High-Quality Electron Beams from Beam-Driven Plasma Accelerators by Wakefield-Induced Ionization Injection

We propose a new and simple strategy for controlled ionization-induced trapping of electrons in a beam-driven plasma accelerator. The presented method directly exploits electric wakefields to ionize electrons from a dopant gas and capture them into a well-defined volume of the accelerating and focusing wake phase, leading to high-quality witness bunches. This injection principle is explained by example of three-dimensional particle-in-cell calculations using the code OSIRIS. In these simulations a high-current-density electron-beam driver excites plasma waves in the blowout regime inside a fully ionized hydrogen plasma of density $5\times {10}^{17}{\mathrm{cm}}^{-3}$. Within an embedded $100\mu \mathrm{m}$ long plasma column contaminated with neutral helium gas, the wakefields trigger ionization, trapping of a defined fraction of the released electrons, and subsequent acceleration. The hereby generated electron beam features a 1.5 kA peak current, $1.5\mu \mathrm{m}$ transverse normalized emittance, an uncorrelated energy spread of 0.3% on a GeV-energy scale, and few femtosecond bunch length.

Figure 1

Schematic of the plasma-cell setup used in OSIRIS 3D simulations. A thin jet of a neutral $\mathrm{H}/\mathrm{He}$ gas mixture is immersed in a laser preionized hydrogen plasma at $n_0$.

Figure 2

Central slices from a 3D OSIRIS simulation depicting the trapping conditions and showing ionization of electrons from a He dopant by wakefields excited by a FACET-type beam in a precreated uniform plasma with a density of $n_e=5\times {10}^{17}{\mathrm{cm}}^{-3}$. (a) Spatial particle density. Plasma (middle palette), driver beam (bottom palette), and electrons ionized from He (top palette). (b) Total electric field and on-axis values (solid line). Light (dark) dashed line shows the ionization threshold of the outer (inner) He electron. (c) ADK ionization probability of the first level of He, the contour at 10% probability (dashed line), and the on-axis values (solid line). (d) The electric potential $\Psi -{\Psi }_t$, its contours in steps of $\Delta \Psi =0.2\times (m{c}^2/e)$, and the on-axis values (solid line). Vertical dotted lines show the limits of the He column.

Figure 3

PIC simulation after 20 mm of beam propagation. (a) Spatial particle density as in Fig. 2a. The curves show currents of the drive beam (right) and injected electrons (left). (b) Longitudinal wakefields.

Figure 4

Witness bunch properties after 20 mm of acceleration. (a) shows the absolute charge distribution of the bunch in longitudinal phase space ($p_z$ vs $\zeta$ plane). The projection of this distribution in $p_z$ is depicted on the left axis. (b) displays the bunch-current dependence on $\zeta$. The relative energy spread and the transverse emittance are plotted for different longitudinal slices along the bunch.