Correlation between He-Ne scatter and 2.7-microm pulsed laser damage at coating defects

A reported correlation between defect-initiated pulsed laser damage and local predamage scatter in multilayer infrared mirror coatings has been analyzed in detail. Examination of a much larger data base confirms the previous result on dielectric-enhanced reflectors with polished substrates over a wide range of energy densities above the damage onset. Scatter signals from individual undamaged defects were detected using a He-Ne scatter probe with a focal spot that nearly coincides with the 150-microm-diam (D1/e(2)) focal spot of the damage-probe beam. Subsequent damage frequency measurements (1-on-1) were made near normal or at 45 degrees incidence with 100-ns pulses at 2.7-microm wavelength. The correlation is characterized by an increase in damage frequency with increasing predamage scatter signal and by equivalence of the defect densities indicated by the two probes. Characteristics of the correlation are compared with a simple model based on focal spot intensity profiles. Conditions that limit correlation are discussed, including variable scatter from defects and background scatter from diamond-turned substrates. Results have implication for nondestructive defect detection and coating quality control.     

Constrained iterative reconstruction by the conjugate gradient method

The conjugate gradient method incorporating the object-extent constraint is applied to image reconstruction of a three-dimensional object using an incomplete projection-data set. The missing information is recovered by constraining the solution with the knowledge of the outer boundary of the object-extent which may be a priori measured or known. The algorithm is derived from the least-squares criterion as an advanced version of conventional iterative reconstruction algorithms such as SIRT (Simultaneous Iterative Reconstruction Technique) and ILST (Iterative Least Squares Technique). In the case of reconstruction from noisy projection data, a method based on the minimum mean-square error criterion is also proposed. Computer simulated reconstruction images of a phantom using limited angle and number of views are presented. The result shows that the conjugate gradient method incorporating the object-extent constraining provides the fastest convergence and the least error.     

Electron microscopic studies of artificial supramolecular structures from collagen solutions

Changes in the molecular structure of collagen fibrils isolated from solutions of native tropocollagen (TC) and of soluble collagen, obtained by enzymic (ESC) and alkaline-salt (ASC) methods, were studied by electron microscopy. It was found that, under certain conditions, it is possible to derive native-like fibrils (NF), segments with long spacings (SLS) and fibrils with long spacings (FLS)_from collagen solutions, these being very similar to structures precipitated from TC. Despite the fact that more structures with clear cross-striations can be isolated from ESC than from ASC, neither enzymic nor alkali-salt treatments cause substantial changes in the molecular structure of solubilized collagen, this determining its orderly aggregation. The data obtained permit the recommendation of new artificial materials for food and medical uses.     

Effect of gaseous environment on the kinetics of the initial stage of sintering of single-crystalline nickel oxide

Conclusions Experiments have confirmed the validity of Eq. (1) and hence the correctness of the method of investigating the sinteringkinetics of spheroidized particles based on electrical conductivity measurements. In the temperature range investigated single-crystalline nickel oxide sinters better in argon than in air, but the mechanisms of mass transfer and energies of activation in both cases are virtually identical. In both gases the kinetic dependence of the initial stage of sintering was found to be of the form x³ _R t.Translated from Poroshkovaya Metallurgiya, No. 2(266), pp. 32–35, February, 1985.     

Submillimeter backward wave oscillators

NASA Lewis Research Center is engaged in a program to develop a series of backward wave oscillators (BWO's) for the frequency range 500 to 2000 GHz. Generically BWO's are electron beam traveling wave tubes operating in a dispersive regime in which the group velocity and the phase velocity of the induced electromagnetic wave are in opposite directions. The oscillation frequency of a BWO is controlled by the electron beam velocity (anode voltage). Such tubes because of their frequency tunability, phase locking capability, and large bandwidth are ideal local oscillators for heterdyne receiver/spectrometers. The design of the BWO's will be discussed with emphasis on the etched slow wave structure, zero compression electron beam, long life cathode, and moderate operating voltages.     

A fonnalism for generating multiparametric encoding gradients in NMR tomography

A general mathematical formalism for generating multiparametric NMR image encoding gradients is introduced. The new schematic approach enables one to construct any desired encoding gradient which may be used in an imaging sequence. Basic gradient waveforms which can be used as building blocks of the desired encoding gradients are presented. A matrix operator for obtaining the encoding gradient for any kind of phase encoding is derived. Specific examples illustrating how to obtain "pure" spatial, velocity, or acceleration encoding gradients for moving spins are presented.