bluuebunny/crossref_metadata_2025 huggingface dataset:
Absolute calibration of the JT-60U neutron monitors using a 252Cf neutron sourcea)
Authors: Nishitani T. , Takeuchi H. , Kondoh T. , Itoh T. , Kuriyama M. , Ikeda Y. , Iguchi T. , Barnes Cris W.
Published: July 2002

Absolutely calibrated measurements of the neutron yield are important for the evaluation of plasma performance such as the fusion gain Q in D–D operating tokamaks. The time-resolved neutron yield is measured with 235U and 238U fission chambers and 3He proportional counters in the JT-60U tokamak. The in situ calibration was performed by moving the 252Cf neutron source toroidally through the JT-60 vacuum vessel. Detection efficiencies of three 235U and two 3He detectors were measured for 92 locations of the neutron point source in toroidal scans at two different major radii. The total detection efficiency for the torus neutron source was obtained by averaging the point efficiencies over the whole toroidal angle. The uncertainty of the resulting detection efficiency for the plasma neutrons is estimated to be ±11%.
High-resolution time- and two-dimensional space-resolved x-ray imaging of plasmas at NOVA
Authors: Landen O. L.
Published: July 2002

A streaked multiple pinhole camera technique, first used by P. Choi and co-workers [C. Deeney and P. Choi, Rev. Sci. Instrum. 60, 3558 (1989); P. Choi and R. Aliaga, ibid. 61, 2747 (1990)] to record time- and two-dimensional space-resolved soft x-ray images of plasma pinches, has been implemented on laser plasmas at NOVA. The instrument is particularly useful for time-resolved imaging of small sources (<150 μm in size) such as implosions for which the necessary alignment accuracy is relaxed from ≤10 μm for a single pinhole, to 200 μm for a nine-pinhole column. Results at 20 μm, 30-ps resolution are presented for ≳2.5-keV imaging, complementing the existing 1–3-keV streaked x-ray microscope capabilities at NOVA.
Light intensity measurements from dark noise analysis of video images
Authors: Kammoun F. , Astruc J. P. , Beysens D. , Hede P. , Guenoun P.
Published: July 2002

It is possible to determine the absolute intensity of a light signal recorded by a video camera with automatic gain by analyzing the background part of the recorded images. The method is based on the determination of the histogram of the gray levels of the dark pixels whose inverse variance appears to be proportional to the camera gain in a large domain of light intensity. An example is given by images recorded by a charge coupled device camera in a space experiment where the light transmittivity is used as a local probe for temperature measurements.
Radio frequency reactance properties of the Bruker ENDOR cavity
Authors: Bender Christopher J. , Babcock Gerald T.
Published: July 2002

The practice of electron-nuclear double resonance (ENDOR) requires that simultaneous microwave and radio frequency irradiation of the sample be performed. A common arrangement for achieving this requirement is to place a nonresonant coil (rf structure) into a cylindrical TM110 resonant microwave cavity. We examined the vector impedance properties of the radio frequency circuit used in these experiments for the commercially available TM110 ENDOR cavity available from Bruker. Our findings indicate that there is a parasitic capacitance between the cavity wall and the coil, and this capacitance undergoes resonance at frequencies that coincide with distortions in the spectral baseline. We are able to eliminate this resonance condition and the associated baseline aberrations by using a broadband impedance match technique that incorporates a series capacitance and an inductive shunt into the rf circuit.
Imaging of lithium pellet ablation trails and measurement of q profiles in TFTR
Authors: Terry J. L. , Marmar E. S. , Snipes J. A. , Garnier D. ,   TFTR Group, Sergeev V. Yu.
Published: July 2002

Video images with 2 μs exposures of the Li+ emission in Li pellet ablation clouds have been obtained in a variety of Tokamak Fusion Test Reactor tokamak discharges. The pellet clouds are viewed from behind the pellet, which is injected from the outside midplane. In this view, the emission forms an elongated cigar shape with the long dimension of the cigar aligned with the local magnetic field. In some cases, two distinct parallel cigars can be seen simultaneously, displaced vertically from one another by ∼5 cm. Measurements using a ten channel array of position sensitive photodiodes show that the mean position of the ablation cloud emission can oscillate vertically by ∼4 cm with periods in the 60–100 μs range, and that these oscillations are highly correlated with ‘‘bursts’’ in the cloud emission. The tilt of the cloud is also measured as a function of time as the pellet traverses the plasma, and in this way the poloidal field profile is obtained. (The total transit time of the pellet is ∼1 ms.) Magnetohydrodynamic equilibrium reconstructions of q profiles have been determined using these measurements.
New type of CD spectropolarimeter with LD option
Authors: Shindo Yohji , Oda Yasunori , Oshima Azuma , Maeda Shiro
Published: July 2002

A new type of circular dichroism (CD) spectrometer has been designed and constructed based on a principle which is entirely different from that of commercial CD spectrometers. The Mueller matrix approach is used to analyze and evaluate important factors determining its performance, which is revealed to be dependent upon (1) the quality of a depolarizer, (2) the residual static birefringence inherent in a collimator lens, a photoelastic modulator, and an analyzer. Furthermore, the polarization characteristics of both a monochromator and a detector are found to have no effect on CD signal in the new CD spectrometer. Thus, a double-grating monochromator can be used instead of a double-prism monochromator. The total performance of the new CD spectrometer is ascertained to be equivalent to that of commercial CD instruments. With using a lock-in amplifier tuned to 100 kHz, the new CD spectrometer can be easily converted into a linear dichroism spectrometer.
A laser three-focus velocimeter using an array of laser diodes for simultaneous measurement of particle size and velocity
Authors: Nakatani Noboru , Oshio Takanori , Sakabe Toshiya
Published: July 2002

