Fig. 1: Experimental setup and generated proton beams.

a Illustration of the experimental setup, showing the laser-water interaction and the primary diagnostics. Each laser pulse contained up to 200 mJ in a pulse length of (57 ± 5)fs (FWHM) and was focused to a focal spot waist of (1.2 × 1.4) μm², giving a peak intensity of I₀ = (3.5 ± 0.4) × 10¹⁹W cm⁻². The laser was focused onto a thin ((600 ± 100) nm) sheet of water generated by a converging nozzle geometry in a vacuum chamber. b Example proton dose distributions for the beams produced from single shots with the 12.7 μm thick Kapton tape target and c the water sheet using comparable laser settings and the same detector screen. The dashed white line highlights the edge of the scintillator screen and the horizontal and vertical stripes are due to aluminium filters which blocked lower energy protons. The dark oval was a hole in the scintillator screen permitting a line-of-sight for the time-of-flight (TOF) diode, as indicated by the red '+'. d The average proton spectra, recorded by the TOF diagnostic, were calculated from several shots at the same conditions as (b) (red—average of 20 shots) and (c) (blue—average of 50 shots). The spectra are normalised to peak flux observed on the scintillator screen. The solid lines show the average spectrum, while the shaded region shows the rms shot-to-shot variations.