  Injection of well-defined, high-quality electron populations into plasma
waves is a key challenge of plasma wakefield accelerators. Here, we report on
the first experimental demonstration of plasma density downramp injection in an
electron-driven plasma wakefield accelerator, which can be controlled and tuned
in all-optical fashion by mJ-level laser pulses. The laser pulse is directed
across the path of the plasma wave before its arrival, where it generates a
local plasma density spike in addition to the background plasma by tunnelling
ionization of a high ionization threshold gas component. This density spike
distorts the plasma wave during the density downramp, causing plasma electrons
to be injected into the plasma wave. By tuning the laser pulse energy and
shape, highly flexible plasma density spike profiles can be designed, enabling
dark current free, versatile production of high-quality electron beams. This in
turn permits creation of unique injected beam configurations such as
counter-oscillating twin beamlets.
