The development of high pulse energy and high repetition rate lasers based on Yb:YAG ceramics is expected to achieve high average power in areas not previously achieved by high energy diode pumped solid state lasers (DPSSL). Such lasers are of interest for advanced materials processing, surface treatments such as laser peening, and pumping ultra-intense lasers for compact radiation and particle sources. The choices of gain media, amplifier geometry, thermal management, and extraction architecture are important aspects for development of a scalable high repetition rate and high energy laser system. We are aiming to develop a pulse energy of 100 J, repetition rate of 100 Hz using conductive-cooled Yb:YAG active-mirror amplifier with a liquid-nitrogen cooling. We report on the status of the development our laser.
We evaluated laser-induced damage thresholds (LIDTs) on silica glasses by two kinds of ultra-short soft X-ray laser pulses (13.5 nm, 70 fs, and 13.9 nm, 7 ps). The comparison of our experimental results and the reported values observed by nanosecond soft X-ray pulse revealed a pulse width dependence of LIDTs on silica glasses in the soft X-ray region. The relationship between the pulse width and LIDTs provided valuable discussions of the laser-induced damage mechanism.
We are investigating the output and temperature characteristics of Yb:YAG TRAM (Total-Reflection Active Mirror) laser using zero-phonon line excitation (969-nm pumping) and direct water jet cooling for efficient heat removal. The TRAM configuration has an advantage of cooling the surface of the Yb:YAG disk without the high-reflection coating. We have developed an efficient hydrodynamic cooling system, where the disk is directly cooled by impinging water jet with flow rate of up to 52 liter/min., while the water temperature can be controlled from 7 to 80 degrees Celsius. For the estimation of operating temperatures of the Yb:YAG, we measured fluorescence spectra from Yb:YAG using a spectrometer. We tested several types of TRAM with different layer thicknesses and doping concentrations, which were designed to absorb more than 80% of the pump power in a single bounce at room temperature. A fiber-coupled CW laser diode (FCLD) with 600 W output power at 969 nm was used as a pump source. The dependences of oscillator output power and the laser medium temperature on the cooling water temperature and flow rate were investigated. The direct impinging water jet at high flow rate was demonstrated to be effective for cooling the laser medium. It was also confirmed that the zero-phonon line excitation at 969-nm resulted in lower laser medium temperature and hence higher output power compared to the 940-nm pumping. In addition, we demonstrated kW-class laser oscillation using the cooling system and achieved slope efficiency of 63 %.
We had developed a unique porous thin films by a special coating method1. In this technique, two dielectric materials A and B having different refractive indices nA and nB ,where nA>nB are simultaneously deposited in vacuum on a substrate such as fused silica or optical glasses. Then the coated surface is processed in ultra-pure water which preferentially dissolves the material B. These processes result in a porous thin film
which has gradient refractive index and has the antireflection (AR) property over broad
bandwidth. The porous coating obtained by this method cannot apply depositing a multilayered dielectric thin film. We have developed a novel method. The present technique, a dielectric material D and a plastic P are simultaneously deposited in vacuum on a heated-substrate such as fused silica, ceramic or optical glasses. Then
the coated surface forms an adaptively mixed thin film ( AMTF ) with dielectric material and plastic. In this coating process, plastics partially evaporate due to the heated-substrate. The refractive index of the coated AMTF mainly decided by the mixing ratio of the dielectric material and plastic. In our samples the damage threshold was confirmed to be 115 J/cm2 at 10 ns and λ=1064 nm. The band width of AMTF with MgF2 and Teflon (AMTF: MgF2 ) was confirmed to cover from 200 to 8000 nm. This AMTF: MgF2 can be applicable not only to AR thin film, but to a high reflectance mirror and polarizer in various high intensity laser syetems.
