bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics simulations of aqueous formamide solution. II. Dynamics of solvent molecules
Authors: Puhovski Yurii P. , Rode Bernd M.
Published: July 2002

Molecular dynamics simulation was employed to study solute–solvent dynamics in water–formamide mixtures over the whole concentration range. Autocorrelation functions were used to investigate the translational/reorientational diffusion of molecules and also vibrational/librational molecular motion. The dynamics of the solution molecules is discussed in relation to structural properties of water–formamide mixtures and a self-consistent picture of the intermolecular structure and motion is obtained. In the water–formamide mixtures rotational diffusion and librational motion of solvent molecules are determined first of all by H bonding whereas translational diffusion and vibrational motion depend to a large extent on spatial solvent arrangements.
Symplectic reversible integrators: Predictor–corrector methods
Authors: Martyna Glenn J. , Tuckerman Mark E.
Published: July 2002

A new fourth order predictor–corrector integration scheme is presented. The unique feature of the new algorithm and what distinguishes it from a Gear predictor–corrector is that the method is derived from the Trotter decomposition of a specially formulated evolution operator and as such, is both symplectic and reversible. In addition, the method retains the useful property of Gear methods that only one force evaluation per time step is required. The new integrator is tested on a harmonic plus quartic oscillator and the Henon–Heiles system. Comparisons are made to the second order velocity Verlet integrator, the true fourth order Yoshida/Suzuki schemes and fourth order Gear. In all cases, the new method works well, giving energy conservation and trajectories of much better quality than velocity Verlet and of comparable quality to the results of the true fourth order schemes for the same computational cost as velocity Verlet.
Electronic shells or molecular orbitals: Photoelectron spectra of Ag−n clusters
Authors: Handschuh H. , Cha Chia-Yen , Bechthold P. S. , Ganteför G. , Eberhardt W.
Published: July 2002

Photoelectron spectra of Ag−n clusters with n=1–21 recorded at different photon energies (hν=4.025, 4.66, 5.0, and 6.424 eV) are presented. Various features in the spectra of Ag−2–Ag−9 can be assigned to electronic transitions predicted from quantum chemical ab initio calculations. While this comparison with the quantum chemical calculations yields a detailed and quantitative understanding of the electronic structure of each individual cluster, a discussion in terms of the shell model is able to explain trends and dominant patterns in the entire series of spectra up to Ag−21.
Multiple volume phase transition of nonionic thermosensitive gel
Authors: Kawasaki Hideya , Nakamura Takato , Miyamoto Keiichi , Tokita Masayuki , Komai Takashi
Published: July 2002

The volume phase transition is studied in the ternary system of the nonionic poly (N-normalpropylacrylamide) gel, water, and various alcohol. The gel shows a reentrant volume phase transition in all mixed solvent systems studied here. It is found that the phase transition is multiple in the mixed solvent systems of water-1-propanol and water-2-propanol. These results are discussed by the mean field theory in which the effects of the chain stiffening due to the adsorption of the solvent molecules are taken into account. These results strongly suggest that the adsorption and/or the desorption of the solvent molecules promote the multiple phase transition of the nonionic poly (N-normalpropylacrylamide) gel.
Femtosecond study of the size-dependent charge carrier dynamics in ZnO nanocluster solutions
Authors: Cavaleri Joseph J. , Skinner David E. , Colombo D. Philip , Bowman Robert M.
Published: July 2002

The dynamics of charge carrier trapping and recombination are measured as a function of ZnO cluster diameter by ultrafast pump–probe absorption spectroscopy. A finite spherical potential well model which shows good agreement with previous experimental work is employed to predict ZnO cluster diameters from absorption onsets. The rate of electron trapping is measured for clusters of 3.2 and 6.2 nm, and is found to increase with increasing cluster size. This increase in trapping rate for increasing cluster size is not consistent with either a diffusional or quantum mechanical picture of electron trapping. A mechanism for electron trapping involving trap-to-trap hopping is discussed whereby the number density of optically accessible deep traps must increase with increasing cluster size. Differences in the dynamics and in the ratio of interior to exterior atoms on the cluster are correlated and discussed. The time-resolved absorption data of the subsequent electron–hole recombination shows the appearance of an early time signal which increases as the cluster size grows. The early time species decays away within the first 50 ps to a diameter-independent plateau value via second-order recombination, and is assigned to electrons trapped in the interior of the cluster. The electron–hole recombination is found to occur faster and to a greater extent in the largest nanoclusters.
Quantum decoherence and the isotope effect in condensed phase nonadiabatic molecular dynamics simulations
Authors: Schwartz Benjamin J. , Bittner Eric R. , Prezhdo Oleg V. , Rossky Peter J.
Published: July 2002

