Hydrogen storage alloys become more and more important in the fields of electric energy production and stage and automobiles such as Ni-MH batteries. The vacancies introduced in hydrogen absorption alloy by charged particle beams were found to be positive effect on the increase in the initial hydrogen absorption reaction rate in the previous study. The initial reaction rates of hydrogen absorption and desorption of the alloy are one of the important performances to be improved. Here, we report on the characterization of the hydrogen absorption reaction rate directly illuminated by a femtosecond and nanosecond lasers instead of particle beam machines. A laser illuminates the whole surface sequentially on a tip of a few cm square LaNi4.6Al0.4 alloy resulting in significant improvement in the hydrogen absorption reaction rate. For characterization of the surface layer, we perform an x-ray diffraction experiment using a monochromatized intense x-ray beam from SPring-8 synchrotoron machine.
Laser induced breakdown spectroscopy (LIBS) and endoscope observation were combined to design a remote probing device. We use this probing device to inspect a crack of the inner wall of the heat exchanger. Crack inspection requires speed at first, and then it requires accuracy. Once Eddy Current Testing (ECT) finds a crack with a certain signal level, another method should confirm it visually. We are proposing Magnetic particle Testing (MT) using specially fabricated the Magnetic Particle Micro Capsule (MPMC). For LIBS, a multichannel spectrometer and a Q-switch YAG laser were used. Irradiation area is 270 μm, and the pulse energy was 2 mJ. This pulse energy corresponds to 5-2.2 MW/cm2. A composite-type optical fiber was used to deliver both laser energy and optical image. Samples were prepared to heat a zirconium alloy plate by underwater arc welding in order to demonstrate severe accidents of nuclear power plants. A black oxide layer covered the weld surface and white particles floated on water surface. Laser induced breakdown plasma emission was taken into the spectroscope using this optical fiber combined with telescopic optics. As a result, we were able to simultaneously perform spectroscopic measurement and observation. For MT, the MPMC which gathered in the defective area is observed with this fiber. The MPMC emits light by the illumination of UV light from this optical fiber. The size of a defect is estimated with this amount of emission. Such technology will be useful for inspection repair of reactor pipe.
We have been developing a new laser cladding system to repair the damages of parts in aging plants. It consists of some devices which are a laser torch, composite-type optical fiber, QCW fiber laser and etc. All devices are installed in a mobile rack, so we can carry it to plants, laboratories or anywhere we want to use. We should irradiate the work with the best accuracy of laser beam and filler wire in laser cladding. A composite-type optical fiberscope is useful. This fiberscope was composed of a center fiber for beam delivery surrounded by 20000 fibers for visible image delivery. Thus it always keeps target on center of gun-sight. We succeeded to make a line laser cladding on an inside wall of 1-inch tube by our system. Before this success, we solved two serious problems which are the contamination of optics and the deformation of droplet. Observing laser cladding process by X-ray imaging with Spring-8 synchrotron radiation, we found that the molten pool depth was formed to be under a hundred micrometers for 10 milliseconds. A Quasi-CW fiber laser with 1 kW was employed for a heat source to generate the shallow molten pool. The X-ray shadowgraph clarified that a molten droplet was formed at the edge of a wire up to a millimeter size. It grew up if the wire didn’t contact with the tube wall in initial state. Here we succeeded to measure the thermo-electromotive force voltage between a wire and a tube metal to confirm whether both came in contact. We propose to apply the laser cladding technology to the maintenance of aging industrial plants and nuclear facilities.
We have developed heat resistant strain sensors using laser processing techniques. The application is aimed at structural health monitoring for high temperature piping systems. This situation requires extraordinary durability such as radiation resistance and noise isolation due to adverse conditions caused by nuclear reactions or electro-magnetic pulses. We proposed that a Fiber Bragg Grating (FBG) sensor made by femtosecond laser processing could be the best candidate. The combination of fabric reinforcement and a heatproof adhesive mold successfully protected the fragile optical fiber once the fiber was installed on the piping material’s surface. To make the best use of the heat-resistant characteristic, we fixed the FBG sensor by metal mold. A groove was processed onto the surface of a SUS metal plate with a grindstone. We used a Quasi-CW laser to weld a filler wire onto the plate. The optical fiber was situated under the filler wire before was heated by laser pulses with 10 joule energy and a duration of 10 ms. A series of weld pool formed a sealing clad on the groove. The FBG sensor was buried at a depth of 1 mm over a length of 1 cm. No degradation in its reflection spectra was detected before and after the processing. The FBG sensor can detect the vibration of the plate caused by impact shocks. In this paper, the Bragg peak shift of the FBG sensor under laser cladding condition has been discussed.
Femtosecond laser pulses successfully ablated the work hardening layers on SUS316L used in boiling water reactors. The measurement of hardness inside the material clarified this new technique to reduce the risk of Stress Corrosion Cracking.
Direct bonding without the use of adhesives was demonstrated on Ti:sapphire laser crystals with a bonding surface of 12 mm x 6 mm and the bonded region was evaluated from the macroscopic to the atomic level by three different methods. Wavefront distortion caused by the bonded region of 10 mm x 5 mm was estimated at 0.031 wavelengths (λ) at 633 nm. Micro defect measurements by a laser tomography method showed that the number of micro defects on the bonded region were much smaller than that of the intrinsic ones inside the crystal. From a magnified inspection, atoms in the bonded region were well arranged with the same regularity as inside the crystal. In addition, micro defects 1 nm in size appeared slightly along the bonded interface where the titanium ion concentration was four times higher than other parts of the crystal.
Thermal lens effects on highly pumped Yb doped phosphate glass was measured by a Shack-Hartmann wavefront sensor for the development of compact chirped pulse amplification systems. High energy pump pulses of 1 - 2 Joules were produced by a flashlamp pumped Ti-sapphire laser. The pumping intensity on the Yb:glass surface exceeded 800 kW/cm2. The pulse energy of 330 mJ from Yb:glass was obtained with 53% slope efficiency with 0.5 Hz reputation. The absorbed pump energy generated the thermal lens effects inside the Yb:glass. The wavefront distortion completely disappeared after 300 ms of pump pulse. Neither heat accumulation nor pumping damage was observed on the Yb:glass.
A high energy flashlamp pumped Ti:sapphire laser has been developed for the pumping source of Yb:glass chirped pulse amplification. The free running oscillator generates 12 Joule/pulse at 793 nm at 1 Hz repetition. The output energy of 6 Joule/pulse at 920 nm has been obtained.
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