1K.Yoshida, H.Yoshida, Y.Kato, and C.Yamanaka, Appl.Phy.Lett.47,911(1985)
Two kinds of pulsed lasers in Japan and Czech Republic were used to irradiate various sample materials to investigate the surface erosion thresholds under very hazardous environments including nuclear fusion chambers. The first was ArF laser in ILT and the second was XUV laser in IPP. These data were in-cooperated with our former data to build up our material strength data for our succeeding applications of various materials to a variety of fields. As an example, we proposed surface erosion monitors to notice the fusion chamber maintenance times with which the facilities can be protected from the collapses under very severe operation conditions. These kinds of monitors are expected to be useful for future different kinds of mechanical structures not only for the fusion chambers but also various chambers for many purposes. Special upconversion phosphors are also newly proposed to be used as the candidate materials to measure the thermal inputs onto the front surfaces of the armor structures. Optical transparent SiC was also newly tested to enrich our data base for our future diagnostic and protection possibilities.
Contamination of optics observed in LFEX compression chamber was a critical problem for maintaining high damage
threshold and high optical performance for mirrors and gratings in the vacuum environments. We conducted a study for
understanding this problem, and we found important knowledge on the nature of contamination, namely materials of
contaminants, source of contamination, the invasion mechanism, and removal method of contamination. We also found
the samples for the damage test is easily contaminated in the storage environment. This means the optical coating
accumulate contaminations even in the air. So we tested in-situ damage test in a gas flowing chamber with controlled
contaminants. The degradation was time-dependent phenomena, and proportional to the vapor
pressure of
contaminants. Several materials were tested, and even in water vapor, the damage threshold was decreased about 10%.
We also found out two methods for removing contaminations from the coatings. According to these studies, our
conclusion is special treatments should be used for knowing the intrinsic damage threshold of the coatings.
We have fabricated a 410 x 468 mm size deformable mirror with 100 Bimorph piezoceramic actuators for the LFEX
laser system at Osaka University. In the case of Bimorph-type deformable mirrors, the mirror surface had to be polished
and coated after bonding the piezoceramic actuators to the rear side of the thin mirror substrate. This provides a good
surface figure, but the coating temperature for the high-reflection mirror was strictly limited because of the thermal
fragility of piezoceramic actuators. The mirror substrate with the actuators was polished, and an ion-assisted multilayer
dielectric coating was produced at 60 degrees Celsius with our 80-inch coating chamber. The flatness of the mirror just
after coating was 7 μm, and reduced by aging to 3.2 μm when the mirror was assembled. The surface figure of the
assembled mirror with 20 piezostack bonded actuators is demonstrated and a laser-induced damage threshold tested with
a witness sample is also reported.
Tungsten and SiC are candidates for the structural materials of the nuclear fusion reactor walls, while CVD poly-crystal diamond is candidate for the window material under the hazardous fusion stresses. We measured the surface endurance strength of such materials with commercial displacement sensors and our recent evaluation method. The pulsed high thermal input was put into the material surfaces by UV lasers, and the surface erosions were diagnosed. With the increase of the total number of the laser shots per position, the crater depth increased gradually. The 3D and 2D pictures of the craters were gathered and compared under various experimental conditions. For example, the maximum crater depths were plotted as a function of shot accumulated numbers, from which we evaluated the threshold thermal input for the surface erosions to be induced. The simple comparison-result showed that tungsten was stronger roughly two times than SiC. Then we proposed how to monitor the surface conditions of combined samples with such diamonds coated with thin tungsten layers, when we use such samples as parts of divertor inner walls, fusion chamber first walls, and various diagnostic windows. We investigated how we might be able to measure the inner surface erosions with the same kinds of displacement sensors. We found out the measurable maximum thickness of such diamond which is useful to monitor the erosion. Additionally we showed a new scheme of fusion reactor systems with injectors for anisotropic pellets and heating lasers under the probable use of W and/or SiC.
