On the efficiency of emission from a plasma column in high-pressure pulse-periodic discharge in cesium

Spectral energy fluxes emerging from the surface of an axially symmetric column of inhomogeneous cesium high-pressure plasma have been calculated by direct integration of the radiation-transport equation. It is shown that, at a specified plasma temperature on the axis, irrespective of the radiation -formation mechanism and the radial distribution of the plasma parameters, the maximum energy fluxes arise when the radial optical thickness τ _R of the plasma column is close to unity. The asymptotic τ _R value, at which the discharge column radiation is the largest part of the Planck radiation, has been found.     

Plastic deformation behaviors and mechanical properties of rolled rings of 20CrMnTi alloy in combined radial and axial ring rolling

In this paper, a new combined radial and axial ring rolling process is proposed, in which a large increase in both the ring diameter and height can be achieved. Using the finite element (FE) method, the plastic deformation behaviors and mechanical properties of the rolled rings of 20CrMnTi alloy in combined radial and axial ring rolling are numerically investigated. It is found that under different axial rolling ratio λ_a and radial rolling ratio λ_r, there are three kinds of plastic deformation behaviors during the process. The first one is that when λ_a = 1 and λ_r > 1, the ring produces the deformation of thickness reduction and diameter expansion and its height basically remains unchanged. The second one is that when λ_a > 1 and λ_r > 1, the ring produces the deformation of thickness reduction, diameter expansion and height increase. The third one is that when λ_a > 1 and λ_r = 1, the ring produces the deformation of thickness reduction, outer diameter constancy and height increase. Owing to these plastic deformation behaviors, the axial and circumferential strain distribution of the rolled rings with the same final geometry is different. To evaluate the mechanical properties of these rolled rings, the compression and tensile tests have been carried out numerically and it is found that the rolled rings with the same final geometry may have different mechanical properties due to the different axial and circumferential strain distribution and the proposed combined radial and axial ring rolling process can thus be adopted to manufacture the rings with the same final geometry but different mechanical properties.     

Variation of transverse and shear stiffness properties of wood in a tree

In this work, the application of the unnotched Iosipescu test for the evaluation of the radial variability of longitudinal–radial stiffness parameters of maritime pine wood was investigated. Rectangular specimens with grain at 45° were tested using the Iosipescu fixture. For this configuration both the transverse (Q₂₂) and the shear (Q₆₆) stiffness parameters can be simultaneously identified by the virtual fields method. Displacement fields were measured by the grid method. The strain fields were reconstructed from the raw displacement fields using a polynomial approximation scheme. For the tested wood material, it was found that both parameters decrease from the centre to about the middle radius of the stem and increase afterwards to the outermost positions. Moreover, a relatively good correlation was obtained between the patterns of radial variability of Q₂₂ and Q₆₆.     

Three-dimensional stress state around quarter-elliptical corner cracks in elastic plates subjected to uniform tension loading

Detailed full-field three-dimensional (3D) finite element analyses have been conducted to study the out-of-plane stress constraint factor T_z around a quarter-elliptical corner crack embedded in an isotropic elastic plate subjected to uniform tension loading. The distributions of T_z are studied in the forward section (0° ? θ ? 90°) of the corner cracks with aspect ratios a/c of 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0. In the normal plane of the crack front line, T_z drops radially from Poisson’s ratio at the crack tip to zero beyond certain radial distances. Strong 3D zones (T_z > 0) exist within a radial distance r/a of about 4.6-0.7 for a/c = 0.2-1.0 along the crack front, despite the stress-free boundary conditions far away. At the same radial distance along the crack front in the 3D zones, T_z increases from zero on one free surface to a peak value in the interior, and then decreases to zero on another free surface. The distributions of T_z near the corner points are also discussed. Empirical formulae describing the 3D distributions of T_z are obtained by fitting the numerical results, which prevail with a sufficient accuracy in the valid range of 0.2 ? a/c ? 1.0 and 0° ? θ ? 90° except very near the free surfaces where T_z is extremely low. Combined with the K-T solution, the transition of approximate plane-stress state near the surfaces to plane-strain state in the interior can be characterized more accurately.     

Influences of carbon content and power density on the uniformity of PECVD grown a-Si1−x:Cx:H thin films

Large area electronics require large size thin films whose eventual inhomogeneities arise as a problem. Hydrogenated amorphous silicon carbide thin films (a-Si_1−xC_x:H) for four different source gas mixtures at two power densities were deposited by plasma enhanced chemical vapor deposition (PECVD) technique. The degree of film homogeneity was investigated through measurements of deposition rate, refractive index and optical energy gap along the radial direction of bottom electrode. Both ellipsometer at various incident angles and optical transmittance at normal incidence were used in mutual control as diagnosing tools. It seems there is a critical power density beyond which inhomogeneities of the deposited films along the radial direction of the electrode are unavoidable.     

