Laser-induced forward transfer (LIFT) is a direct-writing technique enabling deposition of a film. In addition, a single dot smaller than the laser wavelength can be deposited at small shot energy, and the case is called as laser-induced dot transfer (LIDT). In conventional LIDT experiments, multi-shots with step scanning have been used to form array structures, which are useful in plasmonics, pho-chemistry, light harvest, etc..
On the other hand, interference laser processing can achieve an arrayed process and generate a periodic structure in a single shot. In this presentation, the results of LIDT technique which uses a femtosecond laser interference pattern will be presented. As a result, an array of Au nanodots with 3.6 m period was successfully deposited, producing the following unit structures: a single dot, adjoining dots, and stacking dots.
This new technique produces high-purity, catalyst-free nanodots in array that do not require post-cleaning or alignment processes.
X-ray imaging is very useful to investigate a imploded core plasma in inertial fusion experiment. We can obtain many information from X-ray images, such as shape, density and temperature of a plasma. X-ray framing camera (XFC) is capable of taking two dimensional time resolved X-ray images. In previous work, we developed a numerical model of XFC to analyze its X-ray image. Calculated results agreed qualitatively with experimental results. However, it is not enough when we discuss absolute value of the signal. Moreover, in high energy laser experiment, high photon flux may cause signal depletion of XFC. This is a problem for accurate X-ray measurement. In this paper, we report our improved calculation model including signal depletion. Results using the new model shows signal depletion at high applied voltage range. The new model are evaluated by tabletop laser experiments. Calculated results using the new model agree quantitatively with experimental results.
Image sampling is a simple, convenient and working scheme to obtain two-dimensional (2D) images on high-speed streak cameras which have only one-dimensional (1D) slit cathode as an imaging sensor on a streak tube. 1D sampling of a 2D image in one direction was realized as Multi-Imaging X-ray Streak camera (MIXS) with a similar configuration to TV raster scan. 2D sampling of a 2D image was realized as 2-D Sampling Image X-ray Streak camera (2D-SIXS) with a similar configuration to CCD pixels. For optical-UV streak cameras, 2D fiber plate coupled to the output of a streak camera was untied and fibers were rearranged to form a line on the cathode slit. In these schemes, clever arrangement of the sampling lines or points relative to the streaking direction were essential for avoiding overlap of the streaked signals with each other. These streak cameras with image sampling technique were successfully applied to laser plasma experiment, particularly for laser-driven nuclear fusion research with simultaneous temporal- and spatial resolutions of 10 ps and 15 μm, respectively. This paper reviews the concept, history, and such applications of the scheme.
Fast neutrons, which are neutrons with energies greater than 1 MeV, are expected to be a source of nondestructive inspection for a large-size infrastructure such as a bridge girder because of their mean free path exceeding the meter. A neutron-imaging device with 10-ns time resolution can discriminate pulsed neutrons from X-rays via time of flight. For this purpose, we require a fast-response neutron imager with large aperture and high image resolution. A neutron-imaging device with time resolution of 10 ns and aperture size of 40 cm × 60 cm was developed. It was filled with fast response liquid scintillator [1] in an aluminum honeycomb plate, which converts neutrons to optical light images. The scintillation light images were relayed using an optical lens and detected using a fast response image intensified CCD. The detector was tested at an electron linear accelerator (LINAC) facility in Osaka University. A short X-ray pulse (30 ps pulse duration) was generated using LINAC, and X-ray radiograph images were obtained with a 10- ns exposure time duration. The radiograph images were well attenuated within 10-ns from the X-ray injection. A high energy X-ray image and a neutron radiograph image of a 30-cm thick concrete block with iron blocks located behind it were successfully observed. This promising technique could facilitate nondestructive inspection of large concrete constructions.
LFEX is the world’s largest high-energy petawatt laser. So far it delivers 3 kJ/1 ps and is planed to finally deliver 10 kJ/10-20 ps. It has been constructed and became partially operational since 2008, and with full beams since 2014. LFEX is synchronized to nsec Gekko-XII laser for variety of experiments with nsec and psec simultaneous laser beams irradiating the targets for fast ignition and other high-energy density physics.