Photo-thermal deflection (PD) method is one of the typical pump-probe techniques for absorption evaluation. In this
study, the PD method was used for detection of a signature of laser-induced damage prior to critical destruction. A setup
for the PD technique was incorporated into the nanosecond damage testing system and the absorption of some optics was
measured in combination with N-on-1 damage test. The absorption decreased with pulse-laser irradiation at first. Then,
the absorption increased at some point. Finally, the laser damage was caused and the absorption decreased. The
relaxation times of the absorption were also evaluated to reveal the reason for the variation. The measured results
indicated that the decrease came from the disposal of initial contamination and the effect might be considered as laser
conditioning. On the other hand, the increase of the absorption might be attributed to generation of laser-induced defects
(self-trapped excitons).
In high power laser systems, the laser-induced damage threshold (LIDT) in optical coating is very important parameters for obtaining high performances. Recently, LIDT was found to have strong temperature dependences in the bulk, surface of substrates, and in coatings. These temperature dependences of LIDT were carefully measured, and the damage formation model was constructed regarding to this temperature dependence. To explain this temperature dependence of LIDT, the temperature dependences of the initial electron generation and electron multiplication in the avalanche process were taken into account. On the other hand, LIDT in optical coating is very sensitive to organic contaminations accumulated in coating layers during storage and using condition. This paper also introduces the oil-contamination problem in LFEX laser system for First Ignition scheme in the laser fusion. We have analyzed contaminants and evaluated the effects of the contamination. We also developed new cleaning methods to remove contamination from the coating, and we have succeeded to prevent the degradation in LIDT for the duration of evacuation with Silica-gel in the chamber. The quantitative analysis of contamination on LIDT was made. We have investigated the characteristics of LIDT in dielectric coatings under the controlled contamination with several materials.
Electric field intensity formed in a multilayer mirror has been investigated numerically based on the analytical calculation, for the case where two laser beams with different wavelengths enter the mirror simultaneously. In this case, the intensity changes with time due to the interference between the two waves. By taking into account it, a design method of a multilayer mirror for high power lasers is proposed.
The temperature dependence of the laser-induced damage threshold on optical coatings was studied in detail for laser
pulses from 123 K to 473 K at different temperature using Nd:YAG laser (wavelength 1064 nm and pulse width 4 ns)
and Ti:Sapphire laser (wavelength 800 nm and pulse width 100 fs, 2 ps, and 200 ps). The six kinds of optical monolayer
coatings were prepared by electron beam evaporation and the coating materials were SiO2, Al2O3, HfO2, ZrO2, Ta2O5,
and MgF2. For pulses longer than a few picoseconds, the laser-induced damage threshold of single-layer coatings
increased with decreasing temperature. This temperature dependence was reversed for pulses shorter than a few
picoseconds. We describe the physics models to explain the observed scaling. The electron avalanche is essential to
explain the differences in the temperature dependence. In other words, the balance between linear process such as
electron avalanche etc. and nonlinear process such as multiphoton ionization etc. will be able to decide the tendency of
the temperature dependence. The proposed model also gives one of possibility for an extremely high LIDT optics.
In high power laser system, laser-induced damage threshold (LIDT) in optical coating is very important for obtaining
high performances. The dependence of LIDT on the pulse duration and the repetition rate are well known phenomena.
But recently, LIDT was found to have strong temperature dependences in the bulk, surface of substrates, and in coatings.
This temperature dependence of LIDT was carefully measured, and the damage formation model was constructed
regarding to this temperature dependence. This paper introduces LFEX laser system for First Ignition scheme in the
laser fusion. A large-scale pulse compression chamber was designed and constructed, and segmented grating system has
been employed for large-scale pulse compressor. This compressor provided good pulse compression performances, but
we observed a heavy oil-contamination of optics in this chamber. We have analyzed contaminants and evaluated the
effects of the contamination. We also developed new cleaning methods to remove contamination from the coating, and
the quantitative analysis of contamination on LIDT was made. 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.
Studies for temperature dependences of laser-induced damage thresholds for optical devise is introduced in this paper.
