Axis Configuration Research Articles

Recent advances in the LHD experiment

In the first four years of the LHD experiment, several encouraging results have emerged, the most significant of which is that MHD stability and good transport are compatible in the inward shifted axis configuration. The observed energy confinement at this optimal configuration is consistent with ISS95 scaling with an enhancement factor of 1.5. The confinement enhancement over the smaller heliotron devices is attributed to the high edge temperature. We find that the plasma with an average beta of 3% is stable in this configuration, even though the theoretical stability conditions of Mercier modes and pressure driven low-n modes are violated. In the low density discharges heated by NBI and ECR, internal transport barrier (ITB) and an associated high central temperature (>10 keV) are seen. The radial electric field measured in these discharges is positive (electron root) and expected to play a key role in the formation of the ITB. The positive electric field is also found to suppress the ion thermal diffusivity as predicted by neoclassical transport theory. The width of the externally imposed island is found to decrease when the plasma is collisionless with finite beta and increase when the plasma is collisional. The ICRF heating in LHD is successful and a high energy tail (up to 500 keV) has been detected for minority ion heating, demonstrating good confinement of the high energy particles. The magnetic field line structure unique to the heliotron edge configuration is confirmed by measuring the plasma density and temperature profiles on the divertor plate. A long pulse (2 min) discharge with an ICRF power of 0.4 MW has been demonstrated and the energy confinement characteristics are almost the same as those in short pulse discharges.

Ion cyclotron range of frequencies heating and high-energy particle production in the Large Helical Device

Significant progress has been made with ion cyclotron range of frequencies (ICRF) heating in the Large Helical Device. This is mainly due to better confinement of the helically trapped particles and less accumulation of impurities in the region of the plasma core. During the past two years, ICRF heating power has been increased from 1.35 to 2.7 MW. Various wave-mode tests were carried out using minority-ion heating, second-harmonic heating, slow-wave heating and high-density fast-wave heating at the fundamental cyclotron frequency. This fundamental heating mode extended the plasma density range of effective ICRF heating to a value of 1×1020 m−3. This use of the heating mode was its first successful application in large fusion devices. Using the minority-ion mode gave the best performance, and the stored energy reached 240 kJ using ICRF alone. This was obtained for the inward-shifted magnetic axis configuration. The improvement associated with the axis-shift was common for both bulk plasma and highly accelerated particles. For the minority-ion mode, high-energy ions up to 500 keV were observed by concentrating the heating power near the plasma axis. The confinement properties of high-energy particles were studied for different magnetic axis configurations, using the power-modulation technique. It confirmed that with the inward-shifted configuration the confinement of high-energy particles was better than with the normal configuration. By increasing the distance of the plasma to the vessel wall to about 2 cm, the impurity influx was sufficiently reduced to allow sustainment of the plasma with ICRF heating alone for more than 2 min.

Confinement characteristics of high-energy ions produced by ICRF heating in the large helical device

The behaviour of high-energy ions accelerated by an ion cyclotron range of frequency (ICRF) electric field in the large helical device (LHD) is discussed. A better confinement performance of high-energy ions in the inward-shifted magnetic axis configuration was experimentally verified by measuring their energy spectrum and comparing it with the effective temperature determined by an electron slowing down process. In the standard magnetic axis configuration a saturation of the measured tail temperature was observed as the effective temperature was increased. The ratio between these two quantities is a measure of the quality of transfer efficiency from high-energy ions to a bulk plasma; when this efficiency was compared with Monte Carlo simulations the results agreed fairly well.The ratio of the stored energy of the high-energy ions to that of the bulk plasma was measured using an ICRF heating power modulation method; it was deduced from phase differences between total and bulk plasma stored energies and the modulated ICRF heating power. The measured high energy fraction agreed with that calculated using the injected ICRF heating power, the transfer efficiency determined in the experiment and the confinement scaling of the LHD plasma.

A Unified Con/Dis Approach to Assigning Stereogenic Axis Configuration

Abstract A new and unifying approach based on a directional relationship between two arcs joining two ligands at either end of the stereogenic axis is presented for assigning stereogenic axis configuration, which is specified as Con (when both arcs are clockwise or counterclockwise) or Dis (when one arc is clockwise and the other counterclockwise). Con and Dis correspond, respectively, to the descriptors axial R and axial S for a chiral axis, and to E and Z for an achiral alkene axis.

