S. Okihara

Kα spectroscopy to study energy transport in ultrahigh-intensity laser produced plasmas

Abstract K α line emission from partially ionized material was studied to diagnose energy transport in ultrahigh-intensity laser produced plasmas. Dependence of the emission intensity on initial target conductivity was investigated by using a plastic target (C 2 H 3 Cl coated with CH) and a metallic target (Al coated with Mg) irradiated with a 1-TW Ti:sapphire laser pulse of 132 fs at 2×10 17 W/cm 2 . Depth of the heated region was deduced from Cl or Al He- α line intensities given as a function of overcoat thickness. No significant difference was observed between the two types of targets. Density–depth product of the hot region was typically 0.1 mg/cm 2 for both targets and the temperature of hot electrons measured by an electron spectrometer was 50 keV . The experimental results were analyzed with an integration calculation with a one-dimensional (1D) relativistic Fokker–Planck code, a 1D radiation hydrodynamics (rad-hydro) simulation, and an atomic–kinetic model developed for ionization-shifted Cl K α lines. The observed transport features are discussed.

Energetic Proton Generation in a Thin Plastic Foil Irradiated by Intense Femtosecond Lasers

Energetic protons generated in a thin plastic foil under the irradiation of ultra-intense femtosecond lasers have been experimentally and computationally investigated. More energetic protons were emitted forward in the direction of laser propagation than backward. The maximum energy of protons has been scaled via laser intensity I as 1 0.7, up to 2 MeV at 1018 W/cm2. The laser intensity scaling law for the maximum proton energy was reproduced by 2D-PIC simulation.

Numerical study of Kα emission from partially ionized chlorine

Abstract Population kinetics and spectral synthesis codes have been developed to analyze Kα emission from partially ionized chlorine in hydrocarbon plasmas irradiated with high-intensity ultra-short laser pulses. The population kinetics processes are calculated using a bi-Maxwellian electron temperature distribution composed of fast and cold electrons. The fast electrons dominantly contribute to ionize the K-shell bound electrons. i.e., inner-shell ionization, while the cold electrons produce ionization from the outer-shell electrons. The resultant Kα emission provides a distinct spectral signature for each charge state. Also included in the calculation are the opacity effects in the framework of an escape probability method. It is shown that the Kα emission is saturated at a plasma thickness of more than a few microns. Further, we find that unresolved satellite lines overlap significantly with the corresponding parent Kα lines of the next charge state, and make a large contribution to composite spectrum. Finally, time-dependent properties of the Kα emission are also discussed briefly.

X-ray spectroscopy on energy transport and deposition in ultra-intensity laser produced plasmas

Observation of line radiation emanating from inner shell transitions in partially ionized high Z material is suggested as a diagnostic method for heating of dense plasma with an ultra-short high-intensity laser pulse. A new atomic-kinetics code specified for the analysis of the ionization-shift Kα lines was developed to derive electron temperature of the bulk plasma heated by hot electrons. Chlorine Kα to Cl15+ Heα lines from a solid planar target were observed with a 1 TW laser system at ILE Osaka. Comparison of the experimental results with the code prediction infers a ∼100 eV temperature plasma on the target surface with a depth of about 0.6 μm at 1.5×1017 W/cm2. Dependence of the temperature on laser intensity was found to be very weak.

The nonlinear Compton scattering experiment

We constructed an experimental system to observe the nonlinear Compton scattering. As a test experiment, we measure the Compton scattering in a linear region and had a reasonable-agreement with simulation.

Coulomb explosion of benzene induced by an intense laser field

Coulomb explosions of benzene induced by an intense femtosecond laser field were examined in the present study by time-of-flight (TOF) mass spectrometry at laser intensities of 8.0/spl times/10/sup 16/ W cm/sup -2/, with a pulse width of 120 fs. The multiply charged ions of C/sup q+/ (q=1/spl sim/4) and H/sup +/ were detected and their energies were determined to be distributed in the range of 0/spl sim/120 eV. The explosions were concluded to be anisotropic because the kinetic energies of multiply charged carbon ions are high parallel to the laser electric field. Molecular dynamics (MD) simulations, including the effects of tunnel ionization, electron recombination, and the spatial configuration of benzene for laser electric fields were performed to elucidate the kinetic-energy distributions and explosion dynamics. The simulations suggest that the charge-hopping mechanism enhances ionization, finally leading to an anisotropic explosion. Furthermore, we have found that the time profiles of the hopping greatly depend on the spatial configurations in the case of the benzene.