bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic states of thallium clusters and their positive ions [Tln,Tln+ (n=2–5)]
Authors: Vijayakumar M. , Balasubramanian K.
Published: July 2002

Structure and energy separations of low-lying electronic states of Tln (n=3–5) clusters and their positive ions are computed using the complete-active-space multiconfiguration self-consistent field (MCSCF) (CASSCF) and the configuration interaction (CI) methods. The ground state of Tl3 is 4A2 with a near-equilateral geometry. Three near-degenerate states (3B3u, 1Ag, and 1Au) are found as candidates for the ground state of Tl4 with a perfect square geometry. The rhombus and the tetahedral geometries of Tl4 are 5 and 9 kcal/mol higher in energy, respectively. In the case of the Tl5 cluster, three important structures with C2v, C4v, and D3h symmetries are considered in the present study and it is found that the C2v and C4v geometries are the possible candidates for the ground-state geometry. The ionization energies of these clusters are also calculated. Mulliken population analyses and dissociation energies are given.
High resolution infrared spectra of the ν3 vibron in natural sulfur and in the isotopically pure 32S crystal
Authors: Eckert B. , Bini R. , Jodl H. J. , Califano S.
Published: July 2002

We have studied by high resolution infrared spectroscopy the ν3 vibron band in natural and isotopically pure 32S sulfur crystals. From the comparison of the spectra we have assigned the bands due to the different molecules containing one or two atoms of 34S as well as one atom of 33S. We have measured the bandwidth and the frequency of the vibron as a function of temperature from 10 to 140 K both in the natural and in the isotopically pure crystal and we have interpreted the results in terms of decay and scattering processes. In particular we have explained the nonlinear evolution of the bandwidth and of the frequency shifts with temperature in terms of scattering processes controlled by quartic terms of the crystal Hamiltonian.
The Langevin/implicit-Euler/normal-mode scheme for molecular dynamics at large time steps
Authors: Zhang Guihua , Schlick Tamar
Published: July 2002

As molecular dynamics simulations continue to provide important insights into biomolecular structure and function, a growing demand for increasing the time span of the simulations is emerging. Our focus here is developing a new algorithm, LIN (Langevin/implicit-Euler/normal mode), that combines normal-mode and implicit-integration techniques, for large time step biomolecular applications. In the normal-mode phase of LIN, we solve an approximate linearized Langevin formulation to resolve the rapidly varying components of the motion. In the implicit phase, we resolve the remaining components of the motion by numerical integration with the implicit-Euler scheme. Developments of the normal-mode phase of LIN are discussed in this paper. Specifically, we solve two crucial issues of the method. The first involves how to choose and how often to update the Hessian approximation for the linearized Langevin equation. This approximation must be computationally feasible and physically reasonable to capture the motion in the higher end of the vibrational spectrum. Three such general Hessian approximations are discussed. The related issue—the frequency of the Hessian update—is analyzed by projecting the motion onto the different vibrational modes. This analysis demonstrates that a one-picosecond interval is reasonable for updating the Hessian in the model system examined here.

In this connection, we illustrate that the high-frequency motions are highly localized while the low-frequency motions are delocalized. We also show rigorously that the mode amplitudes are inversely proportional to the frequency (consistent with the equipartition theorem), with 90% of the displacement fluctuations coming from a very small group of low-frequency modes. Anharmonic effects essentially influence the low-frequency modes. The second issue involves how to solve the linearized Langevin equation at large timesteps correctly, where the usual discretized formulation of the random force is invalid. This is accomplished by using analytic expressions for the distributions associated with positions and velocities of the individual oscillators as a function of frequency, obtained as the solution of the corresponding Fokker–Planck equation. We apply LIN with these developments to the nucleic acid component deoxycytidine with timesteps ranging from 100 to 1000 fs. We demonstrate that LIN is stable in these simulations, with energies fluctuating about the same values—and possessing overall similar dynamical features—in comparison to 1 fs explicit simulations, though the fluctuations are significantly larger at larger timesteps. Moreover, continuous dynamics is maintained, and pathway information can be obtained. Computational performance is competitive only at very large time steps: a gain factor of 3–4 is obtained for runs with 1000 fs time steps. Larger gains may be achieved for biomolecules, where sparsity and parallelization can be exploited significantly.
A general analysis of approximate nonlinear lumping in chemical kinetics. I. Unconstrained lumping
Authors: Li Genyuan , Tomlin Alison S. , Rabitz Herschel , Tóth János
Published: July 2002

A general analysis of approximate nonlinear lumping for a chemical kinetic system described by an n-dimensional first order ordinary differential equation system dy/dt=f(y) is presented. There is a one-to-one relation between the differential equation system and the linear partial differential operator A=∑ni=1fi(y)(∂/∂yi). The algebraic method in nonlinear perturbation theory is utilized to approximately transform A into some canonical forms in which the new dependent variables are partly separated. These canonical forms of A will give the generalized eigenfunctions or other higher dimensional unconstrained nonlinear lumping schemes of the original system approximately. Unconstrained nonlinear lumping gives a reduced differential equation system describing new variables which are nonlinear functions of the original ones. This approach may supply some purely fast variables. The solutions of original dependent variables can be obtained by the inverse transformation from the lumped variables and the approximate analytical solutions of the purely fast variables. The theoretical basis of this approach is presented. A simple example is used for illustration.
Near-dissociation expansions and dissociation energies for Mg+–(rare gas) bimers
Authors: Le Roy Robert J.
Published: July 2002

