Evidence for cluster emission in fusion reactions between 48Ti and 45Sc at 16 MeV/u

The reaction mechanism in collisions between ⁴⁸Ti and ⁴⁵Sc at 16 MeV/u leading to evaporation-residue-like fusion products (ER) was investigated by measuring and analyzing the ER velocity distributions as a function of ER mass. As expected for this nearly symmetric system the average ER velocities were found to be very close to the center-of-mass velocity. From the widths of the velocity distributions, quantitatively expressed by the variances S_x² and S_z² perpendicular and parallel to the beam direction, information on the recoil momenta of the emitted particles can be obtained. The quantitative analysis yields strong evidence for an incomplete-fusion process during which several correlated or clustered particles are emitted. An estimate of the cluster mass leads to mass numbers ranging from 10 to 20 depending on the ER mass. This interpretation is consistent with the measured ER cross section exceeding the value derived from the stability limit of the compound nucleus by a factor of two.     

The assignment of quantum numbers in the theoretical spectra of the H2 16O, H2 17O, and H2 18O molecules calculated by variational methods in the region 0–26000 cm−1

Quantum numbers have been assigned in the theoretical spectra of three isotopologues of the water molecule: H₂ ¹⁶O, H₂ ¹⁷O, and H₂ ¹⁸O. The spectra were calculated by variational methods in the region 0–26000 cm^?1 at a temperature of 296 K. For each molecule, the quantum numbers are assigned to more than 28000 levels. The quantum numbers are assigned to 216766, 210679, and 211073 spectral lines of the H₂ ¹⁶O, H₂ ¹⁷O, H₂ ¹⁸O molecules, respectively. The theoretical spectra with the assigned quantum numbers are available in the Internet.     

An integral representation and bounds on the effective diffusivity in passive advection by laminar and turbulent flows

Precise necessary and sufficient conditions on the velocity statistics for mean field behavior in advection-diffusion by a steady incompressible velocity field are developed here. Under these conditions, a rigorous Stieltjes integral representation for effective diffusivity in turbulent transport is derived. This representation is valid for all Péclet numbers and provides a rigorous resummation of the divergent perturbation expansion in powers of the Péclet number. One consequence of this representation is that convergent upper and lower bounds on effective diffusivity for all Peclet numbers can be obtained utilizing a prescribed finite number of terms in the perturbation series. Explicit rigorous examples of steady incompressible velocity fields are constructed which have effective diffusivities realizing the simplest upper or lower bounds for all Péclet numbers. A nonlocal variational principle for effective diffusivity is developed along with applications to advection-diffusion by random arrays of vortices. A new class of rigorous examples is introduced. These examples have an explicit Stieltjes measure for the effective diffusivity; furthermore, the effective diffusivity behaves likek ₀(Pe)^1/2 in the limit of large Péclet numbers wherek ₀ is the molecular diffusivity. Formal analogies with the theory of composite materials are exploited systematically.Research partially supported by NSF DMS 90-05799 and ARO DAAL 03-89-K-0039 and AFOSR-90-0090Research partially supported by NSF DMS 87-02864, ARO DAAL 03-89-K-0013 and ONR N 00014-89-J-1044     

A new viscous sublayer influx (VSI) concept for near-wall turbulent momentum,heat and mass transfer

This paper presents a new viscous sublayer influx (VSI) concept to describe near-wall turbulent momentum, heat and mass transfer. Based on visual studies, this concept takes account of a viscous sublayer adjacent to the wall, which is not directly affected by the bursts occurring in the wall region. Fluid penetrates only due to a wallward flow into this viscous sublayer. Thus, in contrast to the known surface renewal concept, the new VSI concept is consistent with visual flow studies and, in addition, makes it possible to meet the experimentally found limiting condition Sh ∼ ³√Sc_Sc→∞ for mass transfer. In this work, two models have been developed from the new VSI concept. The simplified viscous sublayer influx model follows the known models in literature and provides analytical equations for the profiles in the wall region. This model gives an explanation for the varying experimental results on the time intervals between successive bursts and predicts them in quantity by using measured Sherwood numbers at very high Schmidt numbers. The second, more detailed viscous sublayer influx model approximates the wallward flow in the viscous sublayer with a spherical stagnation point flow. The profiles are calculated from two ordinary differential equations. Using measured Sherwood numbers at very high Schmidt numbers, this model provides normal velocity fluctuations at the wall that agree well with experimental data. Furthermore, both models provide axial velocity fluctuations near the wall and Nusselt/Sherwood numbers in the range 0.5 ≤ Pr, Sc≤ 10⁵ that both correspond with experimental data.     