Additionally, the temperature dependence of the laser-induced damage threshold of single-layer optical coatings as
resent progress was clarified using Nd:YAG and Ti:sapphire lasers. The wavelengths of the lasers were 1064 nm and
800 nm and the pulse widths were 4 ns, 200 ps, 2 ps, and 100 fs. For pulses longer than a few picoseconds, the laser-induced
damage threshold of coated substrates increased with decreasing temperature. This temperature dependence was
reversed for pulses shorter than a few picoseconds. A flowchart was presented including the several mechanisms for
laser damage mechanism. The differences in the temperature dependence are explained by the flowchart. As one of
results in theoretical analysis, the electron resistivity i. e. electron mobility is key point to elucidate the temperature
dependence of laser-induced damage threshold.
The new method to estimate the erosion threshold of various material surfaces with high power pulsed electron beam
was extended to the surface erosions with various pulsed laser lights. We are interested in the erosions of candidate
materials to be useful under very severe eovironmental conditions with various laser lights, including ArF and Nd:YAG
laser lights.
Temperature dependence of laser-induced damage thresholds were measured by Nd:YAG laser (1064-nm wavelength, 4-
ns pulse width) and Ti:Sapphire laser (800-nm wavelength, 100-fs, 2-ps, and 200-ps pulse widths) to elucidate the effects
of laser-induced damage mechanisms. As experimental samples, SiO2, MgF2, Al2O3, HfO2, ZrO2, and Ta2O5 were
prepared by electron evaporation. With longer pulses than few picoseconds, laser-induced damage thresholds were
increased with decreasing temperature. Temperature dependence was reversed for shorter pulses than a few picoseconds.
The effects of temperature at different pulse width to laser-induced damage mechanisms were considered with separated
processes. In the conclusions, a temperature effect to free-electron generations by photoionization and multi photon
ionization is negligible. However, the temperature affects to electron multiple (electron avalanche) and critical density.
Electron multiple decreased at low temperature and the laser-induced damage thresholds increased. On the other hand,
critical density decreased at low temperature and the laser damage thresholds decreased. Influence of electron avalanche
is much greater than the impact of critical density. Thus, the trend and the strength of the temperature dependence on
laser-induced damage threshold will be decided by electron avalanche.
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.
Cr3+ co-doped Nd:YAG is very useful for the flashlamp or solar pumped laser through Cr3+ ion has broad absorption
bands in the visible region and an efficient energy transfer caused from Cr3+ to Nd3+. But the process of the energy
transfer is not understood well. It is also a problem that the absorption at near infrared is appeared with changes of Cr3+
to Cr4+. We investigated these problems by measuring the temperature dependence of fluorescence characteristics
(lifetime, excitation fluorescence and absorption) and the change after UV irradiation. The excitation fluorescence
spectra at 450 K increased because the Cr3+ absorption depended on temperature. The fluorescence lifetime excited at the
Cr3+ absorption bands shortened with increase temperature, that is, the energy transfer time will be short. Solarization
was appeared in Nd/Cr:YAG ceramics irradiated by KrF excimer laser pulses at 248 nm.
Laser-induced damage thresholds for dielectric and metal single-layer coatings at different temperature conditions
(123-473 K) were measured by 1064-nm wavelength and 4-ns pulses to elucidate the effects of initial temperature to laser
damage mechanisms. SiO2, MgF2, gold, silver and copper single-layer coatings were prepared as experimental samples.
In the experimental results, temperature dependence of LIDTs for optical substrates and all dielectric single-layer
coatings indicated same trend as that for bulk silica glasses, which increased linearly with decreasing the temperature.
However, all metallic coatings had the inverse trend of the dependence for dielectric coatings. The effects of initial
temperature to laser damage mechanisms were considered with separated processes from the experimental results. In the
conclusions, free-electron generation and electron multiple caused difficultly at low temperature and the laser-induced
damage thresholds increased. On the other hand, plasma heating caused easily at low temperature and the laser-damage
thresholds decreased.