In this paper, we explore in detail the way in which quantum decoherence is treated in different mixed quantum-classical molecular dynamics algorithms. The quantum decoherence time proves to be a key ingredient in the production of accurate nonadiabatic dynamics from computer simulations. Based on a short time expansion to a semiclassical golden rule expression due to Neria and Nitzan [J. Chem. Phys. 99, 1109 (1993)], we develop a new computationally efficient method for estimating the decay of quantum coherence in condensed phase molecular simulations. Using the hydrated electron as an example, application of this method finds that quantum decoherence times are on the order of a few femtoseconds for condensed phase chemical systems and that they play a direct role in determining nonadiabatic transition rates. The decay of quantum coherence for the solvated electron is found to take ≊50% longer in D2O than in H2O, providing a rationalization for a long standing puzzle concerning the lack of experimentally observed isotope effect on the nonadiabatic transition rate: Although the nonadiabatic coupling is smaller in D2O due to smaller nuclear velocities, the smaller coupling in D2O adds coherently for a longer time than in H2O, leading to nearly identical nonadiabatic transition rates. The implications of this isotope dependence of the nonadiabatic transition rate on changes in the quantum decoherence time for electron transfer and other important chemical reactions are discussed.
Comparing ab initio computed energetics with thermal experiments in surface science: CO/MgO(001)
Authors: Nygren Martin A. , Pettersson Lars G. M.
Published: July 2002

Early measurements of the binding energy of CO/MgO(001) gave low values, 0.15–0.17 eV, for the regular site, while later experiments using two independent experimental techniques (temperature programmed desorption and Clausius–Clapeyron plots) have given a much higher value, 0.43–0.45 eV. Theory has shown the opposite trend: early calculations gave a value of 0.38 eV, while the latest results are 0.07–0.11 eV. We have performed a careful theoretical analysis of the Clausius–Clapeyron experiment which is found to be in the assumed equilibrium region. Quantum chemical cluster modelling of regular, step, and corner sites show that only a low-coordinated (corner) site can fit both the measured binding energy and vibrational frequency shift. Accurate embedding techniques with full account of the crystal potential and a high-level treatment of dynamical correlation using large basis sets have been used. Effects of relaxation of the step have been investigated using atomistic simulation techniques. The binding energies (corrected for basis set superposition errors) are computed as 0.08, 0.18, and 0.48 eV for regular, step and corner sites, respectively. Theoretical temperature programmed desorption spectra have been generated and show that desorption from regular and step sites should be expected at 40 and 80 K, respectively. At a corner site the molecule is tilted at 45° to the normal. From angle-resolved near-edge extended x-ray absorption fine structure spectroscopy of CO/NiO(100) the molecule has been found to be vertical. Theoretical spectra have been generated including averaging over the frustrated rotations; agreement with the experimental intensity distribution is obtained also for the tilted (at 0 K) geometry. It is suggested that the oxide films contain a high density of defects.
Absolute partial cross sections for electron-impact ionization of CH4 from threshold to 1000 eV
Authors: Straub H. C. , Lin D. , Lindsay B. G. , Smith K. A. , Stebbings R. F.
Published: July 2002

Absolute partial cross sections for the production of CH4+, CH3+, CH2+ , CH+, C+, H2+, and H+ from electron-impact ionization of CH4 are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ions are completely collected. The overall uncertainty in the absolute cross section values is ±3.5% for singly charged parent ions and is slightly greater for fragment ions. Although previous measurements are generally found to agree well with the present results for CH4+ and CH3+, almost all previous work for the remaining fragment ions lies lower than the present results and in the case of H+ is lower by approximately a factor of 4.
Fragmentation of isolated ions by multiple photon absorption: A quantitative study
Authors: Boissel Pierre , de Parseval Patrick , Marty Philippe , Lefèvre Georges
Published: July 2002

The photofragmentation of anthracene cations under continuous irradiation by visible light is used as a tool to measure the radiative exchanges between the isolated species and its environment. The comparison of the experimental results with a numerical simulation allows an absolute determination of the oscillator strength of the whole set of absorption lines. Experimental evidence is given that, besides infrared vibrational emission, the contribution of electronic transitions has to be accounted for in the evaluation of the cooling rate.
Monte Carlo simulation of athermal mesogenic chains: Pure systems, mixtures, and constrained environments
Authors: Escobedo Fernando A. , de Pablo Juan J.
Published: July 2002

Monte Carlo simulations of mesogenic polymers have been performed for single and binary systems in the bulk and for pure components in constrained environments. An expanded ensemble formalism was employed to facilitate fluctuations of the number of particles in grand canonical, Gibbs, and pseudo-Gibbs ensemble simulations. Phase diagrams were generated for pure systems of athermal 8-mers and 16-mers with varying degrees of flexibility, for a binary mixture of rigid and semiflexible 16-mers (at constant pressure and variable flexibility), and for a bidisperse mixture of rigid chains. Simulations of the partitioning of rigid 12-mer chains between a bulk and a slit pore revealed that, for a certain range of slit gaps, the preferential coplanar orientation of molecules at the proximity of the walls induces the formation of a nematic phase within the pore. A similar induction of a nematic phase was observed for anisotropic molecules dispersed in a stretched, swollen gel. For the bulk systems, our simulation results were compared to predictions of various theories.
Mesoscopic dynamics of copolymer melts: From density dynamics to external potential dynamics using nonlocal kinetic coupling
Authors: Maurits N. M. , Fraaije J. G. E. M.
Published: July 2002

