Present Status and Future Prospects of Laser Fusion Research at ILE Osaka University

Reviewed are the present status and future prospects of the laser fusionresearch at the ILE Osaka. The Gekko XII and Peta Watt laser system have been operated forinvestigating the implosion hydrodynamics, fast ignition, and the relativistic laser plasmainteractions and so on. In particular, the fast ignition experiments with cone shell target havebeen in progress as the UK and US-Japan collaboration programs. In the experiments, the implodedhigh density plasmas are heated by irradiating 500 J level peta-watt laser pulse. The thermalneutron yield is found to increase by three orders of magnitude by injecting the peta-watt laserinto the cone shell target. The Rayleigh-Taylor instability experiment results are also reviewed isthis paper.

Experimental measurements of gamma-photon production and estimation of electron/positron production on the PETAL laser facility

This article reports the first measurements of high-energy photons produced with the high-intensity PETawatt Aquitaine Laser(PETAL)laser.The experiments were performed during the commissioning of the laser.The laser had an energy of about 400 J,an intensity of 8×10^(18)W cm^(−2),and a pulse duration of 660 fs(FWHM).It was shot at a 2 mm-thick solid tungsten target.The high-energy photons were produced mainly from the bremsstrahlung process for relativistic electrons accelerated inside a plasma generated on the front side of the target.This paper reports measurements of electrons,protons and photons.Hot electrons up to35 MeV with a few-MeV temperature were recorded by a spectrometer,called SESAME(SpectreÉlectronS Angulaire MoyenneÉnergie).K-and L-shells were clearly detected by a photon spectrometer called SPECTIX(Spectromètre PetalàCristal en TransmIssion pour le rayonnnement X).High-energy photons were diagnosed by CRACC-X(Cassette de RAdiographie Centre Chambre-rayonnement X),a bremsstrahlung cannon.Bremsstrahlung cannon analysis is strongly dependent on the hypothesis adopted for the spectral shape.Different shapes can exhibit similar reproductions of the experimental data.To eliminate dependence on the shape hypothesis and to facilitate analysis of the data,simulations of the interaction were performed.To model the mechanisms involved,a simulation chain including hydrodynamic,particle-in-cell,and Monte Carlo simulations was used.The simulations model the preplasma generated at the front of the target by the PETAL laser prepulse,the acceleration of electrons inside the plasma,the generation of MeV-range photons from these electrons,and the response of the detector impacted by the energetic photon beam.All this work enabled reproduction of the experimental data.The high-energy photons produced have a large emission angle and an exponential distribution shape.In addition to the analysis of the photon spectra,positron production was also investigated.Indeed,if high-energy photons are generated inside the solid target,some positron/electron pairs may be produced by the Bethe–Heitler process.Therefore,the positron production achievable within the PETAL laser facility was quantified.To conclude the study,the possibility of creating electron/positron pairs through the linear Breit–Wheeler process with PETAL was investigated.