  High energy ion beams (> MeV) generated by intense laser pulses promise to be
viable alternatives to conventional ion beam sources due to their unique
properties such as high charge, low emittance, compactness and ease of beam
delivery. Typically the acceleration is due to the rapid expansion of a laser
heated solid foil, but this usually leads to ion beams with large energy
spread. Until now, control of the energy spread has only been achieved at the
expense of reduced charge and increased complexity. Radiation pressure
acceleration (RPA) provides an alternative route to producing laser-driven
monoenergetic ion beams. In this paper, we show the interaction of an intense
infrared laser with a gaseous hydrogen target can produce proton spectra of
small energy spread (~ 4%), and low background. The scaling of proton energy
with the ratio of intensity over density (I/n) indicates that the acceleration
is due to the shock generated by radiation-pressure driven hole-boring of the
critical surface. These are the first high contrast mononenergetic beams that
have been theorised from RPA, and makes them highly desirable for numerous ion
beam applications.
