A microwave discharge source operating at pressures of several atmospheres

The design of a microwave source in which a discharge is initiated by an electromagnetic surface wave at 2.45 GHz is described. A stable discharge was supported at a gas pressure p ₀ exceeding the atmospheric pressure in He, N₂, and in H₂-Ar, H₂-He, and O₂-He mixtures in a 2-mm inner diameter quartz tube with a 0.15-mm diameter nozzle at a 50- to 115-W microwave power. A degree of dissociation of up to 80% was reached for pure H₂ at p ₀ = 6 Torr and a 6% mixture of H₂ and He at p ₀ = 50 Torr. When p ₀ increases to 19 Torr for H₂ and to 300 Torr for the mixture, the hydrogen-atom beam intensity, in spite of a decrease in the degree of dissociation, increases due to narrowing of the beam particle velocity distribution.     

Methods for detecting events in double-phase argon chambers

Multichannel wire gas electron multipliers in combination with pin anodes are proposed for detection of events in the gas phase of a double-phase argon chamber. Hydrogen with a concentration of 10 vol % is added to argon to eliminate feedbacks by photons emitted by excited argon molecules in avalanche development processes while detecting events in the argon gas. A maximum electron multiplication coefficient of ～300 has been obtained for the multichannel wire gas electron multipliers with a 1-mm gap used to detect α particles in the Ar + 10% H₂ mixture at a pressure of 1 atm (abs.). When a pin anode is used, the maximum electron multiplication factor is ～2.5 × 10⁵ for α particles and 3 × 10⁶ for β particles (⁶³Ni). It has been experimentally shown that adding H₂ with a concentration of 100 ppm to liquid argon has no effect on the singlet component of the scintillation signal in the liquid argon and reduces the emission efficiency relative to the pure argon gas phase only slightly (by 20%).     

Wire GEM detector

A wire GEM (WGEM) detector with a gas gap between meshes was constructed. The detector provides the amplification 5 × 10⁴ for the gas mixture of Ar + 20% CO₂ at atmospheric pressure. As compared with well-known GEM detectors produced by perforation the plastic plate metallized on both sides the WGEM does not suffer from breakdowns between its electrodes and the effect of accumulation of charges on holes’ walls is absent. As a result the WGEM has high reliability and stability.     

Multichannel wire gas electron multipliers

A simple technique for manufacturing electrodes of multichannel wire gas electron multipliers was developed. The maximum proportional multiplication factor of ionization electrons K ^max = 10³ was obtained when the chamber containing a multichannel wire gas electron multiplier was filled with a mixture of Ar + 10%C₂H₄ at a pressure of 1 atm (abs.) and the drift (ionization) gap of the chamber was irradiated with β particles (⁶³Ni). Under irradiation with α particles, K ^max = 10³ at a pressure of 1 atm (abs.) and 3 × 10³ at 0.4 atm (abs.). When the chamber was filled with a mixture of Ne + 10%C₂H₄ at a pressure of 1 atm (abs.) and exposed to β particles, K ^max = 4 × 10³ was obtained.     

Chemical,thermal and dilution effects of carbon dioxide in oxy-fuel combustion of wood in a fixed bed

Experimental and numerical modeling was performed on eucalyptus wood combustion under oxy-fuel conditions using a fixed bed reactor in order to isolate the role of various carbon dioxide effects on the burning rate. Wood combustion was investigated under four different mixtures of O₂ and Ar/CO₂/N₂: 21 % O₂/79 % N₂; 21 % O₂/22.5 % CO₂/56.5 % Ar; 40 % O₂/60 % CO₂; and 40 % O₂/47 % CO₂/13 % Ar. The first three mixtures were designed to have the same peak temperatures in order to isolate chemical and dilution effects of CO₂. This was achieved by substituting some percentage of CO₂ with Ar in O₂/CO₂ mixture while maintaining a constant concentration of O₂. The fourth mixture was meant to isolate the thermal effect of CO₂. The results were obtained from both the experimental rig and numerical simulation for a fixed bed configuration. Wood combustion in the fixed bed was modeled using Lagrange-Euler method, where gas-phase was calculated using computational fluid dynamics (CFD), that is Euler phase, while solid-phase was tracked in Lagrange phase using discrete element method (DEM). The results show that ignition time in CO₂ environment decreases gradually as O₂ concentration is increased. On the other hand, burning rate and flame front speed increase as O₂ concentration is increased. It was established that dilution effect is the most influential parameter on the burning rate of wood combustion in an oxy-fuel system.

