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Journal Articles

Low electron temperature in ablating materials formed by picosecond soft X-ray laser pulses

Ishino, Masahiko; Hasegawa, Noboru; Nishikino, Masaharu; Pikuz, T.*; Skobelev, I. Y.*; Faenov, A.*; Inogamov, N.*; Kawachi, Tetsuya; Yamagiwa, Mitsuru

X-Ray Lasers and Coherent X-Ray Sources; Development and Applications XI (Proceedings of SPIE, Vol.9589), p.958904_1 - 958904_6, 2015/09

 Times Cited Count:1 Percentile:60.64

To study the ablation process induced by the soft X-ray laser pulse, we investigated the electron temperature of the ablating material. Focused soft X-ray laser pulses having a wavelength of 13.9 nm and duration of 7 ps were irradiated onto target surfaces, and we observed the optical emission from surfaces by use of an optical camera. On target surfaces, we could confirm damage structures, but no emission signal in the visible spectral range during ablation could be observed. Then, we estimated the electron temperature in the ablating matter, and we estimated the electron temperature to be lower than 1 eV and the process duration was shorter than 1000 ps. This result is in good accordance with the theoretical prediction. Our investigation implies that the spallative ablation occurs in the low electron temperature region of a non-equilibrium state of warm dense matter. We will present the current state of our eperimental results.

Journal Articles

Observation of dynamics and modification of solid surface using a picosecond soft X-ray laser

Nishikino, Masaharu; Kawachi, Tetsuya; Hasegawa, Noboru; Ishino, Masahiko; Minami, Yasuo*; Suemoto, Toru*; Onishi, Naofumi*; Ito, Atsushi*; Sato, Katsutoshi*; Faenov, A.*; et al.

X-Ray Lasers and Coherent X-Ray Sources; Development and Applications XI (Proceedings of SPIE, Vol.9589), p.958902_1 - 958902_7, 2015/09

Journal Articles

The Observation of a transient surface morphology in the femtosecond laser ablation process by using the soft X-ray laser probe

Hasegawa, Noboru; Nishikino, Masaharu; Tomita, Takuro*; Onishi, Naofumi*; Ito, Atsushi*; Eyama, Tsuyoshi*; Kakimoto, Naoya*; Izutsu, Rui*; Minami, Yasuo*; Baba, Motoyoshi*; et al.

X-Ray Lasers and Coherent X-Ray Sources; Development and Applications XI (Proceedings of SPIE, Vol.9589), p.95890A_1 - 95890A_8, 2015/09

 Times Cited Count:1 Percentile:60.64

We have improved a soft X-ray laser (SXRL) interferometer synchronized with a Ti:Sapphire laser pulse to observe the single-shot imaging of the nano-scaled structure dynamics of the laser induced materials. By the precise imaging optics and double time fiducial system having been installed, the lateral resolution on the sample surface and the precision of the temporal synchronization between the SXRL and Ti:Sapphire laser pulses were improved to be 700 nm and 2 ps, respectively. By using this system, the initial stage (before 200 ps) of the ablation process of the Pt surface pumped by 80 fs Ti:Sapphire laser pulse was observed by the comparison between the soft X-ray reflective image and interferogram. We have succeeded in the direct observation of the unique ablation process around the ablation threshold such as the rapid increase of the surface roughness and surface vibration.

Journal Articles

Soft X-ray laser observation of femtosecond-laser-driven ablation of tungsten

Nishikino, Masaharu; Hasegawa, Noboru; Ishino, Masahiko; Ochi, Yoshihiro; Kawachi, Tetsuya; Yamagiwa, Mitsuru; Kato, Yoshiaki*

Chinese Optics Letters, 13(7), p.070002_1 - 070002_3, 2015/07

 Times Cited Count:1 Percentile:8.29(Optics)

Journal Articles

Observation of the dependence on the fluence and materials in femto-second laser ablation process by using the soft X-ray laser probe

Hasegawa, Noboru; Tomita, Takuro*; Nishikino, Masaharu; Eyama, Tsuyoshi*; Kakimoto, Naoya*; Minami, Yasuo*; Baba, Motoyoshi*; Onishi, Naofumi*; Ito, Atsushi*; Kawachi, Tetsuya; et al.