This paper describes the use of fits to near-dissociation expansions (NDE’s) for performing optimum vibrational level energy extrapolations to determine diatom dissociation energies, together with realistic estimates of the uncertainties due to model-dependence. The imposition of extended near-dissociation theory constraints on the leading deviation from limiting near-dissociation behavior is introduced and applied for the first time. Fits of recently determined vibrational energies for Mg+–Ar, Mg+–Kr, and Mg+–Xe to near-dissociation expansions yield improved estimates of the dissociation energies and realistic predictions for the total number and extrapolated energies of upper vibrational levels for both the A 2Π and X 2Σ+ states. A combined analysis of the data for the A 2Π1/2 and 2Π3/2 states, and of the vibrationally-dependent spin–orbit splittings, yields particularly compact internally consistent results for these systems.
Rotational predissociation dynamics of OH–Ar (A 2Σ+) using the finite range scattering wave function method
Authors: Choi Seung E. , Lester Marsha I. , Jang Hyo Weon , Light John C.
Published: July 2002

Predissociative resonances of OH–Ar are computed up to 300 cm−1 above the Ar+OH (A 2Σ+, v=0, j=0) asymptote for total (rotational) angular momentum states J≤10. The energies, lifetimes, and OH A 2Σ+ product rotational distributions of the predissociative resonances are calculated using a numerical method based on the ‘‘energy independent integral’’ finite range scattering wave function (FRSW) [J. Chem. Phys. 99, 1057 (1993)]. The FRSW method involves evaluation of the scattering matrix and its energy derivative, both of which are only parametrically dependent on energy. The energy independent matrices are determined from the discrete eigenvectors of the ℒ2 Hamiltonian matrix H, which is obtained in discrete variable representation, and an exact (analytical) eigenfunction of the asymptotic Hamiltonian operator Ĥ0. Many long-lived (≳1 ps) resonances are identified for OH–Ar in J=3 with projections of J onto the intermolecular axis of K=0–3. The resonances are characterized with approximate bend and stretch quantum numbers based on the nodal structure of the wave functions. The predissociative states decay by Coriolis coupling to a lower K state and/or through mixing of OH rotor levels induced by the anisotropy of the interaction potential. States that predissociate by Coriolis coupling are identified by their J-dependent lifetimes and the OH product rotational levels accessed. The influence of potential anisotropy on the predissociative resonances is explored by changing the average intermolecular bond length and degree of intermolecular bending excitation. A comparison of the theoretically calculated resonances with those observed experimentally provides a guide for refinement of the adjusted semiempirical potential energy surface [J. Chem. Phys. 98, 9320 (1993)] used in the computations.
Anomalous symmetries of the rovibrational states of HO2: Consequences of a conical intersection
Authors: Barclay V. J. , Dateo Christopher E. , Hamilton I. P. , Kendrick Brian , T Pack Russell , Schwenke David W.
Published: July 2002

We show that the geometric phase arising from a conical intersection of the lowest potential energy surfaces of HO2 causes its bending vibrational wave functions to be double-valued, which enables them to be locally symmetric on one side of the intersection and locally antisymmetric on the other.
Surface directed spinodal decomposition in a partially miscible polymer blend
Authors: Geoghegan M. , Jones R. A. L. , Clough A. S.
Published: July 2002

We have studied the effect of the surface and interface on the phase separation of thin films of a blend of deuterated polystyrene and poly(α-methyl styrene) using 3He nuclear reaction analysis. In the unstable region of the phase diagram, surface directed spinodal decomposition is observed. We describe the growth of the morphology and see that the depletion layer behind the wetting layer at the substrate grows faster than the wetting layer at the vacuum interface with power law growth exponents of 0.27 and 0.14, respectively. The early size of the wetting layer is compared with a theoretical zero-time spinodal wavelength. This wavelength decreases when the film is thinner than a critical thickness in accord with other workers.
Photodissociation spectroscopy of Ca+–rare gas complexes
Authors: Pullins S. H. , Scurlock C. T. , Reddic J. E. , Duncan M. A.
Published: July 2002

Weakly bound complexes of the form Ca+–RG (RG=Ar, Kr, Xe) are prepared in a pulsed nozzle/laser vaporization cluster source and studied with mass-selected resonance enhanced photodissociation spectroscopy. The Ca+ (2P←2S) atomic resonance line is the chromophore giving rise to the molecular spectra in these complexes. Vibrationally resolved spectra are measured for these complexes in the corresponding 2Π←X 2Σ+ molecular electronic transition. These spectra are red shifted from the atomic resonance line, indicating that each complex is more strongly bound in its excited 2Π state than it is in the ground state. Vibronic progressions allow determination of the excited state vibrational constants: Ca+–Ar, ωe′=165 cm−1; Ca+–Kr, ωe′=149 cm−1; Ca+–Xe, ωe′=142 cm−1. Extrapolation of the excited state vibrational progressions, and combination with the known atomic asymptotes and spectral shifts, leads to determination of the ground state dissociation energies Ca+–Ar, D0″=700±100 cm−1 (0.09 eV); Ca+–Kr, D0″=1400±150 cm−1 (0.17 eV); Ca+–Xe, D0″=2300±150 cm−1 (0.29 eV). The spin–orbit splitting in the 2Π1/2,3/2 state for these complexes is larger than expected by comparison to the Ca+ atomic value.
A study of viscosity inhomogeneity in porous media
Authors: Akhmatskaya E. , Todd B. D. , Daivis P. J. , Evans D. J. , Gubbins K. E. , Pozhar L. A.
Published: July 2002

