天野 壮

Mitigation of fast debris and soft x-rays generated from laser-produced xenon plasmas were studied in an argon buffer gas in laser intensities of 10 910 11W cm 2using a cryogenic drum target. Considerable mitigation of debris was confirmed by measurements of material sputtering. From the experimental results, an attenuation parameter of sputtering by the debris and an absorption cross section of soft x-rays at 13.5 nm 2(13.5nm) were derived to be 2.2×10 20m 2and 1.8×10 22m 2, respectively. Moreover, is concluded to be equivalent to the effective collision cross section 1 of a debris particle at kinetic energy of 14 keV. Sufficient debris mitigation can be obtained together with low soft x-ray absorption (less than 10%). These parameters provide a useful design tool for realizing a practical soft x-ray source because they predict the effect of the buffer gas well. © 2012 IOP Publishing Ltd.

We investigated crystallization mechanism of a-Si 0.5Ge 0.5 film by excimer-laser annealing (ELA) at energy density of 50-70mJ/cm 2 in comparison with a method of synchrotron radiation soft X-ray irradiation at storage ring current of 50-220 mA. The nucleation mechanism of Si 0.5Ge 0.5 via ELA will be discussed by considering the soft X-ray method. © 2012 IEEE.

We investigated a low-temperature crystallization of a-Si and a-Ge films by the synchrotron radiation soft X-ray irradiation at storage-ring current of 25-220 mA. It is found that the crystallization temperatures of a-Si and a-Ge were decreased from 680 to 580 degrees C and from 500 to 390 degrees C. These decreasements relate effects of enhancement atomic migration via electron excitation at valence band and core level.

A photonuclear reaction cross section of iodine-129 was measured using a polarized laser-Compton

We investigated a low-temperature crystallization of a- Si, a-Ge and a-Si1-xGex films by the synchrotron radiation (SR) soft X-ray irradiation at storage ring current of 25-220 mA. Crystallization of SiGe caused by the atomic migration during soft X-ray irradiation is effective as compared with the thermal annealing.

Soft X-ray conversion efficiencies in a 5--17 nm wavelength range in xenon and tin plasmas produced by a 1.06 μm Q-switched neodymium-doped yttrium aluminum garnet laser were investigated. They were measured to be 25 and 20% for the xenon and tin plasmas, respectively at a laser intensity of about $2\times 10^{10}$ W/cm2. These results indicate that a stand-alone soft X-ray source using a laser-produced xenon or tin plasma is useful for various applications that require intense, soft X-ray photons in a wide wavelength range.

Energy and flux of positrons and neutrons generated in the electron storage ring NewSUBARU by laser Compton scattering with Nd (1.064 μm,) laser beams were measured. The energy of positrons was measured with an imaging plate. The positron energy has a peak around 10 MeV. With a 3 mm thick Pb target, about 3600 positron sec^-1 can be generated and the maximum generation rate of 0.0436 positron photon^-1 can be obtained under the top-up mode operation. The generation rate is defined to be the number of positrons generated per the incident single gamma-ray photon. The energy of neutrons was measured with a time of flight. The maximum neutrons energy is around 8 MeV which agreed well with the obtained result 8.6 MeV.

The laser plasma soft X-ray source in the wavelength rage of 5-17 nm was developed, which consisted of the rotating drum system supplying cryogenic Xe target and the high repetition rate pulse Nd:YAG slab laser. We found the maximum conversion efficiency of 30% and it demonstrated the soft X-ray generation with the high repetition rate pulse of 320 pps and the high average power of 20 W. The soft X-ray cylindrical mirror was developed and successfully focused the soft X-ray with an energy intensity of 1.3 mJ/cm². We also succeeded in the plasma debris mitigation with Ar gas. This will allow a long lifetime of the mirror and a focusing power intensity of 400 mW/cm² with 320 pps. The high power soft X-ray is useful for various applications.

