Kenjiro Takahashi

30-TW laser–plasma interactions at ILE, Osaka

The 30-TW GEKKO-MII short pulse laser is completed and is used for high field physics studies, including a basic fast ignitor research. The second harmonic (2ω) backscattering shows critical surface recession due to the laser photon pressure. A few hundred protons and deuterons are observed. The 100-TW GEKKO-XII short pulse laser, a petawatt laser module, is constructed to study a feasibility of fast ignition of an imploded target.

Non-ideal detonation of emulsion explosives

Abstract In order to obtain a better understanding of the underwater explosion phenomena related to emulsion explosives, optical measurements using the water tank technique and cylinder expansion test were carried out. Streak photographs were taken using an image converter camera with a usual shadow graph system. Four kinds of emulsion explosives differing in aluminium contents were used in the experiments. These emulsion explosives were placed into copper and PMMA pipes. From the results obtained with the water tank technique, it is recognized that the maximum velocity of underwater shock wave in the case of a copper pipe is about 10% faster than that in the case of a PMMA pipe. From the cylinder expansion test, it is found that the degree of radial displacement of a copper case (Uexp) increases with increased aluminium content and that in the range of more than 10 wt.%, Uexp decreases with increased aluminium content.

Dynamic fracture of tantalum under extreme tensile stress

The dynamic fracture of tantalum is observed at the atomic scale using an x-ray monitoring technique at the SACLA XFEL facility. The understanding of fracture phenomena of a material at extremely high strain rates is a key issue for a wide variety of scientific research ranging from applied science and technological developments to fundamental science such as laser-matter interaction and geology. Despite its interest, its study relies on a fine multiscale description, in between the atomic scale and macroscopic processes, so far only achievable by large-scale atomic simulations. Direct ultrafast real-time monitoring of dynamic fracture (spallation) at the atomic lattice scale with picosecond time resolution was beyond the reach of experimental techniques. We show that the coupling between a high-power optical laser pump pulse and a femtosecond x-ray probe pulse generated by an x-ray free electron laser allows detection of the lattice dynamics in a tantalum foil at an ultrahigh strain rate of ε. ~2 × 108 to 3.5 × 108 s−1. A maximal density drop of 8 to 10%, associated with the onset of spallation at a spall strength of ~17 GPa, was directly measured using x-ray diffraction. The experimental results of density evolution agree well with large-scale atomistic simulations of shock wave propagation and fracture of the sample. Our experimental technique opens a new pathway to the investigation of ultrahigh strain-rate phenomena in materials at the atomic scale, including high-speed crack dynamics and stress-induced solid-solid phase transitions.

Measurements of laser-hole boring into overdense plasmas using x-ray laser refractometry (invited)

We developed a 19.6 nm laser x-ray laser grid-image refractometer (XRL-GIR) to diagnose laser-hole boring into overdense plasmas. The XRL-GIR was optimized to measure two-dimensional electron density perturbation on a scale of a few tens of μm in underdense plasmas. Electron density profiles of laser-produced plasmas were obtained for 1020–1022 cm−3 with the XRL-GIR and for 1019–1020 cm−3 from an ultraviolet interferometer, the profiles of which were compared with those from hydrodynamic simulation. By using this XRL-GIR, we directly observed laser channeling into overdense plasmas accompanied by a bow shock wave showing a Mach cone ascribed to supersonic propagation of the channel front.

Experimental investigation of CFRP ablation with UV and N-IR nanosecond lasers

A carbon fiber reinforced plastic (CFRP) is widely used for automobile, aircraft and so on, because of high strength, lightweight and weather resistance. The cutting of CFRP is difficult since it was composed of complex matrix of carbon fiber and epoxy resin. The laser processing is suitable way to machining tool for CFRP. By applying the laser cutting of CFRP, the epoxy resin was quickly evaporated before the carbon fiber cut, which was caused formation of heat affected zone (HAZ). It is required to decrease the HAZ.In this study, nanosecond lasers at UV and IR were employed to cut the CFRP. The ablation plume of CFRP was observed by spectrometer and laser microscope analysis was conducted to evaluate the HAZ. The results revealed that the HAZ formation depended on the laser wavelength. The HAZ of UV laser became smaller than that of IR laser.A carbon fiber reinforced plastic (CFRP) is widely used for automobile, aircraft and so on, because of high strength, lightweight and weather resistance. The cutting of CFRP is difficult since it was composed of complex matrix of carbon fiber and epoxy resin. The laser processing is suitable way to machining tool for CFRP. By applying the laser cutting of CFRP, the epoxy resin was quickly evaporated before the carbon fiber cut, which was caused formation of heat affected zone (HAZ). It is required to decrease the HAZ.In this study, nanosecond lasers at UV and IR were employed to cut the CFRP. The ablation plume of CFRP was observed by spectrometer and laser microscope analysis was conducted to evaluate the HAZ. The results revealed that the HAZ formation depended on the laser wavelength. The HAZ of UV laser became smaller than that of IR laser.

