Abstract
The collimating effect of self-generated magnetic fields on fast-electron transport in solid aluminium targets irradiated by ultra-intense, picosecond laser pulses is investigated in this study. As the target thickness is varied in the range of 25 μm to 1.4 mm, the maximum energies of protons accelerated from the rear surface are measured to infer changes in the fast-electron density and therefore the divergence of the fast-electron beam transported through the target. Purely ballistic spreading of the fast-electrons would result in a much faster decrease in the maximum proton energy with increasing target thickness than that measured. This implies that some degree of ‘global’ magnetic pinching of the fast-electrons occurs, particularly for thick (>400 μm) targets. Numerical simulations of electron transport are in good agreement with the experimental data and show that the pinching effect of the magnetic field in thin targets is significantly reduced due to disruption of the field growth by refluxing fast-electrons.
Highlights
The collimating effect of self-generated magnetic fields on fastelectron transport in solid aluminium targets irradiated by ultra-intense, picosecond laser pulses is investigated in this study
Ballistic spreading of the fast-electrons would result in a much faster fall in the maximum proton energy, and this implies that some magnetic pinching of the fast-electrons occurs
The laser pulse parameters were fixed throughout the experiment
Summary
The experiment was performed at the Rutherford Appleton Laboratory using the Vulcan laser, delivering pulses with energy (on the target), EL, up to 280 J, duration, τL, equal to 0.7 ps (fullwidth at half-maximum (FWHM)) and wavelength equal to 1.053 μm. The most salient result of our investigation is the measured decrease in the maximum proton energy, Emax, with increasing L, shown in figure 1 (for which good agreement is found between the RCF and Thomson parabola spectrometer measurements) This measurement differs sharply from what is expected on the basis of simple ballistic transport and what has been reported previously for lower laser pulse energies and intensities [11, 12]. If the electron beam divergence decreases with increasing energy, the increased stopping of low-energy electrons within the target may reduce the overall transverse extent of the fast-electron distribution as the target thickness is increased To investigate whether this effect could account for the measured distribution, we calculate the expected Emax as a function of L assuming different distributions of electron divergence angle with energy and incorporating electron stopping.
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