Laser-induced damage threshold (LIDT) in optical coating is very sensitive to organic contaminations accumulated in coating layers during storage and using condition. The sources of contamination are commonly exists, and optical coatings are easily contaminated regardless to the environment pressure, LIDT at ns region decreased largely by contamination, but LIDT at ps seems insensitive. In this study, we have investigated the influence of contamination of optical coating on LIDT and other optical properties. We examined several kinds of coating to clarify the sensitivity to the contamination. Degradations of LIDT were commonly observed in e-beam deposition, IAD and IBS. Some coatings changed spectral characteristics by contamination, and other coatings did not change. Some samples were contaminated as received condition, and some were very clean. Furthermore, we have investigated the characteristics of LIDT in dielectric coatings under the controlled contamination. LIDT of coating drops to 1/2 in the saturated toluene vapor at room temperature.
A heavy oil-contamination was observed on the optical components in LFEX pulse compressor. This contamination
came from the wall of compression chamber, and the damage threshold of the mirror dropped to 1/2 or1/3 of the original
value. The same contamination was observed in different compression chambers in our institute. The contamination
materials were identified as Paraffin-oil and DBP (Di-n-butyl phthalate). Several cleaning schemes were tried, but no
significant improvement was obtained. Finally, we found well-baked silica gel placed in the vacuum chamber improved
the contamination very much. In a small vacuum chamber, the damage threshold increase by 3 times, and this result
indicated the contamination of damage test sample. We also tried to remove contamination with dipping optics in the
water-alcohol mixture, and we obtained almost the same improvements with the silica gel.
Implosion and heating experiments of Fast Ignition (FI) targets for FIREX-1 laser fusion project have been performed
with Gekko-XII and PW/LFEX lasers at the Institute of Laser Engineering, Osaka University. Typical FI target has a
hollow cone for guiding the short-pulse heating laser beam at the time of the maximum compression. The cone is
mounted so as to in one-side penetrate the shell target. Detailed implosion hydrodynamics, FI heating and core plasma
formation of plastic (CD) shell target with gold cone have been clarified by observing those with ultra high-speed
imaging x-ray spectroscopy as well as neutron diagnostics. Multi-channel Multi-Imaging X-Ray Streak Camera
(McMIXS) was improved for observation of time-resolved x-ray images and time-resolved two dimensional temperature
distributions with spatial and temporal resolutions of 20 microns and 24 ps (42 Gfps), respectively. With this instrument,
one can observe heating properties of the imploded core such as spatial distribution of the heated region and its temporal
evolution. Also 2D-SIXS (Two-Dimensional Sampling Image X-ray Streak camera) coupled with an x-ray imager was
improved for time resolved x-ray imaging of the imploded core. Synchronization of the heating beam injection to the
implosion dynamics has been monitored with an x-ray framing camera. It was found that the shape of the core is neither
spherical nor uniform mainly because of the existence of the cone and moving toward the tip of the cone and interacting
with it. Experimental results are compared with two-dimensional hydrodynamic simulations. Target design taking into
account of these phenomena is quite important because such core movement and jet formation can affect the condition of
the cone.
In fast ignition experiment, it is important for effective heating of imploded core to control the injection time of heating laser synchronized with imploded core formation. However, it is difficult to measure the imploded core and heating laser injection at the same time. In this paper, we propose the simultaneous measurement using X-ray framing camera (XFC). In implosion experiment without heating laser, we observed only 2D thermal X-ray images of imploded core. On the other hand, in implosion experiments with heating laser, not only 2D X-ray images but also bright zones were observed on striplines. This zone is considered to show high energy X-ray from hot electron heated by heating laser. It is considered that the heating laser injection time is estimated from the peak position of this bright X-ray intensity profile. To explain the broad X-ray intensity profile, MCP gain calculation is attempted. The calculated results agree with experimental data qualitatively. Although the detailed X-ray energy measurement is needed, it is considered that we can estimate heating laser injection time with about 10 ps resolution from the peak position of high energy X-ray intensity profile.
