Development of high-voltage pulse-slicer unit with variable pulse duration for pulse radiolysis system

A high-voltage pulse-slicer unit with variable pulse duration has been developed and integrated with a 7 MeV linear electron accelerator (LINAC) for pulse radiolysis investigation. The pulse-slicer unit provides switching voltage from 1 kV to 10 kV with rise time better than 5 ns. Two MOSFET based 10 kV switches were configured in differential mode to get variable duration pulses. The high-voltage pulse has been applied to the deflecting plates of the LINAC for slicing of electron beam of 2 μs duration. The duration of the electron beam has been varied from 30 ns to 2 μs with the optimized pulse amplitude of 7 kV to get corresponding radiation doses from 6 Gy to 167 Gy.     

Etched multimode microfiber knot-type loop interferometer refractive index sensor

We propose a novel refractive index sensor based on multimode microfiber knot-type loop (NL) interferometer. The middle portion (~5 cm) of a 15 cm long multimode fiber is etched in 48% hydrofluoric acid to reduce its diameter to ~12 μm. A NL of diameter <1 mm is made from the etched fiber. The ends of etched fiber are spliced with single-mode fibers for launching and detecting light from the NL interferometer. The NL introduces path differences to produce interferometric spectra with free spectral range ~16 nm. The spectrum shifts as the surrounding refractive index of the loop is changed by adding chemicals. We observe the highest sensitivity of the NL interferometer ~172 nm/RIU (refractive index unit) at a refractive index value 1.370 as obtained experimentally using commonly available chemicals. The design could be used as simple, low cost, and highly sensitive biological and chemical sensor.     

Combined optical and acoustical method for determination of thickness and porosity of transparent organic layers below the ultra-thin film limit

Analysis techniques are needed to determine the quantity and structure of materials composing an organic layer that is below an ultra-thin film limit and in a liquid environment. Neither optical nor acoustical techniques can independently distinguish between thickness and porosity of ultra-thin films due to parameter correlation. A combined optical and acoustical approach yields sufficient information to determine both thickness and porosity. We describe application of the combinatorial approach to measure single or multiple organic layers when the total layer thickness is small compared to the wavelength of the probing light. The instrumental setup allows for simultaneous in situ spectroscopic ellipsometry and quartz crystal microbalance dynamic measurements, and it is combined with a multiple-inlet fluid control system for different liquid solutions to be introduced during experiments. A virtual separation approach is implemented into our analysis scheme, differentiated by whether or not the organic adsorbate and liquid ambient densities are equal. The analysis scheme requires that the film be assumed transparent and rigid (non-viscoelastic). We present and discuss applications of our approach to studies of organic surfactant adsorption, self-assembled monolayer chemisorption, and multiple-layer target DNA sensor preparation and performance testing.     

A self-synchronized ADALINE network for on-line tracking of power system harmonics

This paper presents a modified adaptive linear neuron (ADALINE) structure, called self-synchronized adaptive linear neuron (S-ADALINE) network for fast and accurate estimation of power system harmonics. The proposed network relies on the Levenberg gradient descent (LGD) method based parameter updating rule and is capable of dealing with both nominal and off-nominal frequency conditions, rather than the existing modified Widrow-Hoff delta rule based ADALINE network which provides good accuracy only at nominal frequency. Moreover, the S-ADALINE provides faster response and better noise immunity than the conventional approach. The only flaw of the proposed network is its high computational load. Based on simulation studies, performances of the proposed harmonic estimator at different operating conditions have been presented and its accuracy and response time have been compared with the conventional ADALINE structure.     

Fischer–Tropsch Synthesis: Morphology, Phase Transformation and Particle Size Growth of Nano-scale Particles