Radial current in tokamak during neutral beam injection

Generation of radial current I _r during the ionization of fast deuterons in their neutral beam injected into a tokamak is considered. The current I _r and force F determined by the [I _r × B] product vary in a complicated manner along the tokamak radius. For this reason, the torque that arises together with force F also nonuniformly varies along the radius, which can lead to a shear in the rotation velocity. A sufficiently large magnitude of this shear can result in reduction of both micro- and macroinstabilities in tokamak plasma [1, 2]. The influence of injected beam characteristics and other experimental parameters on the spatial distribution of I _r in the TUMAN-3M tokamak is considered.     

Liquid 4He: Contributions to First Principles Theory. II. Quantized Vortices and the λ Transition

Quantized vortices in liquid ⁴He at finite temperatures are treated by first principles theory that extends results presented in paper I of this two-part set. Then the possible role of thermally excited quantized vortices in accounting for the λ transition is studied. This study indicates that vortices are probably not the dominant mechanism responsible for the λ transition, but that they may account for some minor effects near T _λ . A model that identifies critical fluctuations with quantized isothermal fourth sound waves is developed and used to derive formulas for specific heat in He II and He I. Logarithmic divergence of the constant volume specific heat is found. Numerical calculations using those formulas produce close agreement with experimental data. A formula for the radial distribution function near T _λ in He II and He I is derived and numerically evaluated. A formula for the correlation length is derived and numerically evaluated and physical characteristics of the correlation length are discussed. Numerical calculations based on that formula for correlation length are compared with experimental results. Long-range order in the liquid exhibited in the radial distribution function and correlation length is shown not to involve a Bose-Einstein condensate in this theory. The isothermal compressibility is found by integration of the radial distribution function and the result shows that isothermal compressibility is unchanged by critical fluctuations of isothermal fourth sound.     

Application of Different Turbulence Models in Unsteady Flow Simulations of a Radial Diffuser Pump

In this paper, the unsteady flow in a radial pump has been investigated numerically by utilizing different turbulence models at both design and off-design conditions. The numerical results are analyzed and compared on the pressure, relative velocity, and turbulence fields with experimental results from LDV and PIV. The analysis of the results shows that the turbulence model does not show significant influences on the pressure field. At the design condition (Q _des), k-ε predicts generally the best result on the relative velocity magnitude, slightly better than k-ω. However, k-ω has an overwhelming predominance on predicting the velocity vector directions and also on the turbulence kinetic energy. Furthermore, CFD can provide much better agreement to the measurements in the radial gap region than in the impeller region for all examined turbulence models. At off-design condition (0.5Q _des), DES and SST turbulence models predict successfully a “two-channel” stall phenomenon in the impeller detected by the measurements, and other turbulence models failed to predict it.     

Magnetization of a Current-Carrying Superconducting Disk with B-Dependent Critical Current Density

In the frame work of the critical state model (CSM), the magnetic response of a thin type-II superconducting disk that carries a radial transport current and is subjected to an applied magnetic field have been studied. To this end, we have studied the process of the magnetic flux-penetration. For a disk initially containing no magnetic flux but carrying a radial current, when a perpendicular magnetic field is applied, magnetic flux-penetration occurs in three stages: (1) the magnetic flux gradually penetrates from the edges of the disk until an instability occurs, (2) there is a rapid inflow of magnetic flux into the disk’s central region, which becomes resistive, and (3) magnetic flux continues to enter the disk, while persistent azimuthal currents flow in an outer annular region where the net current density is equal to J _c . Also the behavior of a current-carrying disk subjected to an AC magnetic field is calculated. The magnetic flux, the current profiles and the magnetization hysteresis loops are calculated for several commonly used J _c (B) dependences. Finally, the results of the applications of the local field-dependent of the critical current density J _c (B) are compared with those obtained from the Bean model.     