Hydrodynamic instabilities are key issues of the physics of inertial confinement fusion (ICF) targets. Among the instabilities, Rayleigh-Taylor (RT) instability is the most important because it gives the largest growth factor in the ICF targets. Perturbations on the laser irradiated surface grow exponentially, but the growth rate is reduced by ablation flow. The growth rate γ is written as Takabe-Betti formula: γ = [kg/(1+kL)]1/2–βkm/pa, where k is wave number of the perturbation, g is acceleration, L is density scale-length, β is a coefficient, m is mass ablation rate per unit surface, and ρa is density at the ablation front. We experimentally measured all the parameters in the formula for polystyrene (CH) targets. Experiments were done on the HIPER laser facility at Institute of Laser Engineering, Osaka University. We found that the β value in the formula is ~ 1.7, which is in good agreements with the theoretical prediction, whereas the β for certain perturbation wavelengths are larger than the prediction. This disagreement between the experiment and the theory is mainly due to the deformation of the cutoff surface, which is created by non-uniform ablation flow from the ablation surface. We also found that high-Z doped plastic targets have multiablation structure, which can reduce the RT growth rate. When a low-Z target with high-Z dopant is irradiated by laser, radiation due to the high-Z dopant creates secondary ablation front deep inside the target. Since, the secondary ablation front is ablated by x-rays, the mass ablation rate is larger than the laser-irradiated ablation surface, that is, further reduction of the RT growth is expected. We measured the RT growth rate of Br-doped polystyrene targets. The experimental results indicate that of the CHBr targets show significantly small growth rate, which is very good news for the design of the ICF targets.
Hydrodynamic instability in laser-irradiated targets have been investigated in detail by using ultra high-speed x-ray radiographic technique. Recently developed high-resolution x-ray imaging for laser-driven Rayleigh-Taylor (RT) instability experiments as well as data including RT growth rate, ablation density and plasma density profile are desribed. Results are of great importance for comprehensive understanding of the dispersion relation of the laser-driven RT instability. Especially, direct observation of the ablation density was first achieved with temporal and spatial resolutions of 100 ps and 3 μm, respectively. Imaging techniques includes x-ray Moire imaging, x-ray penumbral imaging and Fresnel phase zone plate imaging coupled with x-ray streak cameras or x-ray CCD cameras. Experiments were performed by using Gekko-XII/HIPER laser system at the Institute of Laser Engineering, Osaka University.
We propose a heuristic method for the reconstruction of x-ray penumbral images from a noisy coded source. The ill-posed noisy penumbral image reconstruction problem is modeled as an optimization problem where the reconstructed image can be obtained by minimizing the mean square error between the obtained penumbral image and the estimated penumbral image, and Laplacian values of the estimated image. The Laplacian operator is used here a smoothness constraint. Since with heuristic methods, complicated a priori constraints can be easily incorporated by the appropriate modification of the cost function, the proposed method is well suited to the solution of this ill-posed problem. The proposed method has also been applied to real laser-plasma experiments.
Ultra-fast x-ray imaging is of great importance for diagnosing laser-driven inertial confinement fusion (ICF) plasmas. Typical required spatial and temporal resolutions are 10 micrometer and 10 ps, respectively. We have developed variety of one- (1D) and two-dimensional (2D) image sampling technique for ultrafast time-resolved x-ray imaging with x-ray streak cameras. Moire imaging of an x-ray-backlit target has been developed as 1D image sampling of an objective with 1D repetitive structure with a spatial resolution of 5 micrometer for use in experiments on hydrodynamic instabilities in laser- accelerated targets. With 1D sampling of repetitive 2D images, a multi-imaging x-ray streak camera (MIXS) with temporal- and spatial-resolutions of 10 ps and 15 micrometer, respectively, has been developed and successfully utilized for diagnosing uniformity and heating process of the imploded core plasmas. Two types of spectroscopic applications of the MIXS have been developed. One is multi-channel MIXS (McMIXS) which has three MIXS channels with various spectral responses for time- resolved 2D temperature measurement. Another is monochromatic MIXS (M-MIXS) for temperature, density and material mixing measurement, in which monochromatic images with Bragg crystals are coupled to MIXS. Finally, 2D image sampling of a 2D image on an x-ray streak camera (2D-SIXS) was also developed.
IN order to directly observe low-mode implosion nonuniformities, especially of l equals 1, which prevents stable formation of a hot spark in the compressed core plasma at the final stage of the implosion, a series of direct-drive implosion experiments has been performed at the Gekko-XII glass laser facility by using gas-filled plastic- shell targets. Partially coherent light (PCL) was used as a drive laser to suppress middle- to higher-modes of the irradiation nonuniformity down to approximately 1 percent. A clear shift from the center of the chamber and a slight crosswise structure were seen in the time- and space- resolved shape of the shell in the accelerating phase with an x-ray framing camera and absorbed laser intensity were also estimated by using a rocket equation. In order to confirm the property of these results, separate experiments under similar laser conditions were performed by using Au- coated sapphire spheres as surrogate target. The x-ray intensity distribution on the circumference of the target in the XFCs image, which is strongly dependent on the drive nonuniformity in the accelerating shell resulted from the l equals 1 drive nonuniformity. In our experiments, the l equals 1 drive nonuniformity due to some reproducible factors was found to be of the order of approximately 10-20 percent.
