Monoenergetic Proton Beams Accelerated by a Radiation Pressure Driven Shock

We report on the acceleration of impurity-free quasimononenergetic proton beams from an initially gaseous hydrogen target driven by an intense infrared (lambda = 10 mu m) laser. The front surface of the target was observed by optical probing to be driven forward by the radiation pressure of the laser. A proton beam of similar to MeV energy was simultaneously recorded with narrow energy spread (sigma similar to 4%), low normalized emittance (similar to 8 nm), and negligible background. The scaling of proton energy with the ratio of intensity over density (I/n) confirms that the acceleration is due to the radiation pressure driven shock.