  Plasma wakefield accelerators are capable of sustaining
gigavolt-per-centimeter accelerating fields, surpassing the electric breakdown
threshold in state-of-the-art accelerator modules by 3-4 orders of magnitude.
Beam-driven wakefields offer particularly attractive conditions for the
generation and acceleration of high-quality beams. However, this scheme relies
on kilometer-scale accelerators. Here, we report on the demonstration of a
millimeter-scale plasma accelerator powered by laser-accelerated electron
beams. We showcase the acceleration of electron beams to 130 MeV, consistent
with simulations exhibiting accelerating gradients exceeding 100 GV/m. This
miniaturized accelerator is further explored by employing a controlled pair of
drive and witness electron bunches, where a fraction of the driver energy is
transferred to the accelerated witness through the plasma. Such a hybrid
approach allows fundamental studies of beam-driven plasma accelerator concepts
at widely accessible high-power laser facilities. It is anticipated to provide
compact sources of energetic high-brightness electron beams for
quality-demanding applications such as free-electron lasers.