The theory of transport in highly inhomogeneous systems, developed recently by Pozhar and Gubbins, and the nonequilibrium molecular dynamics (NEMD) technique are employed to study the viscosity of WCA fluids confined in narrow slit pores of width 5.1 and 20σ at reduced densities ρσ3 of 0.422–0.713. Calculated quantities include the equilibrium and nonequilibrium density profiles, equilibrium pair correlation functions, flow velocity profiles, and the viscosity profiles. NEMD simulation results are compared with the theoretical predictions. The agreement is good except for the region within one molecular diameter from the walls. The viscosity was found to vary with position across the pore.
Spectroscopic characterization of the metastable 3pπ 3Π+,0− valence states and the 4s3Σ+ Rydberg states of the MgKr and MgXe van der Waals molecules
Authors: Kaup John G. , Leung Allen W. K. , Breckenridge W. H.
Published: July 2002

The first metastable valence excited states and the first Rydberg states of the MgKr and MgXe molecules have been characterized by resonance two-photon photoionization (R2PI) spectroscopy. The Mg(3s3p 3PJ)⋅RG(3Π0+,0−) metastable states, produced by expanding the products of a laser-ablated magnesium rod in Kr/Ar or Xe/Ar gas mixtures into a supersonic expansion, were excited by a dye laser pulse to several vibrational levels of the Mg(3s4s 3S1)⋅RG(3Σ+) Rydberg states, with detection by ionization with a second dye laser pulse. Spectroscopic constants, bond energies, and bond lengths are reported for both states of MgKr and MgXe. The Σ+3 Rydberg states are much more strongly bound than the lower Π0−3 valence states, and in fact are essentially as strongly bound as the ground states of the analogous MgRG+ ions, characterized previously in the same apparatus. This clearly indicates that the RG atoms can readily penetrate the diffuse Mg(4s) Rydberg electron cloud. The interesting and unusual spin–orbit and “spin–spin” effects observed are attributed to mixing of some RG character into wave functions of predominantly Mg* excited state character. Bonding and spin–orbit interactions in the MgAr, MgKr, and MgXe first triplet metastable and Rydberg states are discussed.
Structure sensitivity of methane dissociation on palladium single crystal surfaces
Authors: Klier Kamil , Hess James S. , Herman Richard G.
Published: July 2002

Palladium single crystals have been found to be active for the C–H bond dissociation of methane in the temperature range 400–600 K, and the activities of the presently investigated Pd(111) and Pd(311) surfaces are compared with previously studied Pd(679). Structure sensitivity is reported that spans an order of magnitude in terms of the rates in the order Pd(111)<Pd(311)<Pd(679), while the effective activation energies range from 32–34 kJ/mol for Pd(111) and Pd(311) to 44 kJ/mol for Pd(679). These data are analyzed with a model that involves unsymmetrical barriers, first by constructing a potential-energy surface for Pd(111), in which the reaction pathway is well-simulated by the Eckart barrier. The Eckart barrier is then shown to obey exactly the Marcus rule for exothermic or endothermic processes. This property is used in comparing the H–CH3 dissociation on the different crystal faces, with the result that the Pd(679) surface provides a driving force of some 26 kJ/mol due to the role of defects compared to smooth planar Pd(111) surface and 22 kJ/mol compared to the Pd(311) surface.
New information on the ozone monomer photochemistry at 266 nm in nitrogen matrix
Authors: Bahou M. , Schriver-Mazzuoli L. , Camy-Peyret C. , Schriver A.
Published: July 2002

The photochemistry of ozone trapped in solid nitrogen and mixed matrices has been reinvestigated using irradiation at 266 nm and infrared spectroscopy between 10 and 25 K. In this temperature range the most probable reaction channel appears to be the formation of metastable excited nitrous oxide molecules that fragment mostly into an oxygen atom and nitrogen. Recombination of O+O2 is negligible, suggesting that the atomic oxygen that is formed from the deactivation of excited N2O is bound to the nitrogen molecule, preventing the reformation of O3 and leading to a fast first-order photodecomposition of ozone in highly diluted samples. The experimental value of the photokinetic constant suggests that the photodissociation cross section of ozone in matrices at 266 nm is weaker than that measured in the gas phase.
Rotational spectrum, structure and modeling of the SO2–CS2 complex
Authors: Peebles Sean A. , Sun Linghong , Kuczkowski Robert L.
Published: July 2002

