Abstract
Thin Al foils (50 nm and 6 μm) were irradiated at intensities of up to 2x 1019 W cm−2 using high contrast (108) laser pulses. Ion emission from the rear of the targets was measured using a scintillator-based Thomson parabola and beam sampling ‘footprint’ monitor. The variation of the ion spectra and beam profile with focal spot size was systematically studied. The results show that while the maximum proton energy is achieved around tight focus for both target thicknesses, as the spot size increases the ion flux at lower energies is seen to peak at significantly increased spot sizes. Measurements of the proton footprint, however, show that the off-axis proton flux is highest at tight focus, indicating that a previously identified proton deflection mechanism may alter the on-axis spectrum. One-dimensional particle-in-cell modelling of the experiment supports our hypothesis that the observed change in spectra with focal spot size is due to the competition of two effects: decrease in laser intensity and an increase in proton emission area.
Highlights
Results, and discuss/interpret the experimental results in light of this
Measurements have been made of the proton spectra produced from thin Al foils when irradiated with a high contrast laser pulse at intensities of up to 2 × 1019 W cm−2
The laser focal spot diameter was varied from a minimum of 4 × 6 μm up to a maximum of 0.5 mm
Summary
The experiment was carried out using the Ti : sapphire Astra laser system, which produced 40 fs pulses of energy up to 500 mJ. A scintillator screen was inserted partially into the ion beam so as not to block the beam going to the main spectrometer and the emission imaged onto a gated CCD to give an indication of the ‘footprint’ distribution [18] of the ion beam size. This scintillator was coated with a 0.2 μm layer of Al to prevent scattered plasma light entering the detector and was situated to look from 6◦ to 50◦ off-target normal. A fast decay (0.5 ns) scintillator was used and the gate time set so that the initial emission due to fast electrons and x-rays was eliminated while protons in the 0.1–1 MeV range were detected
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