JAEA-Conf 2015-001, p.17 - 20, 2015/07

We have succeeded in simultaneous observation of the ablation front and the expansion front with thin filmy structure in the femto-second laser ablation process of a gold target by using the 13.9 nm soft X-ray probe (incident angle to the sample $$sim$$ 70 deg) with soft X-ray interferometer. The dependence on the laser local fluence and materials was obtained by the comparison between gold and tungsten.

Journal Articles

Hydrodynamics driven by ultrashort laser pulse; Simulations and the optical pump - X-ray probe experiment

Inogamov, N. A.*; Zhakhovsky, V. V.*; Hasegawa, Noboru; Nishikino, Masaharu; Yamagiwa, Mitsuru; Ishino, Masahiko; Agranat, M. B.*; Ashitkov, S. I.*; Faenov, A. Y.*; Khokhlov, V. A.*; et al.

Applied Physics B, 119(3), p.413 - 419, 2015/06

 Times Cited Count:6 Percentile:40.44(Optics)

Journal Articles

Observation of femtosecond laser ablation on Tungsten by using soft X-ray reflection and interference imaging

Nishikino, Masaharu; Hasegawa, Noboru; Tomita, Takuro*; Eyama, Tsuyoshi*; Kakimoto, Naoya*; Onishi, Naofumi*; Ito, Atsushi*; Baba, Motoyoshi*; Minami, Yasuo*; Kawachi, Tetsuya; et al.

Reza Gakkai Dai-471-Kai Kenkyukai Hokoku; Tanhacho Ryoshi Bimu Hassei To Sono Oyo, p.9 - 12, 2014/12

no abstracts in English

Journal Articles

Very low electron temperature in warm dense matter formed by focused picosecond soft X-ray laser pulses

Ishino, Masahiko; Hasegawa, Noboru; Nishikino, Masaharu; Pikuz, T. A.*; Skobelev, I. Y.*; Faenov, A.*; Inogamov, N. A.*; Kawachi, Tetsuya; Yamagiwa, Mitsuru

Journal of Applied Physics, 116(18), p.183302_1 - 183302_6, 2014/11

 Times Cited Count:7 Percentile:36.01(Physics, Applied)

We investigated the optical emission from the ablating surfaces induced by the irradiations of soft X-ray laser (SXRL) pulses with the aim of estimation of the maximum electron temperature. No emission signal in the spectral range of 400-800 nm could be observed despite the formation of damage structures on the target surfaces. Hence, we estimated an upper limit for the electron temperature of 0.4-0.7 eV for the process duration of 100-1000 ps. Our results imply that the ablation and/or surface modification by the SXRL is not accompanied by plasma formation but is induced by thermo-mechanical pressure, which is so called a spallative ablation. This spallative ablation process occurs in the low electron temperature region of a non-equilibrium state of warm dense matter.

Journal Articles

Submicron scale image observation with a grazing incidence reflection-type single-shot soft X-ray microscope

Baba, Motoyoshi*; Nishikino, Masaharu; Hasegawa, Noboru; Tomita, Takuro*; Minami, Yasuo*; Takei, Ryota*; Yamagiwa, Mitsuru; Kawachi, Tetsuya; Suemoto, Toru

Japanese Journal of Applied Physics, 53(8), p.080302_1 - 080302_4, 2014/08

 Times Cited Count:8 Percentile:40.13(Physics, Applied)

A grazing incidence reflection-type soft X-ay microscope, using a Fresnel zone plate and a soft X-ray laser with wavelength 13.9 nm and pulse width 7 ps, was developed. Submicron size groove structures made on a Pt film were clearly captured at a single shot exposure, with spatial resolution of about 360 nm. A wide field view of 100 $$mu$$m square was secured under the Kohler illumination. This microscope also had a large depth of focus of more than 100 $$mu$$m and was proven to have a sufficient performance for observing surface morphological changes.

