Proton Acceleration Driven by a Nanosecond Laser from a Cryogenic Thin Solid-Hydrogen Ribbon

Abstract

A high-power pulsed laser is focused onto a solid-hydrogen target to accelerate forward a collimated stream of protons in the range 0.1–1 MeV, carrying a very high energy of about 30 J (∼5% laser-ion conversion efficiency) and extremely large charge of about ∼0.1mC per laser pulse. This result is achieved for the first time through the combination of a sophisticated target system (H2 thin ribbon) operating at cryogenic temperature (∼10K ) and a very hot H plasma (∼300keV “hot electron” temperature) generated by a subnanosecond laser with an intensity of ∼3×1016W/cm2 . Both the H plasma and the accelerated proton beam are fully characterized by in situ and ex situ diagnostics. Results obtained using the ELISE (experiments on laser interaction with solid hydrogen) H2 target delivery system at PALS (Prague) kJ-class laser facility are presented and discussed along with potential multidisciplinary applications.

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Plain Language Summary

Pure streams of accelerated protons are key for both cancer therapies and experiments in condensed-matter physics. However, targets that are irradiated with high-power lasers often yield proton streams contaminated with carbon and other ions. Here, we focus a high-power pulsed laser onto a solid hydrogen target to accelerate a collimated pure proton stream.

We begin by pushing solid H2 through a nozzle to create a ribbon with a width of 1 mm and a thickness that can be modulated between 20 and 100 μm . Our linearly polarized nanosecond laser has an intensity of roughly , and we focus it on the H2 ribbon in vacuum conditions (10−5 to 10−4mbar ) at cryogenic temperatures (10 K). We recover a collimated stream of protons with energies in the MeV range that is free of any contaminants. Using another laser as a probe beam, we examine the expansion of the hydrogen plasma at 3 billion kelvin a few nanoseconds before and after the arrival of the laser pulse. We show that the population of protons we recover is 3 times larger than the number of protons derived from a CH2 target. Additionally, we measure a laser-proton acceleration efficiency 2 to 3 times higher than that of previous experiments.

We expect that the potential applications of our proposed acceleration scheme will include nonconventional cancer therapies and nuclear fusion.