     

Investigation of autoignition of propane and<Emphasis Type="Italic">n</Emphasis>-butane blends using a rapid compression machine

The effects of pressure and temperature on the autoignition of propane andn-butane blends were investigated using a rapid compression machine (RCM), which is widely used to examine the autoignition characteristics. The RCM was designed to be capable of varying the compression ratio between 5 and 20 and minimize the vortex formation on the cylinder wall using a wedge-shaped crevice. The initial temperature and pressure of the compressed gas were varied in range of 720-900 K and 1.6-1.8 MPa, respectively, by adjusting the ratio of the specific heat of the mixture by altering the ratio of the non-reactive components (N₂, Ar) under a constant effective equivalence ratio (ø_f=l.0). The gas temperature after the compression stroke could be obtained from the measured time-pressure record. The results showed a two-stage ignition delay and a Negative Temperature Coefficient (NTC) behavior which were the unique characteristic of the alkane series fuels. As the propane concentration in the blend were increased from 20% and 40% propane, the autoignition delay time increased by approximately 41% and 55% at 750 K. Numerical reduced kinetic modeling was performed using the Shell model, which introduced some important chemical ideas, represented by the generic species. Several rate coefficients were calibrated based on the experimental results to establish an autoignition model of the propane andn-butane blends. These coefficients can be used to predict the autoignition characteristics in LPG fueled SI engines.     

Role of the strengthening phases in abrasive wear resistance of laser-clad NiCrBSi coatings

The influence of the strengthening phases on the tribological characteristics (wear intensity, specific work of wear, coefficient of friction) and the wear mechanisms in two-body abrasion tests with abrasives of different hardnesses (corundum Al₂O₃, ~2000 HV and silicon carbide SiC, ~3000 HV) has been investigated for PG-SR2 (Cr₂₃C₆, 1000–1150 HV), PG-10N-01 (Cr₇C₃, 1650–1800 HV; CrB, 1950–2400 HV), and 75% PG-SR2 + 25% TiC (TiC, 2500–2900 HV; (Cr,Ni)₂₃(C,B)₆ and (Ti,Cr)(C,B), ~2000 HV) coatings. The dominant role of the strengthening phases (compared with the role of the metal matrix) in the abrasive wear resistance of laser-clad NiCrBSi coatings has been estimated. Different wear mechanisms have been identified and, accordingly, different levels of coatings wear resistance have been achieved depending on the ratio between the hardness of the strengthening phases (carbides, borides, carboborides) and abrasive particles.     

A Dispersion Interferometer Based on a CO<Subscript>2</Subscript> Laser

A double-pass dispersion interferometer based on a 9.6-μm CO₂ laser with a sensitivity of 〈 n _e l〉_min ∼ 1 × 10¹³ cm⁻² and a temporal resolution of ∼50 μ s, designed to measure linear plasma density, is described. A ZnGeP₂ nonlinear crystal is used as the frequency doubler. The main advantages of the interferometer are its compactness and a low sensitivity to vibrations of optical elements. The interferometer requires no special vibration isolation. Its main components are arranged compactly on an optical bench outside the apparatus, except for a window for radiation injection and a retroreflector; these are mounted on the wall of the experimental facility's vacuum chamber. The advantages of the dispersion interferometer have been demonstrated in an experiment with a gas-dynamic trap. __________ Translated from Pribory i Tekhnika Eksperimenta, No. 5, 2005, pp. 96–106. Original Russian Text Copyright ? 2005 by Solomakhin, Bagryanskii, Voskoboinikov, Zubarev, Kvashnin, Lizunov, Maksimov, Khil'chenko.     

Nonparametric estimation of a probability density function and its derivatives. <Emphasis Type="Italic">C</Emphasis>-approach

Nonparametric (kernel) estimation of a probability density function f(x) for a sample of finite size is considered using the C-approach. The smoothness parameter β of the estimated probability density is introduced. For the case β > 2, it is shown that the convergence of the density estimate f _n (x) to the function f(x) can be improved by using alternating-sign weight functions (higher-order weight functions). Estimation of the derivatives of a function is briefly considered using the same approach.     

Determining electrophysical characteristics of metal-oxide-semiconductor structures from the data of voltage-capacitance analysis of the depletion region of a semiconductor surface

The oxide layer in nanotransistors with metal-oxide-semiconductor (MOS) structures may be as thin as 20Å. The physical diagnostics of such structures via conventional methods of voltage-capacitance characteristics (VFCs) is impossible without taking into account the usually disregarded effects of degeneracy and dimensional quantization of the electron gas. However, as the oxide-layer thickness decreases, these effects make an increasingly substantial contribution to capacitance C of the MOS structure not only at C?C _i (where C _i is the “oxide capacitance”) but also at C < C _i . In this study, we have developed a general method for determining the principal characteristics of MOS structures from the data of analysis of the VFCs in the region of the Schottky depletion layer. The doping level, the surface potential, the semiconductor surface charge, the voltage of “flat bands,” oxide capacitance C _i , the voltage drop across the oxide, and the sign and density of the charge fixed in it can be found at an accuracy of ?0.1% within the framework of a single experiment regardless of the oxide-layer thickness and without using fitting parameters and a priori assumptions concerning the properties of the electron gas in the accumulation and inversion layers. The stages and results of the implementation of this method are demonstrated by the results of experiments performed on an n-Si-based MOS structure with a 171.2 Å-thick oxide layer.     