Laser-induced damage thresholds of 2.2 picoseconds pulse for HR coatings prepared by Japanese optics makers were
measured. The damage thresholds were compared with that of 10 nanoseconds pulse measured at last year. In
picoseconds pulse, almost HR coatings had an average threshold of about 5 J/cm2. This meant that the damage thresholds
of picoseconds pulse were unconnected to a little defects and/or contaminants in the coatings.
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.
Laser-induced damage thresholds of HR and AR coatings prepared by Japanese optics makers were measured at
10-ns pulse and 1064-nm wavelength. The data would be an important database to improve the damage threshold for makers,
and to guide the designs of laser systems for optics users.
A linear increase of the laser-induced damage thresholds in silica glasses with decreasing the temperature was reported
in this conference at last year. Various nonlinear phenomena should be generated in silica glasses besides the damage in
high intensity. Temperature dependences of the nonlinear refractive indices and the SBS (stimulated Brillouin scattering)
thresholds in silica glasses at temperature 173 K to 473 K were measured with single-mode Q-switched Nd:YAG laser at
fundamental wavelength. As the result, the nonlinear refractive indices increased with decreasing temperature. Because
the change was not enough to explain the temperature dependence of laser-induced damage thresholds, the temperature
dependence of nonlinear refractive indices would be negligible on laser-induced damage thresholds. On the other hand,
the SBS thresholds also increased with decreasing temperature. This result means that acoustic phonons arise easily at
high temperature. Probably, the SBS phenomenon is one of reasons for temperature dependence of laser-induced damage
thresholds.
The laser-induced damage thresholds in silica glasses at different temperature conditions (123 K - 473 K) by Nd:YAG
laser fundamental (wavelength 1064 nm) and third harmonic (wavelength 355 nm) 4 ns of pulses were measured. In the
results, the damage thresholds increased at low temperature. At 1064 nm, the temperature dependence became strong by
the concentration of impurities. However, at 355 nm, the temperature dependences of almost sample were almost the
same for different concentration of impurities.
We have been developing a high average-power laser system for science and industry applications that can generate an output of 20 J per pulse at 10-Hz operation. Water-cooled Nd:glass zig-zag slab is pumped with 803-nm AlGaAs laser-diode modules. To efficiently extract energy from the laser medium, the laser beam alternately passes through dual zig-zag slab amplifier modules. Twin LD modules equipped on each slab amplifier module pump the laser medium with a peak power density of 2.5 kW/cm2. In high power laser system, thermal load in the laser medium causes serious thermal effects. We arranged cladding glasses on the top and bottom of the laser slab to reduce thermal effects.
The laser-induced damage threshold of polished fused silica surfaces is much lower than the damage threshod of its bulk. It is well known that contaminations of polished surface are one of the causes of low threshold of laser-induced surface damage. Particularly, polishing contamination such as cerium dioxide (CeO2) compound used in optical polishing process is embedded inside the surface layer, and cannot be removed by conventional cleaning. For the enhancement of surface damage resistance, various surface treatments have been applied to the removal of embedded polishing compound. In this paper, we propose a new method using slective chemical removal with high-temperature sulfuric acid (H2SO4). Sulfuric acid could dissolve only CeO2 from the fused silica surface. The surface roughness of fused silica treated H2SO4 was kept through the treatment process. At the wavelength of 355 nm, the surface damage threshold was drastically improved to the nearly same as bulk quality. However, the effect of our treatment was not observed at the wavelength of 1064 nm. The comparison with our previous results obtained from other surface treatments will be discussed.
