Abstract
Responses of the uniform near-critical plasma (UNCP) and nano-porous near-critical plasma (NPNCP) upon interaction with a short-intense laser have been scrutinized using two-dimensional (2D) particle-in-cell simulations. Maximum proton energy variation by the deposition of uniform and nano-porous layers in front of a solid target for a wide range of laser intensities (normalized amplitude a0 = 5–25) and average densities of the front layer ne = 0.3 − 3nc (where nc is the critical density) has been parametrically studied. It is found that the proton maximum energy for the front layers with sub-10 µm thicknesses is independent of the target porosity and density. However, in the relatively thick targets, the nano-porous structure decreases the laser energy absorption and, subsequently, the maximum proton energy compared to the uniform one. The results indicate that by employing UNCPs instead of NPNCPs, at the moderate laser intensity, the maximum proton energy reveals a 23% enhancement. This increment could be explained by rapid self-focusing of the laser pulse and dominant direct laser electron acceleration regime on the well-formed plasma channel in the UNCP layer. However, in the case of NPNCPs, the laser scattering from the plasma structure makes it less intense and more disordered, which influences the efficient laser energy coupling to the electrons.
Highlights
Remarkable progress in the high-power laser technology has triggered fast growth of laser driven particle sources
It means that for a specific laser intensity irradiated with the front foam layer at almost near critical density, the proton maximum acceleration at rather low thicknesses is independent of the front layer density
In other words, boosting the laser intensity leads to an increase in the proton maximum energy this enhancement is slower at the higher intensities
Summary
Remarkable progress in the high-power laser technology has triggered fast growth of laser driven particle sources. The plasma critical density increases from nc√to γnc, where γ for the linear polarized laser pulse is defined as γ = 1 + a20/2.23,24 The interaction of the laser beam with electrons is greatly influenced by the target density. There are a number of comprehensive studies in which the effect of thin (few to tens of micrometers) low density foam deposition in the front of the solid foil in the TNSA approach has been examined Their results showed that by employing NCPs, the proton maximum energy increased more than two times compared to the cases with bare solid foils.. This is observed experimentally by Bin et al.27 They reported about three times enhancement in the carbon ion energy by employing a spatially well-defined near-critical density plasma..
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