Journal Articles

Time-resolved soft X-ray imaging of femtosecond laser ablation process in metals

Tomita, Takuro*; Nishikino, Masaharu; Hasegawa, Noboru; Minami, Yasuo*; Takei, Ryota*; Baba, Motoyoshi*; Eyama, Tsuyoshi*; Takayoshi, Shodai*; Kaihori, Takeshi*; Morita, Toshimasa; et al.

Journal of Laser Micro/Nanoengineering, 9(2), p.137 - 142, 2014/06

 Times Cited Count:5 Percentile:33.77(Nanoscience & Nanotechnology)

Femtosecond laser ablation processes on platinum, gold, and tungsten were observed by the single shot pump and probe reflective imaging using a soft X-ray laser probe. To avoid the timing error due to the jitter, we adopted a posteriori correction technique by simultaneous measurement of timing between the pump and probe pulses for every single shot, using a soft X-ray streak camera. A clear difference was found in the temporal behavior of the dynamical response of the soft X-ray reflectivity depending on the irradiated laser fluence in these three materials. On the other hand, the narrow dark rings were found in Pt and W, while an additional bright ring was found outside the dark disk in Au. Our result gives the experimental data comparable with various numerical simulations.

Journal Articles

Development of soft X-ray microscopy using Fresnel zone plate for observation of laser-induce surface dynamics

Nishikino, Masaharu; Baba, Motoyoshi*; Suemoto, Toru; Hasegawa, Noboru; Ishino, Masahiko; Kaihori, Takeshi; Kawachi, Tetsuya; Yamagiwa, Mitsuru

X-Ray Lasers 2012; Springer Proceedings in Physics, Vol.147, p.199 - 202, 2014/00

 Times Cited Count:0 Percentile:0

The dynamical processes of the laser-induced surface modifications such as laser ablation come to attract much attention for the micro processing by the ultra-short laser pulse. It is difficult to observe these phenomena directly, because they are non-repetitive, unrepeatable, and occur very rapidly (pico-second) in a very small (micron) area. In order to observe the detailed laser-ablation process, we have developed the proto-type reflected soft X-ray microscopy using the Fresnel zone plate. The experimental setup and spatial resolution of the X-ray microscopy will be presented.

Journal Articles

Observation of the laser-induced surface dynamics using the single-shot soft X-ray laser probe

Hasegawa, Noboru; Ochi, Yoshihiro; Kawachi, Tetsuya; Nishikino, Masaharu; Ishino, Masahiko; Imazono, Takashi; Kaihori, Takeshi; Morita, Toshimasa; Sasaki, Akira; Terakawa, Kota*; et al.

X-Ray Lasers 2012; Springer Proceedings in Physics, Vol.147, p.117 - 120, 2014/00

 Times Cited Count:0 Percentile:0

We have developed the femto-second laser pump and soft X-ray laser probe system in order to observe the dynamical processes of the femto-second laser ablation. By using this system, we succeed to obtain the temporal evolution of the soft X-ray reflectivity from the laser induced Pt surface. The results lead that the rate of decrease in the reflectivity of the probe beam has a non-linear relation with the pump laser fluence.

Journal Articles

Fabrication and installation of equilibrium field coils for the JT-60SA

Tsuchiya, Katsuhiko; Kizu, Kaname; Murakami, Haruyuki; Kashiwa, Yoshitoshi; Yoshizawa, Norio; Yoshida, Kiyoshi; Hasegawa, Mitsuru*; Kuno, Kazuo*; Nomoto, Kazuhiro*; Horii, Hiroyuki*

Fusion Engineering and Design, 88(6-8), p.551 - 554, 2013/10

 Times Cited Count:8 Percentile:59.34(Nuclear Science & Technology)

