Metal-oxide-junction, triple point cathodes in a relativistic magnetron

Triple point, defined as the junction of metal, dielectric, and vacuum, is the location where electron emission is favored in the presence of a sufficiently strong electric field. To exploit triple point emission, metal-oxide-junction (MOJ) cathodes consisting of dielectric "islands" over stainless steel substrates have been fabricated. The two dielectrics used are hafnium oxide (HfO(x)) for its high dielectric constant and magnesium oxide (MgO) for its high secondary electron emission coefficient. The coatings are deposited by ablation-plasma-ion lithography using a KrF laser (0-600 mJ at 248 nm) and fluence ranging from 3 to 40 J/cm(2). Composition and morphology of deposited films are analyzed by scanning electron microscopy coupled with x-ray energy dispersive spectroscopy, as well as x-ray diffraction. Cathodes are tested on the Michigan Electron Long-Beam Accelerator with a relativistic magnetron, at parameters V=-300 kV, I=1-15 kA, and pulse lengths of 0.3-0.5 micros. Six variations of the MOJ cathode are tested, and are compared against five baseline cases. It is found that particulate formed during the ablation process improves the electron emission properties of the cathodes by forming additional triple points. Due to extensive electron back bombardment during magnetron operation, secondary electron emission also may play a significant role. Cathodes exhibit increases in current densities of up to 80 A/cm(2), and up to 15% improvement in current start up time, as compared to polished stainless steel cathodes.     

An investigation of the degree of randomness in the dispersion of point features (triple junctions) in a plane

The nearest-neighbour distance distribution was used as a criterion to investigate the dispersion of triple junctions. Comparison of the expected and measured distributions by the χ²-test indicated that the triple junctions were probably not randomly located, contrary to an earlier conclusion based on an areal probe of the same specimen. The present results suggest that areal probes are insensitive to large deviations from randomness in the location of points in a plane.     

Magnetic shielding in a low-temperature torsion pendulum experiment

A new type of ether drift experiment searches for anomalous torques on a permanent magnet. A torsion pendulum is used at liquid helium temperature so that superconducting cylinders can be used to shield magnetic fields. Lead shields attenuate the earth's field, while Nb-Sn shields fastened to the pendulum contain the fields of the magnet. The paper describes the technique by which the earth's field can be reduced below 10(-4) G while simultaneously the moment of the magnet can be reduced by a factor of 7 x 10(4).     

Effect of material microstructure and tool geometry on surface generation in single point diamond turning

There is a strong desire in industry to improve surface finish when performing ultra-precision, single point diamond turning (SPDT) to reduce the amount of post process polishing required to meet final product specifications. However there are well known factors in SPDT which limit achievable surface finish. This paper focuses on the role of material microstructure, including grain boundary density and the presence of inclusions, as well as tool design on surface roughness using the concept of size effect. Size effect can be described as an interplay between the material microstructure dimension and the relative size of the uncut chip thickness with respect to the cutting edge radius. Since one of the controllable parameters in size effect is grain size and dislocation density, controlled studies were performed on samples whose microstructure was refined by mechanical strain hardening through rolling and a friction stir process (FSP). The use of the ultra-fine grained workpiece prepared using an FSP was observed to reduce side flow as well as grain boundary and inclusion induced roughness. The role of tool geometry on material induced roughness was investigated using a tool with a rounded primary cutting edge and a flat secondary edge. The use of the flat secondary edge was observed to improve surface finish when machining a flat surface. This improvement was primarily attributed to a reduction in side flow and material microstructural effects. By combining these approaches an average surface roughness R_a value of 0.685 nm was achieved when SPDT a flat surface. Furthermore the custom tool has the potential to significantly improve the productivity of SPDT by allowing for a much higher feed rate while still achieving a high quality surface finish.     

Change in geometry of a cylindrical blank in upsetting with contact friction

The lateral surface distortion of a cylindrical blank during upsetting with contact friction is analyzed. Formulas explicitly describing the change in the convex lateral surface during deformation are obtained for the first time. The accuracy of the formulas is confirmed by comparison with experimental data.     

