Proton acceleration by irradiation of isolated spheres with an intense laser pulse

Abstract

We report on experiments irradiating isolated plastic spheres with a peak laser intensity of 2-3×10^{20}Wcm^{-2}. With a laser focal spot size of 10 μm full width half maximum (FWHM) the sphere diameter was varied between 520 nm and 19.3 μm. Maximum proton energies of ∼25 MeV are achieved for targets matching the focal spot size of 10 μm in diameter or being slightly smaller. For smaller spheres the kinetic energy distributions of protons become nonmonotonic, indicating a change in the accelerating mechanism from ambipolar expansion towards a regime dominated by effects caused by Coulomb repulsion of ions. The energy conversion efficiency from laser energy to proton kinetic energy is optimized when the target diameter matches the laser focal spot size with efficiencies reaching the percent level. The change of proton acceleration efficiency with target size can be attributed to the reduced cross-sectional overlap of subfocus targets with the laser. Reported experimental observations are in line with 3D3V particle in cell simulations. They make use of well-defined targets and point out pathways for future applications and experiments.

Relativistically Intense Laser–Microplasma Interactions

“…much smaller than typically achieved even with laser-driven sources using bulk foil-targets. This includes proton-and X-ray-imaging [34,68], laser-absorption measurements at ultra-high intensities via spectroscopy of all particles contained in a known target (complementary to light-measurements [69]) and could even range to laboratory-scale astrophysics [70] [71]. As an additional motivation, the isolation of target particles has often been suggested in calculations and numerical simulations as an efficient means to positively influence ion kinetic energies and to monochromatize ion-energy distributions from laser driven sources via several mechanisms [e.g.…”

“…A 2014 experimental campaign was carried out at the Texas Petawatt laser performing a target-size scan across the laser focal diameter. This section grew out of our related publication reference [71].…”

“…Noise and background were evaluated and subtracted for each shot individually. The red line in b is the quasi-neutral spectrum for the 10 µm target predicted by[188,214] at characteristic energy of 3.8 MeV[135].Figure adaptedwith permission from original publication[71].…”

Isolated proton bunch acceleration by a petawatt laser pulse

“…The acceleration field is mediated via relativistic electrons, which induce MV/µm electric fields that vary in time and space. The correspondingly broad ion energy distributions could be narrowed by limiting the spatial extent of the ion reservoir on the surface of irradiated opaque foils 6 , 7 or by the use of droplets 8 , 9 . Reducing the foil thickness to the order of the laser skin depth also resulted in non-monotonic, peaked distributions with improved efficiency at higher ion energies 10 , 11 .…”

“…The low density does however limit the accelerated particle numbers severely. Spatially limited targets, so-called mass-limited targets, have attracted severe interest in theory 15 and experiment 9 , 16 , 17 .…”