Lattice dynamics of Hittorf's phosphorus and identification of structural groups and defects in amorphous red phosphorus

Using the force constants and bond polarisabilities of black phosphorus the dispersion, eigenvectors, and density of states for phonons, the sound velocity and the Raman spectra are calculated for polymeric violet Hittorf's phosphorus. Good agreement with Raman and Brillouin (v_s,110,exp=6.95·10⁵cm s^?1) measurements reported for the first time on oriented single crystals is found. Thus we are able to assign specific vibrational modes, such as extended wave-like vibrations and modes localised on the P₈ and P₉ structural subunits. Comparison with vibrational spectra of amorphous red phosphorus shows that the latter contains tubes similar to those of Hittorf's P. The absence of the group of lines (around 373 cm^?1) due to P₈ cages in a-Pproves that the P₈ cages are present in much reduced numbers only there, or that the weaker bonds are broken or distorted. The features at 400 cm^?1 and 455 cm^?1 in a-P might be explained by the occurrence of P₇ cages. Broken P₈ units may also be the origin of the metastable intrinsic defects giving rise to the photo-induced ESR and axial luminescence centers in amorphous phosphorus.     

Ion impact stopping cross sections for various media (Z = 3–100)

ABSTRACT

Stopping cross sections (SCS) for protons, alphas and Li ions are calculated with a modified form of our earlier work by incorporating a different electron density distribution of target materials; this involves four parameters – two projectile dependent and the rest two remain fixed. The prosed model has been tested for three stripped ion (H⁺, He^{2 +} and Li^{3 +}) projectiles and found that it describes quite satisfactorily the experimental SCS data from low energies with projectile velocities nearing v = Z₁v₀ (with Z₁ as the atomic number and v₀ the Bohr velocity) up to 100.0 MeV over a wide range of stopping media with atomic numbers Z₂ =3–100.     

Measurements of Flow Velocity in the Plasma Generator PSI‐2

The plasma flow velocity in the Plasma Generator PSI‐2 has been investigated by using of Mach probe. PSI‐2 is a stationary high‐current arc discharge in which the quasi‐neutral plasma expands along the magnetic field lines. The low‐temperature (T_e < 20 eV), medium density (n_e ∼ 10¹⁸— 10¹⁹ m^—3 ) plasma in the discharge is similar to the plasma in the divertor region of tokamaks. From the ratio of ion saturation currents collected from opposite sides of the probe the flow velocities (Mach numbers) in argon and hydrogen discharges are obtained.     

Near-surface turbulence in the atmospheric boundary layer

Measurements of the near-surface turbulence in the atmospheric boundary layer have been made using hot-wire probes above the salt flats of northwestern Utah, where the momentum thickness Reynolds number, R_θ, is O(10⁶), and the surface is smooth and nearly devoid of flow obstructions. The measurements were made with arrays of up to 24 parallel straight sensors and with a modular 12-sensor probe capable of measuring all of the components of the instantaneous velocity vector and velocity gradient tensor. Measurements were also made in a laboratory wind tunnel at R_θ=1730 using 22 straight sensors. The data analysis focuses on the effects of the Reynolds number on turbulence properties and on the physics of the dissipation rate of turbulent kinetic energy.Some properties are found to be dependent on the Reynolds number when normalized with inner variables, while others are not. Among those that show the significant Reynolds number dependence are the rms and the skewness factor of the streamwise velocity fluctuations.Significant differences in flow structure, particularly those related to high rates of dissipation, are implied by the data. The joint PDF and covariance integrand of streamwise and wall normal vorticity fluctuations show less preferred orientation of the vorticity vector in the buffer layer at R_θ of O(10⁶) than at R_θ=1070. The largest contribution to the dissipation rate, at O(10⁶) is by the ∂w/∂z velocity gradient, while this term makes a quite small contribution to the dissipation rate at low R_θ. Here w and z are the spanwise velocity fluctuations and direction, respectively. Conditional analysis in the streamwise-wall normal (x−y) plane based on high instantaneous dissipation rate shows that the typical high dissipation rate events are generally similar at high and low Reynolds numbers, but display some significant differences.     