In this paper we apply nonlocal kinetic coupling to the dynamic mean-field density functional method, which is derived from generalized time-dependent Ginzburg–Landau theory. The method is applied to the mesoscopic dynamics of copolymer melts, which was previously simulated using a local coupling approximation. We discuss the general theory of time evolution of density variables with general kinetic coefficients developed by Kawasaki and Sekimoto, and especially the limits of the theory that yield the local coupling approximation, the collective Rouse dynamics model, and the reptation dynamics model. We show how a simple approximation to the Rouse dynamics model leads to a feasible numerical model that includes the essential physical features of nonlocal kinetic coupling. This results in a dynamic equation for the external potential instead of the density which allows us to perform calculations of microphase separation in copolymer melts with increased relevance to experimental results. As may be expected from a numerical model that includes nonlocal kinetic coupling, the numerical results show an increased computational efficiency, less defects in the final morphology, and a faster increase of the order parameter compared to local kinetic coupling.
Molecular dynamics simulation of water diffusion in atactic and amorphous isotactic polypropylene
Authors: Fukuda Mitsuhiro , Kuwajima Satoru
Published: July 2002

Molecular dynamics (MD) simulation of the diffusion of water in both atactic polypropylene (aPP) and amorphous isotactic polypropylene (iPP) models has been performed. Constant particle number, constant pressure, and constant temperature (NPT) MD was employed for both systems consisting of three polypropylene chains of 500 monomer units with ten water molecules. The density observed for the iPP model after full equilibration was slightly greater than that for the aPP model, but predicted x-ray scattering curves indicated that both systems were completely amorphous structures. In the aPP model, time duration up to 11 ns showed that the water molecules dynamically came together forming a water cluster with various sizes and again departed to a smaller size cluster or a single water sometimes. In contrast, the clustering was remarkable in the iPP model and maintained its aggregated structure over several nanoseconds. The self-diffusion coefficient, Dself, estimated from the mean-square displacement curve for the aPP model and the iPP one up to 11 ns duration showed 2.2×10−6 cm2/s and 3.1×10−7 cm2/s, respectively. The latter value corresponded well to the range of experimental diffusion coefficients.
Polymer translocation induced by adsorption
Authors: Park P. J. , Sung W.
Published: July 2002

We study the translocation of a flexible polymer through a pore in a membrane induced by its adsorption on the trans side of the membrane. When temperature T is higher than Tc, the adsorption–desorption transition temperature, the attractive interaction between polymer and membrane plays little role in affecting polymer conformation, leading to translocation time that scales as τ∼L3, where L is the polymer contour length. When T<Tc, however, the translocation time undergoes a sharp crossover to τ∼L2 for sufficiently long polymers, following the second-order conformational (adsorption) transition. The translocation time is found to exhibit the crossover around T=Tc′, which is lower than Tc for polymers shorter than a critical length (N<Nc).
Comparison of the performance of local, gradient-corrected, and hybrid density functional models in predicting infrared intensities
Authors: Halls Mathew D. , Schlegel H. Bernhard
Published: July 2002

Density functional theory has been used to calculate infrared (IR) intensities for a series of molecules (HF, CO, H2O, HCN, CO2, C2H2, H2CO, NH3, C2H4, CH2F2, CH2Cl2, and CH2Br2) in an effort to evaluate relative performance of different functionals. The methods employed in this study comprise most of the popular local, gradient-corrected, and hybrid functionals, namely, S-VWN, S-PL, B-LYP, B-P86, B-PW91, B3-LYP, B3-P86, and B3-PW91. Calculations were carried out using various qualities of split valence basis sets augmented with diffuse and polarization functions, both to determine basis set dependence and to evaluate the limit performance. Computed intensities were compared with results from conventional correlated ab initio methods (MP2 and QCISD). Hybrid functionals give results in closest agreement with QCISD over the other methods surveyed. Local and gradient-corrected methods performed remarkably alike, both are comparable to MP2, and outperform Hartree–Fock. Hartree–Fock intensities can be dramatically improved by scaling, making them similar to MP2 results.
Multidimensional femtosecond spectroscopies of molecular aggregates and semiconductor nanostructures: The nonlinear exciton equations
Authors: Chernyak Vladimir , Zhang Wei Min , Mukamel Shaul
Published: July 2002

A unified description of nonlinear optical spectroscopies of molecular aggregates (starting with the Frenkel-Heitler-London Hamiltonian) and semiconductors (starting with the two-band model) is developed using the nonlinear exciton equations (NEE). The equations follow explicitly the complete set of one-, two-, and three-point dynamical exciton variables relevant for the third-order response. Effects of nuclear motions are incorporated through relaxation superoperators calculated perturbatively in exciton-phonon coupling. A Green's function expression for the third-order response is derived by solving the NEE using a new truncation scheme based on factorizing the three-point relaxation kernels. These results set the stage for designing multidimensional spectroscopies of excitons and analyzing them using coherence-transfer pathways.
Linear scaling second-order Moller–Plesset theory in the atomic orbital basis for large molecular systems
Authors: Ayala Philippe Y. , Scuseria Gustavo E.
Published: July 2002

