Toru Kawamura

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.

Properties of EUV emissions from laser-produced tin plasmas

Extreme ultraviolet (EUV) emission from laser produced plasma attracts much attention as a next generation lithography source. The characterization of EUV emission has been carried out using GEKKO XII laser system. The twelve beams irradiated tin or tin-oxide coated spherical targets uniformly and dependence of EUV spectra on laser intensity were obtained with a transmission grating spectrometer and two grazing incidence spectrometers. The EUV Conversion Efficiency (CE, the ratio of EUV energy at the wavelength of 13.5 nm with 2 % bandwidth to incident laser energy) was measured using an absolutely calibrated EUV calorimeter. Optimum laser intensities for the highest conversion were found to be 0.5- 1x1011 W/cm2 with CE of 3 %. The spectroscopic data indicate that shorter wavelength emission increases at higher laser intensities due to excessive heating beyond optimum temperatures (20- 40 eV). The CE was almost independent on the initial coating thickness down to 25 nm.

Estimation of emission efficiency for laser-produced EUV plasmas

Extreme Ultra Violet (EUV) light source produced by laser irradiation emits not only the desired EUV light of 13 ~ 14 nm (about 90 eV) but also shorter x-rays. For example, emissions around 4 ~ 8 nm (about 150 ~ 300 eV) and 1 ~ 2.5 nm (about 0.5 ~ 1.2 keV) are experimentally observed from Sn and/or SnO2 plasmas. These emissions are correspond to the N-shell and M-shell transitions, respectively. From the view point of energy balance and efficiency, these transitions should be suppressed. However, they may, to some extent, contribute to provide the 5p and 4f levels with electrons which eventually emit the EUV light and enhance the intensity. To know well about radiative properties and kinematic of the whole plasma, atomic population kinetics and spectral synthesis codes have been developed. These codes can estimate the atomic population with nl-scheme and spectral shapes of the EUV light. Radiation hydrodynamic simulation have been proceeding in this analysis. Finally, the laser intensity dependence of the conversion efficiency calculated by these codes agrees with that of the corresponding experimental results.

Theoretical simulation of extreme UV radiation source for lithography

A possible design window for extreme ultraviolet (EUV) radiation source has been introduced, which is needed for its realistic use for next generation lithography. For this goal, we have prepared a set of numerical simulation codes to estimate the conversion efficiency from laser energy to radiation energy with a wavelength of 13.5 nm with 2 % bandwidth, which includes atomic structure, opacity and emissibity and hydro dynamics codes. The simulation explains well the observed conversion efficiency dependence of incident power using GEKKO XII laser system as well as spectral shapes. It is found that the conversion efficiency into 13.5 nm at 2% bandwidth has its maximum of a few percent at the laser intensity 1-2 x 1011 W/cm2.

Time- and space-resolved X-ray spectroscopy for observation of the hot compressed core region in a laser driven implosion

Abstract Fusion pellet implosion by direct laser irradiation was investigated by means of time- and space-resolved X-ray spectroscopic measurements. Fusion pellets filled with a deuterium fuel gas including a small amount of Ar dopant were irradiated with 12 beams of intense partially coherent light and line emissions from the dopant were observed. There were two type of implosion studies. First, experimental conditions were carefully chosen to provide implosions that were as stable as possible, these we refer to as the “balanced” cases. Second, the energy balance was manipulated so that two specific beams, diametrically opposed to each other, had relatively large energy differences. This imposes additional low-modal non-uniformity on the pellet, and these experiments are called that the “unbalanced” cases. Experimental results were compared with hydrocode simulations, which were post-processed by the aid of X-ray spectrum analysis code. The experimental results in terms of temporal variations of the Ar He-β line and their spatial profiles obtained with an X-ray monochromatic imager were well replicated by one-dimensional (1-D) simulation for the balanced case, whereas those for the unbalanced case showed large discrepancies. Furthermore, a clear difference was found in the emission of Li-like satellite lines between two cases: In the unbalanced case, the satellite lines became much more intense than the He-β line at late time in the implosion. These experimental results suggest that the imposed low-modal non-uniformities assisted in the quenching of hot, compressed core formation.

Analysis of X-ray spectra of laser produced high density plasmas

Abstract X-ray spectra of laser-produced highly charged gold-ion plasma have been studied theoretically. Based on a precise non-empirical multi-configuration Dirac-Fock calculation, a new model NISEM (Non-Interacting Spectator Electron Model) is proposed to take into account the effect from numerous spectator satellite lines. In the framework of NISEM, experimental spectra are reproduced reasonably well by the present theory.

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.