Asymmetry of π− meson emission in nucleus-nucleus collisions as a measure of the target-to-projectile ratio of the numbers of interacting nucleons

Experimental data on the forward-backward asymmetry of π^- emission in (d,⁴He,¹²C)¹⁸¹Ta interactions atp/A=4.2 GeV/c are presented. The absolute value of the asymmetry coefficient of the inclusive π^- production in the nucleon-nucleonCMS decreases asA _p ^?0.35 with increasing atomic mass of projectile nucleus. A method of obtaining the target-to-projectile ratio of the numbers of participant nucleonsN _t /N _p through measuring the velocity of the symmetric pion emission system is proposed. It has been found that N_t/N_p～A _p ^?0.73 .     

Dynamos at extreme magnetic Prandtl numbers: insights from shell models

We present a magnetohydrodynamic (MHD) shell model suitable for computation of various energy fluxes of MHD turbulence for very small and very large magnetic Prandtl numbers Pm; such computations are inaccessible to direct numerical simulations. For small Pm, we observe that both kinetic and magnetic energy spectra scale as k^?5/3 in the inertial range, but the dissipative magnetic energy scales as k^?11/3exp?(? k/k_η). Here the kinetic energy at large length scale feeds the large-scale magnetic field that cascades to small-scale magnetic field, which gets dissipated by Joule heating. The large-Pm dynamo has a similar behaviour except that the dissipative kinetic energy scales as k^?13/3. For this case, the large-scale velocity field transfers energy to the large-scale magnetic field, which gets transferred to small-scale velocity and magnetic fields; the energy of the small-scale magnetic field also gets transferred to the small-scale velocity field, and the energy thus accumulated is dissipated by the viscous force.     

Untersuchung der Elektronenkomponente von Einzellawinen mit dem Photomultiplier

With a photomultiplier single electron avalanches of 10³ carriers can be observed in nitrogen. 2.5% of all photons (in the range 3000≦λ≦7000 Å) of an avalanche produce a photoelectron at the cathode of the photomultiplier. The influence of the variable solid angle on the photomultiplier pulse is given. Trouble by noise is calculated and it is shown that the information depends on the number of photoelectrons only. The electron components of avalanches in methane, nitrogen, and mixtures have an exponential rise and the measured rise time constant τ_beob is in accordance with the theoretical value 1/αυ_. For carrier numbers >10⁶ space charge influence is observed, as given by theory. The quantum efficiencyQ per ionizing impact is found to beQ～10^?3 in vapours andQ～1 in gases. Values of the electron drift velocity in mixtures of N₂ and CH₄ are given.     

Gas-kinetic numerical studies of three-dimensional complex flows on spacecraft re-entry

The gas-kinetic numerical algorithm solving the Boltzmann model equation is extended and developed to study the three-dimensional hypersonic flows of spacecraft re-entry into the atmosphere in perfect gas. In this study, the simplified velocity distribution function equation for various flow regimes is presented on the basis of the kinetic Boltzmann–Shakhov model. The discrete velocity ordinate technique and numerical quadrature methods, such as the Gauss quadrature formulas with the weight function 2/π^1/2exp(?V²) and the Gauss–Legendre numerical quadrature rule, are studied to resolve the barrier in simulating complex flows from low Mach numbers to hypersonic problems. Specially, the gas-kinetic finite-difference scheme is constructed for the computation of three-dimensional flow problems, which directly captures the time evolution of the molecular velocity distribution function. The gas-kinetic boundary conditions and numerical procedures are studied and implemented by directly acting on the velocity distribution function. The HPF (high performance fortran) parallel implementation technique for the gas-kinetic numerical method is developed and applied to study the hypersonic flows around three-dimensional complex bodies. The main purpose of the current research is to provide a way to extend the gas-kinetic numerical algorithm to the flow computation of three-dimensional complex hypersonic problems with high Mach numbers. To verify the current method and simulate gas transport phenomena covering various flow regimes, the three-dimensional hypersonic flows around sphere and spacecraft shape with different Knudsen numbers and Mach numbers are studied by HPF parallel computing. Excellent results have been obtained for all examples computed.     