We have used Almlöf and Häser’s Laplace transform idea to eliminate the energy denominator in second-order perturbation theory (MP2) and obtain an energy expression in the atomic orbital basis. We show that the asymptotic computational cost of this method scales quadratically with molecular size. We then define atomic orbital domains such that selective pairwise interactions can be neglected using well-defined thresholding criteria based on the power law decay properties of the long-range contributions. For large molecules, our scheme yields linear scaling computational cost as a function of molecular size. The errors can be controlled in a precise manner and our method reproduces canonical MP2 energies. We present benchmark calculations of polyglycine chains and water clusters containing up to 3040 basis functions.
Rydberg and pulsed field ionization-zero electron kinetic energy spectra of YO
Authors: Linton Colan , Simard Benoit , Loock Hans Peter , Wallin Sara , Rothschopf Gretchen K. , Gunion Robert F. , Morse Michael D. , Armentrout Peter B.
Published: July 2002

A spectroscopic study of the Rydberg states of YO accessed from particular rotational levels of the A 2Π1/2, v=0 state has been combined with a pulsed field ionization, zero electron kinetic energy (PFI-ZEKE) investigation. The results provide accurate values of the ionization energy of YO, ionization energy I.E.(YO)=49 304.316(31) cm−1 [6.112 958(4) eV], and of the rotational constant (and bond length) of the YO+ cation in its X 1Σ+, v=0 ground state, B0+=0.4078(3) cm−1 [r0=1.7463(6) Å]. The improved value of I.E.(YO) combined with the known ionization energy of atomic yttrium then leads to the result D00(Y−O)−D00(Y−O)=0.1041±0.0001 eV. Combining this result with the value of D00(Y+−O) obtained from guided ion beam mass spectrometry yields an improved value of D00(Y−O)=7.14±0.18 eV. The PFI-ZEKE spectra display an interesting channel-coupling effect so that all rotational levels with J+⩽J′(A)+0.5 are observed with high intensity, where J+ is the angular momentum of the YO+ cation that is produced and J′(A) is the angular momentum of the A 2Π1/2 state that is reached when the first photon is absorbed. This is thought to result from the interaction between the dipole moment of the rotating YO+ core and the Rydberg electron, which can induce changes in l and J+ subject to the dipolar coupling matrix element selection rule, ΔJ+=±1, Δl=±1. The channel-coupling mechanism also appears to induce an inverse autoionization process in which an unbound electron with a low value of l is captured either by its low-J+ YO+ cation or by a second YO+ cation with the same value of J+. This inverse autoionization process is extremely sensitive to the electron kinetic energy, leading to narrow peaks in the PFI-ZEKE spectrum which are only slightly broader than the laser linewidth employed for this study (0.25 cm−1).
Determining point charge arrays that produce accurate ionic crystal fields for atomic cluster calculations
Authors: Derenzo Stephen E. , Klintenberg Mattias K. , Weber Marvin J.
Published: July 2002

In performing atomic cluster calculations of local electronic structure defects in ionic crystals, the crystal is often modeled as a central cluster of 5–50 ions embedded in an array of point charges. For most crystals, however, a finite three-dimensional repeated array of unit cells generates electrostatic potentials that are in significant disagreement with the Madelung (infinite crystal) potentials computed by the Ewald method. This is illustrated for the cubic crystal CaF2. We present a novel algorithm for solving this problem for any crystal whose unit cell information is known: (1) the unit cell is used to generate a neutral array containing typically 10 000 point charges at their normal crystallographic positions; (2) the array is divided into zone 1 (a volume defined by the atomic cluster of interest), zone 2 (several hundred additional point charges that together with zone 1 fill a spherical volume), and zone 3 (all other point charges); (3) the Ewald formula is used to compute the site potentials at all point charges in zones 1 and 2; (4) a system of simultaneous linear equations is solved to find the zone 3 charge values that make the zone 1 and zone 2 site potentials exactly equal to their Ewald values and the total charge and dipole moments equal to zero, and (5) the solution is checked at 1000 additional points randomly chosen in zone 1. The method is applied to 33 different crystal types with 50–71 ions in zone 1. In all cases the accuracy determined in step 5 steadily improves as the sizes of zones 2 and 3 are increased, reaching a typical rms error of 1 μV in zone 1 for 500 point charges in zone 2 and 10 000 in zone 3.
Mean first passage time solution of the Smoluchowski equation: Application to relaxation dynamics in myoglobin
Authors: Ansari Anjum
Published: July 2002