This paper describes a laser three-focus velocimeter for measurement of particle size and velocity to make the system compact and portable. An array of independently driven laser diodes is used to obtain the stable output of the laser beams without the thermal interference between the laser diodes and to exclude ambiguity of velocity direction. The method measuring velocity and size of a transparent and spherical particle is shown. The optical characteristics are analyzed. Using a flow of glass beads, it is confirmed that this velocimeter is a useful tool for measuring particle size and velocity simultaneously.
A novel bimorph-type actuator for use in scanning probe microscopes
Authors: Hammiche A. , Webb R. P. , Wilson I. H.
Published: July 2002

We describe a novel bimorph-type piezoelectric transducer which achieves three-dimensional positioning in a 70 μm ×70 μm ×10 μm volume with applied voltage ranges less than 100 V. This transducer, appropriately shaped and with appropriately patterned electrodes, has been tested by using it as a scanner in a scanning tunneling microscope. Results obtained in this application of the actuator are presented, including characterization of performance.
Charge-coupled-device based time-of-flight charged particle analyzer
Authors: Germer Thomas A. , Young Rowena Y. , Ho W. , Ravel Mihir K.
Published: July 2002

The time-of-flight technique is well suited for the analysis of electrons photoemitted from a surface by a pulsed laser. In this paper, a novel design for a time-of-flight charged particle analyzer is presented which incorporates a charge-coupled device (CCD) for the direct detection of electrons photoemitted by an ultrashort laser. After traveling through a field-free drift region, the electrons are accelerated, focused, and deflected onto a CCD. The positions at which the electrons strike the CCD determine their flight times through the drift region. The use of this analyzer is demonstrated by detecting electrons emitted from a Cu(111) surface by two-photon photoemission. A computer simulation of the electron trajectories reveals that degradation in resolution and throughput at higher energies can be traced to the chromatic aberrations of the acceleration region just past the drift region.
Diagnostics for real-time plasma control in PBX-M
Authors: Kaita R. , Batha S. , Bell R. E. , Bernabei S. , Hatcher R. , Kozub T. , Kugel H. , Levinton F. , Okabayashi M. , Sesnic S. , von Goeler S. , Zolfaghari A. ,   the PBX-M Group
Published: July 2002

An important issue for future tokamaks is real-time plasma control for the avoidance of magnetohydrodynamic instabilities and other applications that require detailed plasma profile and fluctuation data. Although measurements from diagnostics providing this information require significantly more processing than magnetic flux data, recent advancements could make them practical for adjusting operational settings for plasma heating and current drive systems as well as field coil currents. On the Princeton Beta Experiment-Modification (PBX-M), the lower hybrid current drive phasing can be varied during a plasma shot using digitally programmable ferrite phase shifters, and neural beam functions can be fully computer controlled. PBX-M diagnostics that may be used for control purposes include motional Stark-effect polarimetry for magnetic field pitch angle profiles, soft x-ray arrays for plasma position control and the separation of βp from li, hard x-ray detectors for energetic electron distributions, a multichannel electron cyclotron emission radiometer for ballooning mode identification, and passive plate eddy current monitors for kink stabilization. We will describe the present status of these systems on PBX-M, and discuss their suitability for feedback applications.
Micromachined infrared sensors using tunneling displacement transducers
Authors: Kenny T. W. , Reynolds J. K. , Podosek J. A. , Vote E. C. , Miller L. M. , Rockstad H. K. , Kaiser W. J.
Published: July 2002

Uncooled infrared sensors are important for a number of technological and scientific measurements. We have designed, built, and tested a new uncooled infrared sensor which is made by silicon micromachining. This infrared sensor uses a tunneling displacement transducer to detect the thermal expansion of a small volume of trapped gas. Prototype devices based on this design have been operated with NEP better than 3×10−10 W/√Hz at 25 Hz, which is competitive with the best comparably sized uncooled sensors available. This article will describe the design, fabrication, and operation of all elements of this sensor.
Gallium arsenide as an optical strain gauge
Authors: Weiss Jonathan D. , Lopez Salvador S. , Howard Arnold J.
Published: July 2002

We have investigated the use of gallium arsenide (GaAs) as an optical strain gauge, presuming the shift in its absorption edge with uniaxial stress as the principle of operation. In our experiments, optical fibers guided light of the GaAs samples from a laser diode source and from the samples to an optical detector, as might be the case in a practical application of such a device. Compressive and tensile strains were developed in the samples by flexing a cantilevered beam to which they were bonded. An important understanding of this strain distribution was obtained with the help of a finite-element calculation. The effects of sample width and the input optical energy distribution on the strain-induced change in optical transmission were measured. In the case of the distribution with the shortest median wavelength, over 80% change in transmission was observed for a compressive strain of only 0.05%. In addition, we calculated the strain sensitivity of this device, assuming only a rigid, strain-induced shift in its absorption edge. A comparison between experiment and calculation suggests that strain also caused the band edge of these samples to steepen in compression and broaden in tension. Another calculation suggests that this phenomenon is not intrinsic to the material.
Inductively coupled radio frequency plasma chemical vapor deposition using a ladder-shaped antenna
Authors: Murata Masayoshi , Takeuchi Yosiaki , Sasagawa Eishiro , Hamamoto Kazutoshi
Published: July 2002