Thermal re-emission effects on the LAGEOS I satellite versus spin axis orientation

Since 1976 when the LAGEOS I satellite was launched, the evolution of thermal re-emission effects has been observed and modeled. The effect of sun and earth heating has been analyzed separately and thermal models have been able, almost completely, to explain most of the residuals observed. However, after more than fifteen years of reasonable agreement between theory and observations, unexpected peaks in the along-track perturbations appeared and brought new attention to the problem of thermal force modeling. The efforts to explain these unexpected residual effects have produced interesting works such as the redefinition of the average coefficient of reflectivity (pressure of direct solar radiation), the Yarkovsky-Schach effect (thermal re-emission due to the solar heating), and the effect of asymmetrical reflection at the satellite's surface. Despite the lack of detailed data about the spin axis evolution, changes in the spin axis orientation have been also analyzed, however, none of these effects or corrections seems able to explain the observations. We present in this paper a unified thermal model that includes, simultaneously, the effects of sun and earth heating. The close connection between energy source coordinates (sun and earth) and the periodicity of satellite rotation and translation leads to complex sinusoidal functions that can explain the sudden peaks of maximum and minimum. We show that the residuals observed for LAGEOS I satellite can be predicted and explained by the thermal model presented and we also extend the analysis to a set of test-satellites with different spin axis inclinations concluding that the most stable spin axis configuration is the 90°-03°: spin axis perpendicular to the satellite orbital plane related to the Earth (90°) and spin axis lying on the orbital plane related to the Sun (03°). This configuration is desirable from the point of view of orbit stability because it minimizes the thermal re-emission effects and does not present great peaks of maximum after some years of orbit.

Reciprocal space mapping of silicon implanted with nitrogen by plasma immersion ion implantation

Nitrogen was implanted in (0 0 1) silicon wafers using 12 kV pulses in a glow-discharge plasma immersion ion implantation (PIII) system and at 35 keV in an electron-cyclotron-resonance (ECR) PIII facility. An implantation depth of 80 nm and a retained dose of approximately 3×10 17 cm −2 were found, for both samples, from the nitrogen Auger profiles. Reciprocal space maps (RSMs) around the (0 0 4) and (1 1 3) Si lattice points were measured for the implanted and unimplanted Si wafers, using the high-resolution X-ray diffractometer in the triple axis configuration. An asymmetry in the reciprocal space coordinate Q z (perpendicular to the sample surface) indicates that the implanted atoms force an increase in the Si lattice parameter in this direction. A broadening in the Q x direction (parallel to the sample surface) was also observed, but with a less pronounced effect. For the sample implanted with higher energy, the shape of the map indicates a higher disorder in the crystal structure.

Clusters of ventricular tachycardias signify impaired survival in patients with idiopathic dilated cardiomyopathy and implantable cardioverter defibrillators

OBJECTIVESThis retrospective study was undertaken to provide data on occurrence, significance and therapy of ventricular tachyarrhythmia (VT) clusters (VTCs) in patients with dilated cardiomyopathy (DCM) and implantable cardioverter defibrillators (ICDs).BACKGROUNDData on the clinical significance of VTCs are lacking in patients with DCM and ICDs.METHODSBaseline characteristics of 106 consecutive patients with DCM and ICDs were prospectively collected, and chart reviews and episode data retrospectively analyzed. A VTC was defined as ≥3 sustained VTs/24 h.RESULTSDuring a mean follow-up of 33 ± 23 months, 73 patients (68.9%) had recurrent VT or ventricular fibrillation (VF), 43 patients (40.6%) suffered only single VTs and 30 patients (28.3%) experienced 52 clusters of VTs. Actuarial survival free of VT or VF was 44.6%, 33.0% and 26.5%, and survival free of VTC was 77.3%, 72.2% and 67.1% after one, two and three years, respectively. Independent predictors of VT clusters were heart failure before ICD implantation (p = 0.033), presenting monomorphic VT (p = 0.044), EF <0.40 (p = 0.014) and inducible mVT, especially with right bundle branch block and superior axis configuration (p < 0.001). Survival free of recurrent VTCs was 50.8%, 38.1% and 19.0% after one, two and three years, respectively. Once a VTC had occurred, only 56.7%, 46.4%, 30.9% and 15.5% of patients survived and were not transplanted after one, two, three and four years, respectively. Survival was even more reduced if a VTC was associated with cardiac decompensation: 65.6% and 21.9% after one and two years, respectively.CONCLUSIONSDespite antiarrhythmic intervention, clusters of VTs occur and recur frequently in patients with DCM. They signify impaired survival, especially if they are associated with cardiac decompensation, and may be a harbinger of progressive myocardial deterioration rather than a primarily arrhythmic problem. The benefit of ICD therapy may therefore be low in these patients.

