Abstract
Laser-plasma interactions in the novel regime of relativistically induced transparency (RIT) have been harnessed to generate intense ion beams efficiently with average energies exceeding 10 MeV/nucleon (>100 MeV for protons) at “table-top” scales in experiments at the LANL Trident Laser. By further optimization of the laser and target, the RIT regime has been extended into a self-organized plasma mode. This mode yields an ion beam with much narrower energy spread while maintaining high ion energy and conversion efficiency. This mode involves self-generation of persistent high magnetic fields (∼104 T, according to particle-in-cell simulations of the experiments) at the rear-side of the plasma. These magnetic fields trap the laser-heated multi-MeV electrons, which generate a high localized electrostatic field (∼0.1 T V/m). After the laser exits the plasma, this electric field acts on a highly structured ion-beam distribution in phase space to reduce the energy spread, thus separating acceleration and energy-spread reduction. Thus, ion beams with narrow energy peaks at up to 18 MeV/nucleon are generated reproducibly with high efficiency (≈5%). The experimental demonstration has been done with 0.12 PW, high-contrast, 0.6 ps Gaussian 1.053 μm laser pulses irradiating planar foils up to 250 nm thick at 2–8 × 1020 W/cm2. These ion beams with co-propagating electrons have been used on Trident for uniform volumetric isochoric heating to generate and study warm-dense matter at high densities. These beam plasmas have been directed also at a thick Ta disk to generate a directed, intense point-like Bremsstrahlung source of photons peaked at ∼2 MeV and used it for point projection radiography of thick high density objects. In addition, prior work on the intense neutron beam driven by an intense deuterium beam generated in the RIT regime has been extended. Neutron spectral control by means of a flexible converter-disk design has been demonstrated, and the neutron beam has been used for point-projection imaging of thick objects. The plans and prospects for further improvements and applications are also discussed.
Highlights
Research on intense ion-beam generation got a significant impetus as a result of groundbreaking results on proton acceleration[1] obtained by irradiating $lm-thick foils with intense (>1020 W/cm2) $0.5 ps laser pulses at the Nova PW laser,[2] where $10% of the laser energy was transferred to the beam with proton energies up to 58 MeV
This mode yields an ion beam with much narrower energy spread while maintaining high ion energy and conversion efficiency
We have utilized a new generation of metal-channel photomultiplier tube (PMT) with a planar photoelectron collection structure developed for nuclear-safeguard work at LANL.[50]
Summary
Research on intense ion-beam generation got a significant impetus as a result of groundbreaking results on proton acceleration[1] obtained by irradiating $lm-thick foils with. RIT comes about in a laser-heated electron plasma when, because of the increased relativistic electron rest mass, the plasma frequency decreases below the laser frequency, so that there is no longer a critical surface to reflect the light Accessing this regime with existing lasers required sub-micron-thick laser-target foils, which in turn required the development of laser-pulse cleaning[10] to achieve unprecedentedly high levels of contrast. Operation was found by the LANL relativistic laser-plasma research team where the plasma self organizes in a way that results in efficient ion acceleration and a narrow ion-energy distribution, both qualities result from separate processes This new regime does not require multi-layered targets, it works in such targets as well. In addition to the work described we acknowledge the research by other groups that have considered and are experimenting with relativistic laser-plasmas in the RIT regime.[35–45]
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