NOx emission characteristics in turbulent hydrogen jet flames with coaxial air

The characteristics of NOx emissions in pure hydrogen nonpremixed jet flames with coaxial air are analyzed numerically for a wide range of coaxial air conditions. Among the models tested in simple nonpremixed jet flame, the one-half power scaling law could be reproduced only by the Model C using the HO₂/H₂O₂ reaction, implying the importance of chemical nonequilibrium effect. The flame length is reduced significantly by augmenting coaxial air, and could be represented as a function of the ratio of coaxial air to fuel velocity. Predicted EINOx scaling showed a good concordance with experimental data, and the overall one-half power scaling was observed in coaxial flames with Model C when flame residence time was defined with flame volume instead of a cubic of the flame length. Different level of oxygen mass fraction at the stoichiometric surface was observed as coaxial air was increased. These different levels imply that the coaxial air strengthens the nonequilibrium effect. This paper was recommended for publication in revised form by Associate Editor Haecheon Choi Hee-Jang Moon received his B.S. degree in Aeronautical Engineering from Inha University, Korea in 1986. He then received his M.S. and Doctoral degrees from Universite de Rouen, France in 1988 and 1991, respectively. Dr. Moon is currently a Professor at the School of Aerospace and Mechanical Engineering at Korea Aerospace University in Koyang, Korea. He serves on the Editorial Board of the Korean Society of Propulsion Engineers. His research interests are in the area of turbulent combustion, hybrid rocket combustion and nanofluids. Youngbin Yoon received his B.S. and M.S. degrees in Aerospace Engineering from Seoul National University, Korea in 1985 and 1987, respectively. He received a Ph.D. degree from the University of Michigan in 1994. Dr. Yoon is currently a professor at the School of Mechanical and Aerospace Engineering in Seoul National University, Korea. He is currently on the Editorial board and executive of ILASS-KOREA. The research areas of Dr. Yoon are liquid rocket injectors, combustion instability and control, ram and gas turbine combustor and laser diagnostics.     

Applications of comber-type coaxial lines with a built-in charging transformer for generating high-power nanosecond pulses

A high-power nanosecond pulse generator, based on a comber-type coaxial line section, which is charged by a high-voltage transformer built into this line, is described. Experimental data on its test results are given. It is stated that the considered approach can be used for decreasing the geometrical sizes of the generators. In the experiment, when 2-GW power pulses were generated, an extension of produced pulses by a factor of 1.8 was reached without substantial deterioration of the generated pulse shapes, as compared with a standard coaxial forming line of the same length.     

A quasi-optical exciter of the whispering gallery mode in a coaxial waveguide

A combination of a flat horn and a profiled quasi-coaxial section was designed to match standard and coaxial waveguides. The rotating mode H_15.1 was excited within a 50 MHz band around the frequency of 36 GHz with an efficiency over 90%.     

Analysis of flame shapes in turbulent hydrogen jet flames with coaxial air

This paper addresses the characteristics of flame shapes and flame length in three types of coaxial air flames realizable by varying coaxial air and/or fuel velocity. Forcing coaxial air into turbulent jet flames induces substantial changes in flame shapes and NOx emissions through the complex flow interferences that exist within the mixing region. Mixing enhancement driven by coaxial air results in flame volume decrease, and such a diminished flame volume finally reduces NOx emissions significantly by decreasing NOx formation zone where a fuel/air mixture burns. It is found that mixing in the vicinity of high temperature zone mainly results from the increase of diffusive flux than the convective flux, and that the increase of mass diffusion is amplified as coaxial air is increased. Besides, it is reaffirmed that non-equilibrium chemistry including HO₂/H₂O₂ should be taken into account for NOx prediction and scaling analysis by comparing turbulent combustion models. In addition, it is found that coaxial air can break down the self-similarity law of flames by changing mixing mechanism, and that EINOx scaling parameters based on the self-similarity law of simple jet flames may not be eligible in coaxial air flames. This paper was recommended for publication in revised form by Associate Editor Haecheon Choi Hee-Jang Moon received his B.S. degree in Aeronautical Engineering from Inha University, Korea in 1986. He then received his M.S. and Doctoral degrees from Universite de Rouen, France in 1988 and 1991, respectively. Dr. Moon is currently a Professor at the School of Aerospace and Mechanical Engineering at Korea Aerospace University in Koyang, Korea. He serves on the Editorial Board of the Korean Society of Propulsion Engineers. His research interests are in the area of turbulent combustion, hybrid rocket combustion and nanofluids.     

