We apply independent component analysis (ICA) to real data from a gravitational wave detector for the first time. Specifically, we use the iKAGRA data taken in April 2016, and calculate the correlations between the gravitational wave strain channel and 35 physical environmental channels. Using a couple of seismic channels which are found to be strongly correlated with the strain, we perform ICA. Injecting a sinusoidal continuous signal in the strain channel, we find that ICA recovers correct parameters with enhanced signal-to-noise ratio, which demonstrates the usefulness of this method. Among the two implementations of ICA used here, we find the correlation method yields the optimal results for the case of environmental noise acting on the strain channel linearly.

The chemical structures of various ion-beam irradiated isotactic-polypropylene samples were studied. Results of micro-Fourier transform infrared spectroscopy and ultraviolet–visible spectroscopy suggest not only the linear energy transfer, but also the fluence is effective in local transformation of the isotactic-polypropylene.

We discuss prospects for direct measurement of stochastic gravitational wave background around 0.1--1 Hz with future space missions. It is assumed to use correlation analysis technique with the optimal time-delay-interferometry (TDI) variables for two sets of LISA-type interferometers. The signal to noise for detection of the background and the estimation errors for its basic parameters (amplitude, spectral index) are evaluated for proposed missions.

The Laser Interferometer Space Antenna (LISA) is expected to detect N similar to 22 x 10(+/-1) close white dwarf binaries in the Large Magellanic Cloud ( LMC) through their gravitational radiation with signal-to-noise ratios greater than similar to 10 in observational durations of 3 yr or more. In addition to chirp mass, location on the sky, and other binary parameters, the distance to each binary is an independent parameter that can be extracted from an analysis of gravitational waves from these binaries. Using a sample of binaries, one can establish the mean distance to the LMC as well as the variance of this distance. Assuming no confusion noise at frequencies above 2 mHz, for data collected over 10 yr, LISA might determine the LMC distance to similar to 4.5(N/22)(1/2) (1 yr/T(obs))(1/2)% and the line-of-sight extent of the LMC to similar to 15(N/22)(1/4)%, relative to its distance, at the 1 sigma confidence. For a 3 yr observational window, these estimates are degraded by a factor of similar to 2.5. In general, the estimates based on LISA are competitive with some of the proposed direct geometric techniques of measuring the LMC distance in the future with missions such as the Space Interferometry Mission and the Global Astrometric Interferometer for Astrophysics.

We study how the angular resolution of the Laser Interferometer Space Antenna for merging massive black-hole binaries would be improved if we observe multiple gravitational wave ``images'' due to strong gravitational lensing. The correlation between fitting parameters is reduced by the additional information of the second image which significantly reduces the error box on the sky. This improvement would be very helpful for identifying the host galaxy of a binary. The angular resolution expected with multiple detectors is also discussed.

(2003). Porosity formation mechanism and its prevention in laser welding. Welding International: Vol. 17, No. 6, pp. 431-437.

We calculate how accurately parameters of the short-period binaries (10-4 Hz ≲ f ≲ 10-2 Hz) will be determined from the gravitational waves by the Laser Interferometer Space Antenna (LISA). In our analysis, the chirp signal, , is newly included as a fitting parameter, and dependence on the observation period or wave frequency is studied in detail. Implications for gravitational wave astronomy are also discussed quantitatively.

Pulsed YAG laser spot welding has been noted as high-quality and good-flexibility joining technology. However, porosity is likely to form in a deeply penetrated spot weld. In this study, therefore, the keyhole behavior and porosity formation situation during YAG laser spot welding of Type 304 and A5083 were observed by the microfocused X-ray transmission method. It was consequently measured that the drilling and collapse speeds of the keyhole in Type 304 molten pool were about 0.5 m/s and 1 m/s, respectively, under the laser irradiation of a rectangular pulse shape. Namely, the collapse of the keyhole was so rapid as to form a bubble and porosity. It was also confirmed that the porosity was formed from the middle or bottom part of a deep and narrow keyhole as a result of its abrupt collapse. Moreover, spot welding was tried under the conditions of proper pulse shapes devised for the porosity reduction. Consequently the optimum saw-teeth-like pulse shape with tailing power was revealed for the prevention of porosity, and the mechanism of porosity formation and prevention was proved by the X-ray transmission observation method.