Radio-frequency (rf) excited SiH4 plasma is produced with an electrode of a ladder-shaped antenna which is positioned within a plasma chamber. The negative self-bias potential on the electrode is a few volts under a gas pressure 50 mTorr, rf power of 20–100 W. It was observed that the phase of the potential precedes the current fed to the electrode. Hydrogenated amorphous silicon films are formed on a 300 mm×300 mm substrate with a uniformity of ±15%.
Charge exchange recombination spectroscopy on the stellarator W7-AS
Authors: Baldzuhn J. , Ohlendorf W. ,   W7-AS-Team
Published: July 2002

In the advanced stellarator W7-AS, active charge exchange recombination spectroscopy CXRS is performed on intrinsic plasma impurities. Measurements are done on the doublet lines of the singly charged species of He II, B V, C VI, and N VII. A pulsed diagnostic neutral beam injector excites CX spectral lines in the visible or near UV region. By switching the beam off and on, the electron impact excited spectral light intensity from the cold plasma edge can be subtracted from the signal of interest. The spectral lines are observed after transmission by imaging mirrors and glass fibers by means of Czerny–Turner monochromators with high spectral resolution of down to 0.3 Å per spectral channel. A turnable mirror device allows for the observation at well defined spatial locations within the plasma. As detectors, photomultiplier arrays, and charge coupled device camera systems are used. By spectral measurement of the CX Doppler line intensity, line shift, and line broadening, we evaluate the impurity density and rotation velocity profile and the impurity ion temperature. These parameters provide the transport relevant radial electric field and the ion temperature profile. The radial ion particle and heat transport together with the ambipolar electric field play a decisive role for the optimization of the three-dimensional stellarator plasma.
Generation of ultrabroadband femtosecond pulses in the mid-infrared by optical rectification of 15 fs light pulses at 100 MHz repetition rate
Authors: Bonvalet A. , Joffre M. , Martin J. L. , Migus A.
Published: July 2002

Quasi-single-cycle near-infrared light pulses with a measured spectrum extending from 7 to 15 μm have been generated, opening up new perspectives in IR spectroscopy. The method is based on the rectification of 0.8 μm 10–15 fs light pulses from a 100 MHz oscillator, using the instantaneous second-order polarizability of bulk semiconductors such as GaAs.
Recent developments of the ISOLDE laser ion source
Authors: Lettry J. , Catherall R. , Focker G. J. , Jonsson O. C. , Kugler E. , Ravn H. , Tamburella C. ,   The ISOLDE Collaboration, Fedoseyev V. , Mishin V. I. , Huber G. , Sebastian V. , Koizumi M. , Köster U.
Published: July 2002

The ISOLDE laser ion source (LIS), in which stepwise resonant laser ionization is performed inside a high-temperature cavity, has been extended to include the elements Be, Zn, Cu and Cd. The ionization efficiencies obtained are discussed with respect to the level of saturation of each step achieved with the available laser beam intensities. Because of a high ionization potential of about 9 eV the first resonant transition of Be, Zn and Cd is in the far ultraviolet (UV) region. The UV laser beam needed for the first resonant step of Be, Zn and Cd is achieved in a new laser setup via frequency tripling of the dye laser light. The ions created during the 30 ns laser pulse are extracted from the hot cavity of the ion source by the electrical field resulting from the ohmic heating of the cavity. The time distribution of the laser-ionized ion bunches was measured for various cavities and temperatures. The time structure of the ion bunch is discussed in order to gain insight into the development of LIS cavities.
Movable magnetic probe array for toroidal Alfven eigenmodes study in the compact helical system heliotron/torsatron
Authors: Takechi M. , Toi K. ,   CHS Group
Published: July 2002

A movable magnetic probe array is installed on the compact helical system (CHS) heliotron/torsatron to study Alfven eigenmodes exited by energetic beam ions. The array consists of six sets of magnetic probes for measurements of radial and poloidal magnetic field fluctuations. This probe array is put inside a thin stainless pipe protected with a c/c-composite carbon sleeve containing 20% boron. The carbon sleeve also acts to minimize deleterious effects on a plasma, evaporating boron during a plasma shot. By using this probe array, radial profiles of magnetic fluctuations in the range of f<200 kHz can be obtained even inside the last closed flux surface up to 〈r〉/〈a〉∼0.8 without serious disturbances to the neutral beam injection (NBI) heated CHS plasmas. High frequency magnetic fluctuations less than 200 kHz related to the toroidal Alfven eigenmodes (TAE) are detected with these magnetic probe arrays in NBI heated plasmas. The mode numbers of the observed TAE can be successfully determined by combination of the radial profiles of fluctuations obtained with the movable probe array and a set of poloidal and toroidal arrays of Mirnov probes.
A summary of the U.S. contribution to ITER diagnostics design and research and development during the engineering design activity (abstract)
Authors: Young Kenneth M. ,   Members of the U.S. Home Team
Published: July 2002

During the ITER engineering design activity (EDA) phase the U.S. Home Team has participated in very many aspects of the credited design and R&D being carried out to demonstrate the feasibility of providing sufficiently good measurements for plasma control and physics studies. This work has made use of the requirements set by the ITER Diagnostics Expert Group and has been done with the active participation of the members of the ITER Joint Central Team. U.S. team members have carried out design for high frequency magnetic probes, edge Thomson scattering, toroidal interferometer/polarimeter, a reflectometer for plasma position, visible continuum array, charge exchange spectroscopy, motional Stark effect system, pulsed diagnostic neutral beam, neutron flux monitors, and activation system, lost-alpha-particle detectors, fast-wave reflectometer, knock-on tail alpha-particle system, and a CO2 laser collective scattering system. Most of these designs will be relatively advanced at the end of the EDA but some will merely have demonstrated the concept. Among the principal activities in R&D have been studies of radiation sensitivity to induced electrical effects in ceramics and fiberoptics, a full radiation sensitivity test of a magnetic probe, and testing in support of the systems in design.
Laser activation of diamond films for electroless metal plating: Valence band modifications
Authors: Pimenov S. M. , Shafeev G. A. , Loubnin E. N.
Published: July 2002