400-kV trigatrons for high-power low-inductance pulse generators

A series of four-channel trigatrons operating at a voltage of up to 400 kV and a current of 280 kA per channel have been developed and tested. The control electrode is coaxially arranged in a hole of the main (positive) high-voltage electrode. This design ensures a small spread (jitter) of the operation delay time (Δt _d < 1 ns) for the discharge gap filled with elegas (SF₆). The electric strength of lateral surfaces of the device body is increased without using dielectric fluids. The service life of switches is increased by using tubular metal (steel) screens in the working volume. For the parallel operation of trigatrons, the Δt _d value is also below 1 ns, but it sharply increases if the gap between the main electrodes exceeds 11–12 mm.     

Investigating the change in wear behaviour of a tool steel after surface melting and gaseous alloying

In this study it is shown that surface melting in the presence of a reactive gas can be used as a method of changing the tribological properties of a hot forging die steel. A H13 tool steel surface was melted in the presence of a gaseous shield of pure argon, nitrogen, carbon dioxide and a mixture of 80% carbon dioxide–20% argon. Microhardness profile measurements and metallographic examination was used to study the changes in wear behaviour after surface modification. The results indicate a significant increase in surface hardness after the melting and gaseous alloying process with the most wear resistant surfaces produced under a shield of CO₂ and CO₂–Ar gases. This was attributed to the formation of a fine dendritic microstructure consisting of chromium–vanadium carbides. The presence of these hard phases in the surface reduce the degree of plastic deformation and wear by adhesive mechanisms.     

Experimental investigation of the preferred Strouhal number used in self-resonating pulsed waterjet

Self-resonating pulsed waterjet (SRPW) is superior to plain waterjet in many ways and is being employed in numerous applications. To further improve the performance of SRPW, the optimal value of the preferred Strouhal number (S_d), which is used to determine the chamber length of a self-resonating nozzle, was experimentally studied at inlet pressures of 10 MPa and 20 MPa. The axial pressure oscillation peak and amplitude were used to evaluate the performance of SRPW, in order to find the optimum S_d value. Results show that S_d value determines the self-resonance behavior of an organ-pipe nozzle and greatly affects the intensity of the axial pressure oscillation. Under the experimental conditions, the optimum S_d values are 0.315 and 0.278 respectively, corresponding to inlet pressures of 10 MPa and 20 MPa. Compared with the default value of 0.3 obtained from air jet experiment, the optimum S_d value at inlet pressure of 10 MPa is a little larger and oppositely a bit smaller at inlet pressure of 20 MPa. Thus, if the inlet pressure is not considered, S_d value of 0.3 is reasonable for determining the chamber length of a self-resonating nozzle for generating effective SRPW.     

A Hemispherical Fission Chamber for the Neutron-Flux Monitoring

A fission ionization chamber has been designed to monitor neutron fluxes produced by the proton beam of the Van de Graaf accelerator in the ⁷Li(p, n)⁷Be nuclear reaction. A target producing neutrons is placed at the center of the chamber, which has a hemispherical shape. The neutrons escaping from the target are detected within the limits of a space angle a little larger than 2 sr. A ²³⁵U isotope is used as a fissile material. A U₃O₈ layer with a thickness of 300 g/cm² and an area of 80 cm² has already been deposited on the surface of the inner hemisphere using electrolysis. The chamber is filled with a gas mixture of 97% Ar + 3% CO₂. The detection efficiency of the chamber for neutrons with 1- to 150-keV energies is found to be 1.6 × 10^–5. Using this chamber, it is possible to measure (with a statistical accuracy of 1%) the integrated neutron yield in a thick target exposed for 20 min to a proton beam with a current of 3 A and an energy of 1942 keV, which is 60 keV above the threshold of the ⁷Li(p, n)⁷Be reaction. The design of the fission chamber is described, and the test results are presented.     

A setup for measurements of free-electron lifetime in a gas phase

A setup for measuring the lifetime of free electrons in a neutral buffer gas that contains an electronegative impurity is described. This setup is intended for prompt diagnostics of carcinogenicity of chemical compounds and consists of an ionization chamber with laser photoionization of the cathode, a gas system, and a system for collection and processing of experimental data based on an HBJI45 amplitude-to-digital converter. The techniques for preparing the tested gaseous mixtures and processing the experimental results are described. The results of measurements of the lifetimes and calculated free-electron capture constants for a CO₂ + O₂ calibrating mixture and mixtures of CO₂ with organic molecules are presented. The setup allows measurements of the lifetimes of free electrons in mixtures in the range 0.5–70 μs at measurement errors of 0.2–6%, respectively, and study of two or three potentially carcinogenic substances a week.     