The rotational spectra of seven isotopomers of the SO2–CS2 van der Waals dimer have been observed with a Fourier transform microwave spectrometer. The rotational constants for the normal species were determined to be A=2413.2000(3) MHz, B=1105.3803(3) MHz and C=884.9885(2) MHz. They are consistent with the SO2 straddling the CS2 molecule and Cs symmetry for the complex. The centers of mass of the two monomers are separated by 3.4287(2) Å. Two structures were found that are consistent with this symmetry which differ in the relative tilt of the CS2 and SO2. In both structures, the C2 axis of the SO2 is aligned close to parallel to the CS2 molecular axis with the oxygen end of the SO2 tipped closer to the CS2. In one structure the deviation from parallel is 9.8(8)° while in the other it is 17.7(11)°. The dipole moment components have been determined to be μa=0.0137(5) D and μb=1.1961(9) D. A semi-empirical model employing electrostatic, dispersion and repulsion interactions was employed to analyze the system and resulted in a reasonable reproduction of the angular geometry. A comparison of the results for the SO2–CS2 complex with the closely related SO2–CO2 and SO2–OCS complexes is presented.
The utility of many-body decompositions for the accurate basis set extrapolation of ab initio data
Authors: Mielke Steven L. , Garrett Bruce C. , Peterson Kirk A.
Published: July 2002

We present a powerful new technique for the extrapolation of ab initio data based on many-body decompositions. Using the new methodology and subtle modifications of the standard correlation consistent basis sets, the H+H2 barrier height is estimated at 9.603 kcal/mol with a precision of about 0.003 kcal/mol; this extremely accurate result is all the more striking as it can be obtained using basis sets no larger than aug-cc-pVQZ. The method is also used to yield highly accurate energies for the H+H2 system on a grid of points previously calculated by quantum Monte Carlo. The three-body energy, summed with exact one- and two-body energies, is observed to yield a useful approximate lower bound for the total energy. The highly accurate energies afforded by this method can also be used to assess the accuracy of previously calculated data that has been used to construct potential energy surfaces. As an example, we make a detailed comparison between the new results and the quantum Monte Carlo results for H+H2.
Experimental determination of the thermal accommodation and condensation coefficients of water
Authors: Shaw Raymond A. , Lamb Dennis
Published: July 2002

The condensation coefficient of water vapor on liquid water and the thermal accommodation coefficient of air on liquid water are poorly known despite their importance in many applications, such as cloud physics. We have developed a new technique for determining the condensation and thermal accommodation coefficients experimentally. The technique consists of simultaneously measuring the homogeneous nucleation rate of ice and the evaporation rate of liquid water droplets as a function of pressure (droplet Knudsen number). As the Knudsen number increases, surface kinetic processes limit mass and energy fluxes and, as a result, the equilibrium temperature of an evaporating droplet is a function of the condensation and thermal accommodation coefficients. The homogeneous freezing nucleation rate is used as a sensitive measure of the droplet temperature. The nucleation and evaporation rates are determined by observing the scattered light from evaporating water droplets suspended in an electrodynamic levitation system housed within a controlled environment. The observed rates are consistent with a condensation coefficient between 0.04 and 0.1, with 0.06 being most probable, and a thermal accommodation coefficient between 0.1 and 1, with 0.7 being most probable.
Sensitive measurement of absolute two-photon absorption cross sections
Authors: Sengupta P. , Balaji J. , Banerjee S. , Philip R. , Kumar G. Ravindra , Maiti S.
Published: July 2002

We develop a method for measuring absolute two-photon absorption cross sections (σ2) and employ it to determine the σ2 of Rhodamine-6G in methanol (16.2±2.4 GM at 806 nm). Our measurement calibrates the relative excitation spectrum previously reported for this chromophore. The method is based on our derivation of an analytical expression describing the transmission of Gaussian laser pulses through a two-photon absorbing medium. The expression is valid for arbitrary absorber thickness, at all distances from the focus. This generalizes the prevalent “z-scan” (translation of the sample along the beam direction) technique for measuring two-photon absorbance, removing the requirements of a “thin” (thickness ≪ Rayleigh range of the focused laser beam) sample and of placing the sample at the focus. This leads to an improvement of the sensitivity of the technique by over two orders of magnitude, enabling measurement of the two-photon absorption cross sections of even weakly absorbing specimens at moderate intensities. The results are significant for applications such as nonlinear microscopy, optical data storage and optical power limiting.
An experimental study of poly(9,9-dioctyl-fluorene) and its interfaces with Li, Al, and LiF
Authors: Greczynski G. , Fahlman M. , Salaneck W. R.
Published: July 2002

The chemical and electronic structure of a new conjugated polymer, poly(9,9-dioctyl-fluorene), has been studied by photoelectron spectroscopy. The polymer films were free of oxygen contamination (within the detection limits of photoelectron spectroscopy). X-ray and ultraviolet photoelectron spectroscopy was carried out on Li- and LiF-deposition on poly(9,9-dioctyl-fluorene), as well as, on LiF deposition on aluminum. In the case of Li-deposition on poly(9,9-dioctyl-fluorene) films, doping occurred resulting in the formation of polaronic charge carriers at low doping levels and bipolaronic charge carriers at high doping levels. LiF-deposition on poly(9,9-dioctyl-fluorene) did not cause doping of the polymer films, nor did the LiF dissociate at the interface. No significant shifts in a binding energy of the core levels nor any changes in the work function occurred. LiF-deposition on sputter-cleaned aluminum may cause the surface dipole formation, though LiF dissociation was not detected. Al-deposition on LiF/PFO films did not cause dissociation of LiF, unlike the case in Alq3.
Generalized conditions for the decoupling theorem of quantum field theory in Minkowski space with particles of vanishingly small masses
Authors: Manoukian Edward B.
Published: July 2002