Experimental results are presented on the modification of the valence band of diamond under a KrF excimer laser etching in the process of the activation of a diamond film surface for electroless copper plating. The Auger electron spectroscopy (AES) is used to obtain the valence-band density-of-states spectra of the diamond film before and after laser treatment. According to AES data, a characteristic feature of the laser-treated diamond surface is the appearance of nonzero density of states in the gap of diamond which promotes the electron transfer from the reducing agent to the metal ion during electroless deposition. The results are also discussed on the electroless metallization of diamond films under a load-induced activation of the film surface with a diamond indentor.
Manganese perovskites: Thick-film based position sensors fabrication
Authors: Balcells Ll. , Enrich R. , Mora J. , Calleja A. , Fontcuberta J. , Obradors X.
Published: July 2002

In this letter we report on the growth of thick films of magnetoresistive La2/3Sr1/3MnO3 films using a spray printing technique. The as-prepared films display a room-temperature magnetoresistance of 0.0012%/Oe in the 1 kOe field region. We will show that this field sensitivity is high enough to fabricate devices which, operated under a bias magnetic field, can be used as a sensitive and low-cost magnetic sensors.
Au49+, Pb50+, and Ta48+ ions from laser-produced plasmas
Authors: Woryna E. , Parys P. , Wol/owski J. , Láska L. , Krása J. , Mašek K. , Pfeifer M. , Králiková B. , Skála J. , Straka P. , Rohlena K.
Published: July 2002

Results of generation of intense currents of highly charged ions of heavy elements from laser-produced Au, Pb, and Ta plasmas are reported. High-power iodine laser PERUN operating with λ=1.315 μm was used for that purpose. Using a parabolic mirror with a hole in the center instead of a focusing lens increased the efficiency of the illumination system and made the ion measurements along the target normal possible. Multiply charged ions were recorded with an ion energy analyzer in a distance of 240 cm.
80 W cw TEM00 1064 nm beam generation by use of a laser-diode-side-pumped Nd:YAG rod laser
Authors: Konno Susumu , Fujikawa Shuichi , Yasui Koji
Published: July 2002

We have demonstrated stable and efficient 80 W cw TEM00 operation (M2=1.1) of a diode-side-pumped Nd:YAG rod laser. The laser has a simple and scalable configuration consisting of a diffusive pumping reflector and a polarization-dependent bifocusing compensation.
Intense visible photoluminescence in amorphous SiOx and SiOx:H films prepared by evaporation
Authors: Rinnert H. , Vergnat M. , Marchal G. , Burneau A.
Published: July 2002

Visible photoluminescence (PL) can be observed in a-SiOx and a-SiOx:H alloys prepared by evaporation of SiO in ultrahigh vacuum and under a flow of hydrogen ions, respectively. The hydrogen and oxygen bonding is studied by infrared spectrometry. The hydrogen stability is followed by thermal desorption spectrometry experiments. The evolution of the PL with annealing treatments shows that the PL can be attributed to a quantum confinement effect in a-Si clusters embedded in the matrix of a-SiOx. Hydrogen does not greatly contribute to the PL efficiency and to the thermal evolution of the a-Si clusters.
Characterization of a semi-insulating GaAs photoconductive semiconductor switch for ultrawide band high power microwave applications
Authors: Islam N. E. , Schamiloglu E. , Fleddermann C. B.
Published: July 2002

Simulation results depicting physical conditions in a photoconductive semiconductor switch in the pulse charging state, prior to high power switching, are analyzed. Results show that surface conditions and EL2 traps in semi-insulating GaAs influence the conduction process, specifically at high bias. Formation of trap-filled regions renders the device inhomogeneous for stable conduction and premature breakdown occurs, due to a large extent on unstable current filamentation within the device. Unlike insulators, the breakdown of desorbed gas from the surface (surface flashover) does not contribute to premature breakdown.
Quasi-ideal strontium titanate crystal surfaces through formation of strontium hydroxide
Authors: Koster Gertjan , Kropman Boike L. , Rijnders Guus J. H. M. , Blank Dave H. A. , Rogalla Horst
Published: July 2002

In recent years, well-defined and nearly perfect single crystal surfaces of oxide perovskites have become increasingly important. A single terminated surface is a prerequisite for reproducible thin film growth and fundamental growth studies. In this work, atomic and lateral force microscopy have been used to display different terminations of SrTiO3. We observe hydroxylation of the topmost SrO layer after immersion of SrTiO3 in water, which is used to enhance the etch-selectivity of SrO relative to TiO2 in a buffered HF solution. We reproducibly obtain perfect and single terminated surfaces, irrespective of the initial state of polished surfaces and the pH value of the HF solution. This approach to the problem might be used for a variety of multi-component oxide single crystals. True two-dimensional reflection high-energy electron diffraction intensity oscillations are observed during homo epitaxial growth using pulsed laser deposition on these surfaces.
Low temperature periodic electrical poling of flux-grown KTiOPO4 and isomorphic crystals
Authors: Rosenman G. , Skliar A. , Eger D. , Oron M. , Katz M.
Published: July 2002

