  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.