A robust numerical approach to solving the Smoluchowski equation describing a diffusive process is presented for the case where standard procedures are not so useful, in particular, diffusion along a spatially rough potential. The approach developed here makes use of an analytical expression for the mean first passage time for a system to get from one point to another along an arbitrary rough potential, and reduces the solution of the Smoluchowski equation to the solution of a relatively small number of first-order coupled differential equations. The results of this approach are compared with a discrete approximation solution of the Smoluchowski equation as well as with the analytical solution for the special case of a smooth harmonic potential. A significant reduction of computational time is achieved over the discrete approximation method. A model of configurational diffusion along a one-dimensional harmonic potential with coordinate-dependent diffusion coefficient is used to fit the highly nonexponential relaxation dynamics observed in myoglobin following the photodissociation of the bound carbon-monoxide. The relaxation is well described with an effective diffusion coefficient that decreases exponentially along the reaction coordinate. This decrease can arise from either an increase in the roughness of the potential surface or an increase in the friction along the reaction coordinate as the system approaches equilibrium.
The ultraviolet photofragmentation of doubly charged transition metal complexes in the gas phase: Initial results for [Cu.(pyridine)n]2+ and [Ag.(pyridine)n]2+ ions
Authors: Puskar Ljiljana , Barran Perdita E. , Wright Rossana R. , Kirkwood David A. , Stace Anthony J.
Published: July 2002

Results are presented of a study of the UV photofragmentation patterns of the doubly charged complexes [Cu.(pyridine)n]2+ and [Ag.(pyridine)n]2+ for n=4–7. The two systems behave very differently, with Ag(II) being dominated by the presence of charge transfer products at small values of n. This ease of reduction of Ag(II) can be equated to behavior seen in the condensed phase. At all values of n both types of complex display a remarkable tendency to fragment to the most stable configuration, which can correspond to either a singly or doubly charged ion.
Polyelectrolyte adsorption on heterogeneously charged surfaces
Authors: Ellis M. , Kong C. Y. , Muthukumar M.
Published: July 2002

The adsorption transition of a uniformly charged polyelectrolyte onto heterogeneously charged surfaces has been investigated using off-lattice Monte Carlo simulations. Each of these surfaces contains both positive and negative charges. In addition to the usual case of adsorption of a polyelectrolyte to a surface with net charge opposite to that of the polymer, we show that a polyelectrolyte can adsorb onto a surface with net surface charge density similar to that of the polyelectrolyte. This adsorption is caused by the spatial inhomogeneity of the surface charges, which creates attractive regions with charge density different from the overall charge density of the surface. The spatial inhomogeneity of the surface charges also leads to differences in the conformation of the adsorbed polyelectrolyte. The critical conditions of strength and range of electrostatic interactions and chain length necessary for adsorption of a polyelectrolyte to a heterogeneously charged surface are demonstrated.
On invariance properties of the wave equation
Authors: Bluman George , Kumei Sukeyuki
Published: July 2002

A complete group classification is given of both the wave equation c2(x)uxx−utt=0 (I) and its equivalent system vt=ux, c2(x)vx=ut (II) when the wave speed c(x)≠const. Equations (I) and (II) admit either a two- or four-parameter group. For the exceptional case, c(x)=(Ax+B)2, equation (I) admits an infinite group. Equations (I) and (II) do not always admit the same group for a given c(x): The group for (I) can have more parameters or fewer parameters than that for (II); moreover, the groups can be different with the same number of parameters. Separately for (I) and (II), all possible c(x) that admit a four-parameter group are found explicitly. The corresponding invariant (similarity) solutions are considered. Some of these wave speeds have realistic physical properties: c(x) varies monotonically from one positive constant to another positive constant as x goes from −∞ to +∞.
Summing the spectral representations of Pöschl–Teller and Rosen–Morse fixed-energy amplitudes
Authors: Kleinert H. , Mustapic I.
Published: July 2002

The spectral representations of the fixed-energy amplitude of the symmetric and the general Pöschl–Teller potentials are summed via a Sommerfeld–Watson transformation which leads to a simple closed-form expression. The result is used to write down a similar expression for the symmetric and general Rosen–Morse potentials, exploiting the close correspondence that exists between the two systems within the Schrödinger theory and the path integral formalism via a Duru–Kleinert transformation. From the singularities of the latter amplitude the bound and continuum states of the general Rosen–Morse potential are extracted.
Exact results and universal asymptotics in the Laguerre random matrix ensemble
Authors: Forrester P. J.
Published: July 2002

For various values of the parameters in the Laguerre random matrix ensemble, the distribution of the smallest eigenvalue and the scaled n-level distribution function are calculated exactly in terms of generalized hypergeometric functions. In certain cases these functions are expressed as multidimensional integrals, from which asymptotic formulas are calculated and predictions of universal behavior verified.
Diagrammatic method of integration over the unitary group, with applications to quantum transport in mesoscopic systems
Authors: Brouwer P. W. , Beenakker C. W. J.
Published: July 2002

A diagrammatic method is presented for averaging over the circular ensemble of random-matrix theory. The method is applied to phase-coherent conduction through a chaotic cavity (a ‘‘quantum dot’’) and through the interface between a normal metal and a superconductor.
Motion of curves and surfaces and nonlinear evolution equations in (2+1) dimensions
Authors: Lakshmanan M. , Myrzakulov R. , Vijayalakshmi S. , Danlybaeva A. K.
Published: July 2002

