ALBERT

Notes: A multichannel motional Stark effect polarimeter system, which is capable of simultaneous measurement of a radial electric field, has recently been developed on the JFT-2M. The diagnostic can measure the polarization angle at 18 radial locations, which cover a region between just inside from the magnetic axis and the outboard edge of the plasma. By viewing two neutral beam lines (one is co-parallel to the plasma current and the other is counter parallel) simultaneously and near tangentially to the toroidal magnetic field from only one spectroscopic instruments, it provides the best sensitivity in radial electric field measurements with good spatial resolution. The magnetic field pitch angle is also measured with the smallest uncertainty. Preliminary data for L-mode plasma has been obtained. It is found that the statistical uncertainty of the magnetic field pitch angle and radial electric field is about 0.1° and 4 kV/m, respectively, with a time resolution of 10 ms. © 2001 American Institute of Physics.

Notes: Since 1994 at the National Institute of Radiological Sciences HIMAC, the NIRS Penning ionized-gauge ion source (PIGIS) has so far been used for radiotherapy with carbon beams and for basic research with several ion species. The source can produce beams from H2 to Fe with a charge-to-mass ratio of between 1/7 and 1/2. It is a hot-cathode type with a filament, of which the lifetime is on the order of a week and the cathode on the order of 1 month, under a low-duty pulsed operation (≤0.5%) synchronized to the ring operation. This PIGIS can produce highly charged ions: for example, 1 e mA of Ar7+ or 500 eμ A of Si5+ ions (by sputtering) under the typical arc power of 5 kW in peak. This article describes the present status of the NIRS-PIGIS. © 2000 American Institute of Physics.

Notes: A novel two-beam method is proposed and applied for the first time, to characterize photorefractive damage (PRD) in a LiNbO3 quasiphase-matched (QPM) wavelength converter. In the proposed method, irradiation light from a Ti sapphire laser and a broadband probe beam from an erbium-doped fiber amplifier are coupled into a LiNbO3 QPM waveguide. The PRD effect caused by the irradiation is studied by monitoring the generated second-harmonic light spectrum of the probe light. It is shown that PRD in the LiNbO3 QPM waveguide can be qualitatively characterized by the proposed method, and relevant information relating to the QPM wavelength conversion can be extracted directly. © 2000 American Institute of Physics.

Notes: We have developed a high-current laser ion source that can provide low-charge-state ions for induction accelerators. A frequency-doubled Nd:YAG laser (λ=532 nm) irradiated a copper target with relatively low intensities of 108–109 W/cm2. The laser-produced plasma supplied a large number of Cu+ and Cu2+ ions over 1014 ions/cm2 at a distance of 10 cm from the target. We have successfully extracted high-current copper beams over 0.1 A/cm2 using an inductive acceleration module. The beam emittance measured by a pepper-pot method was about 40 πmm mrad. © 2000 American Institute of Physics.

Notes: We studied the effect of growth interruption time and growth temperature on the natural formation of InGaAs/AlGaAs quantum disk structures grown on high-index GaAs (311)B substrates by metalorganic vapor phase epitaxy at about 800 °C. The InGaAs/AlGaAs quantum disks were uniformly grown only with growth interruption time of 0 s. With increasing growth interruption time, the self-organized growth mode disappeared with the growth interruption time of 20 s. During growth interruption, strain for the natural formation of quantum disks was released by intermixing between In and Al. With decreasing growth temperature, the quantum disk did not show a hexagonal shape with clear facets. The quantum disks were formed at temperature range between 700 and 830 °C. The self-organization appeared through strain-driven mass transport. The strain-driven mass transport seems to have two characteristic lengths such as the amplitude and the periodicity determined by In composition and the InGaAs layer thickness from electron-beam induced mass transport phenomena. © 2000 American Institute of Physics.

Notes: The chromium doped spinel crystals, MgAl2O4:Cr3+, have been grown using floating zone technique and its application for the fiber-optic thermometer is discussed on the basis of the temperature dependence of the fluorescence lifetime. Lifetimes of the spinel crystals (τ=10 ms), which decrease with temperature, were about 3 times larger than that of the ruby crystals (τ=4 ms) at room temperature. Temperature coefficients of the photoluminescence (PL) lifetimes for spinel crystals (37 μs/K) are also 3 times longer than that for the ruby crystals (10 μs/K). The spinel crystals are potentially useful sensor probes for the fiber-optic thermometer, which have an advantage of high sensitivity due to long PL lifetime and large temperature coefficient. © 2002 American Institute of Physics.

