  Laser-driven ion acceleration is an attractive technique for compact
high-energy ion sources. Currently, among various physical and technical issues
to be solved, the boost of ion energy and the reduction of energy spread
represent the key challenges with this technique. Here we present a scheme to
tackle these challenges by using a hundred-terawatt-class laser pulse
irradiating a cone target. Three-dimensional particle-in-cell simulations show
that a large number of electrons are dragged out of the cone walls and
accelerated to hundreds of MeV by the laser fields inside the cone. When these
energetic dense electron beams pass through the cone target tip into vacuum, a
very high bunching acceleration field, up to tens of TV/m, quickly forms.
Protons are accelerated and simultaneously bunched by this field, resulting in
quasi-monoenergetic proton beams with hundred MeV energy and low energy spread
of ~2%. Results exploring the scaling of the proton beam energy with laser and
target parameters are presented, indicating that the scheme is robust. This
opens a new route for compact high-energy proton sources from fundamental
research to biomedical applications.