Indirect/direct-hybrid drive scheme to suppress the initial imprint of the laser irradiation nonuniformities has been proposed and investigated as a new drive scheme for inertial fusion. In direct drive inertial confinement fusion, initial imprinting of laser irradiation nonuniformity is considered to cause seeding of the perturbation on target surface in the very beginning of the irradiation which may be amplified by Rayleigh-Taylor instability in the acceleration phase of the implosion and be deleterious to efficient heating of the hot spark at the center of the compressed fuel core plasma. In indirect/direct-hybrid drive scheme, the target is first irradiated very uniformly with low-intensity soft x-ray prepulse from external sources apart from the target. Indirect x-ray pre-irradiation of the surface causes a pre- expansion layer of the plasma before the irradiation of the direct drive laser beam. When the drive beam comes later, the target has a substantial stand-off distance between the ablation front and the beam absorption region. Thus the thermal smoothing effect is expected to occur in this transport layer, and the initial imprint can be significantly reduced. We have demonstrated planar target experiments on the indirect/direct hybrid scheme and observed reduction of the initial imprint. Implosion experiments the indirect/direct hybrid drive spherical capsules with external x-ray sources has been started. Overall implosion was performed successfully.
The initial imprint of density perturbation due to spatial nonuniformity of laser intensity is one of the most important issues in laser fusion research. Several imprint mitigation scheme by means of soft x-ray radiation have been proposed to reduce the induced perturbation through the thermal conduction region. One of the schemes uses an external x-ray source prior to laser incidence to produce preformed plasma. Another has a low-density foam layer and high-Z material to heat the foam radiatively and make it uniform. We present the dynamics of these schemes and the perturbation growth with nonuniform laser from the results of 2D simulation using our integrated code.
In the direct-drive scheme implosion of the inertial confinement fusion, the hot spark formation is critically affected by laser irradiation non-uniformities and subsequent hydrodynamic instabilities. Influence of the low- modal irradiation non-uniformities on the hot spark formation was investigated by means of the time- and space- resolved x-ray spectroscopic measurements. Experimental results were compared with post-processed hydro-code simulations by the aid of x-ray spectrum analysis code.
Reviewed are the progress in direct drive implosion researches with Gekko XII laser system. Precise observation of the growth rate of Rayleigh-Taylor instability and the suppression of imprints using indirect-direct hybrid implosion have been investigated. Theoretical and experimental researchers on the fast ignition scheme are also studied. Relativistic laser plasma interaction experiments with Peta-Watt Module and Gekko XII are also described. Finally, the future direction of the research including the development of solid state laser for fusion reactor is discussed.
From 1995, ILE has started development of a high power chirp pulse amplification Nd:glass laser for investigating the fast ignitor concept as a new approach toward high gain inertial confinement fusion. The output of the new glass laser line is estimated to be 100 TW, 100 J, 1 ps and is focused on a high density compressed plasma by the GEKKO-XII laser for the additional fast heating.
We have successfully demonstrated double pass enhancement of amplified spontaneous emission of soft x rays, 23.2 and 23.6 nm of 3p - 3s transitions in Ne-like Ge, using an x-ray multilayer mirror. In this paper, we report on the fabrication of the mirror and analysis of its damage suffered during the experiments. The mirror used was a Mo-Si multilayer mirror with the reflectivity of 35% at the wavelength of 23.6 nm, deposited by an rf-sputtering system. In the damaged area of the mirror, only the multilayer was locally evaporated and the bare substrate underneath appeared. The size of the damaged area corresponded to the aperture size. We carried out the simulation on the spatial and temporal distribution of the mirror temperature during the experiment. Assuming that thermal x rays enter the mirror with the largest amount of energy among all the fluxes at the early stage of the enhancement, the result of the simulation can explain the damage feature and the temporal profile of the intensity of the amplified spontaneous emissions.
Basic characteristics of soft x-ray lasers generated as amplified spontaneous emission are described. Experimental results on soft x-ray amplification in neon-like germanium ions, obtained recently at the Institute of Laser Engineering, are reported. By comparing the experimental results with a simplified model on amplified spontaneous emission, basic parameters such as coherence and brightness of the Ge soft x-ray laser are evaluated.
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