Proton spectra from ultraintense laser–plasma interaction with thin foils: Experiments, theory, and simulation

A beam of high energy ions and protons is observed from targets irradiated with intensities up to ${\text{5×10}}^{19}\hspace{0.17em}{\mathrm{W/cm}}^2.$ Maximum proton energy is shown to strongly correlate with laser-irradiance on target. Energy spectra from a magnetic spectrometer show a plateau region near the maximum energy cutoff and modulations in the spectrum at approximately 65% of the cutoff energy. Presented two-dimensional particle-in-cell simulations suggest that modulations in the proton spectrum are caused by the presence of multiple heavy-ion species in the expanding plasma.