  A scheme with gold cone-capillary is proposed to improve the protons
acceleration and involved problems are investigated by using the
two-dimensional particle-in-cell simulations. It is demonstrated that the
cone-capillary can efficiently guide and collimate the protons to a longer
distance and lead to a better beam quality with a dense density $\geq10n_c$,
monoenergetic peak energy $E_k \sim 1.51~\mathrm{GeV}$, spatial emittance
$\sim0.0088~\mathrm{mm}~\mathrm{mrad}$ with divergence angle $\theta \sim
1.0^{\circ}$ and diameter $\sim 0.5\mathrm{\mu m}$. The enhancement is mainly
attributed to the focusing effect by the transverse electric field generated by
the cone as well as the capillary, which can prevent greatly the protons from
expanding in the transverse direction. Comparable to without the capillary, the
protons energy spectra have a stable monoenergetic peak and divergence angle
near to $1.0^{\circ}$ in longer time. Besides, the efficiency of acceleration
depending on the capillary length is explored, and the optimal capillary length
is also achieved. Such a target may be benefit to many applications such as
ions fast ignition in inertial fusion, proton therapy in medicine and so on.