Analysis of laser ablation dynamics of CFRP in order to reduce heat affected zone

A carbon fiber reinforced plastic [CFRP], which has high strength, light weight and weather resistance, is attractive material applied for automobile, aircraft and so on. The laser processing of CFRP is one of suitable way to machining tool. However, thermal affected zone was formed at the exposure part, since the heat conduction property of the matrix is different from that of carbon fiber. In this paper, we demonstrated that the CFRP plates were cut with UV nanosecond laser to reduce the heat affected zone. The ablation plume and ablation mass were investigated by laser microscope and ultra-high speed camera. Furthermore, the ablation model was constructed by energy balance, and it was confirmed that the ablation rate was 0.028 μg/ pulse in good agreement with the calculation value of 0.03 μg/ pulse.

Study of laser channeling into an overdense plasma

Abstract Channel formation in a laser produced plasma was studied with the density profile observed by an UV probe beam, X-ray pinhole pictures, and time resolved spectra of back scattered laser light. When a 1053 nm laser light was focused on a preformed plasma created on a 100 μm thick plastic at a laser intensity of 2 × 10 17 W/cm 2 , the UV probed picture showed a hole on the density profile at the spot of laser focus. The X-ray pinhole picture showed a 30 μm diameter spot on the surface of plastic target, even though the best focus was placed at more than 100 μm in front of the surface. The back scattered light spectra showed a red shift much larger than a normal SBS or Doppler shift. The focused laser beam appeared to self focus over 100 μm distance in a preformed plasma and reached beyond a critical density.

XUV laser grid image refractometry applied in laser hole boring experiments

We measured laser channeling into an overdense plasma by using a 19.6 nm Ne-like Ge XUV laser. One micrometer/100 ps laser light at 1017 W/cm2 interacted with a long scale length plasma preformed on a CH slab target. Grid image refractometry (GIR) with the x-ray laser was applied to obtain the deflection information in the plasma, which provided two dimensional density profiles (2-D) of the overdense plasmas.

Experimental investigation of CFRP cutting with nano second laser under air and Ar gas ambience

A carbon fiber reinforced plastic (CFRP) is widely used for automobile, aircraft and so on, because of having high strength, lightweight and weather resistance. A laser is one of useful tools for cutting CFRP. However, a matrix evaporated zone (MEZ) is formed around the laser irradiation area since heat property of the resin is different from that of carbon fiber. It is required for optimizing the laser processing condition to minimize the MEZ. In our experiment, the CFRP plate was cut with a nanosecond laser under air and Ar gas ambience. The ambient gas is an important factor for reduction of MEZ since formation of MEZ might be caused due to an oxidization of carbon fiber and epoxy resin. In order to evaluate the oxidization, spectroscopic analysis was carried out to investigate an ablation plume under air and Ar gas. Furthermore, a surface on CFRP plate was observed with a scanning electron microscope (SEM). As the results, the cutting quality for argon is better than that for air, and the MEZ for Ar gas is smaller than than that for air.

Nanosecond laser processing of CFRP in Ar gas ambience for HAZ reduction

A cross type of carbon fiber reinforced plastic (CFRP) plate, which composed of a carbon fiber and an epoxy resin, was put on the X-Y stage in the processing chamber. Ar gas was filled at 0.1 MPa in the chamber after exhaust at 1Pa. In this condition, the laser was irradiated the CFRP passing the attenuator for adjusting the laser intensity. After laser irradiation, the matrix evaporated zone (MEZ) was investigated with a confocal laser microscope. In Ar gas ambience, the MEZ was occurred few delamination area from 1.0 × 108 to 1.0 × 1010 W/cm2 of the laser fluence. The results revealed that the laser cutting of the CFRP in Ar gas ambience might be a useful method to suppression of the oxidization of the carbon fiber.A cross type of carbon fiber reinforced plastic (CFRP) plate, which composed of a carbon fiber and an epoxy resin, was put on the X-Y stage in the processing chamber. Ar gas was filled at 0.1 MPa in the chamber after exhaust at 1Pa. In this condition, the laser was irradiated the CFRP passing the attenuator for adjusting the laser intensity. After laser irradiation, the matrix evaporated zone (MEZ) was investigated with a confocal laser microscope. In Ar gas ambience, the MEZ was occurred few delamination area from 1.0 × 108 to 1.0 × 1010 W/cm2 of the laser fluence. The results revealed that the laser cutting of the CFRP in Ar gas ambience might be a useful method to suppression of the oxidization of the carbon fiber.