The proof of the decoupling theorem of quantum field theory given earlier [E. B. Manoukian, J. Math. Phys. 26, 1065 (1985)] in Minkowski space, in the distributional sense, for theories involving particles with vanishingly small masses as well is extended under more general conditions, thus being applicable to a larger class of graphs. All subtractions of renormalization are carried out at the origin of momentum space with the degree of divergence of a subtraction coinciding with the dimensionality of the corresponding subdiagram.
On the zeta-function regularization of a two-dimensional series of Epstein–Hurwitz type
Authors: Elizalde E.
Published: July 2002

As a further step in the general program of zeta-function regularization of multiseries expressions, some original formulas are provided for the analytic continuation, to any value of s, of two-dimensional series of Epstein–Hurwitz type, namely, ∑∞n1,n2=0[a1(n1+c1)2 +a2(n2+c2)2]−s, where the aj are positive reals and the cj are not simultaneously nonpositive integers. They come out from a generalization to Hurwitz functions of the zeta-function regularization theorem of the author and Romeo [Phys. Rev. D 40, 436 (1989)] for ordinary zeta functions. For s=−k,0,2, with k=1,2,3,..., the final results are, in fact, expressed in terms of Hurwitz zeta functions only. For general s they also involve Bessel functions. A partial numerical investigation of the different terms of the exact, algebraic equations is also carried out. As a by-product, the series ∑∞n=0exp[−a(n+c)2], a,c>0, is conveniently calculated in terms of them.
Solutions for the vector k-constrained KP hierarchy
Authors: Zhang You-Jin , Cheng Yi
Published: July 2002

A class of (1+1)-dimensional integrable systems which are constrained from the Kadomtsev–Petviashvili (KP) hierarchy is studied. The bilinear equations of these integrable systems are shown first, and then by using the free fermion operators their rational and soliton solutions which are closely related to the τ functions of the KP hierarchy are obtained. The result implies that a large family of solutions of the (2+1)-dimensional KP hierarchy can be obtained by solving these (1+1)-dimensional integrable systems; in particular, the N-soliton solutions of the KP hierarchy can be deduced from the soliton solutions of these (1+1)-dimensional integrable systems.
Submanifolds and quotient manifolds in noncommutative geometry
Authors: Masson Thierry
Published: July 2002

We define and study noncommutative generalizations of submanifolds and quotient manifolds using the derivation-based differential calculus introduced by M. Dubois-Violette and P. Michor. We give examples to illustrate these definitions.
d=2, N=2
superconformally covariant operators and super W-algebras
Authors: Gieres François , Gourmelen Stéphane
Published: July 2002

We construct and classify superconformally covariant differential operators defined on N=2 super Riemann surfaces. By contrast to the N=1 theory, these operators give rise to partial rather than ordinary differential equations which leads to novel features for their matrix representation. The latter is applied to the derivation of N=2 super W-algebras in terms of N=2 superfields.
Effect of hydrostatic pressure on the resistance and critical temperature of YBa2Cu3O7−δ single crystals
Authors: Balla D. D. , Bondarenko A. V. , Vovk R. V. , Obolenskii M. A. , Prodan A. A.
Published: July 2002

The effect of hydrostatic pressure on the resistance and resistive transition to the superconducting state in YBa2Cu3O7−δ single crystals with different oxygen contents is studied experimentally. It is found that, following the application (or removal) of pressure, the resistance relaxes to an equilibrium value that depends on pressure, the relaxation time at room temperature being about two days. It is shown that the value of the superconducting transition temperature depends on the pressure, but the width and shape of the transition depend mainly on the degree of resistance relaxation to the equilibrium value. It is concluded that the decrease in resistance under hydrostatic pressure is associated with the ordering of labile oxygen in the Cu–O plane. The possible mechanisms of variation of the superconducting transition temperature are discussed.
Numerical solutions for the deformation of a bubble rising in dilute polymeric fluids
Authors: Noh D. S. , Kang I. S. , Leal L. Gary
Published: July 2002

The steady-state deformation of a bubble rising in polymeric liquid has been investigated using a general numerical technique for the solution of free-boundary problems in non-Newtonian fluid mechanics. The technique is based on a finite-difference solution of the governing equations on an orthogonal curvilinear coordinate system, which is constructed numerically and adjusted to fit the boundary shape exactly at any time. The problem was analyzed based on the constitutive equation proposed by Chilcott and Rallison [J. Non-Newtonian Fluid Mech. 29, 381 (1988)], which models a dilute polymer solution as a suspension of dumbbells with finite extensibility. Computations were carried out to investigate the effect of conformation change of polymer chains on the bubble deformation for various values of the Deborah number, maximum chain extensibility (i.e., roughly proportional to molecular weight), capillary number, and the Reynolds number. Numerical results show good agreement with existing experimental findings reported elsewhere. Especially, the tendency of transition from a prolate shape to a cusped shape has been observed in the creeping flow limit.
Kinetics of a one-dimensional granular medium in the quasielastic limit
Authors: McNamara Sean , Young W. R.
Published: July 2002