The effect of vacuum on weld penetration and porosity formation was investigated in high-power CW CO2 and YAG laser welding. It was consequently confirmed in welding with both lasers that the penetration was slightly deeper in aluminum alloys and was improved in austenitic stainless steel with a decrease in the ambient pressure. It was also revealed that no porosity was present in the materials welded at lower pressures. The reason for no porosity formation in vacuum was examined by observing keyhole behavior, bubble and porosity formation situation and liquid flow in the molten pool during high power YAG laser welding under various conditions through the microfocused X-ray real-time observation system. It was confirmed in the coaxial Ar or He shielding gas that a lot of bubbles were generated near the bottom part of molten pool from the tip of a fluctuated keyhole and resulted in large pores. On the other hand, under the vacuum conditions, no bubbles were formed in the melt pool from the keyhole, although the middle and bottom parts of the keyhole swelled in the molten pool probably because the evaporation of metals was so intense. Moreover, quite different liquid flows were observed between the normal and vacuum welding. Namely, there was a strong molten flow from the bottom of molten pool near the keyhole tip along the solidification interface to the upper rear part in the normal welding, while the liquid flowed upwards along the rear keyhole wall probably due to the strong stream of metallic vapors in vacuum. It is considered in vacuum welding that the liquid flow into the bottom part of the molten pool from the keyhole does not occur because of the direction of evaporated metals toward the upper keyhole outlet. This may exert a beneficial effect on the reduction or prevention of pores or porosity.

(2001). Porosity formation mechanism and suppression procedure in laser welding of aluminium alloys. Welding International: Vol. 15, No. 3, pp. 191-202.

In order to apply aluminum alloys to structural components, they should be joined with higher strength than that of the original materials. However, blowholes are apt to form in aluminum welding, because the process of melting and solidifying the material is unstable. Aluminum alloys were welded using 2-kW and 3-kW continuous wave Nd:YAG lasers with the aim of obtaining a stable welding process. Two beams were delivered by optical cables 0.6 mm in diameter and focused on the surface of the specimens as dual spots. At a shorter beam distance of 0.36 mm, the weld bead surface was humped, making it unacceptable in terms of quality. Sound weld beads were obtained at beam distances of 0.6 mm and 1.0 mm. X-ray observation was carried out in order to investigate the mechanism of process stabilization. A large keyhole opening that was observed at a large beam distance is thought to result in a stable welding process.

Laser welding can produce a deeply penetrated bead at high speed. However, in high power cw CO2 laser welding, the characteristic porosity is easily formed in the weld metal, but its formation mechanism has not been well understood. Therefore, the authors have conducted systematic studies of the elucidation of porosity formation mechanism and the development of preventive remedies. They have revealed that many bubbles are formed, mainly from the bottom tip of the keyhole by intense evaporation of the metal. It has also been revealed that the keyhole fluctuates frequently and changes its size and shape, corresponding to the intermittent bubble formation. The majority of bubbles are trapped at the solidifying front in the rear part of the molten pool. However, there are few reports that deal with the simultaneous observation of keyhole and plasma dynamic behavior as well as the formation of bubbles and porosity. In this study, therefore, the interrelationship between keyhole and plasma behavior was examined by using two synchronized high-speed cameras and an x-ray transmission observation system. Especially, the effect of shielding gas on porosity formation was investigated in terms of plasma and keyhole behavior. In the case of He shielding gas, metallic plasma emanated from a keyhole, and the keyhole was open continuously. On the other hand, in the case of N2 shielding gas, a big gas plasma was formed above the weld bead periodically, and metallic plasma and keyhole disappeared just like in the pulsed laser welding. Such periodical interval and duty were different depending on the materials used and exerted an effect on porosity formation tendency.

A deep cavity called keyhole is formed in the laser weld pool due to the intense recoil pressure of evaporation. The formation of keyhole leads to a deep penetration weld with high aspect ratio. However, a hole drilled in a liquid pool is primarily unstable by its nature and the instability of keyhole also causes the formation of porosity in the weld metal. The porosity formation is one of the serious problems in the very high power laser welding, but its mechanism has not been well understood. The authors have conducted systematic studies on observation of keyhole as well as weld pool dynamics and their related phenomena to reveal the mechanism of porosity formation and its suppression methods. The paper describes the real time observation of keyhole and laser plasma/plume behaviors in the high power CW CO2 laser welding by the high speed optical and X-ray transmission methods, cavity formation process and its suppression measures.

