Multi-keV proton energies from exploding hydrogen clusters

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Abstract
We report the first direct experimental measurements of the proton energies produced from explosions of isolated hydrogen (H2) clusters in an intense laser field. The measured proton energy distribution from the interaction of a 35 mJ laser pulse of duration 90 fs with a cluster of mean radius 100 Å was measured and has a mean proton energy of 7±1 keV. A maximum proton energy of >20 keV was recorded. We show the cluster expansion in this experiment is not due to a purely Coulomb explosion but a combination of Coulomb and hydrodynamic mechanisms. These findings are relevant to fusion studies in laser-heated clusters.