Journal Article Micron-size Hydrogen Cluster Target for Laser-Driven Proton Acceleration

神野, 智史  ,  金崎, 真聡  ,  宇野, 雅貴  ,  松井, 隆太郎  ,  上坂, 充  ,  岸本, 泰明  ,  福田, 祐仁

60 ( 4 )  , pp.044021-1 - 044021-9 , 2018-03 , IOP Publishing
In laser-driven ion acceleration techniques, we propose the way to produce impurity free, highly reproducible, and robust proton beams exceeding 100 MeV using Coulomb explosion of micron-size hydrogen clusters. In this study, the micron-size hydrogen clusters were generated by expanding the cooled high-pressure hydrogen gas into a vacuum via a conical nozzle connected to a solenoid valve cooled by a mechanical cryostat. The size distributions of the hydrogen clusters were evaluated by measuring the angular distribution of laser light scattered from the clusters. The data were analyzed mathematically based on Mie scattering theory combined with Tikhonov regularization method. The maximum size of the hydrogen cluster at 25 K, 6 MPa in the stagnation state was recognized to be 2.15 ± 0.10 μm. The mean cluster size decreased with increasing the temperature and was found to be much larger than that given by Hagena’s formula. This discrepancy suggests that the micron-size hydrogen clusters were formed by atomization (spallation) of liquid or supercritical fluid phase of hydrogen. In addition, the density profiles of gas phase were evaluated for 25 to 80 K at 6 MPa using a Nomarski interferometer. Based on the measurement results and the equation of state for hydrogen, the cluster mass fraction was obtained. The 3D Particles-in-Cell (PIC) simulations concerning interaction processes of micron-size hydrogen clusters with high power laser pulses predict the generation of protons exceeding 100 MeV, accelerated in a laser propagation direction via an anisotropic Coulomb explosion mechanism, demonstrating a future candidate in laser-driven proton sources with the upcoming multi-PW lasers.

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