  Plasma-based accelerators (PBAs) driven by either intense lasers (laser
wakefield accelerators, LWFAs) or particle beams (plasma wakefield
accelerators, PWFAs), can accelerate charged particles at extremely high
gradients compared to conventional radio-frequency (RF) accelerators. In the
past two decades, great strides have been made in this field, making PBA a
candidate for next-generation light sources and colliders. However, these
challenging applications necessarily require beams with good stability, high
quality, controllable polarization and excellent reproducibility. To date, such
beams are generated only by conventional RF accelerators. Therefore, it is
important to demonstrate the injection and acceleration of beams first produced
using a conventional RF accelerator, by a PBA. In some recent studies on LWFA
staging and external injection-acceleration in PWFA only a very small fraction
(from below 0.1% to few percent) of the injected charge (the coupling
efficiency) was accelerated. For future colliders where beam energy will need
to be boosted using multiple stages, the coupling efficiency per stage must
approach 100%. Here we report the first demonstration of external injection
from a photocathode-RF-gun-based conventional linear accelerator (LINAC) into a
LWFA and subsequent acceleration without any significant loss of charge or
degradation of quality, which is achieved by properly shaping and matching the
beam into the plasma structure. This is an important step towards realizing a
high-throughput, multi-stage, high-energy, hybrid conventional-plasma
accelerator.