It is shown that a class of important integrable nonlinear evolution equations in (2+1) dimensions can be associated with the motion of space curves endowed with an extra spatial variable or equivalently, moving surfaces. Geometrical invariants then define topological conserved quantities. Underlying evolution equations are shown to be associated with a triad of linear equations. Our examples include Ishimori equation and Myrzakulov equations which are shown to be geometrically equivalent to Davey-Stewartson and Zakharov-Strachan (2+1) dimensional nonlinear Schrödinger equations, respectively.
Monotone Riemannian metrics and relative entropy on noncommutative probability spaces
Authors: Lesniewski Andrew , Ruskai Mary Beth
Published: July 2002

We use the relative modular operator to define a generalized relative entropy for any convex operator function g on (0,∞) satisfying g(1)=0. We show that these convex operator functions can be partitioned into convex subsets, each of which defines a unique symmetrized relative entropy, a unique family (parametrized by density matrices) of continuous monotone Riemannian metrics, a unique geodesic distance on the space of density matrices, and a unique monotone operator function satisfying certain symmetry and normalization conditions. We describe these objects explicitly in several important special cases, including g(w)=−log w, which yields the familiar logarithmic relative entropy. The relative entropies, Riemannian metrics, and geodesic distances obtained by our procedure all contract under completely positive, trace-preserving maps. We then define and study the maximal contraction associated with these quantities.
Heat/mass transfer from surface films to shear flows at arbitrary Peclet numbers
Authors: Stone Howard A.
Published: July 2002

An efficient numerical method is described for studying the combined conductive and convective transport from a surface film on a planar boundary to a fluid in simple shear flow. Such problems arise most commonly in the use of hot film anemometers, electrochemical shear probes, or in simple models of chemical reactions. The method is illustrated by calculating the total flux (Nusselt number) and variation of the flux along the surface for isothermal circular disks at arbitrary values of the Peclet number (dimensionless shear rate) and is compared with asymptotic results valid at high and low Peclet numbers. The theoretical low Peclet number results of Phillips [Q. J. Mech. Appl. Math. (in press)] lie within 2% of the numerical results for Peclet numbers as high as P=2. At high Peclet numbers, a theoretical estimate for the neglected flux from the edge regions is used, together with the numerical simulations, to propose a correction to the standard one-term asymptotic expression. This approximate relationship remains within 7% of the numerical calculations for Peclet numbers as small as P=5. In addition, results for the total heat transfer as a function of Peclet number are presented for isolated elliptical disks at two orthogonal orientations with respect to the flow and an asymptotic expression for high Peclet numbers is presented for arbitrary disk orientations.
Stokes problems of multiparticle systems: Periodic arrays
Authors: Tran-Cong T. , Phan-Thien N. , Graham A. L.
Published: July 2002

A boundary element method is developed and used to compute the volume-average rheological properties of layered suspensions in a mean simple shearing flow up to a volume concentration of 0.5. The suspension is made up of a periodic array of neutrally buoyant, monosized spheroids in the x and y directions suspended in a Newtonian fluid that is subjected to a mean shear flow in the xz plane. The results for a monolayered suspension of spheres agree well with other simulations and some experimental results. Some new results for prolate and oblate spheroids are also given. Both the volume-average first and second normal stress differences are found to be zero in the simulation in all the cases considered.
Eddy shocklets in decaying compressible turbulence
Authors: Lee Sangsan , Lele Sanjiva K. , Moin Parviz
Published: July 2002

The existence of eddy shocklets in three-dimensional compressible turbulence is controversial. To investigate the occurrence of eddy shocklets, numerical simulations of temporally decaying isotropic turbulence are conducted. Dilatation statistics from simulations with different initial fluctuation Mach numbers, Mt, show that dilatation is more intermittent and more negatively skewed for higher Mt. By studying instantaneous flow fields, shocklets are found and verified to have all the characteristics of a typical shock wave, such as proper jumps in pressure and density along with a local entropy peak inside the high-compression zone. Although overall compressible dissipation contributes to less than one-tenth of the total dissipation, compressible dissipation around shocklets is about an order of magnitude larger than typical values of incompressible dissipation. In the zones of eddy shocklets, pressure is highly correlated with dilatation to convert kinetic energy into internal energy. These mechanisms near shocklets should be accounted for in phenomenological modeling for highly compressible turbulence. Three-dimensional turbulence is found to be less sensitive to the initial compressibility, and requires higher initial Mt for eddy shocklets to form than for two-dimensional turbulence. Higher Mt and higher Reynolds number are found to increase the probability of shocklet occurrence.
Ion temperature-gradient-driven modes and anomalous ion transport in tokamaks
Authors: Romanelli F.
Published: July 2002

The stability of the ion temperature gradient modes is investigated using the kinetic ion response without expansions in ωD/ω. A systematic parameter study is carried out using a low-beta circular flux surface equilibrium in order to determine the stability boundaries in ηi vs εn space (ηi=d ln Ti/ d ln n, εn=Ln/R). Particular attention is devoted to the consequences of the presence of these modes for anomalous ion transport.
Spheromak injection into a tokamak
Authors: Brown M. R. , Bellan P. M.
Published: July 2002