The dynamics of a one-dimensional granular medium has a finite time singularity if the number of particles in the medium is greater than a certain critical value. The singularity (‘‘inelastic collapse’’) occurs when a group of particles collides infinitely often in a finite time so that the separations and relative velocities vanish. To avoid the finite time singularity, a double limit in which the coefficient of restitution r approaches 1 and the number of particles N becomes large, but is always below the critical number needed to trigger collapse, is considered. Specifically, r→1 with N∼(1−r)−1. This procedure is called the ‘‘quasielastic’’ limit. Using a combination of direct simulation and kinetic theory, it is shown that a bimodal velocity distribution develops from random initial conditions. The bimodal distribution is the basis for a ‘‘two-stream’’ continuum model in which each stream represents one of the velocity modes. This two-stream model qualitatively explains some of the unusual phenomena seen in the simulations, such as the growth of large-scale instabilities in a medium that is excited with statistically homogeneous initial conditions. These instabilities can be either direct or oscillatory, depending on the domain size, and their finite-amplitude development results in the formation of clusters of particles.
Radiative collapse of a Z pinch in hydrogen and helium
Authors: Robson A. E.
Published: July 2002

The radiative collapse of a Z pinch in hydrogen or helium is examined from a practical viewpoint. The maximum density that may be achieved is calculated by modifying the Pease–Braginskii current to include the effect of opacity and by incorporating the pinch into a realistic circuit. It is found that the maximum density is increased by the addition of series inductance inside the vacuum chamber. In most cases the density limit is determined by the pinch becoming optically thick, although for low line density and large series inductance the limit may be set by the onset of electron degeneracy. Pinches in hydrogen lead to denser collapsed states than pinches in helium.
Progress with energy confinement time in the CTX spheromak
Authors: Jarboe T. R. , Wysocki F. J. , Fernández J. C. , Henins I. , Marklin G. J.
Published: July 2002

Large improvements in spheromak parameters and new understanding have been obtained from the CTX experiment at Los Alamos [Phys. Rev. Lett. 51, 39 (1983); 61, 2457 (1988)]. In one experiment the global energy confinement time has been increased an order of magnitude over previous experiments to 0.2 msec and the magnetic-energy decay time increased to 2 msec. These results were achieved in a decaying spheromak by reducing the helicity dissipation in the edge. In another smaller spheromak, record electron temperatures (∼400 eV) and record magnetic field strengths (∼30 kG) have been obtained.
Generalized gyroviscous force and its effect on the momentum balance equation
Authors: Chang Zuoyang , Callen J. D.
Published: July 2002

The perturbed nonlinear gyroviscous force (∇⋅Π̃g, where Π̃g is the gyroviscous stress tensor) is calculated through order δ2 [δ∼(1/Ω)(∂/∂t), k⊥ρ∼δ1/2, where Ω is the particle gyrofrequency, k⊥ρ denotes the finite Larmor radius effect] by solving the stress tensor evolution equation. The result shows that ∇⋅Π̃g mostly cancels the diamagnetic convective term nmV*⋅∇⊥ũ (where V* is a generalized total diamagnetic flow) in the momentum balance equation. This formula generalizes the usual, widely used ‘‘gyroviscous cancellation’’ [in which the ∇⋅Π̃g cancels mostly the total time derivative of the diamagnetic flow (dṼd/dt)] to include temperature variations and a perturbed stress tensor drift. It is proved that when temperature variations are neglected, the new formula reduces to the conventional form. When the temperature variations are considered, the new formula is simpler and more rigorous than the conventional one. A new polarization drift formula deduced from the new gyroviscous force is also derived.
On resonant destabilization of toroidal Alfvén eigenmodes by circulating and trapped energetic ions/alpha particles in tokamaks
Authors: Biglari H. , Zonca F. , Chen L.
Published: July 2002

Toroidal Alfvén eigenmodes are shown to be resonantly destabilized by both circulating and trapped energetic ions/alpha particles. In particular, the energetic circulating ions are shown to resonate with the mode not only at the Alfvén speed (vA), but also at one-third of this speed, while for trapped ions, the dominant instability mechanism is shown to be due to the resonance between the precessional magnetic drift and the wave. Implications of the theory for present and future tokamaks are discussed.
Nonlinear stabilization of the Rayleigh–Taylor instability by external velocity shear
Authors: Hassam A. B.
Published: July 2002

It is shown that the Rayleigh–Taylor instability of magnetized plasma is nonlinearly stabilized by the external imposition of transverse velocity shear with zero spatial second derivative. The critical velocity shear required for stabilization is of order γg(ln Ra)1/2, where γg is the linear growth rate and Ra is the Rayleigh number.
Numerical comparison of strong Langmuir turbulence models
Authors: Shen Mei-Mei , Nicholson D. R.
Published: July 2002

Two models of Langmuir turbulence, the nonlinear Schrödinger equation and the Zakharov equations, are solved numerically for an initial value problem in which the electric field evolves from an almost flat initial condition via the modulational instability and finally saturates into a set of solitons. The two models agree well with each other only when the initial dimensionless electric field has an amplitude less than unity. An analytic soliton gas model consisting of equal-amplitude, randomly spaced, zero-speed solitons is remarkably good at reproducing the time-averaged Fourier spectra in both cases.
A model for the loop voltage of reversed field pinches
Authors: Jarboe T. R. , Alper B.
Published: July 2002

