Fig. 2: The effect of the μ-bar width.

a Experimentally measured differential proton energy spectra emitted from d = 0.2 μm thick Au foils (black) and μ-bar targets (color scale). The insets in (a) present example raw TPIS traces resulting from irradiation of a foil target and a w = 1.8 μm wide μ-bar. b Compilation of the proton cutoff energies from experiments of various laser and target parameters, as a function of the laser pulse energy (gray) adapted from ref. ⁸. The results of this work are shown with the μ-bar width indicated in color. c Simulated differential proton energy spectra for the same experimental parameters as in (a), overlaid with snapshots of the transverse electric field (blue-to-red), electron density (green) and space averaged electron and proton energy (color scale). d–f Measured and simulated proton energy cutoffs, peak-values of the electric field in the sheath, and the cone angles of protons emitted with E > 0.5 MeV plotted vs. w. Systematic uncertainties of 5% and 12% are associated with the fitting procedures of the energy cutoffs and cone angles respectively. g Energy resolved virtual source distribution of the protons and (h) the RMS of the emittance for a d = 0.2 μm, w = 2 μm μ-bar.