A coaxial chopping gap filled with air at atmospheric pressure with a pulse decay time ≤100 ps

The results of experimental studies of a coaxial chopping gap filled with air at atmospheric pressure are presented. At a pulse rise time of 100–300 ps and a voltage pulse amplitude of 30–145 kV, the obtained duration of the voltage-pulse decay time is ≤ 100 ps. The high stability of the gap operation is attained in the presence of the edge with a small radius of curvature on one of the electrodes. The fast voltage decay across the gap is determined by the preionization of the gap by runaway electrons.     

An electromagnetic generator of focused shock waves with a coaxial radiator excited by the axial current

The device hereafter described forms focused acoustic pressure pulses excited by a pulsed magnetic field affecting a cylindrical shell. A specific feature of the instrument is its coaxial radiator connected to a capacitive storage through a pulse transformer with a single-turn secondary winding that is used to excite the acoustic pulse. Design versions of the coaxial radiator are considered and estimates for setting design parameters are presented along with the test results obtained on a prototype model. The device can be used in shock-wave medical apparatuses for fragmentation of nephroliths and gall-stones.     

LDV measurements of turbulent flow behavior of droplets in a two-phase coaxial jet

Coaxial nozzles are frequently utilized for the atomization of liquids in sprays. The performance of a nozzle is generally evaluated by its atomizing characteristics, which are actually governed by the turbulence interactions of two fluids. With this point of view, this experimental study was carried out to investigate the turbulent behavior of the droplets atomized in a two-phase coaxial jet. Air and water have been used as the working fluids, and the measurements have been made by an on-line data acquisition system connected to a two-channel LDV set(DISA, 5W, Argon laser, blue: 488 nm, green: 514.5 nm). In order to generate a two-phase mixing jet, two types of coaxial nozzles (liquid column type, liquid sheet type) were used. For the investigations of the turbulent flow structure of this two-phase mixing jet, the spreading rates, mean and fluctuating components, intermittency factors and the iso-contours of joint probability densities were measured and analyzed. The results from the both types of nozzles did not show remarkable differences in mean and fluctuating velocity distributions, intermittency factors or the iso-joint probability density contours. Since the measurements were made in the fully developed turbulent mixing regions, the mean velocity distribution profiles showed good similarities and agreed well with the semi-empirical curves. The RMS values were represented as high order levels and so were the intermittency factors. The typical development trends of turbulent components ofu′ andv′ for both types were illustrated in the iso-joint probability density contours.     

Cutting mechanism of fine ceramics with a single point diamond

Circular cutting with a single point diamond cutting edge, in which the depth of cut is very small but reaches a maximum at one point, has been carried out in order to investigate the cutting of sialon and/or partially stabilized zirconia under conditions similar to grinding. This has been carried out by investigating the sliding phenomenon of a diamond cutting edge, the normal cutting force, groove profile, stock removal and so on. It has become clear that there are three different regions within the cut: elastic, plastic and cutting regions are all present when there is interference between a diamond grain and a fine ceramic. A cutting groove is generated by plastic deformation as well as brittle fracture.     