Studies of dielectric spectroscopy, dc conductivity, and polarization switching allowed the observation of phase transition from the superionic to the insulating state in flux-grown KTiOPO4 (KTP) crystals at low temperature where the high mobility of potassium ions is suppressed. A low temperature method of fabrication of engineered periodic domain structures in superionic KTP and isomorphic crystals is proposed. It enabled us to tailor homogeneous domain gratings with various periods in the range 4–39 μm for quasiphase-matched nonlinear optical converters in KTP plates over whole area of 30×30 mm2.
Triplet luminescent dinuclear-gold(I) complex-based light-emitting diodes with low turn-on voltage
Authors: Ma Yuguang , Zhou Xuemei , Shen Jiacong , Chao Hsiu-Yi , Che Chi-Ming
Published: July 2002

The electroluminescence (EL) from a dinuclear-gold(I)-chlorate compound containing bridging phosphine ligands [Au2(dppm)Cl2] as emitting layer is reported. Devices with a structure Al/Au2(dppm)Cl2/indium–tin–oxide demonstrated a uniform emission under the driving voltage below 1 V. The EL emission was from triplet excited state and the emission color of the device was found to depend on the deposition rate of Au2(dppm)Cl2, which can be explained as the different aggregation forms of the stacking compound in the deposition process.
Polymer solution light-emitting devices
Authors: Chang Shun-Chi , Yang Yang , Pei Qibing
Published: July 2002

Traditional conjugated polymer electroluminescent devices are thin-film solid-state devices consisting of a thin polymer film sandwiched between two electrodes. In this letter, we demonstrate the generation of luminescence from polymer solutions in a compact polymer solution configuration. This unique polymer solution light-emitting device (SLED) consists of a thin layer of a polymer solution sandwiched between two transparent indium–tin–oxide/glass substrates. When biased, the device turns on at slightly above the band-gap energy and emits bright luminescence. The emission spectrum is consistent with the photoluminescence spectrum obtained from the polymer solution. We suggest that the mechanism of the SLED is due to the electrogenerated chemiluminescence effect. The SLED combines the advantages of low operating voltage, and easy and low-cost fabrication. The SLED is also a highly transparent emissive device when transparent materials are used for the electrodes and the substrates.
Indium tin oxide contacts to gallium nitride optoelectronic devices
Authors: Margalith T. , Buchinsky O. , Cohen D. A. , Abare A. C. , Hansen M. , DenBaars S. P. , Coldren L. A.
Published: July 2002

We have fabricated GaN-based light-emitting diodes using transparent indium tin oxide (ITO) p contacts. ITO-contacted devices required an additional 2 V to drive 10 mA, as compared to similar devices with metal contacts. However, ITO has lower optical absorption at 420 nm (α=664 cm−1) than commonly used thin metal films (α=3×105 cm−1). Uniform luminescence was observed in ITO-contacted devices, indicating effective hole injection and current spreading.
Hydrogen-assisted pulsed-laser deposition of (001)CeO2 on (001) Ge
Authors: Norton D. P. , Budai J. D. , Chisholm M. F.
Published: July 2002

The growth of epitaxial (001)CeO2 on a (001) Ge surface using a hydrogen-assisted pulsed-laser deposition method is reported. Hydrogen gas is introduced during film growth in order to reduce or eliminate the presence of the GeO2 from the semiconductor surface during the initial nucleation of the metal–oxide film. The hydrogen partial pressure and substrate temperature are selected to be sufficiently high such that the germanium native oxides are thermodynamically unstable. The Gibbs free energy of CeO2 is larger in magnitude than that of the Ge native oxides, making it more favorable for the metal–oxide to reside at the interface in comparison to the native Ge oxides. By satisfying these criteria, the metal–oxide/semiconductor interface is shown to be atomically abrupt with no native oxide present.
Luminescence enhancement of EuS nanoclusters in zeolite
Authors: Chen Wei , Zhang Xinhui , Huang Yining
Published: July 2002

Large luminescence enhancement was observed in EuS clusters formed in ultrastable zeolite-Y by solid-state diffusion in vacuum at 600 °C. The emission of EuS clusters shifts to higher energies relative to that of bulk EuS powder. The blueshift and the emission enhancement are attributed to quantum-size confinement and are discussed based on the magnetic exciton and the magnetic polaron models, respectively.
Femtosecond polarization spectroscopy in molecular gas mixtures: Macroscopic interference and concentration measurements
Authors: Hertz Edouard , Lavorel Bruno , Faucher Olivier , Chaux Roland
Published: July 2002

Raman-induced polarization spectroscopy (RIPS) experiments combined with homodyne detection have been conducted with a femtosecond laser at room temperature and low pressure (p<2 atm) in CO2–N2 mixtures as well as in air (O2–N2 mixtures). Each molecule of the mixture produces its own time-dependent signal, measured as a series of recurring transients. Macroscopic interference is observed when transients of both molecules overlap in the time domain. This interference leads to a large modification of the signal, which is well reproduced by calculations. The total signal recorded in CO2–N2 or O2–N2 mixtures of known concentration is analyzed in order to measure the polarizability anisotropy ratio of the two components at 800 nm. The ratio measured in an O2–N2 (air) mixture is compared with values of previous works. The knowledge of the ratio for CO2/N2 allows us to determine the concentration of CO2–N2 unknown gas mixtures. The method is presented as a relevant technique for concentration measurements in the picosecond time domain. The accuracy of the measurements and the influence of macroscopic interference in the results are discussed. The present work can be easily extended to other gas mixtures and a broad concentration range.
Depletion interaction between spheres immersed in a solution of ideal polymer chains
Authors: Tuinier Remco , Vliegenthart Gerrit A. , Lekkerkerker Henk N. W.
Published: July 2002