Strain determination in PbEuTe/PbTe multi-quantum wells

A series of Pb1−xEuxTe/PbTe multi-quantum well (MQW) samples were grown on (111) cleaved BaF2 substrates by molecular beam epitaxy. The Eu content was maintained at x∼0.05–0.06 and the PbTe well width was varied from 23 to 206 Å. The samples were characterized structurally by high resolution x-ray diffraction in the triple axis configuration. The ω/2Θ scans of the (222) Bragg reflection showed very well resolved satellite peaks up to the tenth-order for all samples indicating that sharp interfaces were obtained. Reciprocal space mapping around the (224) lattice point indicated that the MQW structure tended to the free-standing condition. The (222) ω/2Θ scans were calculated by dynamical theory of x-ray diffraction and compared to the measured ones. Using the in-plane lattice constant as the main fitting parameter, the strain in the PbTe well inside the MQW structure was obtained as a function of its width. It decreased monotonically from an almost fully strained layer to 26% of strain relaxation as the PbTe well increased from 23 to 206 Å.

Uniaxial locked growth of high-quality epitaxial ZnO films on [formula omitted]-Al 2O 3

ZnO epitaxial films have been grown on sapphire substrates using molecular beam epitaxy (MBE). Elemental sources of Zn and O were used with a RF radical source being employed to increase the reactivity of the oxygen source gas. High-sensitivity pole figure measurements indicated that the films were uniquely (0 0 0 1) oriented with no trace of secondary orientations. The unique orientation is a consequence of the coincidental near zero lattice mismatch of the ZnO a lattice constant of 0.3250 nm and the sapphire c lattice constant over four or 0.3248 nm leading to the term uniaxial locked epitaxy. Atomic force microscopy of as-grown samples indicated that the films were flat with a RMS roughness of less than 0.4 nm. Two dimensional X-ray reciprocal space mapping of the ZnO (1 0 1 ̄ 4) asymmetric reflection using a triple axis configuration indicated that the lateral correlation lengths increased from several hundred nanometers for the case of (0 0 0 1)ZnO grown on sapphire(0 0 0 1) substrates to about 0.5 μm for growth on (1 1 ̄ 2 0) sapphire substrates. This is interpreted as being a consequence of less in-plane twisting of domains due to stress from lattice mismatch. Preliminary photoluminescence measurements indicate a dramatic increase in intensity with bound exciton features less than 0.7 meV.

A new weight function expression for through cracks

The application of weight functions for the determination of stress intensity factors can be greatly enhanced if simple but accurate expressions are available. A simple weight function expression for through cracks under Mode I (opening mode) loading is proposed in this study. The requirement that weight functions should be applied to stress fields that do not possess fewer symmetries than the reference loading case used in the derivation of the weight function, is first examined. It is found that this restriction can be largely removed by using a more general coordinate axis configuration and an unambiguous definition of crack length in deriving a weight function expression. A new expression is then proposed for through cracks by considering the Taylor series expansion of the crack surface displacement as a function of distance from the crack tip. The form of the proposed expression is verified, with different numbers of terms, by using it to curve-fit reference weight function values for the following three crack configurations: (a) an array of collinear through cracks in an infinite plate; (b) a central through crack in a rectangular plate; and (c) an eccentric through crack in a rectangular plate. Such reference values are generated either from closed-form expressions or by finite element analysis. The proposed expression is found to produce accurate values for both centrally and eccentrically located cracks.

Reciprocal space maps of PbTe/SnTe superlattices

PbTe/SnTe superlattices were grown on (111) BaF2 substrates by molecular beam epitaxy using PbTe as buffer layers. The individual layer thickness and number of repetitions were chosen in order to change the strain profile in the superlattices from completely pseudomorphic to partially relaxed. The superlattices structural properties were investigated by making reciprocal space maps around the asymmetric (224) Bragg diffraction points and ω/2Θ scans for the (222) diffraction with a high resolution diffractometer in the triple axis configuration. With the strain information obtained from the maps, the (222) ω/2Θ scan was simulated by dynamical diffraction theory. The simulated spectra of the pseudomorphic superlattices, in which the in-plane lattice constant is assumed to be the same as the PbTe buffer throughout the superlattice, fitted in a remarkably good agreement with the measured data, indicating that almost structurally perfect samples were obtained. For the thicker superlattices, the (224) reciprocal space maps revealed a complex strain profile. Our results show the importance of detailed structural characterization on the interpretation of the electrical properties.

