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 (PIC) calculations using the code OSIRIS. In these simulations a high-current-density electron-beam driver excites plasma waves in the blow-out regime inside a fully-ionized hydrogen plasma of density $5\times10^{17} \mathrm{cm^{-3}}$. Within an embedded $100 \mathrm{\mu m}$ 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 $1.5 \mathrm{kA}$ peak current, $1.5 \mathrm{\mu m}$ transverse normalized emittance, an uncorrelated energy spread of 0.3% on a GeV-energy scale, and few femtosecond bunch length.