Gas flow between coaxial tubes: impedance to gas flow in an endotracheal tube increases with a catheter within

The insertion of a suction catheter or a bronchoscope down an endotracheal tube increases the resistance to gas flow down the tube. The extent to which this occurs depends on the relative diameters of the endotracheal tube and the coaxially introduced catheter. This study utilises a laboratory model to quantify this effect, using a steady flow down an annulus between two tubes whose long axes lie co-axially. Two diameters of an endotracheal tube were modelled to represent flow down adult and neonatal endotracheal tubes; these were of internal diameter (d(o)) 6.3 mm and 3.2 mm, and of length (L) 555 mm. A steady flow of air was generated to pass through the model 'endotracheal' tube. Flowrates were calculated to give Re of approximately 5000 for the larger endotracheal tube, and of approximately 1300 for the smaller. These values correspond to clinically appropriate flowrates in adult and neonatal patients, respectively. The pressure drop deltaPo down the endotracheal tube was measured initially without any obstruction, using a calibrated pressure transducer. Catheters of diameter (d(i)) 0.8 mm, 1.6 mm, and 3.2 mm were introduced into the larger diameter endotracheal tube, while catheters of 0.8 mm and 1.6 mm were introduced into the smaller one, and flow was restored to its original value. The pressure drops deltaP down the endotracheal tubes were measured with the catheters introduced a length 'x' into the tube, to x = L/2 and to x = L. Results are compared with a theoretical calculation on the basis of laminar flow for concentric tubes. If a sampling tube or suction catheter is used down the length of an infant's endotracheal tube, the results show that for most values of do/di, there is a significant rise in deltaP/deltaPo. Where a flexible bronchoscope is used down an endotracheal tube or a telescope down a rigid bronchoscope, the value of deltaP/deltaP(o) may also increase unacceptably where d(o)/d(i) is low. The results show that for equal d(o)/d(i), and equal values of x, deltaP/deltaPo are lower for higher values of Re than for lower; and that for lower values of Re there is a more rapid increase in deltaP/deltaPo as x increases, than for higher Re, especially at low values of d(o)/d(i). This result quantifiably confirms clinical experience; that care must be taken in introducing a catheter down a neonatal endotracheal tube. Deviation of these results from the theoretical calculation is less for the smaller Reynolds numbers and smaller values of d(o)/d(i), because under these conditions the flow is more likely to be laminar, with a greater degree of concentricity.     

Characterization of a new open cylindrical ion cyclotron resonance cell with unusual geometry

A new cylindrical ion cyclotron resonance cell with electrodes of different diameters is characterized. It consists of a central segmented electrode for ion trapping and detection and two planar trapping electrodes with a center bore, to which two small tube electrodes are fitted. The cell can trap either positive or negative ions or both ion polarities in the center region. For trapping both ion polarities, an unsymmetrical double well potential can be generated. Ions generated from SF(6) by electron impact or electron attachment are investigated. In depth analysis of radial excitation patterns of positive and negative ions trapped simultaneously in different stability regions reveals sharp discrimination in the extent of radial acceleration. SIMION simulations of the radial excitation show different trajectories of positive and negative ions. Axial component of radial dipolar excitation field exists in the terminal stability regions.     

Reduced stress shielding with limited micromotions using a carbon fibre composite biomimetic hip stem: a finite element model

Total hip arthroplasty (THA) enjoys excellent rates of success in older patients, but younger patients are still at risk of aseptic loosening and bone resorption from stress shielding. One solution to the stress shielding problem is to use a hip stem with mechanical properties matching those of cortical bone. The objective of the present study was to investigate numerically the biomechanical performance of such a biomimetic hip stem based on a hydroxyapatite (HA)-coated carbon fibre composite. A finite element model (FEM) of the biomimetic stem was constructed. Contact elements were studied to model the bone-implant interface in a non-osseointegrated and osseointegrated state in the best way. Three static load cases representing slow walking, stair climbing, and gait in a healthy individual were considered. Stress shielding and bone-implant interface micromotions were evaluated and compared with the results of a similar FEM based on titanium alloy (Ti-6Al-4V). The composite stems allowed for reduced stress shielding when compared with a traditional Ti-6Al-4V stem. Micromotions were slightly higher with the composite stem, but remained below 40 microm on most of the HA-coated surface. It is concluded that a biomimetic composite stem might offer a better compromise between stress shielding and micromotions than the Ti-6Al-4V stem with the same external geometry.     