The influence of laser plasma soft X-ray (LPX) irradiation on crystallization of a-Si film by infrared (IR) furnace annealing is investigated. The crystallization temperature by LPX irradiation followed by IR annealing is lowered down to 420°C and the grain size increases up to 270 nm. This phenomenon is related with the change in characteristics of a-Si film which is generated by the bond distortion and relaxation during the LPX irradiation. It was found that the LPX-irradiated film is constituted by the two different layers and the refractive index of upper layer was lower than that of under layer. The dangling bond density of a-Si film was also decreased by LPX irradiation to a-Si film. From these results, the crystallization mechanism is discussed.

We have developed a laser-Compton gamma-ray source at beamline (BL1) in NewSUBARU, an electron storage ring operating at electron energies of 0.7-1.5 GeV. Initially, we generated 17-37 MeV gamma rays using a Nd:YVO(4) laser operating at 1.06 mu m and 0.53 mu m, achieving a luminosity of 6000 photons/(W.mA.s) and a flux of 5x10(6) photons/s with a laser power of 4 W and a beam current of 200 mA. We then installed a CO(2) laser operating at 10.6 mu m, and generated gamma rays in the energy range of 1-4 MeV. In this range, the gamma-ray luminosity is 7300 photons/(W.mA.s) and the flux is 5.6x10(6) photons/s when operating the laser at 4 W and 200 mA. These performances are in good agreement with calculations. We will next add a Tm-fiber laser operating at 2 mu m to generate gamma rays in the energy range of 4-21 MeV, using a laser-focusing design to generate the maximum flux. The luminosity of this stage is calculated to be 6400 photons/(W.mA.s). Nd:YVO(4), CO(2), and Tm-fiber laser are all capable of operating in a high-power regime and can generate kW-level output power. At present, the maximum gamma-ray flux is limited only by the laser power. When operating the laser at I kW of output power with a beam current of 200 mA, we expect a flux of up to similar to 10(9) photons/s. At NewSUBARU, up to several dozen MeV gamma-ray sources are capable of generating such high power.

Information on the photonuclear reactions in the energy region below the El giant resonances is vital for understanding astrophysical processes relevant to nucleosynthesis. Several topics from recent experimental studies of the photonuclear reactions with a high-quality monochromatic gamma-ray beam will be presented.

Soft X-ray and gamma-ray beam Sources are developed by using laser and high-energy electron beams. Laser plasma soft X-ray Source is a cryogenic Xe target irradiated by a high average power (> 100 W) and high peak power (30 MW, 0.3 J/10 ns) laser. A conversion efficiency from the laser energy to the soft X-ray (70 to 200 eV) energy of 22% was achieved. Laser Compton gamma-ray source on a synchrotron radiation facility, New SUBARU, is a new quasi-monochromatic high-energy photon beam. The world's highest class gamma-ray flux of more than 106 photons/s is generated at a photon energy range of 6 to 17 MeV. (C) 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 92(10): 13-23, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10134

A new laser Compton gamma ray source in the energy range of several-MeV was developed using a CO(2) laser at the NewSUBARU electron storage ring. When the electron energies were 974, 1220 and 1460MeV, the maximum gamma ray energies were measured to be 1.72, 2.72, and 3.91 MeV. The luminosity of the gamma rays was 7300 photon/mA/W/s and a flux of 5.8 x 10(6) photon/s was achieved. A gamma beam divergence of 0.5 mrad and its pattern were also measured. These performances agreed with calculations. This generation of gamma rays is in a no-loss mode for the storage electrons, and the maximum flux was limited only by the power of the laser. A flux of up to 10(11) photon/s will be achieved using a 30 kW class CO(2) laser. Several-MeV gamma ray sources at NewSUBARU are capable of generating such high power. (C) 2009 Elsevier B.V. All rights reserved.