Superconducting properties of internal-tin route Nb3Sn wires with radially arranged filaments : Development that realizes high Jc and low hysteresis loss

An internal-tin route Nb₃Sn superconducting wire that has both remarkably low hysteresis loss (Q_h) and high critical current density (J_c) was developed according to a new design idea. The wire was constructed by arranging the filaments in a radial layout, enlarging the outer filaments along the radial direction, narrowing the filament spacing in the radial direction intentionally and enlarging the filament spacing in tangential direction. Thus, the electromagnetic coupling among the filaments in tangential direction due to the bridging and/or proximity effect was suppressed without decreasing the volume fraction of Nb. As a result, excellent properties such as J_c(12 T) = 1.15 × 10³ A/mm² and Q_h = 301 mJ/cm³ (for 1 cycle of B = ±3 T) were obtained. We also evaluated the transition temperature (T_c) and upper critical field (B_c2) of the wire. The values for T_c and B_c2 were 17.3 K and 24.1 T, respectively, which were much better than those of usual internal-tin route wires. Moreover, electron probe micro-analyses confirmed that the good T_c and B_c2 were the result of the qualitative improvement of the Nb₃Sn compound based on the effects of arranging the Nb filaments radially, increasing the ratio of Sn-to-Nb and shortening the diffusion length for Sn. This wire is promising for use with conduction-cooled high-field magnets, in which there is a need to decrease the load of the cryocooler, and also for the strands of fusion coils.     

Tunneling radial cracks in layered structures from contact loading

Glass/epoxy laminates glued onto a compliant substrate are indented with a hard ball. The damage is characterized by a set of transverse cracks which pop out from the subsurface of the glass layers due to flexure and propagate stably in the radial direction with load in a bell-shape front under a diminishing stress field. Compliant interlayers, even extremely thin ones, are effective in inhibiting crossover fracture. This leads to crack tunneling and crack multiplication in the hard layers, which enhances energy dissipation and reduces the spread of damage relative to the basic bilayer configuration. The experiments show that the fracture in a given layer is well approximated by a power-law relation of the form c^3/2K_C/P = δ, where P, c, and K_C are the indentation load, crack length and fracture toughness, in that order, and δ an implicit function of the layer position and material and geometric variables, derived with the aid of available tunnel crack solutions.The model specimen studied provides a useful insight into the fracture behavior of natural, biological and synthetic layered structures from concentrated loading. The analysis shows that the crack arrest capability of a thin interlayer increases in proportion to the modulus misfit ratio between the layer and interlayer, and that the spread of radial cracks in a laminate of given thickness reduces in proportion to n^1/3, where n is the number layers in the laminate.     

An exact solution of the governing equation of a fluid of second-grade for three-dimensional vortex flow

Three-dimensional vortex flow of a fluid of second-grade, for which the velocity field is in the form of υ_r = f(r), υ_θ = g(r), υ_z = zh(r), where r, θ, z are cylindrical polar coordinates, is considered and an exact solution of the governing equation is given. It is an important fact that for this type of flow of a Newtonian fluid, the axial gradient of radial distribution of pressure does not exist and this is unrealistic in many problems of rotational flow. It is found that the axial gradient of radial distribution of pressure exists for this type of flow of a fluid of second grade. It is emphasized that there are exact solutions for the velocity field considered of the governing equation for an Oldroyd type fluid and a Maxwell type one. For some special cases of the velocity field closed form solution of the governing equation are investigated.     

Vickers indentation tests in a Zr62.55Cu17.55Ni9.9Al10 bulk amorphous alloy

Vickers indentation tests were conducted on a Zr_62.55Cu_17.55Ni_9.9Al₁₀ bulk amorphous alloy to investigate the evolution of shear bands and its plastic deformation dimension via a bonded interface technique. Under all indentation loads, the plastic deformation is accommodated through semi-circular and radial shear bands. The plastic deformation dimension increases with increasing the indentation loads. A simplified λ = C(P)^0.5 model was put forward to predict and estimate the plastic deformation dimension characterized by shear bands in the subsurface. For the Zr_62.55Cu_17.55Ni_9.9Al₁₀ amorphous alloy, C is about 15.314 µm/N^0.5. The normalized shear band zone is independent to the indentation load.     

Structure of isolated vortex lines in rotating3He-A and3He-B

First we calculate the texture of a Mermin-Ho vortex in a field of 284 Oe and compare its energy with the energies of singular vortices in³He-A as a function of rotation speed and temperature. Then we calculate the order parameter components ψ _r , ψ_φ and ψ _z (r, ф,z are cylindrical coordinates) for an isolated vortex line in³He-B. We find that in the core region the transverse components ψ _r and ψ_φ are strongly reduced in comparison to the component ψ _z along the rotation axis. A strong magnetic field has the reverse effect: ψ _z is reduced in comparison to ψ _r and ψ_φ. Finally we calculate the corrections of order (1?T/T _c ) to the Ginzburg-Landau solutions. These yield an interesting structure of the vortex core. For radial distances large compared to the coherence length, ψ_φ is reduced as the temperature decreases.     

