Abstract
A new, maximum proton energy, $e$, scaling law with the laser pulse energy, $E_L$ has been derived from the results of 3D particle-in-cell (PIC) simulations. Utilizing numerical modelling, protons are accelerated during interactions of the femtosecond relativistic laser pulses with the plain semi-transparent targets of optimum thickness [Esirkepov {\it et al.} Phys. Rev. Lett. {\bf 96}, 105001 (2206)]. The scaling, $e \sim E_L^{0.7}$, has been obtained for the wide range of laser energies, different spot sizes, and laser pulse durations. Our results show that the proper selection of foil target optimum thicknesses, results in a very promising increase of the ion energy with the laser intensity even in the range of parameters below the radiation pressure (light sail) regime. The proposed analytical model is consistent with numerical simulations.
Highlights
Ion acceleration by intense ultrashort laser pulses has led to many original applications such as triggering of nuclear reactions [1,2]; research of warm dense matter [3]; laboratory astrophysics [4]; radiography [5,6,7]; fast ignition [8]; and hadron therapy [9]
-called “break-out afterburner” (BOA) [18], is possible but it is relevant to somewhat thicker targets and longer pulses which require laser hole boring during the pulse-foil interaction
The 3D PIC simulations to find optimum target thickness for the laser pulse energy must examine the dependence on different spot sizes and pulse durations. Such optimization should include a systematic study of laser light absorption in semitransparent targets, a study that will form an important part of our paper
Summary
Ion acceleration by intense ultrashort laser pulses has led to many original applications such as triggering of nuclear reactions [1,2]; research of warm dense matter [3]; laboratory astrophysics [4]; radiography [5,6,7]; fast ignition [8]; and hadron therapy [9]. The high contrast pulses of modern lasers [10,11] have enabled effective acceleration of ions from ultrathin foils that are semitransparent to laser light In this regime, where target thickness does not exceed laser skin depth, a high-intensity laser light pulse expels electrons from the irradiated area of the foil in a forward direction. The 3D PIC simulations to find optimum target thickness for the laser pulse energy must examine the dependence on different spot sizes and pulse durations Such optimization should include a systematic study of laser light absorption (i.e., laser to electron energy conversion efficiency) in semitransparent targets, a study that will form an important part of our paper. We will show that our model, which is valid for arbitrary λDe=l, qualitatively explains numerical simulations when the ponderomotive dependence [14] of the effective electron temperature on the absorbed laser energy is used
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