The depletion interaction between two spheres due to nonadsorbing ideal polymers is calculated from the polymer concentration profile using the excess (negative) adsorption. Computer simulations show that the polymer concentration profiles around two spheres are well described by the product function of the concentration profile around a single sphere. From the interaction potential between two spheres the second osmotic virial coefficient, B2, is calculated for various polymer-colloid size ratios. We find that when the polymers become smaller than the spheres, B2 remains positive in the dilute regime. This shows that the depletion interaction is ineffective for relatively small spheres.
On the structureless shape of the optical absorption bands of the β-oxygen cryocrystal
Authors: Loktev V. M. , Sharapov S. G.
Published: July 2002

An attempt is made to calculate the absorption spectrum of the β phase of solid oxygen with allowance for the absence of long-range magnetic order in it. It is shown that the correlational character of the spin ordering in β-O2 leads to a substantial change in the spectral function describing the electronic spectrum of this cryocrystal and, as a consequence, results in a loss of the fine structure of the corresponding optical absorption bands, which become broad and structureless.
Calculated absolute electron-impact ionization cross sections for AlO, Al2O, and WOx (x=1–3)
Authors: Deutsch H. , Hilpert K. , Becker K. , Probst M. , Märk T. D.
Published: July 2002

The Deutsch–Märk (DM) formalism was used to calculate absolute electron impact ionization cross sections for the metal oxide molecules AlO, Al2O, and WOx (x=1–3). These molecules are important in materials research and they are also found as impurities in the plasma edge of fusion reactors. We also calculated ionization cross sections for the atoms Al and W. In the case of the Al-containing compounds, we find an unexpected ordering of the maximum ionization cross section σmax, σmax(AlO)<σmax(Al)<σmax(Al2O). Furthermore, the maximum ionization cross section for all four W-containing compounds W, WO, WO2, and WO3 is roughly the same with σmax values in the range of 6–7×10−16 cm2. These findings can be understood by analyzing the DM calculations for these species as well as on the basis of semi-classical arguments. In addition, calculations using the semi-empirical modified additivity rule confirm the trends in the cross section ordering for, respectively, the Al-containing and W-containing compounds that was predicted by the DM formalism, at least qualitatively.
Neoclassical tearing modes and their stabilization by electron cyclotron current drive in ASDEX Upgrade
Authors: Zohm H. , Gantenbein G. , Gude A. , Günter S. , Leuterer F. , Maraschek M. , Meskat J. , Suttrop W. , Yu Q. ,   ASDEX Upgrade Team,   ECRH-Group (AUG)
Published: July 2002

Neoclassical tearing modes (NTMs) are studied in the ASDEX Upgrade tokamak [O. Gruber, H.-S. Bosch, S. Günter et al., Nucl. Fusion 39, 1321 (1999)]. The onset βN for NTMs scales with the normalized ion gyroradius ρ*, independent of the normalized collisionality ν* for a wide range of ν*. This scaling is in accordance with both polarization current and χ⊥/χ∥ model, if for the latter, the heat flux limit on parallel heat conductivity is taken into account. Analysis of NTM stabilization experiments indicates that the typical seed island size is 1–1.5 cm, again in agreement with both models. Mode coupling to the q=1 surface is a crucial element in determining the impact of the NTM on the discharge performance. Complete stabilization using electron cyclotron current drive (ECCD) has been demonstrated at βN=2.5 and with about 10% of the total heating power. The results are in good qualitative agreement with modeling using the Rutherford equation and in quantitative agreement with a two-dimensional nonlinear cylindrical tearing mode code. A precise positioning of the ECCD microwave beam, so far achieved by feedforward variation of Bt, is required for efficient stabilization. This calls for feedback control of the stabilization scheme. Our plans to implement such a scheme on ASDEX Upgrade are discussed.
Coupled-perturbed Hartree–Fock theory for infinite periodic systems: Calculation of static electric properties of (LiH)n, (FH)n, (H2O)n, (–CNH–)n, and (–CH=CH–)n
Authors: Bishop David M. , Gu Feng Long , Kirtman Bernard
Published: July 2002

Previously we have shown how to obtain the electric properties of a polymer or other periodic system at the coupled Hartree–Fock level by direct, analytical calculation rather than by extrapolation of oligomer results. Here we add computationally simpler noniterative formulas and test the methodology for the longitudinal dipole moment (μ), polarizability (α), first hyperpolarizability (β), and second hyperpolarizability (γ) of five quasilinear polymers: (LiH)n, (FH)n, (H2O)n, trans-polymethineimine (–CNH–)n, and trans-polyacetylene (–CH=CH–)n. The polymer values are in good agreement with large-oligomer calculations. In this connection the role of phase factors, particularly in determining the dipole moment, is elucidated. We are now in a good position to include electron correlation using methods analogous to those employed for molecular systems.
Effect of growth temperature on luminescence and structure of self-assembled InAlAs/AlGaAs quantum dots
Authors: Liu H. Y. , Xu B. , Qian J. J. , Ye X. L. , Han Q. , Ding D. , Liang J. B. , Zhong X. R. , Wang Z. G.
Published: July 2002

