An analytical approach to the design of electrodes in high-power, fast-transverse-flow lasers

Usually, the design of the electrodes of fast-transverse-flow CO₂ lasers relies more on experimental data than on theoretical analysis. Traditional systems sustain a stable, high-power discharge but the current distribution generates a peaked, non-uniform small-signal-gain pattern. We present a theoretical model suitable for the design of electrodes that achieves a quasi uniform gain distribution. The analysis, based on a two-temperature model and the gas-transport equations, computes the electron density that supports a flat gain pattern. Combining the desired electron density with the electron-balance equations, the model determines the required electric field. The results were used to design a new set of electrodes for a home-made fast transverse flow CO₂ laser. A stable, large volume discharge () with very good uniformity was obtained. The resulting gain distribution was registered in a bi-dimensional map. The peak gain rose from , attained with the old electrode set, to with the new one and the gain showed a homogeneous profile.     

Parameter estimation for 1D PWL chaotic maps using noisy dynamics

Nonlinear Dynamics - Many physical situations involve chaotic systems implemented in hardware. Among them one-dimensional piecewise linear maps are popular candidates for such applications because...     

Elimination of motion,pulsatile flow and cross-talk artifacts using blade sequences in lumbar spine MR imaging

The purpose of this study is to evaluate the ability of T2 turbo spin echo (TSE) axial and sagittal BLADE sequences in reducing or even eliminating motion, pulsatile flow and cross-talk artifacts in lumbar spine MRI examinations. Forty four patients, who had routinely undergone a lumbar spine examination, participated in the study. The following pairs of sequences with and without BLADE were compared: a) T2 TSE Sagittal (SAG) in thirty two cases, and b) T2 TSE Axial (AX) also in thirty two cases. Both quantitative and qualitative analyses were performed based on measurements in different normal anatomical structures and examination of seven characteristics, respectively. The qualitative analysis was performed by experienced radiologists. Also, the presence of image motion, pulsatile flow and cross-talk artifacts was evaluated. Based on the results of the qualitative analysis for the different sequences and anatomical structures, the BLADE sequences were found to be significantly superior to the conventional ones in all the cases. The BLADE sequences eliminated the motion artifacts in all the cases. In our results, it was found that in the examined sequences (sagittal and axial) the differences between the BLADE and conventional sequences regarding the elimination of motion, pulsatile flow and cross-talk artifacts were statistically significant. In all the comparisons, the T2 TSE BLADE sequences were significantly superior to the corresponding conventional sequences regarding the classification of their image quality. In conclusion, this technique appears to be capable of potentially eliminating motion, pulsatile flow and cross-talk artifacts in lumbar spine MR images and producing high quality images in collaborative and non-collaborative patients.     

Comparison of PD BLADE with fat saturation (FS), PD FS and T2 3D DESS with water excitation (WE) in detecting articular knee cartilage defects

The purpose of this study, is to compare the sequences: 1) proton density (PD) BLADE (BLADE is a PROPELLER-equivalent implementation of the Siemens Medical System) with fat saturation (FS) coronal (COR), 2) PD FS COR, 3) multi-planar reconstruction (MPR) with 3 mm slice thickness and 4) multi-planar reconstruction (MPR) with 1.5 mm slice thickness, both from the T2 3D-double-echo steady state (DESS) with water excitation (WE) sagittal (SAG), regarding their abilities to identify changes in the femorotibial condyle cartilage in knee MRI examinations. Thirty three consecutive patients with osteoarthritis (18 females, 15 males; mean age 56 years, range 37–71 years), who had been routinely scanned for knee examination using the previously mentioned image acquisition techniques, participated in the study. A quantitative analysis was performed based on the relative contrast (ReCON) measurements, which were taken both on normal tissues as well as on pathologies. Additionally, a qualitative analysis was performed by two radiologists. Motion and pulsatile flow artifacts were evaluated. The PD BLADE FS COR sequence produced images of higher contrast between Menisci and Cartilage, Fluid and Cartilage, Pathologies and Cartilage as well as of the Conspicuousness Superficial Cartilage and it was found to be superior to the other sequences (p < 0.001). The sequences T2 3D DESS 1.5 mm and T2 3D DESS 3 mm were significantly superior to the PD BLADE FS COR and the PD FS COR sequences in the visualization of Bone and Cartilage and the Conspicuousness Deep Surface Cartilage. This pattern of results is also confirmed by the quantitative analysis. PD FS BLADE sequences are ideal for the depiction of the cartilage pathologies compared to the conventional PD FS and T2 3D DESS sequences.     

