Results 1 - 10 of 15
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[en] The generation and diffusion dynamics of the optically induced lattice distortion in the relaxed excited state of SrTiO3 are studied by a transient birefringence experiment. The observed signal of the lattice distortion has a component which decays in the millisecond region. Its amplitude shows a nonlinear dependence on the pump power that indicates the relaxed excited state is generated through a multiphoton absorption process. We observed the lattice distortion induced by a separated pump beam from the probe beam. The signal of the lattice distortion appears later as the separation between the pump and probe beams becomes larger. Experimental results suggest that the optically induced lattice distortion diffuses spatially and its direction is along the crystal axes (copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
[en] A 40keV 1.2MW Neutral Beam Injector for the Compact Helical System (CHS) device has been constructed for the purpose of producing high pressure plasma. The beam line consists of one ion source, and is characterized by changeable injection angle for optimizing beam deposition, and for investigating confinement of high energy ions. A new technology for manufacturing cooling channels of extracting grids (both copper and molybdenum) is developed. The physical requirement on the neutral beam and the design features of the injector are presented. 4 refs., 10 figs
[en] Propagation of nanosecond light pulses in a resonantly absorbing medium is studied. Deviation from the conventional group velocity and superluminal-to-subluminal transition were observed for the natural atomic transitions of Rb. The observed frequency and atomic density dependences of the propagation delay in the anomalous dispersion region, where the conventional concept of the group velocity breaks down, are reproduced well by the theory of the net group delay
[en] Propagation of optical pulses in a resonantly absorbing medium is studied. Propagation time of nanosecond pulses was measured for the Rb D1 transition. At the center of two absorption lines, delay of the pulse peak which is about ten times as large as the pulse width was observed, where zero delay is defined for the propagation with the light velocity in vacuum. On the other hand, at the peak of an absorption line, negative delay was observed for large absorption, where the advance time is as large as 25% of the pulse width. Simulation including the effect of absorption and phase shift reproduced well the experimental results
[en] In Large Helical Device (LHD) experiments, an electron temperature (Te) more than 15 keV has been observed by the yttrium-aluminum-garnet (YAG) laser Thomson scattering diagnostic. Since the LHD Thomson scattering system has been optimized for the temperature region, 50 eV≤Te≤10 keV, the data quality becomes worse in the higher Te region exceeding 10 keV. In order to accurately determine Te in the LHD high-Te experiments, we tried to increase the laser pulse energy by simultaneously firing three lasers. The technique enables us to decrease the uncertainties in the measured Te. Another signal accumulation method was also tested. In addition, we estimated the influence of high-energy electrons on Te obtained by the LHD Thomson scattering system.
[en] Optically induced magnetization and its fast dynamics in aqueous solutions of transition-metal ions is studied by the polarization spectroscopy with the pump-probe technique. The fast spin dynamics in solutions at room temperature, which cannot be detected by the conventional magnetic-resonance methods, was observed by an all-optical method. A circularly polarized pump pulse creates the magnetization in the ground state of the copper ions in aqueous solutions of copper sulfate, and the time evolution of the magnetization in the subnanosecond region is monitored through the change of the polarization of the probe pulse. Quantum-beat free-induction decay signals are observed in transverse magnetic fields, whose Fourier transform gives the ESR spectra. The observed concentration dependence of the relaxation rate in no external magnetic field is interpreted by the spin cross relaxation due to the magnetic dipole interaction between the copper ions
[en] Photo-induced effect in Ca-doped SrTiO3 was investigated by observing coherent phonons. Coherent phonons of soft modes were studied by using ultrafast polarization spectroscopy. Under an ultraviolet (UV) illumination, a shift of the ferroelectric phase transition point at 28 K toward the lower temperature side was observed, and a decrease of phonon frequencies after the UV illumination was found.
[en] The large helical device (LHD) Thomson scattering system measures electron temperature (Te) and density (ne) profiles of LHD plasmas, along the LHD major radius (R). The total length of plasma measured is 3 m (R = 2.325–5.386 m), the number of observation points is 144, and the spatial resolution is 12–25 mm. The sampling frequency is 10–100 msec (10–100 Hz). The measurable temperature and density ranges have been estimated to be 5 eV–20 keV and 1018–1022 m−3, respectively. The LHD Thomson scattering system consists of several subsystems, yttrium-aluminum-garnet (YAG) lasers, light collection optics, polychromators, and data acquisition system. In usual plasma experiments, we use three types of YAG lasers: 2 J/10 Hz, 1.6 J/30 Hz, and newly developed 1.2/50 Hz YAG lasers. Thomson scattering signals are analyzed with the FASTBUS-based data acquisition system. Recently, a hardware technique and three data analysis methods have been tested to improve data quality. By using these methods, the data quality has been increased by more than an order of magnitude in high-Te, low-ne plasma experiments. In the paper, we describe the current status of the LHD Thomson scattering system.
[en] A self-consistent analytical solution for net erosion of a divertor plate which is set perpendicular to magnetic field lines is presented. The primary flux profile of hydrogen and impurities except redepositing particles is externally given as well as the return ratio of sputtered atoms to the plate. In the direction along the divertor trace, all conditions are uniform. The ionization mean free path is assumed constant to simplify equations. The analytical solution is compared with net erosion experiments carried out in compact helical system (CHS) by exposing a graphite target to a neutral beam heated plasma column introduced perpendicularly to the target along the magnetic field lines through a 2 cm slit opend on a graphite limiter. After exposure to 98 discharges, the target surface is analyzed with Rutherford backscattering method. Deposition profiles of Ti and O impurities are very well explained with the analytical predictions. (orig.)
[en] We measured an absolute ortho-D2 concentration of gas samples taken from irradiated liquid D2 samples as a function of the integrated radiation dose and the irradiation time by means of a frequency-resolved Raman spectroscopy to study the ortho-D2 conversion mechanism under irradiation. The measurement was carried out by irradiating liquid ortho-D2 at a temperature of 25 K with Bremsstrahlung photons, which were produced by bombarding tantalum with 33 MeV electrons, and whose absolute fluxes were determined experimentally by employing a foil activation method. The obtained ortho-D2 concentration was found to decrease from 98% to 82% monotonically with increasing the radiation dose, irrespective of the electron-beam power. This fact clearly indicates the important role of a radiation-induced breakup effect of the ortho D2 under the present experimental conditions of radiation dose and irradiation time. The obtained conversion rate is ten times faster than the evaluated value based on an existing model, requiring an alternate mechanism to enhance the conversion