The programme of constructing JT-60SA device is progressing under the framework of the Broader Approach project. Superconducting poloidal field (PF) coil system, which was decided to be procured by Japan, consists of a central solenoid (CS) with four solenoid modules and six equilibrium field (EF) coils. Each of EF coil has individual diameters, 4.5 to 12 m. Fabrication of EF4 coil, which is set at the lowermost of torus, was started from the beginning of 2009 as a first EF coil. EF4 coil has ten double pancake (DP) coils, and sizes of circularity were measured for all DP coil after curing process. Maximum error of circularity was 3.1 mm, which was nearly a half of the design tolerance, 6 mm. After stacking these DP coils, winding pack of EF4 was completed in the spring of 2012. After optimizing the positions of DP coils to cancel the error of circulation which each DP coil has, error of radial current centre of DP coils will be achieved in the range between + 0.2 to - 0.4 mm. Structural analysis of terminal structure was also performed. Terminal part has a pair of conductors bended toward the lower side of winding pack. A side of them (positive terminal) was covered by stainless steel armor to prevent the movement by electromagnetic force because a length of conductor was longer due to starting from the top of winding pack. Another side (negative terminal) was not covered by armor in the first design because this length was relatively short. However, it was clear on the structural analysis that mechanical strength of insulation around this terminal was not sufficient. Therefore, we also reinforced this side with stainless steel. From this April, fabrication of EF coils with large bore (larger than 8 m of diameter) will be started at the facility built in JAEA Naka site. In this paper, we will discuss about technological problem during the fabrication of large bore EF coils, such as temperature control at the winding process.

Journal Articles

Measurement of femtosecond laser ablation by soft X-ray laser interferometory and reflective imaging

Nishikino, Masaharu; Hasegawa, Noboru; Ishino, Masahiko; Yamagiwa, Mitsuru; Kawachi, Tetsuya; Minami, Yasuo*; Terakawa, Kota*; Takei, Ryota*; Baba, Motoyoshi*; Suemoto, Toru; et al.

JAEA-Conf 2013-001, p.16 - 19, 2013/09

The dynamics of the femtosecond laser ablation is still not clear at the early stage. Several studies on the time-resolved imaging of femtosecond laser ablation process have been performed on various materials. In this study, we have demonstrated a pump and probe imaging of the metal surface morphology during the femtosecond laser ablation by using the laser-driven plasma induced soft X-ray laser (SXRL) as a probe beam. The pumping laser used for ablation was a Ti:Sapphire laser system based on chirped pulse amplification. The pump beam had a Gaussian profile and the focal spot size on the metal surface was about 70 micron at FWHM. The dependence for the pump laser fluence of the ablation surface expansion and the surface condition (roughness and density gradient) were observed from the soft X-ray interferogram and reflective image, respectively.

Journal Articles

Nano-meter scale modifications on material surfaces induced by soft X-ray laser pulse irradiations

Ishino, Masahiko; Faenov, A.*; Tanaka, Momoko; Tamotsu, Satoshi*; Pikuz, T.; Hasegawa, Noboru; Nishikino, Masaharu; Inogamov, N.*; Skobelev, I.*; Fortov, V.*; et al.

Proceedings of SPIE, Vol.8849, p.88490F_1 - 88490F_8, 2013/09

 Times Cited Count:2 Percentile:76.86

Journal Articles

Observation of the nano-scale surface dynamics of femtosecond laser ablation by time-resolved soft X-ray imaging technique

Nishikino, Masaharu; Hasegawa, Noboru; Tomita, Takuro*; Minami, Yasuo*; Takei, Ryota*; Baba, Motoyoshi*; Eyama, Tsuyoshi*; Takayoshi, Shodai*; Kawachi, Tetsuya; Hatomi, Daiki*; et al.

Proceedings of SPIE, Vol.8849, p.88490E_1 - 88490E_6, 2013/09

 Times Cited Count:2 Percentile:76.86

We have developed the pump and probe interferometer and reflective imaging technique of the metal surfaces during the femtosecond laser ablation by using the laser-driven soft X-ray laser at the wavelength of 13.9 nm. The pumping laser used for ablation was a Ti: Sapphire laser pulse with the duration of 80 fs pulse at a central wavelength of 795 nm, and had a gaussian spatial profile. By using the X-ray imaging technique, the time resolved image of nano-scaled ablation dynamics of the platinum and the gold pumped by a femtosecond laser pulse was obtained.

