Observation of a Narrow Spectral Bandwidth in Proton Beam with High Energy Conversion Efficiency Driven by an Ultrashort High-Intensity Ti:Sapphire Laser

Energetic protons with energies as high as 2.4 MeV and energy conversion efficiency from laser to proton of >1.4% are produced by focusing an ultrashort high-intensity Ti:sapphire laser pulse. We observe a narrow bandwidth structure in the proton spectra having a small energy spread with full width at half maximum of about 7% of its absolute value near 1.9 MeV by using a collimator in front of proton energy spectrometer.