Notes: Effects of PH3/H2 (PH3/H2=10%) plasma passivation of GaAs grown on Si substrate have been investigated in detail. It is observed that both the surface phosphidization and defect hydrogenation can be realized simultaneously with a reduced plasma-induced damage. The optical and electrical properties of GaAs on Si are effectively improved by PH3/H2 plasma exposure due to the passivation of bulk and surface defects-related nonradiative recombination centers by incorporation of hydrogen (H) and phosphorous (P) atoms. As a result, the PH3/H2 plasma exposed GaAs Schottky diodes on Si show an increase in the reverse breakdown voltage by a factor of about 1.6, and the as-passivated GaAs solar cell grown on Si shows an increase in the conversion efficiency from 15.9% to 18.6% compared to that of the as-grown samples. The passivated GaAs devices on Si show outstanding thermal stability, which is probably due to the active participation of both H and P atoms in the PH3/H2 plasma passivation process. © 2000 American Institute of Physics.

Notes: Photorefractive damage (PRD) of LiNbO3 quasiphase matched (QPM) wavelength converters is studied by a novel two-beam method, in which the second-harmonic generation (SHG) tuning curve of a broadband probe beam is monitored. The QPM condition and wavelength conversion efficiency, which are characterized by the peak wavelength and peak intensity of the SHG tuning curve respectively, are investigated in detail with respect to the irradiation power and irradiation time. It is found that the QPM condition and wavelength conversion efficiency change markedly even for low irradiation powers (〈20 mW), indicating that the PRD effect is non-negligible in LiNbO3 QPM wavelength converters intended for practical applications. © 2000 American Institute of Physics.

Notes: Sm(Zr)Fe7Nx+αFe composite thin films were prepared on various substrates with differing values of thermal expansion from SmxFebalZry (11.3[less, double equals]x[less, double equals]17.8,0[less, double equals]y[less, double equals]3.2) targets by the rf sputtering method followed by annealing and nitrogenation. Their phase formation and magnetic properties were studied. All of the as-deposited films were amorphous, and the only films synthesized from the target with x=84 were crystallized into a composite of Sm(Zr)Fe7+αFe by annealing. Successive nitrogenation of the films with x=84 formed Sm(Zr)Fe7Nx+αFe composites whose grain diameter was estimated to be 30–50 nm. Despite the small coercivity of the prepared films, their recoil curves showed a remarkable spring-back phenomenon which originates in the exchange coupling between Sm(Zr)Fe7Nx and α Fe grains. The addition of Zr decreased the lattice constant, and choice of substrate did not affect the grain size. © 2000 American Institute of Physics.

Notes: Previous studies have shown that nanoparticle chain aggregates (NCA) of titania are elastic [S. K. Friedlander, H. D. Jang and K. H. Ryu, Appl. Phys. Lett. 72, 1 (1998)]. The NCA were a few tenths of a micron long and composed of (approximately) 7 nm primary particles. They were produced by thermal decomposition of titanium tetraisopropoxide vapor in nitrogen. The goal of this study was to see whether the elastic behavior depends on (a) the material properties, (b) primary particle size, and (c) method of NCA formation. For this purpose, titania, alumina, and iron oxide NCA were generated by laser ablation. Rotating metal foil targets were mounted in a small cylindrical chamber and exposed to an excimer laser beam. The resulting aerosol was swept out by an oxygen stream. The generator was operated to produce NCA with similar mobility diameter and primary particle size. The NCA were deposited on the carbon or formvar films of an electron micrograph grid. Under the electron beam a hole develops in the carbon film in the neighborhood of the deposited NCA. The NCA then stretch and contract as described in our earlier study [S. K. Friedlander, H. D. Jang and K. H. Ryu, Appl. Phys. Lett. 72, 1 (1998)]. The titania, alumina, and iron oxide NCA generated by laser ablation all showed elastic behavior for primary particles smaller than about 10 nm. However, titania NCA composed of 36 nm primary particles did not exhibit elastic behavior indicating that very small primary nanoparticles are needed for this phenomenon to occur. The small scale stretching and contraction of chain segments were studied by measuring changes in the bond angles between adjoining particles and in the lengths of the segments studied. The elastic behavior is probably associated with local folding of chain segments due to van der Waals forces. Under tension, folded chains straighten but when the tension is relaxed, folds tend to reform but not reversibly. Rotation and sliding probably occur at the boundaries between particles during stretching. We hypothesize that elastic behavior is a general property of NCA composed of transition metal oxides with primary particles smaller than 10–15 nm; the phenomenon has now been observed for NCA produced in two ways, thermal decomposition and laser ablation. These phenomena may play a role in the action of nanoparticle additives such as fumed silica and carbon black used to improve the properties of rubber. NCA elasticity may also contribute to the ductile properties of nanoparticle compacts. © 2000 American Institute of Physics.