Journal Articles

Experimental verification of femtosecond laser ablation schemes by time-resolved soft X-ray reflective imaging

Tomita, Takuro*; Yamamoto, Minoru*; Hasegawa, Noboru; Terakawa, Kota*; Minami, Yasuo*; Nishikino, Masaharu; Ishino, Masahiko; Kaihori, Takeshi*; Ochi, Yoshihiro; Kawachi, Tetsuya; et al.

Optics Express (Internet), 20(28), p.29329 - 29337, 2012/12

 Times Cited Count:18 Percentile:69.28(Optics)

Pump and probe reflective imaging using a soft X-ray laser probe was applied to the observation of the early stage of femtosecond laser ablation process on platinum. In strongly excited area, drastic and fast reflectivity drop was observed. In moderately excited area, the decay of the reflectivity is slower than that in the strongly excited area, and the reflectivity reaches its minimum at t = 160 ps. In weakly excited area, laser-induced reflectivity change was not observed. These results give the critical information about the femtosecond laser ablation.

Journal Articles

Resonance characteristics and maximum turn voltage of JT-60SA EF coil

Murakami, Haruyuki; Kizu, Kaname; Tsuchiya, Katsuhiko; Yoshida, Kiyoshi; Yamauchi, Kunihito; Shimada, Katsuhiro; Terakado, Tsunehisa; Matsukawa, Makoto; Hasegawa, Mitsuru*; Minato, Tsuneaki*; et al.

IEEE Transactions on Applied Superconductivity, 22(3), p.9501405_1 - 9501405_5, 2012/06

 Times Cited Count:4 Percentile:31.53(Engineering, Electrical & Electronic)

The withstand voltage of turn insulation is essential issues for the superconducting magnet. The actual turn voltage is larger than the turn voltage under the ideal condition because of the voltage fluctuations of the power supply and the resonance phenomenon in the magnet. In this paper, the voltage measurement of the JT-60U power supply and the resonance characteristics of the EF4 are described. The actual maximum turn voltage is almost same as the voltage under the ideal condition.

Journal Articles

Manufacture of the winding pack and development of key parts for the JT-60SA poloidal field coils

Tsuchiya, Katsuhiko; Kizu, Kaname; Murakami, Haruyuki; Yoshida, Kiyoshi; Kurihara, Kenichi; Hasegawa, Mitsuru*; Kuno, Kazuo*; Nomoto, Kazuhiro*; Horii, Hiroyuki*

IEEE Transactions on Applied Superconductivity, 22(3), p.4202304_1 - 4202304_4, 2012/06

 Times Cited Count:7 Percentile:44.38(Engineering, Electrical & Electronic)

no abstracts in English

Journal Articles

Manufacturing of JT-60SA equilibrium field coils

Hasegawa, Mitsuru*; Horii, Hiroyuki*; Nomoto, Kazuhiro*; Imai, Yoshio*; Murai, Takashi*; Minato, Tsuneaki*; Kuno, Kazuo*; Tsuchiya, Katsuhiko; Murakami, Haruyuki; Kizu, Kaname; et al.

Proceedings of 24th International Cryogenic Engineering Conference (ICEC 24) and International Cryogenic Materials Conference 2012 (ICMC 2012) (CD-ROM), p.571 - 574, 2012/05

JT-60U magnet system will be upgraded to the superconducting coils (JT-60SA) in the Broader Approach project. JT-60SA magnet system has 18 Toroidal Field coils, a Central Solenoid with 4 modules and 6 Equilibrium Field (EF) coils. This paper describes the manufacturing procedure of EF4 coil, that is the first manufactured EF coil of JT-60SA. The winding pack of EF4 coil was successfully manufactured within geometrical tolerance requirements.

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