Ion acceleration from microstructured targets irradiated by high-intensity picosecond laser pulses.

Structures on the front surface of thin foil targets for laser-driven ion acceleration have been proposed to increase the ion source maximum energy and conversion efficiency. While structures have been shown to significantly boost the proton acceleration from pulses of moderate-energy fluence, their performance on tightly focused and high-energy lasers remains unclear. Here, we report the results of laser-driven three-dimensional (3D)-printed microtube targets, focusing on their efficacy for ion acceleration. Using the high-contrast (∼10^{12}) PHELIX laser (150J, 10^{21}W/cm^{2}), we studied the acceleration of ions from 1-µm-thick foils covered with micropillars or microtubes, which we compared with flat foils. The front-surface structures significantly increased the conversion efficiency from laser to light ions, with up to a factor of 5 higher proton number with respect to a flat target, albeit without an increase of the cutoff energy. An optimum diameter was found for the microtube targets. Our findings are supported by a systematic particle-in-cell modeling investigation of ion acceleration using 2D simulations with various structure dimensions. Simulations reproduce the experimental data with good agreement, including the observation of the optimum tube diameter, and reveal that the laser is shuttered by the plasma filling the tubes, explaining why the ion cutoff energy was not increased in this regime.

Author Correction: Quantitative phase contrast imaging of a shock-wave with a laser-plasma based X-ray source.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Quantitative phase contrast imaging of a shock-wave with a laser-plasma based X-ray source.

X-ray phase contrast imaging (XPCI) is more sensitive to density variations than X-ray absorption radiography, which is a crucial advantage when imaging weakly-absorbing, low-Z materials, or steep density gradients in matter under extreme conditions. Here, we describe the application of a polychromatic X-ray laser-plasma source (duration ~0.5 ps, photon energy >1 keV) to the study of a laser-driven shock travelling in plastic material. The XPCI technique allows for a clear identification of the shock front as well as of small-scale features present during the interaction. Quantitative analysis of the compressed object is achieved using a density map reconstructed from the experimental data.

Soft X-ray backlighter source driven by a short-pulse laser for pump-probe characterization of warm dense matter.

Here we propose a pump-probe X-ray absorption spectroscopy temperature measurement technique appropriate for matter having temperature in the range of 10 to a few 100 eV and density up to solid density. Atomic modeling simulations indicate that for various low- to mid-Z materials in this range the energy and optical depth of bound-bound and bound-free absorption features are sensitive to temperature. We discuss sample thickness and tamp layer considerations. A series of experimental investigations was carried out using a range of laser parameters with pulse duration ≤5 ps and various pure and alloyed materials to identify backlighter sources suitable for the technique.

Chemical-vapor deposited ultra-fast diamond detectors for temporal measurements of ion bunches.

This article reports on the development of thin diamond detectors and their characterization for their application in temporal profile measurements of subnanosecond ion bunches. Two types of diamonds were used: a 20 µm thin polycrystalline chemical vapor deposited (CVD) diamond and a membrane with a thickness of (5 ± 1) µm etched out of a single crystal (sc) CVD diamond. The combination of a small detector electrode and an impedance matched signal outlet leads to excellent time response properties with a signal pulse resolution (FWHM) of τ = (113 ± 11) ps. Such a fast diamond detector is a perfect device for the time of flight measurements of MeV ions with bunch durations in the subnanosecond regime. The scCVD diamond membrane detector was successfully implemented within the framework of the laser ion generation handling and transport project, in which ion beams are accelerated via a laser-driven source and shaped with conventional accelerator technology. The detector was used to measure subnanosecond proton bunches with an intensity of 108 protons per bunch.

Studying the Dynamics of Relativistic Laser-Plasma Interaction on Thin Foils by Means of Fourier-Transform Spectral Interferometry.

We apply Fourier-transform spectral interferometry (FTSI) to study the interaction of intense laser pulses with ultrathin targets. Ultrathin submicrometer-thick solid CH targets were shot at the PHELIX laser facility with an intensity in the mid to upper 10^{19} W/cm^{2} range using an innovative double-pulse structure. The transmitted pulse structure was analyzed by FTSI and shows a transition from a relativistic transparency-dominated regime for targets thinner than 500 nm to a hole-boring-dominated laser-plasma interaction for thicker targets. The results also confirm that the inevitable preplasma expansion happening during the rising slope of the pulse, a few picoseconds before the maximum of the pulse is reached, cannot be neglected and plays a dominant role in laser-plasma interaction with ultrathin solid targets.

Maximum Proton Energy above 85 MeV from the Relativistic Interaction of Laser Pulses with Micrometer Thick CH_{2} Targets.

We present a study of laser-driven ion acceleration with micrometer and submicrometer thick plastic targets. Using laser pulses with high temporal contrast and an intensity of the order of 10^{20} W/cm^{2} we observe proton beams with cutoff energies in excess of 85 MeV and particle numbers of 10^{9} in an energy bin of 1 MeV around this maximum. We show that applying the target normal sheath acceleration mechanism with submicrometer thick targets is a very robust way to achieve such high ion energies and particle fluxes. Our results are backed with 2D particle in cell simulations furthermore predicting cutoff energies above 200 MeV for acceleration based on relativistic transparency. This predicted regime can be probed after a few technically feasible adjustments of the laser and target parameters.