A simple model is presented that uses the concept of helicity balance to predict the toroidal loop voltage of reversed field pinches (RFP’s). Data from the RFP’s at Culham [Plasma Phys. Controlled Fusion 27, 1307 (1985)] are used to calibrate and verify the model. The model indicates that most of the helicity dissipation occurs in edge regions that are outside the limiters or in regions where field lines contact the walls. The value of this new interpretation to future RFP and spheromak experiments is discussed.
Laser-intensity scaling experiments in long-scalelength, laser-produced plasmas
Authors: Drake R. P. , Young P. E. , Williams E. A. , Estabrook Kent , Kruer W. L. , Lasinski B. F. , Darrow C. B. , Baldis H. A. , Johnston T. W.
Published: July 2002

An experimental technique is demonstrated that allows variation of the average laser intensity by more than two orders of magnitude while producing much smaller changes in the other parameters that determine the laser–plasma interactions. By irradiating exploding-foil targets with 0.35 μm laser light in flat-topped pulses of variable duration, the high-power Nova laser [Rev. Sci. Instrum. 57, 2101 (1986)] produced plasmas with electron temperatures of order 1 keV and with scale lengths of the (radial and axial) electron-density gradients of order 1000 laser wavelengths. By using a constant target thickness and systematically decreasing the pulse length and spot size as laser intensity increased, the changes in the temperature and in the scale lengths were minimized. The time-resolved spectrum of the Raman-scattered light was used to measure both the electron temperature and the maximum density of the expanding plasmas. In this paper, these measurements are compared to both 1-D models and 2-D simulations. The expected trends of slower burnthrough and lower temperature with lower laser intensity were observed. However, the inferred temperatures are lower and the burnthrough of the target (at high laser intensities) is slower than the modeling predicts. Possible sources of these discrepancies are discussed.
Power requirements for current drive
Authors: Boozer Allen H.
Published: July 2002

General formulas for the efficiency of current drive in toroidal plasmas are derived using entropy arguments. The highest possible efficiency for current drive in which a high-energy electron tail is formed is shown to be p=Erj, with p and j the power and current densities and Er≊0.09n14 V/m with n14 the electron density in units of 1014/cm.3 The electric field required to maintain the current in a runaway discharge is also shown to equal Er. If the plasma current is carried by near-Maxwellian electrons, waves that have a low phase velocity, compared to the energy of the electrons with which they interact, can drive a current with Ohmic efficiency, p=ηj2. Such waves were first discussed in the context of current drive by Fisch [Rev. Mod. Phys. 59, 175 (1987)].
Direct numerical simulation of transition to turbulence from a high-symmetry initial condition
Authors: Boratav Oluş N. , Pelz Richard B.
Published: July 2002

The three-dimensional (3-D) time evolution of a high-symmetry initial condition [J. Phys. Soc. Jpn. 54, 2132 (1985)] is simulated using a Fourier pseudospectral method for Re=1/ν=500, 1000, 2000, and 5000 with an effective resolution of 10243 collocation points (1713 independent modes, maximum wave number kmax=340). It is found that much before the peak enstrophy is reached, there is a short interval when the local quantities increase sharply. It is also found that during this interval, six vortex dipoles (at the origin) and three dipoles (at the π/2 corner) collapse toward two separate vorticity null points at the opposite corners of the domain in a nearly self-similar fashion. The coherent vortices break up afterward, followed by a sharp decrease in local quantities. The singularity analysis shows that, within the limits of the resolution, the maximum vorticity scales approximately as (T−Tc)−1, shortly before the breakup. However, the increase in peak vorticity stops at a certain time, possibly due to viscous dissipation effects. The temporal evolution of the width of the analyticity strip shows that δ approaches zero at a rate faster than exponential, but reaches a minimum value and starts to increase. This suggests that the solution remains uniformly analytic, as is the case in the viscous Burgers equation.
On one-dimensional planar and nonplanar shock waves in a relaxing gas
Authors: Sharma V. D. , Radha Ch.
Published: July 2002

The paper examines the evolutionary behavior of shock waves of arbitrary strength propagating through a relaxing gas in a duct with spatially varying cross section. An infinite system of transport equations, governing the strength of a shock wave and the induced discontinuities behind it, are derived in order to study the kinematics of the shock front. The infinite system of transport equations, when subjected to a truncation approximation, provides an efficient system of only finite number of ordinary differential equations describing the shock propagation problem. The analysis, which accounts for the dynamical coupling between the shock fronts and the flow behind them, describes correctly the nonlinear steepening effects of the flow behind the shocks. Effects of relaxation on the evolutionary behavior of shocks are discussed. The first-order truncation approximation accurately describes the decay behavior of weak shocks; the usual decay laws for weak shocks in a nonrelaxing gas are exactly recovered. The results concerning shocks of arbitrary strength are compared with the characteristic rule. In the limit of vanishing shock strength, the transport equation for the first-order induced discontinuity leads to an exact description of an acceleration wave. In the strong shock limit, the second-order truncation criterion leads to a propagation law for imploding shocks which is in agreement (within 5% error) with the Guderley’s exact similarity solution.
Mixing enhancement in a lobed injector
Authors: Smith L. L. , Majamaki A. J. , Lam I. T. , Delabroy O. , Karagozian A. R. , Marble F. E. , Smith O. I.
Published: July 2002

