Energy spectra of fast electrons, generated when high-intensity laser pulses irradiated hollow conical targets, have been measured experimentally. It is shown here that the slope temperature of the fast electrons is strongly dependent on the opening angle of the cone, and has a maximum value at 25°. The data confirms optical guiding of the laser pulse, by comparison of the measured electron temperature with ray-tracing calculations that include absorption in plasmas. The enhanced energy flow and intensity induced by optical guiding of the laser pulse inside the cone as a function of the opening angle as well as the f-number of the focusing optics is discussed.
High density energetic electrons that are created by intense laser plasma interactions drive MeV proton acceleration. The correlation between accelerated MeV protons and escaped electrons is experimentally investigated at laser intensities in the range of 10 18-10 19 W / cm 2 with S-polarization. Observed proton maximum energies are linearly proportional to escaped electron slope temperatures with a scaling coefficient of about 10. In the context of the simple analytical fluid model for transverse normal sheath acceleration, hot electron sheath density near the target rear surface can be estimated if an empirical acceleration time is assumed.
Multi MeV proton beam is generated via target normal sheath acceleration when the target is irradiated with an ultra-intense laser pulse. In addition, a unique structure, "zonal pattern", of energetic protons is observed in the perpendicular directions of the target edges using triangular targets. The sheath field production on the target edges may be responsible for this zonal pattern. Two dimensional particle-in-cell simulations show that the electrostatic field initially produced at around the cross point at the laser axis and the rear surface expands on the target surface in time. The field enhancement occurs at the target edges when the sheath field reaches there. The enhanced field can accelerate protons in a zonal pattern.
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