Enhancement of Proton Acceleration by Hot-Electron Recirculation in Thin Foils Irradiated by Ultraintense Laser Pulses

Abstract

MeV-proton production from solid targets irradiated by 100-fs laser pulses at intensities above 1x10(20) W cm(-2) has been studied as a function of initial target thickness. For foils 100 microm thick the proton beam was characterized by an energy spectrum of temperature 1.4 MeV with a cutoff at 6.5 MeV. When the target thickness was reduced to 3 microm the temperature was 3.2+/-0.3 MeV with a cutoff at 24 MeV. These observations are consistent with modeling showing an enhanced density of MeV electrons at the rear surface for the thinnest targets, which predicts an increased acceleration and higher proton energies.

High-intensity laser-driven proton acceleration: influence of pulse contrast

“…While the laser pulse is present, electrons generated at the laser focus are reflected from the Debye sheath built up at the target surfaces and recirculate within the target, enhancing the electrostatic fields responsible for ion acceleration. With a peak intensity of 1!10 20 W cm K2 , Mackinnon et al (2002) observed proton energies up to 24 MeV from Al targets with thickness down to 3 mm. Such an enhancement due to recirculation in thin foil targets motivates research using even thinner targets.…”

“…With this contrast, and a minimum pedestal duration of 1.0 ns, we observe protons with energy extending beyond 4.5 MeV, from Al target foils of thickness down to 0.2 mm. Motivated by the observed enhancement in proton numbers and energies reported by Mackinnon et al (2002), and attributed to electron recirculation within thin foils, we extend our investigation to even higher contrast and thinner targets.…”

“…A measurement of proton acceleration with laser pulses with exceptional temporal contrast of 10 10 was reported (Mackinnon et al 2002) using the JanUSP laser (at the Lawrence Livermore National Laboratory, USA). With this contrast, a dramatic enhancement in measured proton energy was observed when decreasing target thickness below a certain threshold.…”

Enhanced proton beams from ultrathin targets driven by high contrast laser pulses

“…4 In addition, if the thickness is less than c /2 ͑ is the laser pulse duration and c the speed of light͒, electron recirculation within the target during the laser pulse may further enhance the acceleration field at the rear surface. 5 Finally, for very high intensities and ultrathin targets, relativistic transparency allows part of the laser pulse to be transmitted through the target and contribute to increased electron heating. 6 However, there are limits to this decrease in target thickness in order to optimize proton acceleration.…”

Investigations of Field Dynamics in Laser Plasmas with Proton Imaging

“…When the electrons reach the rear side of the target only the fastest electrons (precursor electrons) can escape since a potential will be build up which hinder the electrons to escape. The electrons which can not escape turn around reentering the target surface and starting to oscillate [64,65]. These electrons which leak into the vacuum (as discussed in Chapter 2.3) create a so-called electron sheath.…”