An unpromoted ultrafine iron nano-particle catalyst was used for Fischer–Tropsch synthesis (FTS) in a CSTR at 270 °C, 175 psig, H₂/CO = 0.7, and a syngas space velocity of 3.0 sl/h/g Fe. Prior to FTS, the catalyst was activated in CO for 24 h which converted the initial hematite into a mixture of 85% χ-Fe₅C₂ and 15% magnetite, as found by Mössbauer measurement. The activated catalyst results in an initial high conversion (ca. 85%) of CO and H₂; however the conversions decreased to ca. 10% over about 400 h of synthesis time and after that remained nearly constant up to 600 h. Mössbauer and EELS measurement revealed that the catalyst deactivation was accompanied by gradual in situ re-oxidation of the catalyst from initial nearly pure χ-Fe₅C₂ phase to pure magnetite after 400 h of synthesis time. Experimental data indicates that the nucleation for carbide/oxide transformation may initiates at the center of the particle by water produced during FTS. Small amount of ?′-Fe_2.2C phase was detected in some catalyst samples collected after 480 h of FTS which are believed to be generated by syngas during FTS. Particle size distribution (PSD) measurements indicate nano-scale growth of individual catalyst particle. Statistical average diameters were found to increase by a factor of 4 over 600 h of FTS. Large particles with the largest dimension larger than 150 nm were also observed. Chemical compositions of the larger particles were always found to be pure single crystal magnetite as revealed by EELS analysis. Small number of ultrafine carbide particles was identified in the catalyst samples collected during later period of FTS. The results suggest that carbide/oxide transformation and nano-scale growth of particles continues either in succession or at least simultaneously; but definitely not in the reverse order (in that case some larger carbide particles would have observed). EELS-STEM measurement reveals amorphous carbon rim of thickness 3–5 nm around some particles after activation and during FTS. Well ordered graphitic carbon layers on larger single crystal magnetite particles were found by EELS-STEM measurement. However the maximum thickness of the carbon (amorphous or graphitic) rim does not grow above 10 nm suggesting that the growths of particles are not due to carbon deposition.     

Testing for a Unit Root in Autoregressive Moving-average Models with Missing Data

Testing for a single autoregressive unit root in an autoregressive moving-average (ARMA) model is considered in the case when data contain missing values. The proposed test statistics are based on an ordinary least squares type estimator of the unit root parameter which is a simple approximation of the one-step Newton–Raphson estimator. The limiting distributions of the test statistics are the same as those of the regression statistics in AR(1) models tabulated by Dickey and Fuller (Distribution of the estimators for autoregressive time series with a unit root. J. Am. Stat. Assoc . 74 (1979), 427–31) for the complete data situation. The tests accommodate models with a fitted intercept and a fitted time trend.     

Comments, with reply, on `A generalization of Van den Berg'sintegral-square error computational technique for scattering' by A.J.Mackay and A. McCowen

The commenters note that if the conjugate gradient method were used in the above named work (ibid., vol.AP-35, p.213-20, Feb. 1987) its use automatically enforces the orthogonality of the authors' functions, without requiring any additional storage. In replying the authors agree that it was perhaps an oversight not to compare Van den Berg's method with the conjugate gradient method. However, they note that it was not their aim to provide such a comparison, but only to show how improvements could be made to the spectral iteration technique (SIT)     

Generalized pencil-of-function method for extracting poles of an EMsystem from its transient response

A generalized pencil-of-function (GPOF) method is developed for extracting the poles of an electromagnetic system from its transient response. The GPOF method needs the solution of a generalized eigenvalue problem to find the poles. Subspace decomposition is also used to optimize the performance of the GPOF method. The GPOF method has advantages over the Prony method in both computation and noise sensitivity, and approaches the Cramer-Rao bound when the signal-to-noise ratio (SNR) is above threshold. An application of the GPOF method to a thin-wire target is presented     

RCS of two-dimensional structures consisting of both dielectricsand conductors of arbitrary cross section

The problem of determining the electromagnetic field scattered by two-dimensional structures consisting of both dielectric and conducting cylinders of arbitrary cross section is considered. The conductors may be in the form of strips and the dielectrics may be in the form of shells. The conductors may be partially or fully covered by dielectric layers, while the dielectrics may be partially covered by conductors. Only homogeneous dielectrics are studied. Both the transverse electric (TE) and the transverse magnetic (TM) cases are considered. The problem is formulated in terms of a set of coupled integral equations involving equivalent electric and magnetic surface currents radiating in unbounded media. The method of moments is used to solve the integral equations. Simple expansion and testing procedures are used. Numerical results for scattering cross sections are given for various structures     

Suboptimal approximation/identification of transient waveforms from electromagnetic systems by pencil-of-function method

A noniterative method for approximating signals by a linear combination of exponentials is presented. Although the technique results in a suboptimal approximation, the continuous dependence of the suboptimal exponentssim{s}_{i}on the integral square errorepsilonis such that lim(epsilon = 0) sim{s}_{i} rightarrow {s}_{i}, the best least squares exponents. The method is also useful for system identification, where the system is modeled by a black box and one has access only to the input and output terminals. A technique is demonstrated for finding the multiple poles of a system along with the residues at the poles when the system output to a known input is given. Advantages of the method are natural insensitivity to noise in the data and a capability for approximately determining signal order. Representative computations are made of the poles from the transient response of a conducting pipe tested at the ATHAMAS-I EMP simulator.