The effects of laser plasma X-ray (LPX) irradiation on amorphous silicon (a-Si) film structure and the density of states around the edge of the valence and conduction bands were investigated for low-temperature crystallization. An LPX with a photon energy of 115 eV was generated by a Nd:YAG excitation laser using solid Xe. The Raman spectra of a-Si films were not changed by LPX irradiation. However, the optical transmittance of a-Si film was increased by LPX irradiation at approximately 570 nm. This phenomenon relates to the movement of Si atoms to the quasi-nuclei phase in a-Si film.

We have proposed an approach to use a gamma-ray for nuclear transmutation by Compton scattering of laser photons with high current accelerator. We demonstrated laser Compton scattering to obtain 20 MeV gamma-ray to address transmutation rates with the giant resonance of Au-197 and (121)Iodine. Experiments for measurements of neutron spectrum and pair production were performed for energy balance of this scheme and for target conditions. The results showed reaction rate was about 2-4% for appropriate photon energies and neutron energy was up to 4 MeV in the calculation. (C) 2007 Elsevier Ltd. All rights reserved.

The half-life of an isomer in Ho164 was measured by using photodisintegration reactions. The photons were generated by Compton scattering of laser photons and relativistic electrons at the electron storage ring NewSUBARU in the super photon ring 8-GeV (SPring-8) facility. The half-life is 36.4±0.3 min. This value is about 3% shorter than the previous value 37.5-0.5+1.5 min reported in 1966. © 2008 The American Physical Society.

A laser Compton scattering gamma-ray based nuclear transmutation is proposed to reduce the hazards of long-lived activity nuclear waste. In accordance with this proposal, a laser Compton scattering gamma-ray facility has been built on NewSUABARU storage ring. The facility provides 17.6 MeV gamma-ray photons, which is applicable to the nuclear transmutation research. In order to investigate the reaction rate of Iodine material, the (NaI)-Na-23-I-127 target is adopted for the irradiation experiment. The results show that the experimental data is close to the simulation result.

We measure half-lives of the ground state of 184Re and an isomer in 164Ho, which are populated by (γ,n) reactions with laser Compton scattering (LCS) γ-ray source at NewSUBARU. These neutron-deficient isotopes are located on nucleosynthesis flows of the supernova γ process. The measured half-life of 35.4 γ 0.3 d for 184Re is shorter than the previous half-life of T1/2 = 38.0 γ 0.5 by about 7%. The half-life of the 164Ho isomer is 36.4 γ 0.3 min. This is about 3% shorter than a recommended value T 1/2 = 37.5-0.5 min. These results indicate that half-lives of all unstable nuclei near the β stability line may be not robust and that the LCS γ-rays are useful for precise determination of the half-lives. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlikeLicence.

As for deposition-free-targets, cryogenic Xe and Li targets of laser- produced-plasma have been studied for extreme ultraviolet (EUV) light sources. A fast rotating cryogenic drum system, which can continuously supply a solid Xe target, has been developed. It successfully generated about 1 W of EUV power by continuous irradiation of Nd: YAG slab laser at 320 pps, with a maximum conversion efficiency of 0.9%. Experimental results for the Xe target of the double-pulse and short-wavelength irradiation have also been reported. In addition, a new Li target using a forced-recombination scheme was demonstrated. It was found that the maximum conversion efficiency was 2.3% with a laser irradiation of 532nm in Li planar targets. By using both direct EUV emissions and forced-recombination emissions, higher efficiency can be achieved.

We investigated the crystallization by Laser Plasma Soft-X-ray (LPX) irradiation followed by Excimer Laser Annealing (ELA) at low-energy densities of 40-60 mJ/cm(2). The crystalline fraction of poly-Si films prepared at 50 and 60 mJ/cm(2) for the LPX followed by the ELA was 75-85 %. The LPX irradiation before ELA decreases the critical energy density for the start of the crystallization. It is indicated experimentally that this phenomenon is not due to a rise in heat by the LPX irradiation.

