witness electron bunch, shown by vertical grey dash-dotted lines in both plots.$ $Evolution of the Gaussian witness beam characteristics as functions of the beam propagation distance $s$. (a) The average beam energy (blue lines) and the relative energy spread $\sigma_{\gamma_e}/\left<\gamma_e\right>$ (black lines). $\left<\gamma_e\right>$ is average Lorentz gamma factor that represents the beam energy. (b) The RMS transverse size $\sigma_r$ (blue lines) and the normalized projected emittance $\epsilon_n$ (black lines). Results of both the low-charge (120 pC, denoted by dash-dotted lines) and high-charge (400 pC, denoted by solid lines) cases are shown.$ Show more plots

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