Recent results from the Caltech spheromak injection experiment [to appear in Phys. Rev. Lett.] are reported. First, current drive by spheromak injection into the ENCORE tokamak as a result of the process of magnetic helicity injection is observed. An initial 30% increase in plasma current is observed followed by a drop by a factor of 3 because of sudden plasma cooling. Second, spheromak injection results in an increase of tokamak central density by a factor of 6. The high-current/high-density discharge is terminated by a sharp peaking of the density profile followed by an interchange instability. In a second experiment, the spheromak is injected into the magnetized toroidal vacuum vessel (with no tokamak plasma) fitted with magnetic probe arrays. An m=1 (nonaxisymmetric) magnetic structure forms in the vessel after the spheromak undergoes a double tilt; once in the cylindrical entrance between gun and tokamak, then again in the tokamak vessel. In the absence of net toroidal flux, the structure develops a helical pitch (the sense of pitch depends on the helicity sign). Experiments with a number of refractory metal electrode coatings have shown that tungsten and chrome coatings provide some improvement in spheromak parameters. Design details of a larger, higher-current spheromak gun with a new accelerator section are also discussed.
Magnetohydrodynamic wave drag
Authors: Newcomb William A.
Published: July 2002

A calculation is made of the energy loss by emission of shear-Alfvén and slow magnetoacoustic waves by a perfectly conducting solid spherical projectile moving with uniform velocity through an ideal magnetoplasma.
The energy-momentum tensor for the linearized Maxwell–Vlasov and kinetic guiding center theories
Authors: Pfirsch D. , Morrison P. J.
Published: July 2002

A modified Hamilton–Jacobi formalism is introduced as a tool to obtain the energy-momentum and angular-momentum tensors for any kind of nonlinear or linearized Maxwell-collisionless kinetic theories. The emphasis is on linearized theories, for which these tensors are derived for the first time. The kinetic theories treated—which need not be the same for all particle species in a plasma—are the Vlasov and kinetic guiding center theories. The Hamiltonian for the guiding center motion is taken in the form resulting from Dirac’s constraint theory for nonstandard Lagrangian systems. As an example of the Maxwell-kinetic guiding center theory, the second-order energy for a perturbed homogeneous magnetized plasma is calculated with initially vanishing field perturbations. The expression obtained is compared with the corresponding one of Maxwell–Vlasov theory.
Toroidal kinetic ηi-mode study in high-temperature plasmas
Authors: Dong J. Q. , Horton W. , Kim J. Y.
Published: July 2002

A new kinetic integral equation for the study of the ion-temperature-gradient-driven mode in toroidal geometry is developed that includes the ion toroidal (curvature and magnetic gradient) drift motion ωD, the mode coupling from finite k∥ due to the toroidal feature of the sheared magnetic configuration. The integral equation allows the stability study for arbitrary k∥vi/(ω − ωD) and k⊥ ρi. A systematic parameter study is carried out for the low β circular flux surface equilibrium. Possible correlations between the unstable mode characteristics and some experimental results such as the fluctuation spectrum and the anomalous ion thermal transport measurements are discussed.
Thermomagnetic instability in a magnetized plasma
Authors: Fruchtman A. , Strauss H. R.
Published: July 2002

The thermomagnetic instability, driven by gradients of the plasma density and temperature, which are parallel to each other and orthogonal to the magnetic field, is studied. The focus of this paper is on a magnetized plasma, in which the electron cyclotron frequency is larger than the electron collision frequency. The nature of the transverse heat conductivity in the magnetized plasma causes the instability to be driven by opposite density and temperature gradients. The growth rate in the magnetized plasma is much smaller than in the unmagnetized plasma. In a very strong magnetic field heat convection is larger than heat conduction, and the instability is further stabilized by the Hall field.
Theory of kinetic ballooning modes excited by energetic particles in tokamaks
Authors: Tsai Shih-Tung , Chen Liu
Published: July 2002

Resonant excitations of kinetic ballooning modes by the energetic ions/alpha particles in tokamaks have been analyzed theoretically. The theoretical analysis includes finite-size orbit effects of both circulating and trapped particles. With energetic particle contributions suppressed in the singular inertial layer, an analytic dispersion relation can then be derived via an asymptotic matching analysis. The dispersion relation, in particular, demonstrates the existence of two types of modes; that is, the magnetohydrodynamic gap mode and the energetic-particle continuum mode. Specific expressions for real frequencies, growth rates and threshold conditions are also derived for a model slowing-down beam ion distribution function.
Compact toroid formation, compression, and acceleration
Authors: Degnan J. H. , Peterkin R. E. , Baca G. P. , Beason J. D. , Bell D. E. , Dearborn M. E. , Dietz D. , Douglas M. R. , Englert S. E. , Englert T. J. , Hackett K. E. , Holmes J. H. , Hussey T. W. , Kiuttu G. F. , Lehr F. M. , Marklin G. J. , Mullins B. W. , Price D. W. , Roderick N. F. , Ruden E. L. , Sovinec C. R. , Turchi P. J. , Bird G. , Coffey S. K. , Seiler S. W. , Chen Y. G. , Gale D. , Graham J. D. , Scott M. , Sommars W.
Published: July 2002

