  We report stable laser-driven proton beam acceleration from ultrathin foils
consisting of two ion species: heavier carbon ions and lighter protons.
Multi-dimensional particle-in-cell (PIC) simulations show that the radiation
pressure leads to very fast and complete spatial separation of the species. The
laser pulse does not penetrate the carbon ion layer, avoiding the proton
Rayleigh-Taylor-like (RT) instability. Ultimately, the carbon ions are heated
and spread extensively in space. In contrast, protons always ride on the front
of the carbon ion cloud, forming a compact high quality bunch. We introduce a
simple three-interface model to interpret the instability suppression in the
proton layer. The model is backed by simulations of various compound foils such
as carbon-deuterium (C-D) and carbon-tritium (C-T) foils. The effects of the
carbon ions' charge state on proton acceleration are also investigated. It is
shown that with the decrease of the carbon ion charge state, both the RT-like
instability and the Coulomb explosion degrade the energy spectrum of the
protons. Finally, full 3D simulations are performed to demonstrate the
robustness of the stable two-ion-species regime.