The effect of growth temperature on the optical properties of self-assembled In0.65Al0.35As/Al0.35Ga0.65As quantum dots is studied using photoluminescence and electroluminescence spectra. With the growth temperature increasing from 530 to 560 °C, the improvement of optical and structural quality has been observed. Furthermore, edge-emitting laser diodes with three stacked InAlAs quantum dot layers grown at different temperature are processed, respectively. For samples with quantum dots grown at 560 °C, the continuous wave operation is obtained up to 220 K, which is much higher than that of ones with InAlAs islands grown at 530 °C and that of the short-wavelength quantum-dot laser previously reported.
Nuclear spin–spin coupling in silane and its isotopomers: Ab initio calculation and experimental investigation
Authors: Sauer Stephan P. A. , Raynes William T. , Nicholls Robert A.
Published: July 2002

Ab initio calculated symmetry coordinate and internal valence coordinate coefficients for the two spin–spin coupling surfaces of the silane molecule—J1(Si, H) and J2(H,H)—are presented. Calculations were carried out at the level of the second-order polarization propagator approximation involving coupled-cluster singles and doubles amplitudes [SOPPA(CCSD)] using a large basis set for a total of 78 different geometries corresponding to 133 distinct points on the J1(Si, H) surface and 177 distinct points on the J2(H,H) surface. The results were fitted to fourth order in Taylor series expansions and are presented to second order in the coordinates. Both couplings are sensitive to geometry—more so than found for methane in earlier calculations. The surfaces are averaged over a very accurate, recent ab initio force field to give values for the couplings in silane and its variously deuterated isotopomers over a range of temperatures. For J(Si, H) in SiH429 both stretching and bending contribute to the nuclear motion effects with the former being considerably larger numerically. For J(H,D) in SiH328D the bending and stretching contributions are both substantial but, being of opposing sign, cancel each other out, leaving the bending–stretching cross terms to give most of the remaining contributions. The calculated values are in excellent agreement with new experimental values presented in this work; for J1(Si, H) in SiH429 and SiHD329 at 298 K we calculate −199.9 Hz and −198.5 Hz, respectively, to be compared with experimental values of −201.3 (±0.4) Hz and −199.9 (±0.4), Hz respectively. For (γH/γD) J(H,D) we predict a value of 2.58 Hz, to be compared with 2.61 (±0.08) Hz obtained by experiment at 298 K. Calculation of the tensor components of all parts of the one-bond and two-bond couplings are reported for equilibrium geometry and compared to newly calculated values of the corresponding components of methane. The principal finding for the one-bond coupling is that K∥>K⊥ for silane and K∥<K⊥ for methane. For J(H, H) each component of the contributory parts of the coupling is numerically smaller for silane than for methane.
Mode coupling in lead zirconate titanate/epoxy 1–3 piezocomposite rings
Authors: Or S. W. , Chan H. L. W.
Published: July 2002

Lead zirconate titanate (PZT)/epoxy 1–3 piezocomposite rings were fabricated with PZT volume fractions φ ranging from 0.82 to 0.94 and with a small epoxy width of 77 μm in order to investigate their resonance characteristics and to reveal the mode coupling. Four major resonance modes were observed, namely the coupled longitudinal-thickness fH and lateral fL1 and fL2 mode resonances of individual PZT elements inside the rings as well as the radial fR and wall-thickness fW mode resonances of the whole rings. No stopband resonances were observed in the frequency range of 1 to 10 MHz. fH was found to increase linearly with the decrease in element height while fL1 and fL2 remained constant. When the height and width of the elements became comparable, coupling of fH with fL1 and fL2 occurred. The observed fH, fL1, and fL2 for all samples agreed with those calculated by the mode-coupling theory. fR and fW were almost independent of the ring thickness but increased as φ increased. A guide of operating fH in the rings without causing mode coupling was presented to optimize the composite structure for transducer design.
Formation of chain-folded structures in supercooled polymer melts
Authors: Meyer Hendrik , Müller-Plathe Florian
Published: July 2002

Molecular dynamics simulations of supercooled polymer melts are reported for long chains. This is the first simulation study at melt densities where the formation of chain-folded structures resembling the lamellae of polymer crystals is observed. Crystallization and subsequent melting temperatures are related linearly to the inverse lamellar thickness. Analysis of the single chain conformations in the crystal shows that most chains reenter the same lamella by tight backfolds. Simulations are performed with a mesoscopic bead-spring model including a specific angle bending potential. They demonstrate that chain stiffness alone, without an attractive inter-particle potential, is a sufficient driving force for the formation of chain-folded lamellae.
Material-specific mapping of metal/semiconductor/dielectric nanosystems at 10 nm resolution by backscattering near-field optical microscopy
Authors: Hillenbrand R. , Keilmann F.
Published: July 2002

We report that three main constituents of nanosystems—metals, semiconductors, and dielectrics—can be categorically distinguished by their specific optical near-field contrast at 633 nm wavelength. The decisive property is the local dielectric constant as we show by calculations based on dipolar coupling theory. Experiments with Au/Si/PS(polystyrene) nanostructures using an apertureless scattering-type near-field optical microscope yield optical images at 10 nm resolution, with clear material contrast close to predicted levels.
The brightest x-ray source: A very long undulator at SPring-8
Authors: Hara Toru , Yabashi Makina , Tanaka Takashi , Bizen Teruhiko , Goto Shunji , Maréchal Xavier M. , Seike Takamitsu , Tamasaku Kenji , Ishikawa Tetsuya , Kitamura Hideo
Published: July 2002