The surface-damage resistance of fused silica was enhanced 2.8-fold by removing a subsurface damage. For the conventionally polished fused silica surface, μm-scale subsurface damage and a shallow (20 nm to 100 nm) structurally modified zone produced during grinding and polishing were formed on the top of surface. Several surface etching techniques and super-precise polishing process were used to remove subsurface damage from a fused silica surface. First the conventionally polished surfaces were chemically etched in a buffered HF solution to remove 300μm of surface material, and then super-precise polishing was performed to obtain an optical surface. After that, the polishing compound was removed by using ion-beam etching. The effect of subsurface damage on laser damage resistance was characterized by the measuring of the laser-induced damage threshold (LIDT) for the laser radiations of 1064 nm and 266 nm respectively. For the wavelength of 1064 nm, the effect of the removal of subsurface damage wasn't clearly seen, although the enhancement of surface-damage resistance by the ion-beam etching could be confirmed. However, in the case of 266 nm, enhanced LIDT of 28 J/cm2 was obtained from the subsurface damage removed surface. The surface LIDT increased by 2.8 times compared to that of conventionally polished fused silica surfaces.
We developed a novel coating method using chemical reactions of gaseous reactants on a surface. A self-limiting nature of surface chemical reactions allows precisely controlled growth of films with high uniformity and controllability of thickness over large area. The nonuniformity of thickness distribution was under 1% over 240 mm in diameter. The film thickness had proportional relationship with a number of chemical reactions. TiO2 films at growth temperature of 25 degree(s)C had a laser-induced damage threshold of 5 J/cm2 for 1-ns, 1.06-micrometers laser pulses. The laser damage resistance of TiO2 films decreased at higher growth temperature. TiO2 films grown at the high temperature had higher crystallinity. We clarified that the laser damages resulted from the local sites that absorbed the laser energy.
The gray-tracking of KTiOPO4 nonlinear crystals, which is used for second harmonic generation of solid state lasers, have been investigated. The optical absorptions were measured, and then the susceptibilities of gray-tracking were observed by measuring the reduction of transmittance of the crystal by irradiating of the laser pulses with the second harmonics of Nd:YAG laser. In addition, impurities in the crystals were analyzed. In comparison with the susceptibilities and the absorptions, the susceptibility of the gray-tracking of KTP crystals indicated a dependence on the initial absorption at 532-nm wavelength. Moreover, it is prevented with increasing OH concentration in the crystals. We propose a new approach toward the improvement of gray-tracking for KTP crystals using annealing process under control of the humidity.
KEYWORDS: Laser damage threshold, Laser induced damage, Chemical reactions, Thin films, Thin film growth, High power lasers, Molecules, Refractive index, Crystals, Signal detection
We prepared optical thin films grown with surface chemical reactions using TiCl4 and H2O for TiO2. The nonuniformity of thickness distribution was under 1% over 240 mm in diameter. The structure of TiO2 film grown at 25 degrees Celsius was amorphous. The structure changed into polycrystalline with an increase of growth temperature up to 400 degrees Celsius. Secondary ion mass spectrometry showed that chloride residents presented in the films at every growth temperature. However, these chloride residents could be removed by thermal annealing at 400 degrees Celsius. The TiO2 film at the growth temperature of 25 degrees Celsius had a laser-induced damage threshold of 5 J/cm2 for 1-ns, 1064 nm laser pulse. The damage threshold of TiO2 films decreased at higher growth temperature. Chloride in the films had no influence on the laser-induced damage threshold.
The gray-tracking of KTiOPO4 nonlinear crystals, which is used for second harmonic generation of solid state lasers, have been investigated. The susceptibilities of gray-tracking were evaluated by measuring the reduction of transmittance of the crystal with the second harmonics pulse of Nd:YAG laser, and were compared with the optical absorption coefficients and contaminants in each crystal. As result, the susceptibility indicated a dependence on the initial absorption at 532-nm wavelength. Moreover, it was prevented with increasing hydroxyl concentration in the crystals. We propose a new approach toward the improvement of gray-tracking for KTP crystals annealing process under control if the humidity.
