Effect of fluctuations in the down ramp plasma source profile on the emittance and current profile of the self-injected beam in a plasma wakefield accelerator

C Zhang,F Li,V Yakimenko,C Joshi,W B Mori,S Corde,X L Xu,C.-K Huang,K A Marsh,M Hogan

doi:10.1103/physrevaccelbeams.22.111301

Abstract

With its extreme beam parameters, the FACET II facility enables the test of the down ramp injection scheme in the laboratory for the beam-driven plasma wakefield accelerator approach. In addition to the ideal cases studied in previous theoretical work, we investigate the effect of fluctuations in the down ramp plasma source profile on the emittance and current profile of the self-injected beam using 2D and 3D particle-in-cell simulations. We show that down ramps with a length of $\ensuremath{\sim}10\text{ }\text{ }c/{\ensuremath{\omega}}_{p}$ can be reproducibly created by generating a shock in a supersonic flow. Simulations show that the emittance of the injected beam using such down ramps is $\ensuremath{\sim}0.1\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. A gentler ramp can further reduce the slice emittance of the beam to $\ensuremath{\sim}0.03\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. The emittance of the injected beam depends on the ramp length but is insensitive to the shape of the ramp. However, the current profile of the injected beam can be manipulated by changing the ramp shape. A simulation shows that, when a noncylindrical driver is used, the emittance of the injected beam differs in the two orthogonal transverse planes and is a few times larger than that of the cylindrical driver case. A full-scale simulation where we use a realistic density profile that includes the up ramp, the injecting down ramp, and the exiting down ramp is presented to show that the beam emittance is preserved during the acceleration and extraction of the beam.

Highlights

The rapid development of plasma-based accelerators is opening new possibilities in both fundamental and applied research by providing tools of scientific discovery
In addition to the ideal cases studied in previous theoretical work, we investigate the effect of fluctuations in the down ramp plasma source profile on the emittance and current profile of the self-injected beam using 2D and 3D particle-in-cell simulations
We have investigated the generation of ultralowemittance, high-brightness beams using a high-energy, high-current beam driver such as that available at FACET II to excite a wake across a density down ramp

Summary

INTRODUCTION

The rapid development of plasma-based accelerators is opening new possibilities in both fundamental and applied research by providing tools of scientific discovery. It was proposed that self-injection triggered by a density down ramp has the potential of generating electron beams with a slice emittance as low as ∼10 nm and a brightness up to 1021 A=m2=rad2 [23]. We present simulation results to address the latter part of the question, namely, to show the effects of the plasma source itself on the sensitivity and reproducibility of the down ramp injection To this end, we have characterized the reproducibility of the density down ramps of interest and used particle-in-cell code simulations to quantify fluctuations of the emittance of the injected beam and its growth during the acceleration process and eventual extraction using a second longer down ramp that acts as a matching section. We show through a 2D simulation that the emittance of the injected beam is preserved in the acceleration and the matching section, which makes it possible to capture, transport, and characterize the beam downstream using conventional beam diagnostics

REPRODUCIBLE DOWN RAMP PLASMA SOURCE

SENSITIVITY OF RAMP VARIATIONS ON EMITTANCE AND CURRENT PROFILE

NONCYLINDRICAL DRIVER

FULL-SCALE SIMULATION

CONCLUSIONS