Soft x-ray and gamma-ray beam sources are developed by using laser and high energy electron beams. Laser plasma soft x-ray source is a cryogenic Xe target irradiated by a high average power (>100 W) and high peak power (30MW, 0.3 J/10 ns) laser. A conversion efficiency from the laser energy to the soft x-ray (70-200eV) energy of 22% was achieved. Laser Compton gamma-ray source on a synchrotron radiation facility, NewSUBARU, is new quasi-monochromatic high energy photon beam. World highest class gamma-ray flux of more than 10⁶ photons/s is generated at photon energy range of 6 to 17 MeV.

An investigation of Smith–Purcell radiation is carried out with a three-dimensional particle-in-cell simulation. The simulation model consists of a rectangular grating with limited length and width, which is driven by a single electron bunch, a train of periodic bunches, and a continuous beam. Smith–Purcell radiation is observed and distinguished from an evanescent wave, which travels near the surface of the grating. We found that the evanescent wave has a frequency lower than the allowed minimum Smith–Purcell frequency, and does not radiate until it reaches the ends of the grating. It is also shown that superradiant radiation can be excited by periodic electron bunches. With the present parameters, superradiant radiations are emitted at higher harmonics of the bunch frequency and at corresponding Smith–Purcell angles. The initially continuous beam can be bunched by the interaction with the evanescent wave when the beam current is above a certain threshold, and consequently induces the superradiant Smith–Purcell radiation. The threshold current is derived as well.

Emissions of fast ion and neutral debris from laser-produced lithium plasmas have been investigated. We found that Li+, Li2+, and Li3+ ion energy spectra look similar. The peak positions and maximum energies in their energy spectra were about 700–800 eV and about 800–900 eV, respectively, which were much lower than those of such high-Z targets as Xe and Sn. These indicate that most of the fast Li+ and Li2+ ions were produced by the recombination of Li3+ ions while the plasma expanded, and that they were not accelerated to a high energy range as much as the high-Z targets. We did not observe any fast neutral Li atoms. These results suggest that the use of the Li target for the extreme ultraviolet (EUV) source would be advantageous for practical applications.

The effect of laser plasma soft X-ray (LPX) irradiation on crystallization by excimer laser annealing (ELA) was investigated at low ELA energy densities. The crystalline fraction at energy densities of 50 and 60 mJ/cm2 for LPX followed by ELA is nearly equal to that at 80 to 100 mJ/cm2 for the ELA method with non-LPX irradiation. The results obtained indicate that LPX irradiation before ELA reduces the critical energy density for the start of crystallization. The combined method of LPX irradiation and ELA will enable us to realize a low-temperature process for ELA crystallization.

The stability of a continuously growing solid-Xe target (thickness 500 Em) on a rotating drum (diameter 17 cm, rotation speed 1000 rpm and up-down length 3 cm) chilled by liquid-N₂ is investigated. The thermal load and the propagation process of a laser-induced shockwave in a solid-Xe target irradiated at a high-repetition rate and a high laser power are calculated theoretically. The thermal load is calculated to be approximately 0.7 W/cm² for a pulse energy of 1 J, a repetition rate of 30 kHz and an average power of 30 kW. Under these conditions the solid-Xe target should be chilled to ≤ 84 K for it to attain thermal equilibrium. The target system has the ability to supply solid-Xe target sufficiently rapidly that foliation of the target due to an accumulation of shockwaves does not occur under these irradiation conditions, since the lifetimes of the shockwaves were estimated to be about 1 μs.

The conventional cavity for a free-electron laser (FEL) oscillator usually forms an optical beam of Gaussian mode, which undergoes transverse spread along the interaction region. The transverse divergence of an optical beam will induce reduction of the FEL gain from three aspects: degenerating filling factor, causing diffraction loss and limiting the effective interaction distance. Bessel optical beam has been experimentally demonstrated diffraction-free characteristics in its propagation, which provides a possibility of improvement of FEL gain. In this paper, we present a conceptual design of a Bessel beam cavity for the free-electron laser oscillator. This cavity generates nondiffracting optical beam in the wiggler, resulting in improving the filling factor, decreasing the diffraction loss and elongating the effective interaction distance.