Machinability and set-up orientation for five-axis numerically controlled machining of free surfaces

In this paper, we investigate the problems of determining machinability and part set-up orientation for a given surface model. We first develop kinematic models of the five-axis machines, based on the axis configuration, then develop algorithms for determining the feasibility of one-set-up machining (machinability) and the part orientation for types C-A and A-B tool configurations. Machinability is determined by a computationally efficient procedure for finding the intersection of the feasible tool motion on the sphere and a visibility map which we call the binary spherical map (BSM). The part set-up is so chosen as to give the smallest rotational range among all feasible configurations. The algorithms developed have been tested via numerical simulations. The results show that they can be readily implemented in a CAD/CAM system, as an automated process planner selecting the efficient machine type and part set-up for NC machining.

Study of MHD Equilibrium and Stability for Heliotron/Torsatron with Helical Axis

MHD equilibrium and stability of LHD (Large Helical Device) are studied when circular plane magnetic axis is distorted into spatial helical one by introducing unbalance between two helical coil currents. The helical axis effect increases rotational transform with enhancing magnetic hill at zero beta, where beta is the ratio of plasma pressure to magnetic pressure. The significant effect of helical magnetic axis is reduction of average radius of the outermost flux surface or elimination of edge region with large magnetic hill. For finite beta plasmas this reduction of average plasma radius has substantial stabilizing effect on pressure driven MHD modes through enhancement of magnetic well produced by both Shafranov shift and helical shift of magnetic axis. However, by comparing the helical axis configuration of LHD to planer axis configuration with the same radius produced by using limiter, the change of equilibrium and stability due to the helical axis is found weak in the LHD case. The merit of the helic...

The molecular orientation in copper phthalocyanine thin films deposited on metal film surfaces

The molecular orientation in copper phthalocyanine (CuPc) films deposited on metal films of Au, Ag, Cu and Cr has been studied by reflection-absorption infrared spectroscopy and X-ray diffraction. The molecular planes of CuPc in the ultra-thin films (corresponding to several tens of monolayers) deposited on Au, Ag and Cu films were found to be nearly parallel to the metal film surfaces. Further deposition (up to 100 nm) on such thin films produces a molecular orientation with the molecular planes inclined away from the metal film surfaces. This corresponds to a crystal structure of the α phase with a standing b axis configuration. However, the molecular planes of the CuPc deposited on Cr films are rather perpendicular to the Cr film surfaces. These CuPc films are in the α phase with a parallel b axis configuration.

On the mechanism of cyclogenesis as deduced from vertical axis tilts

The relative strengths of 2 mechanisms to explain early stages of cyclogenesis are examined with an initial value model. Either mechanism can be emphasized by different choices of initial trough axis configuration. In our model, if initial conditions have an upper level trough upstream of the surface trough, and both troughs have little tilt, the growth is primarily by normal mode instability. Some evidence is presented suggesting that observed troughs are initially separate and have little along-flow tilt. A case study is shown where an observed storm develops while maintaining a fixed structure for a short period of time. The evidence suggests that the normal mode concept is a relevant motif for cyclogenesis.

Cw photon echo: Theory and observations.

A novel coherent transient spectroscopy, cw photon echo, is described. A medium is exposed to broadband cw laser radiation and then to a pulsed laser, resulting in a continuous emission signal from the medium. The temporal profile of this signal is studied using full density-matrix theory for the cases of a two-level system and a multilevel system. It is shown that such signal decays with a homogeneous dephasing time ${\mathit{T}}_{2}$ and can have a quantum-beat-type modulation, reproducing in a single shot the conventional two-pulse photon echo plot versus first-to-second pulse separation. Experimental demonstrations are given for the $^{7}$${\mathit{F}}_{0}$${\mathrm{\ensuremath{-}}}^{5}$${\mathit{D}}_{0}$ transition of ${\mathrm{Eu}}^{3+}$ and the $^{3}$${\mathit{H}}_{4}$${\mathrm{\ensuremath{-}}}^{1}$${\mathit{D}}_{2}$ transition of ${\mathrm{Pr}}^{3+}$; both ions are doped in ${\mathrm{YAlO}}_{3}$. The Eu ions exhibit a single exponential decay with a slight modulation due to the ground-state hyperfine splittings. The temperature dependence of the signal is also studied. The Pr ions show a very deep modulation pattern, which provides excellent agreement with the independent two pulse echo measurement. A numerical simulation based on the modulation theory also agrees with the observed decay curves and the nuclear axis configuration in this crystal is discussed.