Langmuir probe diagnostics of microwave electron cyclotron resonance (ECR) plasma

A Langmuir probe diagnostics is done on the microwave ECR generated plasma in a 2.45 GHz, 1.5 kW facility set up in our laboratory (for thin film deposition) by inserting a probe in the plasma close to substrate location (640 mm away from main ECR zone). A program using Graphical User Interface (GUI) was used for data analysis of I-V probe characteristics to obtain the radial electron energy distribution function (EEDF) in plasma. Plasma parameters such as charged particle density (n_e and n_i), electron temperature (T_e), plasma potential (V_pl) and floating potential (V_float) were estimated at substrate location for two incident microwave power levels at a fixed operating pressure. These parameters were estimated by different methods like orbital motion limited (OML) theory, electron energy distribution function (EEDF) and conventional method. The results obtained by the different methods are compared and observed differences are explained. The results indicate that even though the diffusion of plasma at the substrate location is mainly forced by particle collisions that lead to radial plasma uniformity, it still shows a non-Maxwellian behavior for the electrons with two groups having different energies.     

The induction of a propagating normal zone (quench) in a superconductor by local energy release

Experiments were performed to measure the minimum energy, E_c of an instantaneous point source, needed to induce a propagating normal transition (quench) in a superconductor. The energy pulse was supplied to the superconductor by a miniaturized heater element which had a thermal time constant of ～ 20 μs. E_c depends on the current distribution in the conductor and is lower for a nonhomogeneous distribution. A computer calculation solving the nonhomogeneous heat equation agrees within a factor of two with the results for adiabatic conditions (no radial heat transfer) without adjustable parameters. The results obtained with the conductor placed in liquid helium could be fitted into the calculation with parameters close to measured transient heat transfer data.     

Evidence for a Local Structural Change in La2CuO4.1 Across the Superconducting Transition

Polarized Cu—K edge XAFS (X-ray absorption fine structure) on La₂CuO_4.1 indicate that the radial distribution function of the copper in plane oxygen pairs is a two-site distribution, in agreement with the results found by Bianconi et al. for temperatures below the appearance of a pseudogap. Additionally we find evidence of a change in this distribution across the superconducting transition, suggesting coupling between the local lattice structure and the charged particles involved in the superconductivity.     

Optimum strain gage location for evaluating stress intensity factors in single and double ended cracked configurations

A general methodology has been proposed for calculation of optimum radial strain gage locations for measurement of the stress intensity factors using strain gage technique. The upper bound (r_max) of strain gage locations for complex single ended and double ended cracked configurations has been determined using the proposed method. Further, dependency of the r_max on the crack length to width ratio and on the state of stress is investigated. Numerical results obtained from the present investigation are observed to be in accordance with the theoretical predictions. Using the proposed approach the correctness of strain gage locations used by the earlier researchers is also verified.     

Superfluidity in Hydrogen-Deuterium Mixed Clusters

Path-integral Monte Carlo calculations have been performed on para-H₂ clusters mixed with several ortho-D₂ molecules, (D₂) $_{N_{D}}$ (H₂) $_{N_{H}}$ with 1≤N _D ≤5 and 10≤N _H ≤20, to study their superfluid behavior at low temperatures. It is found that heavier D₂ molecules are located near the center of the mixed cluster, and radial density distributions of D₂ and H₂ tend to separate from each other as the number of D₂ molecules increases. We have also found significant suppression of superfluidity at specific cluster sizes when compared to those of the neighboring sizes. This can be understood in terms of the magic number stability previously reported in hydrogen clusters. Finally we present the local superfluid density distributions in the mixed clusters, which shows uniform local superfluidity of H₂ except near D₂ molecules.     

Fracture in lamellar TiAl simulated with the cohesive model

Fracture behaviour of lamellar TiAl is studied by means of experiments and numerical simulations using a cohesive model. The fracture process can be described by two cohesive parameters: traction, T₀, and separation work, Γ₀. These are identified for polysynthetically twinned crystals (PST) and a polycrystalline TiAl alloy by comparison of the numerical simulations with experimental fracture tests. The appropriate shape of the traction-separation law for quasi-brittle fracture was identified. The simulation of the PST crystal deformation is based on crystal plasticity including the specific lamellar orientation relations. In a PST crystal, fracture occurs by inter- and translamellar fracture. In an extruded polycrystal, the lamellae of the colonies are randomly aligned in radial direction but more uniformly in extrusion direction for which different fracture toughness values are obtained. For the polycrystalline γTiAl conventional J₂ plasticity is applied, which seems appropriate to predict macroscopic effects either for stable crack extension or unstable failure. The cohesive separation, δ₀, can be interpreted as a characteristic length for a representative microstructural volume.