S. Fujioka

Characterization of extreme ultraviolet emission from laser-produced spherical tin plasma generated with multiple laser beams

Spherical solid tin targets were illuminated uniformly with twelve beams from the Gekko XII laser system to create spherical plasmas, and the extreme ultraviolet (EUV) emission spectra from the plasmas were measured. The highest conversion efficiency of 3% to 13.5nm EUV light in 2% bandwidth was attained for an irradiance of around 5×1010W∕cm2. The experimental results were reproduced fairly well using a theoretical model taking the power balance in the plasma into consideration.

Characterization of density profile of laser-produced Sn plasma for 13.5nm extreme ultraviolet source

We investigated the electron density profile corresponding to the dominant extreme ultraviolet (EUV) emission from a laser-produced Sn plasma using a combination of a green and an UV interferometer. A comparison between experimental results and a one-dimensional radiation hydrodynamic simulation shows reasonable agreement, and the discrepancy could be attributed to three-dimensional plasma expansion. It was found that, due to opacity effects, most of the EUV light comes from an under-dense plasma region.

Monochromatic imaging and angular distribution measurements of extreme ultraviolet light from laser-produced Sn and SnO2 plasmas

Properties of extreme ultraviolet (EUV) emission from laser-produced Sn and SnO2 plasmas were investigated. EUV emission images were taken with a monochromatic imager for 13.5nm with 4% bandwidth. It was found that the EUV emitting plasma is not formed symmetrically with respect to the target surface normal but extends toward laser incident axis. This result is consistent with the angular distribution of EUV emission peaked toward the direction nearly perpendicular to the laser axis. The asymmetric plasma can be attributed to the interaction of a long laser pulse with expanding plasma along the path of laser incidence.

High-energy-density plasmas generation on GEKKO-LFEX laser facility for fast-ignition laser fusion studies and laboratory astrophysics

The worlds largest peta watt (PW) laser LFEX, which delivers energy up to 2?kJ in a 1.5?ps pulse, has been constructed beside the GEKKO XII laser at the Institute of Laser Engineering, Osaka University. The GEKKO-LFEX laser facility enables the creation of materials having high-energy-density which do not exist naturally on the Earth and have an energy density comparable to that of stars. High-energy-density plasma is a source of safe, secure, environmentally sustainable fusion energy. Direct-drive fast-ignition laser fusion has been intensively studied at this facility under the auspices of the Fast Ignition Realization Experiment (FIREX) project.In this paper, we describe improvement of the LFEX laser and investigations of advanced target design to increase the energy coupling efficiency of the fast-ignition scheme. The pedestal of the LFEX pulse, which produces a long preformed plasma and results in the generation of electrons too energetic to heat the fuel core, was reduced by introducing an amplified optical parametric fluorescence quencher and saturable absorbers in the front-end system of the LFEX laser. Since fast electrons are scattered and stopped by the strong electric field of highly ionized high-Z (i.e. gold) ions, a low-Z cone was studied for reducing the energy loss of fast electrons in the cone tip region. A diamond-like carbon cone was fabricated for the fast-ignition experiment. An external magnetic field, which is demonstrated to be generated by a laser-driven capacitor-coil target, will be applied to the compression of the fuel capsule to form a strong magnetic field to guide the fast electrons to the fuel core. In addition, the facility offers a powerful means to test and validate astronomical models and computations in the laboratory. As well as demonstrating the ability to recreate extreme astronomical conditions by the facilities, our theoretical description of the laboratory experiment was compared with the generally accepted explanation for astronomical observations.

Tin laser-produced plasma source modeling at 13.5nm for extreme ultraviolet lithography

Extreme ultraviolet lithography semiconductor manufacturing requires a 13.5nm light source. Laser-produced plasma emission from Sn V–Sn XIV ions is one proposed industry solution. The effect of laser pulse width and spatial profile on conversion efficiency is analyzed over a range of power densities using a two-dimensional radiative magnetohydrodynamic code and compared to experiment using a 1.064μm, neodymium:yttrium aluminium garnet laser on a planar tin target. The calculated and experimental conversion efficiencies and the effects of self-absorption in the plasma edge are compared. Best agreement between theory and experiment is found for an 8.0ns Gaussian pulse.

