SU-GG-T-462: Observation of Quasi-Monoenergetic Laser Accelerated Proton and Carbon Beams

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Purpose: To perform preliminary experiments to achieve the Directed Coulomb Explosion (DCE) regime of proton acceleration to therapeutic energies in high-intensity laser-matter interactions. Method and Materials: Particle-in-Cell (PIC) simulations of the planned experiments at HERCULES laser at the University of Michigan have predicted a new regime of attainable laser-target interactions for proton acceleration. The laser was recently upgraded to 300 TW with Amplified Spontaneous Emission (ASE) intensity contrast ratio of 10−11, allowing intensities of 2×1022 W/cm2 to be achieved in a near diffraction limited, 1.3 micron, focal spot. Dual plasma mirrors have been installed and characterized to reduce the prepulse at < 30 ps (from the uncompensated dispersion of optical elements during the pulse compression) before the main pulse providing 3 orders of magnitude contrast improvement. This allowed experiments on thin foil membranes (50 nm) with 50TW temporally clean pulses without compromising the target. Results: We found for the first time that for all target thicknesses proton spectra exhibit quasi-monoenergetic features, which are more pronounced for ultra-thin (50 nm Si3N4) targets resulted in AE/E∼30%. Moreover for these Si3N4 targets spectra for all the charge states of carbon ions C3+-C6+ are also found to be quasi-monoenergetic. Maximum proton energy drops from 6 MeV for 1 ⌈ m Mylar foil to 4 MeV for 50 nm Si3N4 membranes. Conclusion: Implementation of dual plasma mirrors substantially improved laser contrast and created more favorable proton and carbon flux-energy distributions. Further improvements to the plasma mirrors are required, using better antireflection coatings on glass substrates, to achieve the DCE regime of proton acceleration.