An experimental investigation of the non-reactive mixing processes associated with a lobed fuel injector in a coflowing air stream is presented. The lobed fuel injector is a device which generates streamwise vorticity, producing high strain rates which can enhance the mixing of reactants while delaying ignition in a controlled manner. The lobed injectors examined in the present study consist of two corrugated plates between which a fuel surrogate, CO2, is injected into coflowing air. Acetone is seeded in the CO2 supply as a fuel marker. Comparison of two alternative lobed injector geometries is made with a straight fuel injector to determine net differences in mixing and strain fields due to streamwise vorticity generation. Planar laser-induced fluorescence (PLIF) of the seeded acetone yields two-dimensional images of the scalar concentration field at various downstream locations, from which local mixing and scalar dissipation rates are computed. It is found that the lobed injector geometry can enhance molecular mixing and create a highly strained flowfield, and that the strain rates generated by scalar energy dissipation can potentially delay ignition in a reacting flowfield.
Qualitative flow visualization using colored lights and reflective flakes
Authors: Thoroddsen S. T. , Bauer J. M.
Published: July 2002

We present a novel flow-visualization technique utilizing reflective flakes in combination with color illumination. Three differently colored columated light beams are used to illuminate the flow, each color being directed from a separate direction. In this way, the color of the light reflected from the flakes gives an indication of the local flake orientation. The flake orientation in complex three-dimensional (3-D) flow is in general a complicated function of the local velocity gradient tensor, but can be calculated if the underlying velocity field is known. This has recently been demonstrated by Gauthier et al. [Phys. Fluids. 10, 2147 (1998)] using monochome light. In complex flow fields the distribution of flakes may, however, be rearranged by the motion, thus making the local intensity of reflection depend on both orientation and flake concentration. The color is, however, immune to the local number density of flakes inside the flow, making quantitative information possible. This technique is demonstrated by visualizing the finer details of vortices in a Taylor–Couette device.
Flow state multiplicity in convection
Authors: Hof B. , Lucas P. G. J. , Mullin T.
Published: July 2002

Pattern formation in a layer of fluid heated from below is an example of macroscopic ordering in continuous media. Here we show that in a relatively compact experimental version of the problem, a rich and diverse set of stable flows can be found. These flows, many of which are novel, can be categorized and understood in terms of their symmetry properties. This approach shows promise for providing insight into the more complicated fluid motion that occurs as the lateral dimension of the layer is increased.
Spectroscopic investigations of the plasma behavior in a plasma opening switch experiment
Authors: Sarfaty M. , Maron Y. , Krasik Ya. E. , Weingarten A. , Arad R. , Shpitalnik R. , Fruchtman A. , Alexiou S.
Published: July 2002

The electron density, the electron kinetic energy, the particle motion, and electric fields in a coaxial positive-polarity plasma opening switch (POS) were studied using spectroscopic diagnostics. A gaseous source that injects the plasma radially outward from inside the inner POS electrode was developed. The plasma was locally seeded with various species, desired for the various measurements allowing for axial, radial, and azimuthal resolutions both prior to and during the 180 ns long current pulse. The electron density was determined from particle ionization times and the electron energy from line intensities and time dependent collisional-radiative calculations. Fluctuating electric fields were studied from Stark broadening. The ion velocity distributions were obtained from emission-line Doppler broadenings and shifts. The early ion motion, the relatively low ion velocities and the nearly linear velocity dependence on the ion charge-to-mass ratio, leads to the conclusion that the magnetic field penetrates the plasma early in the pulse. The ion velocity dependence on the axial location were thus used to infer the time dependent axial distribution of the magnetic field, indicating the formation of a relatively high current density at the load-side edge of the plasma. This is expected to cause plasma acceleration towards the load, found to be supported by charge-collector measurements. The fast magnetic field penetration could be explained by mechanisms based on the Hall effect.
Particle-in-cell simulations of fast magnetic field penetration into plasmas due to the Hall electric field
Authors: Swanekamp S. B. , Grossmann J. M. , Fruchtman A. , Oliver B. V. , Ottinger P. F.
Published: July 2002

Particle-in-cell (PIC) simulations are used to study the penetration of magnetic field into plasmas in the electron-magnetohydrodynamic (EMHD) regime. These simulations represent the first definitive verification of EMHD with a PIC code. When ions are immobile, the PIC results reproduce many aspects of fluid treatments of the problem. However, the PIC results show a speed of penetration that is between 10% and 50% slower than predicted by one-dimensional fluid treatments. In addition, the PIC simulations show the formation of vortices in the electron flow behind the EMHD shock front. The size of these vortices is on the order of the collisionless electron skin depth and is closely coupled to the effects of electron inertia. An energy analysis shows that one-half the energy entering the plasma is stored as magnetic field energy while the other half is shared between internal plasma energy (thermal motion and electron vortices) and electron kinetic energy loss from the volume to the boundaries. The amount of internal plasma energy saturates after an initial transient phase so that late in time the rate that magnetic energy increases in the plasma is the same as the rate at which kinetic energy flows out through the boundaries. When ions are mobile it is observed that axial magnetic field penetration is followed by localized thinning in the ion density. The density thinning is produced by the large electrostatic fields that exist inside the electron vortices which act to reduce the space-charge imbalance necessary to support the vortices. This mechanism may play a role during the opening process of a plasma opening switch.