Advanced laser-produced EUV light source for HVM with conversion efficiency of 5-7% and B-field mitigation of ions

We propose a new scheme for high conversion efficiency from laser energy to 13.5 nm extreme ultra violet emission within 2 % band width, a double pulse laser irradiation scheme with a tin droplet target. We consider two-color lasers, a Nd:YAG laser with 1.06 µm in wavelength as a prepulse and a carbon dioxide laser with 10.6 µm in wavelength for a main pulse. We show the possibility of obtaining a CE of 5 - 7 % using a benchmarked radiation hydro code. We have experimentally tested the new scheme and observed increase of CE greater than 4 %. We show many additional advantages of the new scheme, such as reduction of neutral debris, energy reduction of debris ions, and decrease of out of band emission. We also discuss debris problems, such as ion sputtering using newly developed MD simulations, ion mitigation by a newly designed magnetic coil using 3-PIC simulations and tin cleaning experiments.

Development of wide-field, multi-imaging x-ray streak camera technique with increased image-sampling arrays

In order to enlarge the field of view of a multi-imaging x-ray streak (MIXS) camera technique [H. Shiraga et al., Rev. Sci. Instrum. 66, 722 (1995)], which provides two-dimensionally space-resolved x-ray imaging with a high temporal resolution of ∼10 ps, we have proposed and designed a wide-field MIXS (W-MIXS) by increasing the number of image-sampling arrays. In this method, multiple cathode slits were used on the photocathode of an x-ray streak camera. The field of view of the W-MIXS can be enlarged up to 150–200 μm instead of ∼70 μm for a typical MIXS with a spatial resolution of ∼15 μm. A proof-of-principle experiment with the W-MIXS was carried out at the Gekko-XII laser system. A cross-wire target was irradiated by four beams of the Gekko-XII laser. The data streaked with the W-MIXS system were reconstructed as a series of time-resolved, two-dimensional x-ray images. The W-MIXS system has been established as an improved two-dimensionally space-resolved and sequentially time-resolved technique.

Quantitative measurement of hard x-ray spectra for high intensity laser produced plasma

X-ray line spectra ranging from 17 to 77 keV were quantitatively measured with a Laue spectrometer, composed of a cylindrically curved crystal and a detector. Either a visible CCD detector coupled with a CsI phosphor screen or an imaging plate can be chosen, depending on the signal intensities and exposure times. The absolute sensitivity of the spectrometer system was calibrated using pre-characterized laser-produced x-ray sources and radioisotopes. The integrated reflectivity for the crystal is in good agreement with predictions by an open code for x-ray diffraction. The energy transfer efficiency from incident laser beams to hot electrons, as the energy transfer agency for specific x-ray line emissions, is derived as a consequence of this work.

Charge exchange spectroscopy in Snq+(q = 6-15)-He collisions

Extreme ultra-violet (EUV) spectra of multiply charged tin ions were measured in the wavelength range of 5-38 nm following the electron capture into excited states of slow Snq+(q = 6-15) ions passing through He gas target. The charge-state dependence of 4d-nl (nl = 4f, 5p and 5f) transitions was obtained with assumption of the single-electron capture. Identi.cation of the transitions has been carried out by comparison with the results of the theoretical calculation.

Time-resolved spectroscopic observations of shockinduced silicate ionization

We conducted time-resolved spectroscopic observations of shock-heated quartz and forsterite using a high-power laser. The results revealed that ionization occur easily under shockinduced warm dense conditions. We compare the obtained temperatures on the Hugoniot with a few theories. The comparison suggests that the contribution of shock-induced ionization to the isochoric specific heat during shock compression is ~1 kb/atom for quartz and forsterite. Shock-induced ionization and subsequent electron recombination leads to a larger amount of vapor and may lead to dynamical and chemical evolution of silicate vapor clouds different from our current understandings.