It is well known that porosity is easily formed in high power laser welding, which is quite a serious problem to be solved. At present, there are few reports studying interrelationship between keyhole and plasma behavior with the objective of understanding the effect of shielding gas on porosity formation. In this study, therefore, the relationship between keyhole and plasma behavior was observed directly by using two synchronized ultra high-speed cameras and X-ray transmission observation system. In the case of He gas, metallic plasma was continuously formed, and the keyhole was always open. It was observed that many large bubbles, which were formed from the tip of a keyhole, were trapped at the solidifying front in the rear part of the molten pool, and lead to the porosity formation. On the other hand, in the case of N2 gas, big nitrogen plasma was formed above the weld bead periodically, and its absorption of laser caused the disappearance of metallic plasma and keyhole. This periodical interval and duty were different among materials used and affected the bubble and porosity suppression beneficially.

With the objectives of clarifying the formation mechanism of porosity and producing a sound weld bead, welding conditions of porosity formation were investigated in A5083 alloy and Type 304 steel welded with a high power YAG laser, and the behavior of a keyhole, bubbles and porosity as well as liquid flows were observed during laser welding through X-ray transmission imaging system using markers. It was confirmed that a lot of bubbles and pores were formed in 3.5 kW YAG laser weld beads produced in Ar, He and N-2 gases except Type 304 in N-2 gas. Porosity was reduced at high welding speed in Type 304 steel even in He and Ar gases. A lot of bubbles were formed by the evaporation of metals from the bottom tip of the keyhole and flowed upwards in front of the solid-liquid interface. Some bubbles disappeared out of the molten surface especially in A5083 alloy welded at low welding speed, but the majority of bubbles were trapped at the solidifying front of the weld beads in most cases. The shielding gas was also included in the porosity. This mechanism is similar to that in high power CO2 laser welding. Fast liquid flows occurred circularly from the bottom keyhole to the rear upper part of the molten pool, from the rear to the front near the pool surface, and from the top to the bottom behind the keyhole in weld molten pools of both A5083 alloy and Type 304 steel in He, Ar or N-2 shielding gas. Slightly different flows were noticed in the molten pool of Type 304 steel between YAG and CO2 lasers.

It is generally acknowledged in any steels or alloys that porosity is liable to be formed in a keyhole type of deeply penetrated fusion zones made with a high power laser. Therefore, this investigation was carried out with the objectives of elucidating the formation mechanism of porosity and developing preventive procedure of pores in high power laser welding.The formation behavior of keyhole, bubbles and porosity was observed during butt-joint or bead-on-plate CO2 laser welding of stainless steel by microfocused X-ray transmission in-situ imaging system with high speed video camera. The influence of various laser welding conditions on porosity formation was investigated. As a result, it was observed that many bubbles were predominantly frequently formed at the bottom of a molten pool from the tip part of a deep keyhole in consequence of its dynamic motion due to intense evaporation, and some bubbles were formed from the middle part of the keyhole in the case of focal point under the plate surface or high power laser irradiation. It was also seen that most of the bubbles were soon captured or trapped into pores by the solidifying solid-liquid interface during floating up in a stainless steel. SEM observation result of a fractured surface demonstrated traces that liquid was' penetrated and solidified inside pores, probably because evaporated materials trapped in the bubble solidified and/or the temperature dropped to render their inside pressure lower. According to Q mass-spectroscopic analysis of gas content of porosity, it was revealed that He shielding gas and H2 gas were included inside a pore near the bottom of the weld fusion zone. It was observed to be feasible to produce a sound full-penetration weld bead without porosity in steel plates of 10 mm thickness, since no bubbles were formed from the bottom part of the fully penetrated keyhole; Bubbles, porosity or pores were confirmed to be reduced by utilizing N2 shielding gas and forward keyhole-inclination welding in He shielding gas even in the case of partially penetrated weld.