Abstract
In this study, ion acceleration from thin planar target foils irradiated by ultrahigh-contrast (1010), ultrashort (50 fs) laser pulses focused to intensities of 7×1020 W cm−2 is investigated experimentally. Target normal sheath acceleration (TNSA) is found to be the dominant ion acceleration mechanism when the target thickness is ⩾50 nm and laser pulses are linearly polarized. Under these conditions, irradiation at normal incidence is found to produce higher energy ions than oblique incidence at 35° with respect to the target normal. Simulations using one-dimensional (1D) boosted and 2D particle-in-cell codes support the result, showing increased energy coupling efficiency to fast electrons for normal incidence. The effects of target composition and thickness on the acceleration of carbon ions are reported and compared to calculations using analytical models of ion acceleration.
Highlights
We report on an experimental investigation of carbon ion acceleration by the Target normal sheath acceleration (TNSA) mechanism using one of the currently available highest power (115 TW), ultrahigh-contrast (1010), ultrashort pulse (50 fs) laser systems, operating at average intensities of 7 × 1020 W cm−2
We report on an experimental investigation of the optimization of carbon ion acceleration driven by ultrahigh-contrast (1010), ultrashort (50 fs) laser pulses focused to an average intensity equal to 7 × 1020 W cm−2—about an order of magnitude higher intensity than previous ion acceleration experiments using laser pulses with tens of femtoseconds duration
Higher laser energy transfer to ions is obtained for irradiation at normal incidence compared to oblique incidence at 35◦
Summary
The experiment was performed using the Astra-Gemini laser at the Rutherford Appleton Laboratory. The charge-to-mass ratio and energy distributions of the accelerated ions were measured using two Thomson parabola ion spectrometers, positioned along the target normal direction for each incident angle, as shown in figure 1. They had a line of sight to the laser focal spot in the plane of the laser beam axis and the target normal axis. The spatial intensity distribution of the lower half (just below the plane of the spectrometers and the target normal axis) of the proton beam was measured, for protons with energy above a lower detection limit of 5 MeV (defined by the thickness of a light-shield aluminium filter), using a plastic scintillator and gated CCD imaging system. The aluminium filter stops heavier ions from reaching the scintillator and protects it from the target debris
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