Energetic protons (∼ 1.2 MeV) were generated by irradiation of ultra-short high-power laser pulses onto a 5 μm thick copper tape target and the dependence of the proton energies on the laser intensity and pulse duration were investigated. The laser intensity was varied between 8.5×1017 W/cm2 and 6.6×1018 W/cm2, and the pulse duration was varied from 55 fs to 400 fs. The maximum proton energy Ep_max and proton temperature were proportional to laser intensity, and they increased with the pulse duration when the laser intensity was kept constant. Thus far, Ep_max has been usually scaled as a function of laser intensity, but Ep_max depends not only on the laser intensity, but also on the pulse duration in an ultra-short pulse regime such as several tens of femto-seconds.
