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

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 ∼10 c=ω 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 ∼0.1 μm. A gentler ramp can further reduce the slice emittance of the beam to ∼0.03 μm. 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.

All-optical density downramp injection in electron-driven plasma wakefield accelerators

“…However, the witness beam emittances produced from the more realistic, asymmetric driver beam exceed the particularly low emittance produced for the perfectly symmetric drive beam substantially. Similar emittance increase has been observed in [48] for asymmetric driver beams. This configuration and the analysis of resulting witness beam emittance in different planes is discussed further in Fig.…”

“…LWFA ionization injection methods [41], successful experimental realization of downramp injection in the dephasing-free PWFA has not been achieved until very recently [17]. This is despite that a large fraction of seminal downramp injection theory work was delivered in context of PWFA [31,44], its potential as highbrightness electron beam source [45] had been discovered, and many further theoretical and simulation-based studies with gentle [46][47][48][49] and steeper [50] ramps have since been carried through.…”

Controlled density-downramp injection in a beam-driven plasma wakefield accelerator

“…If trapping is avoided except on the density downramp, the injection process can be controlled and densitydownramp injection characterised in detail. Numerical studies with 3D particle-in-cell (PIC) codes predict that density-downramp injection with a sufficiently sharp gradient produces electron beams with transverse normalized emittances below 1 µm [35][36][37][38]. Such sharp density downramps can be achieved either hydrodynamically, as commonly used in laser-driven plasma wakefield acceleration [39][40][41], or by ionization of distinct gas species with two perpendicularly focused laser arms [28,29]; the latter is the approach pursued in this work.…”

Terawatt attosecond X-ray source driven by a plasma accelerator

“…The variation of the chirp due e.g. to variations in the plasma density or the length of the accelerating section [50], is reduced in the asymptotic limit. This reduces the fluctuations of the bunch length after compression and consequently improves the stability of the X-ray pulses whose properties are determined by the incoming electron beam profile.…”