Large electron polarization of H-like20Ne-ions traversing gd foils at very high velocities

It is shown that transient magnetic fields in Gd-host for²⁰Ne ions at a mean velocity of 12.5v₀(v₀=c/137), considerably beyond the Bohr velocity of 1s electrons of Ne ions (v_1s=10v₀), are dominated by the Fermi contact field of these electrons. The first excited 2⁺-state of²Ne was used as probe. The derived value for the mean degree of polarization p_1s=0.32(13), is surprisingly large, though still consistent with values obtained at lower velocities. Present calculations of spin exchange cross sections severely underestimate the observed polarization at this high velocity.The authors are grateful to the operating staff of the UNILAC for providing excellent beam conditions. They also thank Dr. P. Maier-Komor for preparing the delicate targets. Support by the BMFT is acknowledged.     

Micro-machined super-miniature hot-film multi-array probe for field experiments with sub-Kolmogorov resolution

This is a report on the first successful experiments with a unique micro-hot-film multi-array probe. The main aim of this part of the project was to design, fabricate, implement and test in laboratory conditions a micro-hot-film multi-array probe for measurements in turbulent flows in field experiments with sub-Kolmogorov resolution. The key feature is that the probe is enabling to access, along with all three components of turbulent velocity fluctuations, also all nine components of the spatial velocity gradients tensor, including the option of obtaining the stream-wise velocity derivatives without employing the Taylor hypothesis, as well as velocity temporal derivatives. Hence the multi-array probe (typically five arrays) is built with each array consisting of four hot-film sensors, similarly to that of larger multi-array probe, based on arrays with four hot wires used in the atmospheric surface layer at Taylor micro-scale Reynolds number up to Re_λ ≈ 10⁴. This part of work relates to Reynolds numbers Re_λ < 500 and employs several laboratory flows including flow in a channel of rectangular cross section, in its bulk and in the boundary layers in the proximity of its smooth and rough walls, and in the confined jet of circular cross section.     

Subluminal and superluminal propagation in Er<Superscript>3+</Superscript> doped fiber Bragg grating

The method to pump the FBG written into an Er³⁺-doped optical fiber is proposed to decrease or increase the group velocity of a probing pulse based on the fact that a pump-induced process changes the refractive index and dispersion associated with the ⁴I_15/2-⁴I_13/2 transition in Er³⁺-doped optical fiber. The system equations are derived. The group velocity modification is numerically demonstrated and discussed with the effects of an optical pump power, fiber Bragg grating length, doping concentration of Er³⁺ ions, and modulation amplitude of the grating.     

Lokale Messung von Driftgeschwindigkeit und Temperatur der Ionen einer Argon-Niederdruckentladung mit resonanter Laserstreuung

The radial ion drift velocity and temperature in an argon low-pressure discharge have been measured by a resonant laser scattering method with high spatial resolution. The detection limit for the drift velocities was 2 · 10⁴ cm · s^?1. In the vicinity of the discharge axis the drift velocity grows linear with the distance from the axis. The measurements have shown the existence of a radial dependence of the ion temperature T¹(r). Comparisons of axial ion temperature T_? with T_‖(r = 0) are an experimental demonstration of the anisotropy of the ion velocity distribution in this kind of discharges.     