The laser ablative figuring of the silica glass has been investigated experimentally. F2 laser and ArF excimer lasers are used as the laser light source for efficient ablation of silica glass material. The output beam of F2 and ArF laser were focused on to the surface of silica glass plate. The ablation rate were measured by a surface profilometer. The process on the surface was done by scanning in X-Y direction and a uniform ablation was observed. However the surface roughness was large as compared with the case of PMMA. The waveform of incident and transmitted laser light was measured by high speed photo-tubes to observe the time dependence of the absorption. The measured waveform indicates that the absorption was small at the leading edge of the laser pulse, and a strong ablation was induced at the latter part of laser pulse due to the excited state absorption. These phenomena are quite similar to both in F2 and ArF laser light. We have developed a simple model in which the instantaneous absorption is defined by the absorbed energy prior to the moment.
The effectiveness of multi-wedge plates array for the improvement of the laser irradiation uniformity on the pellet target in inertial confinement fusion has been investigated. A multi-wedge plates array of 90-mm in diameter was fabricated with wedge plates of 7 pairs, which ad a size of 30-mm diameter and a wedge angle of about 0.1 degrees. Also, to prevent the diffraction caused by each wedge plates, a soft aperture with the transmission profile of 16th super gaussian was designed and fabricated. As the result, it was confirmed that the array decreased the low- mode uniformity and is able to control the beam profile.
We prepared optical thin films grown with surface chemical reactions using TiCl4 and H2O for TiO2, Al(CH3)3 and H2O2 for Al2O3 as reactants. The uniformity of thickness distribution was good over 240 mm in diameter. The film thickness could be preciously controlled only by the number of cycles. Two-layer TiO2/Al2O3 anti-reflection coating with less than 0.2 percent reflection loss at 532nm was fabricated. Laser induced threshold strongly depend on the growth temperature and the introduced gas pressure. They increasing with decreasing the growth temperature and with increasing the introduced gas pressure.
The progress of implosion physics research and relevant technologies enable us to examine technical and economical feasibility, and to plan the realistic strategy to the commercial power plant. The most important key issue for IFE is the driver technology. The development of the laser fusion driver is opening new industrial technologies based on the photon processes and new fields of high energy physics.
Laboratory and field experiments for laser triggered lightning were performed in order to induce so called triggered lightning, in which an electric leader initiates from tall objects on the ground to thunderclouds, using a plasma channel produced by CO2 lasers and Nd lasers. Significant milestone was achieved in technological developments and verification of a scientific feasibility of the techniques such as determination of laser trigger timing which lead to laser triggered natural lightning. In the laboratory experiments technologies to produced long plasma channels and characteristics of discharge process induced by plasma channels were extensively investigated. In the field experiments developed are an automatic laser trigger system using an RF burst emission by a preliminary breakdown, methods to measure lightning path, thundercloud movement and field strength, and to make a continuous plasma channel at the top of the lightning tower which acts effectively for leader initiation.
A series of field experiments for laser triggered lightning have been carried out targeting the winter thunderstorms in Fukui, Japan. A combination of a 2 kJ CO2 laser system, a 600 J Nd-glass laser and a 4th harmonics 100 mJ YAG laser was used to produce a plasma channel which are effective to trigger and guide an electrical leader from the lightning tower. A new scheme of autotriggering laser system by preliminary breakdown (PB) has been developed. The PB triggering system demonstrated its capability of irradiating the laser at the right timing for triggering the leader. On February 11th 1997, in the field experiment, the electric leader was initiated by the laser plasma using the PB triggering system for the first time. A number of diagnostics including UHF interferometers, a tower current monitor, a capacitive antennas verified the initiation of leader triggered by the laser plasma both in time and space.
We have developed a super-lightweight mirror usable for laser fusion for the first time. The porous fused silica with density of 0.2 g/cm3 was sandwiched between two fused silica plates. The porous silica and the fused silica plates were contacted under high temperature of about 1500 - 1600 degree(s)C. The weight and optical flatness of the mirror (3500 mm(phi ) X 36 mmt) were about 1.7 kg and (lambda) /8, respectively. A larger diameter mirror of 530 mm(phi ) is being fabricated.
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