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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 10μm full width half maximum (FWHM) the sphere diameter was varied between 520 nm and 19.3μm. Maximum proton energies of ∼25 MeV are achieved for targets matching the focal spot size of 10μm 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.
Highlights
Localized plasmas produced by intense laser pulses played an important role in the early development and understanding of laser plasma physics [1,2,3,4]
The ponderomotive scaling for electron temperatures has been shown via simulations to be applicable for mass-limited targets that are comparable in size to the laser focal spot size [7]
The presented study constitutes the first experimental study confirming the dependency of ion acceleration on the ratio of target diameter to laser spot size, which so far has only been treated theoretically [7,8,33]
Summary
Localized plasmas produced by intense laser pulses played an important role in the early development and understanding of laser plasma physics [1,2,3,4]. Recent experimental approaches to realize mass-limited targets include water droplets with diameters typically in the 10 μm range [12,13,14], gas-cluster targets in the 100 nm range [15,16,17], and objects held by thin structures in the 100 nm to few μm range [18,19] In all these cases the range of accessible target diameters is limited, while the solid structure is either directly connected or near the target at distances of few micrometers and/or the target is surrounded by significant amounts of gas. In this paper we present the first experiment using truly isolated and well-defined polystyrene (C8H8) spheres as targets, spanning an unprecedented range of target diameters Using such targets with high-energy laser pulses enables the experimental test of several theoretically
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