The Bethe Equation at q = 0, The Möbius Inversion Formula,and Weight Multiplicities: III. The X

It is shown that the numbers of off-diagonal solutions to the U _q (X ^(r) _N ) Bethe equation at q = 0 coincide with the coefficients in the recently introduced canonical power series solution of the Q-system. Conjecturally, the canonical solutions are characters of KR (Kirillov–Reshetikhin) modules. This implies that the numbers of off-diagonal solutions agree with the weight multiplicities, which is interpreted as a formal completeness of the U _q (X ^(r) _N ) Bethe ansatz at q = 0.     

POD-based wall boundary conditions for the numerical simulation of turbulent channel flows

Simulation of turbulent wall-bounded flows requires a high spatial resolution in the wall region, which limits the range of Reynolds numbers which can be effectively reached. In previous work, we proposed proper orthogonal decomposition (POD) based wall boundary conditions to bypass the simulation of the inner wall region. Tests were carried out for direct numerical simulation at a low Reynolds number Re_τ = 180. The boundary condition is based on the POD spatial eigenfunctions which are determined a priori in the full channel. It consists of a three-component velocity field on the plane y⁺ = 50 which is reconstructed at each instant from a combination of selected eigenfunctions. The coefficients of the combination are estimated from the simulation in the reduced domain using the threshold-based reconstruction method described in Podvin et al. The study is now extended to large-eddy simulation at higher Reynolds numbers Re_τ = 295 and Re_τ = 590. Two versions of the reconstruction method are considered. In the first version, both the phases and the moduli of the coefficients are allowed to vary. In the second version, only the phases are adjusted. We find that the latter method is associated with improved statistics and is relatively robust with respect to the reconstruction threshold. However, it is sensitive to the details of the numerical simulation, unlike the former method, which is associated with less accurate statistics and is more dependent on the reconstruction threshold.     

Molecular dynamics simulation of femtosecond ablation and spallation with different interatomic potentials

Fast heating of target material by femtosecond laser pulse (fsLP) with duration τ_L∼40-100 fs results in the formation of thermomechanically stressed state. Its unloading may cause frontal cavitation of subsurface layer at a depth of 50 nm for Al and 100 nm for Au. The compression wave propagating deep into material hits the rear-side of the target with the formation of rarefaction wave. The last may produce cracks and rear-side spallation. Results of MD simulations of ablation and spallation of Al and Au metals under action fsLP are presented. It is shown that the used EAM potentials (Mishin et al. and our new one) predict the different ablation and spallation thresholds on absorbed fluence in Al: ablation F_a=60{65} mJ/cm²and spallation F_s=120{190} mJ/cm², where numbers in brackets { } show the corresponding values for Mishin potential. The strain rate in spallation zone was 4.3×10⁹ 1/s at spallation threshold. Simulated spall strength of Al is 7.4{8.7} GPa, that is noticeably less than 10.3{14} GPa obtained from acoustic approximation with the use of velocity pullback on velocity profile of free rear surface. The ablation threshold F_a≈120 mJ/cm² and crater depth of 110 nm are obtained in MD simulations of gold with the new EAM potential. They agree well with experiment.     

EFFECTS OF FREE STREAM VELOCITY ON TURBULENT NATURAL CONVECTION FLOW OVER A VERTICAL FORWARD-FACING STEP

Measurements of heat transfer and fluid flow of turbulent boundary-layer air flow in natural and mixed convection over an isothermal two-dimensional, vertical forward-facing step are reported. The upstream and downstream walls and the step itself were heated to a uniform and constant temperature. Air velocity and temperature distributions and their turbulent fluctuations are measured simultaneously using a two-component laser-Doppler velocimeter (LDV) and a cold wire anemometer, respectively. The present study treats buoyancy-dominated mixed convection over a vertical forward-facing step and examines the effect of a small free stream velocity on turbulent natural convection. The experiment was carried out for a step height of 22 mm, for a range of free stream air velocities 0 m/s ? u_∞ ? 0.55 m/s (corresponding to a range of Reynolds numbers of 0 ? Re\abinf{s} ? 712), and a temperature difference, ΔT, of 30°C between the heated walls and the free stream air (corresponding to a local Grashof number Gr_xi = 6.45 × 10¹⁰). It was found that the reattachment length increases while the heat transfer rate from the downstream heated wall decreases as the small free stream velocity increases.