A long undulator is a direct way to create a brilliant synchrotron radiation source. However, the length of undulators has been bounded by technical limitations and available space in accelerators. The first long undulator was developed at SPring-8, which is a third-generation synchrotron radiation facility equipped with four 30-m-long straight sections, making it the most brilliant x-ray source in the world. The magnets are placed inside a vacuum chamber and this makes it possible to arrange 780 periods continuously for 25 m. The absolute flux and spectrum of the output radiation were measured and compared with theory. The observed spectrum is in a good agreement with theory once the electron beam orbit was corrected for geomagnetic fields.
Electron energy-loss spectroscopy characterization of the boron p-like density of states in MgB2
Authors: Kong X. , Wang Y. Q. , Li H. , Duan X. F. , Yu R. C. , Li S. C. , Li F. Y. , Jin C. Q.
Published: July 2002

The microstructure and boron K-edge fine structure in MgB2 produced under high pressure are studied using high-resolution transmission electron microscopy and electron energy-loss spectroscopy in a transmission electron microscope. A pre-peak located at 188 eV in the boron K-edge energy-loss near edge structure (ELNES) provides a direct evidence for the existence of a high and unfilled p-like density of states (DOS) across the Fermi level. The σ(px+py) states dominate in this unfilled p-like DOS. The theoretical simulation results of the ELNES using the linearized augmented plane wave method can explain the experimental spectrum very well.
Effect of surface anisotropy on the hysteretic properties of a magnetic particle
Authors: Dimian M. , Kachkachi H.
Published: July 2002

We study the influence of surface anisotropy on the zero-temperature hysteretic properties of a small single-domain magnetic particle, and give an estimation of the anisotropy constant for which deviations from the Stoner–Wohlfarth model are observed. We consider a spherical particle with simple cubic crystalline structure, a uniaxial anisotropy for core spins and radial anisotropy on the surface, and compute the hysteresis loop by solving the local Landau–Lifshitz equations for classical spin vectors. We find that when the surface anisotropy constant is at least of the order of the exchange coupling, large deviations are observed with respect to the Stoner–Wohlfarth model in the hysteresis loop and thereby the limit-of-metastability curve, due to the nonuniform cluster-wise reversal of the magnetization.
High density, high performance high-confinement-mode plasmas in the Joint European Torus (JET)
Authors: Suttrop W. , Ongena J. , Bécoulet M. , Cordey J. G. , Dumortier P. , Huysmans G. T. A. , Lang P. T. , Loarte A. , Lomas P. J. , Saibene G. , Sartori R. , Parail V. V. , Valovic M. , Andrew Ph. , Andrew Y. , Beurskens M. N. A. , Budny R. , Charlet M. , Coffey I. , Eich T. , Gowers C. , Hillis D. L. , Hogan J. , Ingesson L. C. , Jachmich S. , Kallenbach A. , Koslowski H. R. , Lawson K. D. , Maddison G. P. , Maraschek M. E. , McDonald D. C. , Messiaen A. , Milani F. , Monier-Garbet P. , Nave M. F. F. , Puiatti M. E. , Rapp J. , Righi E. , Sauter O. , Sartori F. , Schweinzer J. , Stamp M. , Strachan J. D. , Stober J. , Telesca G. , Unterberg B. , Valisa M. , de Vries P. , Weyssow B. , Zastrow K. D. ,   EFDA-JET workprogramme
Published: July 2002

Recent experiments at the Joint European Torus [Rebut et al., Fusion Eng. Des. 22, 7 (1993)] aim to improve confinement quality in high-confinement-mode (H-mode) plasmas at high densities. Energy confinement time as predicted by the International Thermonuclear Experimental Reactor ITER-H98(y,2) scaling at densities near or in excess of 85% of the Greenwald density limit scaling has been obtained by (i) strong plasma shaping (triangularity 0.35<δ<0.5), or (ii) impurity seeding, or (iii) high-field side pellet injection. Slow peaking of central density without confinement degradation is observed. Loss of sawteeth and core impurity accumulation is prevented by central ion cyclotron resonance heating. In high triangularity and impurity seeded plasmas, reduction of average power loss associated with type I edge localized modes (ELMs) is found which is attributed to the occurrence of additional losses in between ELMs. Broad band magnetic fluctuations are seen which are reminiscent of regimes with small ELMs in other tokamaks. Plasma configurations have been varied to find best combinations of edge pedestal parameters and small ELM losses.
Effect of impurity oxygen on the low-temperature plasticity of solid normal hydrogen
Authors: Alekseeva L. A. , Butenko Yu. V.
Published: July 2002

The features of the low-temperature plastic deformation of solid normal hydrogen (n-H2) of 99.99% purity (the main impurity is oxygen) are investigated at liquid-helium temperature by the method of stepwise uniaxial loading under conditions of creep. It is shown that molecular oxygen has a significant influence on the parameters of the strain curves of n-H2. A softening of the crystal lattice of solid n-H2 is observed, which may be due to acceleration of the ortho–para conversion in the hydrogen samples as a result of the presence of impurity O2 molecules.
MgB
2
:
Synthesis, sound velocity, and dynamics of the vortex phase
Authors: Ignatova T. V. , Zvyagina G. A. , Kolobov I. G. , Masalitin E. A. , Fil’ V. D. , Paderno Yu. V. , Bykov A. N. , Paderno V. N. , Lyashenko V. I.
Published: July 2002

The sound velocity is measured in polycrystalline MgB2 synthesized from the constituent elements, and the bulk modulus in compression and the Debye temperature are calculated. The conversion of an elastic wave into electromagnetic radiation is investigated in the mixed state. The dynamic parameters of the vortex lattice are estimated.