The development of neural responsiveness to mechanical stimuli at the location of nerve injury requires axonal transport

Footwear-generated biomechanical manipulations (e.g., wedge insoles) have been shown to reduce the magnitude of adduction moment about the knee. The theory behind wedged insoles is that a more laterally shifted location of the center of pressure reduces the distance between the ground reaction force and the center of the knee joint, thereby reducing adduction moment during gait. However, the relationship between the center of pressure and the knee adduction moment has not been studied previously. The aim of this study was to examine the association between the location of the center of pressure and the relative magnitude of the knee adduction moment during gait in healthy men.A novel foot-worn biomechanical device which allows controlled manipulation of the center of pressure location was utilized. Twelve healthy men underwent successive gait analysis testing in a controlled setting and with the device set to convey three different para-sagittal locations of the center of pressure: neutral, medial offset and lateral offset.The knee adduction moment during the stance phase significantly correlated with the shift of the center of pressure from the functional neutral sagittal axis in the coronal plane (i.e., from medial to lateral). The moment was reduced with the lateral sagittal axis configuration and augmented with the medial sagittal axis configuration.The study results confirm the hypothesis of a direct correlation between the coronal location of the center of pressure and the magnitude of the knee adduction moment.

Pitch-Modulatedl=1 Torsatron

The pitch modulation on an l =1 torsatron field leads to the improvement of the confinement properties due to the formation of a local magnetic well, the reduction of the field ripple and the increase of the rotational transform. The pitch-modulated l =1 torsatron has the simplest coil structure and generates a compact helical magnetic axis configuration with a much smaller excursion amplitude than the coil radius. Here, the vacuum configuration and the finite beta equilibrium of this system are described.

Region of slow conduction in sustained ventricular tachycardia: Direct endocardial recordings and functional characterization in humans

Direct endocardial recording from a discrete region of slow conduction in the left ventricle was performed in a patient during sustained ventricular tachycardia. The tachycardia had a right bundle branch block and superior axis configuration with the earliest site of endocardial activation in the posterolateral left ventricle. At this site, the left ventricular electrogram during the tachycardia displayed two deflections with distinctly different responses to rapid pacing. During rapid pacing from the right ventricular apex, one deflection was transiently entrained, representing activation of the ventricle distal to a region of slow conduction in the reentrant circuit. However, the other deflection was not entrained and arose from activation proximal to this region, At a critically rapid pacing rate, interruption of the tachycardia was associated with conduction block in the region of slow conduction, as demonstrated by dissociation of the two deflections on the posterolateral left ventricular electrogram. At pacing rates that transiently entrained but did not terminate the tachycardia, decremental properties of conduction were demonstrated in the region of slow conduction, but not in the rest of the reentrant circuit.These responses of the posterolateral left ventricular electrogram to pacing during ventricular tachycardia strongly suggest that the recordings bracketed a discrete region of slow conduction in the left ventricle that was critical for the maintenance of ventricular tachycardia. Furthermore, these data demonstrate that this region of slow conduction in the left ventricle had decremental conduction properties and was the site of block during rapid pacing at rates sufficient to interrupt the tachycardia.

Performance of laser dye solutions under high signal fluxes: An analysis of signal propagation and excited-state energy extraction in mobile and rigid media

As a continuous-wave signal builds up in an energized laser dye solution, the lifetimes for the excited state of the molecules become shorter than their orientational relaxation time. Physical consequences of the phenomomenon are considered for two extreme situations—transmission through pure mobile and pure rigid media continua, respectively. The dye is treated as a linear oscillator and spectral properties are assumed to be unchanged between the media. Two problems are examined, namely signal propagation and the extraction of energy at a point in the medium for a number of configurations of pump and signal polarizations, transverse or longitudinal excitations. Signal gradients and steady-state conversion efficiencies are presented algebraically and graphically. A mobile medium has one of the steepest gradients, the largest energy storage, and one of the highest conversion efficiencies. Nevertheless, performance of a transversely excited, single-pass amplifier for the commonly used configuration where pump and signal are plane polarized and have a shared polarization axis (Configuration C) shows little difference between rigid or mobile media, according to this analysis. For a rigid Configuration C, only about 21% of the dye molecules carry most of the signal load, in contrast with a mobile medium where all molecules work equally, and such a restricted distribution undergoes rapid turnover of its excited state. Twisting of the signal plane of polarization in a dye amplifier is confirmed.