The target backsheath field acceleration mechanism is one of the main mechanisms of laser-driven proton acceleration (LDPA) and strongly depends on the comprehensive performance of the ultrashort ultra-intense lasers used as the driving sources. The successful use of the SG-II Peta-watt (SG-II PW) laser facility for LDPA and its applications in radiographic diagnoses have been manifested by the good performance of the SG-II PW facility. Recently, the SG-II PW laser facility has undergone extensive maintenance and a comprehensive technical upgrade in terms of the seed source, laser contrast and terminal focus. LDPA experiments were performed using the maintained SG-II PW laser beam, and the highest cutoff energy of the proton beam was obviously increased. Accordingly, a double-film target structure was used, and the maximum cutoff energy of the proton beam was up to 70 MeV. These results demonstrate that the comprehensive performance of the SG-II PW laser facility was improved significantly.

We report on experimental observation of non-laminar proton acceleration modulated by a strong magnetic field in laser irradiating micrometer aluminum targets. The results illustrate the coexistence of ring-like and filamentation structures. We implement the knife edge method into the radiochromic film detector to map the accelerated beams, measuring a source size of 30–110 μm for protons of more than 5 MeV. The diagnosis reveals that the ring-like profile originates from low-energy protons far off the axis whereas the filamentation is from the near-axis high-energy protons, exhibiting non-laminar features. Particle-in-cell simulations reproduced the experimental results, showing that the short-term magnetic turbulence via Weibel instability and the long-term quasi-static annular magnetic field by the streaming electric current account for the measured beam profile. Our work provides direct mapping of laser-driven proton sources in the space-energy domain and reveals the non-laminar beam evolution at featured time scales.

The interaction of ultra-intense high-power lasers with solid-state targets has been largely studied for the past 20 years as a future compact proton and ion source. Indeed, the huge potential established on the target surface by the escaping electrons provides accelerating gradients of TV/m. This process, called target normal sheath acceleration, involves a large number of phenomena and is very difficult to study because of the picosecond scale dynamics. At the SPARC_LAB Test Facility, the high-power laser FLAME is employed in experiments with solid targets, aiming to study possible correlations between ballistic fast electrons and accelerated protons. In detail, we have installed in the interaction chamber two different diagnostics, each one devoted to characterizing one beam. The first relies on electro-optic sampling, and it has been adopted to completely characterize the ultrafast electron components. On the other hand, a time-of-flight detector, based on chemical-vapour-deposited diamond, has allowed us to retrieve the proton energy spectrum. In this work, we report preliminary studies about simultaneous temporal resolved measurements of both the first forerunner escaping electrons and the accelerated protons for different laser parameters.

Using the example of the PHELIX high-energy short pulse laser we discuss the technical preconditions to investigate ion acceleration with submicrometer thick targets. We show how the temporal contrast of this system was improved to prevent pre-ionization of such targets on the nanosecond timescale. Furthermore the influence of typical fluctuations or uncertainties of the on-target intensity on ion acceleration experiments is discussed. We report how these uncertainties were reduced by improving the assessment and control of the on-shot intensity and by optimizing the positioning of the target into the focal plane. Finally we report on experimental results showing maximum proton energies in excess of 85 MeV for ion acceleration via the target normal sheath acceleration mechanism using target thicknesses on the order of one micrometer.