收发同轴脉冲激光引信在水雾中的后向散射偏振特性

考虑激光引信的探测性能易受大气气溶胶的散射作用和吸收作用的影响,本文基于Mie散射理论,采用Monte Carlo方法模拟了脉冲激光在水雾中的后向散射偏振特性。对1~10μm的19种粒径的水雾进行仿真,获得了典型波长和水雾粒径下激光后向散射的平均偏振对比度。建立了收发同轴脉冲激光在水雾下的后向散射偏振特性实验系统,通过改变超声波加湿器的浓度,得到了5种典型粒径的模拟水雾粒子,并对其进行了实验。研究结果表明:当水雾粒径在1~10μm时,激光后向散射的平均偏振对比度为0.30~0.65,实验结果与仿真结果具有较好的一致性。得到的结果证明了激光后向散射理论模型和Monte Carlo仿真的正确性,为激光引信的抗水雾干扰研究提供了理论支撑。     

Change point estimation of high-yield processes with a linear trend disturbance

In this paper, the maximum likelihood estimator (MLE) of the change point in a high-yield process when a linear trend disturbance occurs in the proportion nonconformity of the process is first derived. Then, the performances of the proposed change point estimator in terms of both accuracy and precision are compared to the MLE of the change point designed for step changes. The results of the comparison analysis that is performed using Monte Carlo simulation experiments show that not only the average estimates of the change point estimator designed for linear trends are closer to the real change point, but also its mean square error is smaller than the one of the estimator designed for step changes for almost all shifts. In addition, better precisions are obtained by the proposed estimator than the ones obtained by the estimator designed for step changes. In short, in the presence of a linear trend disturbance, the MLE of the change point designed for linear trend disturbances outperforms the MLE of the change point designed for step changes.     

EMAC of Horizontally Polarized Waves in a Conductive Coating on a Dielectric Substrate

General expressions have been obtained for elastic displacements that appear in a coating–substrate system due to generation of vibrations by electromagnetic acoustic conversion in the coating by any conversion mechanism. Direct and inverse EMAC in a thin conductive coating on a dielectric substrate system has been considered for the magnetostriction–magnetoelastic conversion of vibrations. It has been found that Love waves and SH waves, the velocities of which differ from the classical velocities, are generated in this system. These velocity deviations have been determined for the first mode of Love waves and for the zero mode of SH waves as functions of the thickness and acoustic properties of the coating and substrate. It has been shown that the use of the EMAC provides information that can be employed for contactless estimation of the longitudinal (along the polarizing field) magnetostriction of a coating.     

A scaling method for combustion stability rating of coaxial gas-liquid injectors in a subscale chamber

A scaling method to examine combustion-stability characteristics of a coaxial injector is devised based on the acoustics and combustion dynamics in a chamber. The method is required for a subscale test of stability rating with a model chamber, which is cost-effective compared with an actual full-scale test. First, scaling and similarity rules are considered for stability rating and thereby, three conditions of acoustic, hydrodynamic, and flame-condition similarities are proposed. That is, for acoustic similarity, the natural or resonant frequencies in the actual chamber should be maintained in the model chamber. And, two parameters of density ratio and velocity ratio are derived for the requirement of hydrodynamic and flame-condition similarities between the actual and the model conditions. Next, one example of an actual combustion chamber with high performance is selected and the proposed scaling method is applied to the chamber for understanding of the method. The design operating condition for a model test is presented by mass flow rates of propellants. Stability boundaries can be identified on the coordinate plane of chamber pressure and mixture ratio of fuel and oxidizer by applying the scaling method.