Metal-water interactions and hydrogen bonding in dittmarite-type compounds M'M''PO4.H2O (M'=K+, NH4+; M''=Mn2+, Co2+, Ni2+). Correlations of IR spectroscopic and structural data

The vibrational behavior of the uncoupled nu(OD) modes and of the water librations in dittmarite-type compounds M'M'PO4.H2O (M'=K+, NH4+; M'=Mn2+, Co2+, Ni2+) is analyzed in terms of the influence of two types of metal-water interactions (M+-H2O and M2+-H2O), the hydrogen bonding and the repulsion potential of the lattices. The M+-H2O interaction is found to be the main factor, which influences nuOD. The strong K+-H2O interaction weakens in a higher degree the intramolecular O-H bonds than the corresponding M2+-H2O interactions (M2+=Mn, Co, Ni). As a result nuOD is shifted to lower wavenumbers in the potassium series than in ammonium one, irrespective of the synergetic effect of M2+, the hydrogen bond lengths and the repulsion potential of the lattices. The analysis of the spectroscopic data evidences for the dominating influence of the M2+-H2O interaction on the wagging mode. The blue shift of nuwag strictly follows the increasing synergetic effect of M2+, i.e. nuM'/Mn相似文献   

Influence of the characteristic curve of the material on the diffraction efficiency of holographic volume phase gratings

The behaviour of the diffraction efficiency of holographic gratings is discussed on the basis of the multiwave coupled-wave approach, taking into consideration the photoinduced change in the refractive index of the recording medium. The cases of Bragg and off-Bragg reconstruction in Bragg and mixed diffraction are considered. The influence of the higher harmonics of the refractive index modulation profile on the diffraction efficiency is shown.     

Hypergroups and binary relations

The paper deals with a binary relation R on a set H, where the Rosenberg partial hypergroupoid H _R is a hypergroup. It proves that if H _R is a hypergroup, S is an extension of R contained in the transitive closure of R and S ², then H _S is also a hypergroup. Corollaries for various extensions of R, the union, intersection and product constructions being employed, are then proved. If H _R and H _S are mutually associative hypergroups then is proven to be a hypergroup. Lastly, a tree and an iterative sequence of hyperoperations where k = 1, 2, ...) on its vertices are considered. A bound on the diameter of is given for each k such that is associative. Received December 18, 1998; accepted in final form February 8, 2000.     

New route for the synthesis of benzimidazoles by a one-pot multistep process with mono and bifunctional solid catalysts

One-pot multistep reactions involving a new environmentally friendly catalytic procedure have been developed for the synthesis of benzimidazoles. Benzimidazole derivatives with biological and pharmaceutical interest have been prepared by a one-pot four step process with a solid catalyst containing basic and oxidation sites. The four steps refer to: (a) oxidation of the alcohol; (b) cyclocondensation of the aldehyde formed with ortho-phenylenediamines, (c) oxidation of the carbon-nitrogen bond, (d) N-alkylation reaction. The process is illustrated by the synthesis of 1,2-disubstituted benzimidazole derivative with antiviral activity.     

Theoretical study on structural and mechanistic aspects of synthesis of a 3-aminopyrazole derivative

The structure of 5-hydroxy-3,5-dimethyl-1-S-methylisothiocarbamoyl-2-pyrazolinium iodide (HDMCPI), a cyclic intermediate for a 3-aminopyrazole derivative, was determined by means of X-ray analysis and spectroscopic techniques. In a treatment of HDMCPI in alkaline aqueous solution, 4-acetyl-3(5)-amino-5(3)-methylpyrazole (AAMP) was unexpectedly yielded. The reaction of HDMCPI was monitored by ¹H and ¹³C NMR spectroscopy. It was shown that keto-imine tautomer appears as the only tautomeric form. Density functional theory explained the spontaneous formation of keto-imine tautomer, whose existence is the main condition for generating a carbanion in alkaline medium. The carbanion further undergoes cyclization and elimination of MeSH, thus yielding AAMP. In the reaction of acetylacetone with thiosemicarbazide instead of S-methylisothiosemicarbazide, there were no traces of AAMP. This result can be attributed to the absence of keto-imine form in the tautomeric equilibrium, which would provide the formation of a carbanion for a nucleophilic attack and further cyclization.     

Structural and electrical characteristics of InGaAsN layers grown by LPE

Crystallographic and transport properties of nominally undoped and Sn-doped InGaAsN layers grown by low-temperature LPE have been studied and related to the growth conditions.In the case of lattice matching, flat and uniform mirror-like layers of 8–10 μm in thickness are obtained. The compositions of the layers under study have been determined by combination of X-ray microanalysis and X-ray diffraction methods to be In_0.035Ga_0.065As_0.086N_0.014. The lattice mismatch between layer and substrate Δa_l/a_s calculated from X-ray diffraction curves is less than?7×10^?4 for all samples. The layers grown at lower epitaxy temperatures exhibit the highest crystalline quality, better lattice match and better homogeneity. This is in good agreement with the results of morphological study by atomic force microscopy which show root mean-square surface roughness of 0.18 nm for the best layers.CV and Hall measurements reveal that intentionally undoped InGaAsN layers are n-type with free carrier concentration about one order of magnitude higher in comparison to layers not containing nitrogen and high electron mobility values over 2000 cm²/Vs. A dramatic reduction in the free carrier concentration and slightly increase in mobility are observed for Sn-doped InGaAsN layers.