  Simulations of proton-driven plasma wakefield accelerators have demonstrated
substantially higher accelerating gradients compared to conventional
accelerators and the viability of accelerating electrons to the energy frontier
in a single plasma stage. However, due to the strong intrinsic transverse
fields varying both radially and in time, the witness beam quality is still far
from suitable for practical application in future colliders. Here we
demonstrate efficient acceleration of electrons in proton-driven wakefields in
a hollow plasma channel. In this regime, the witness bunch is positioned in the
region with a strong accelerating field, free from plasma electrons and ions.
We show that the electron beam carrying the charge of about 10% of 1 TeV proton
driver charge can be accelerated to 0.6 TeV with preserved normalized emittance
in a single channel of 700 m. This high quality and high charge beam may pave
the way for the development of future plasma-based energy frontier colliders.