Study on the radiative transfer through non-gray gas mixtures within an irregular 3-D enclosure by using the modified weighted sum of gray gas method

In this study the modified weighted sum of gray gases model (WSGGM) using the gray gas regrouping technique and the discrete ordinates interpolation method (DOIM) are applied to analyze the radiative transfer within an irregular 3-D enclosure filled with non-gray gas mixture of CO₂, H₂O and N₂. The computer code developed in this study is successfully applied for solving the non-gray gas radiation within a 3-D rectangular enclosure and the gray gas radiation within an irregular 3-D enclosure by showing fairly good agreements with the existing results. In this paper the radiative transfer within an irregular 3-D enclosure filled with non-isothermal non-gray gases with uniform mixtures of CO₂, H₂O and N₂ is studied to demonstrate the applicability of the modified sum of gray gases model for irregular systems and to examine the effect of the concentrations of CO₂ and H₂O on the radiative transfer within modern combustors. Results show that the wall heat fluxes and the radiative heat source terms are increased as the concentrations of CO₂ and H₂O are increased. Results also show that the radiative fluxes caused by the mixture gases with high concentrations CO₂ and H₂O which can be observed in oxy-fuel combustion systems can reach up to nearly twice of those found in ordinary air-fuel combustion systems.     

Single-Frequency TEA CO<Subscript>2</Subscript> Laser for Experiments on Non-Resonant Radiation Interaction with Matter

The design philosophy and output radiation parameters of single frequency TEA CO₂ laser with bleaching intracavity longitudinal modes selector (cell filled with SF₆) are described. At cavity tuning to 10P(16) line and choosing optimum SF₆ pressure in the cell the stable single frequency lasing is realized with scatter of radiation peak power in a series of “shots” less than ±7% of average value. The radiation energy density and intensity gradually tuned in the ranges 0.36–12.5 J/cm² and 2.9–100 MW/cm² correspondingly were realized in the focal plane of a lens with f = 127 mm.     

Measurements of the solidification point of the GKGh-136 silicone liquid

Two methods for measuring solidification point T _S of the GKGh-136 silicone liquid are described. T _S ≌ 125 K is determined by the first method from specific features of the temperature dependence of the resistance of an organic quasi-2D conductor in a GKGh-136 droplet with a size of ～1 mm. T _S ≌ 130 K is assessed by the second method from the occurrence of a specific feature in the temperature dependence of the resistance of the GKGh-136 and fine-dispersed graphite mixture, which is caused by desorption of helium in the sample volume during warming-up >T _S .     

Fast design of the QP-based optimal trajectory for a motion simulator

The main difficulty in realizing a motion simulator comes from the constraints on its workspace. The so-called washout filter prevents a simulator from being driven to go off its pre-determined boundaries and generate excessive torques. By noting that the existing washout filters are conservative and more aggressive motions may be accommodated, this paper presents a novel approach that fully exploits the simulator workspace and thereby reproduces the real-world sensations with high fidelity. The washout filter converts the real-world input trajectory as a realizable one that satisfies the spatial and dynamic constraints while minimizing the sensation error and fidelity between the motions experienced in the real world and on the motion simulator. The control objective is to reduce the computational burdens by using the QP algorithm. The proposed approach formulates the task of designing a washout filter as a quadratic programming (QP). The direct approach to the solution of the QP often results in a computational burden that amounts toO(N ³) flops andO(N ²) storage space (N=10⁴ ～ 10⁵, typically). By judiciously exploiting the Toeplitz structures of the underlying matrices, an orders-of-magnitude faster algorithm is obtained to reduce the computational burdens toO(Nlog₂ N) flops andO(N) storage space. The extensive simulation studies on the Eclipse-II motion simulator at Seoul National University assure that the QP-based fast algorithm outperforms the existing ones in reproducing the real-world sensations.     

The use of new resonance integrals for determining the parameters of the epithermal neutron spectrum

A simple method for determining parameters α and b in the presentation of the epithermal neutron spectrum in the form of φ_epi(E) = b/E ^{1 + α} using expansion of function φ_epi(E) in Taylor series with respect to parameter α is described. The problem of determining the parameters is reduced to finding a solution to a system of algebraic equations. The approximation used describes the spectrum with an adequate accuracy up to energies of ～1–50 keV, depending on value α ～ 0.2?0.1, with the help of the first two or three expansion terms (resonance integrals), respectively. The first two resonance integrals for reactions ¹⁹⁷Au(n, γ)¹⁹⁸Au and ⁵⁵Mn(n, γ)⁵⁶Mn are estimated. The measured rates of the appropriate reactions and the values of parameters α and b for the case of an assembly with a graphite moderator are presented.