Research on forming, compressing, and accelerating milligram-range compact toroids using a meter diameter, two-stage, puffed gas, magnetic field embedded coaxial plasma gun is described. The compact toroids that are studied are similar to spheromaks, but they are threaded by an inner conductor. This research effort, named marauder (Magnetically Accelerated Ring to Achieve Ultra-high Directed Energy and Radiation), is not a magnetic confinement fusion program like most spheromak efforts. Rather, the ultimate goal of the present program is to compress toroids to high mass density and magnetic field intensity, and to accelerate the toroids to high speed. There are a variety of applications for compressed, accelerated toroids including fast opening switches, x-radiation production, radio frequency (rf) compression, as well as charge-neutral ion beam and inertial confinement fusion studies. Experiments performed to date to form and accelerate toroids have been diagnosed with magnetic probe arrays, laser interferometry, time and space resolved optical spectroscopy, and fast photography. Parts of the experiment have been designed by, and experimental results are interpreted with, the help of two-dimensional (2-D), time-dependent magnetohydrodynamic (MHD) numerical simulations. When not driven by a second discharge, the toroids relax to a Woltjer–Taylor equilibrium state that compares favorably to the results of 2-D equilibrium calculations and to 2-D time-dependent MHD simulations. Current, voltage, and magnetic probe data from toroids that are driven by an acceleration discharge are compared to 2-D MHD and to circuit solver/slug model predictions. Results suggest that compact toroids are formed in 7–15 μsec, and can be accelerated intact with material species the same as injected gas species and entrained mass ≥1/2 the injected mass.
Rotation in Ohmically heated tokamaks: Experiment and theory
Authors: Rowan William L. , Meigs A. G. , Solano E. R. , Valanju P. M. , Calvin M. D. , Hazeltine R. D.
Published: July 2002

Comparison of experiments with theory leads to a reasonably complete understanding of impurity rotation in Ohmic plasmas in a tokamak. In the experiments, the components of the impurity rotation velocity were derived from measurements of the Doppler shift of carbon spectral emission. Neoclassical theory for rotation was evaluated using measured plasma profiles and compared with experiment. The comparisons suggest that plasma toroidal rotation is damped (but not quenched) by momentum loss due to ion–neutral charge exchange and that the damping can now be accurately predicted.
Resistive pressure-gradient-driven turbulence with self-consistent flow profile evolution
Authors: Carreras B. A. , Lynch V. E. , Garcia L. , Diamond P. H.
Published: July 2002

Resistive pressure-gradient-driven turbulence causes a modification of the average poloidal flow profile through Reynolds stress. The poloidal flow profile modification can be induced through a diamagnetic term and/or ‘‘seed’’ flow. This leads to several possible results for the steady-state poloidal flow profile. If this profile is such that V″θ≠0 at the singular surface, there is a reduction in linear growth rate and suppression of the fluctuation. When V″θ = 0 and V′θ≠0, there is linear stabilization but the fluctuation suppression is weak.
Reduction of order in the geometric optics of plasmas
Authors: Friedland L. , Goldner G.
Published: July 2002

Small amplitude waves in plasmas are usually described by large systems of linearized Maxwell and kinetic equations. A procedure of elimination of components and reduction of the order of the system is discussed within the geometric optics approximation. In some cases characterized by Hermitian generalized dielectric tensors ℰ describing the unreduced problem, the successive reduction procedure yields, at each step, energy conserving reduced systems that preserve the general form and first-order nature of the equations. The number of equations in the final reduced system is equal to the number of degenerate vanishing eigenvalues of ℰ. The theory is applied in the case of transverse waves, propagating along the magnetic field in plasmas with plane parallel stratification. Both cold streaming plasma and kinetic problems are considered by using the same-order reduction procedure. The cold plasma case, at low densities, becomes doubly degenerate at cyclotron resonance, reflecting mode coupling between the vacuum electromagnetic and electron cyclotron modes. Mode conversion coefficients found from the solution of the reduced system of two first-order differential equations, characterizing this case, are in an excellent agreement with the results of the numerical solutions of the full unreduced system of equations. The kinetic case is approached by viewing the plasma as consisting of many beamlets each governed by the cold fluid approximation. The problem represents a multiply degenerate situation as many beamlets are in resonance at a time. Renormalized perturbation analysis of the partially reduced system in the low-density case predicts results similar to those found in the cold plasma, with possible broadening of the resonance region. At large densities, for propagation from the lower magnetic field side, the wave is reflected and the cyclotron resonance is inaccessible. In contrast, for propagation from the high magnetic field side, the electromagnetic energy is transferred to the electrons via the mechanism of Landau cyclotron damping.
High peak power K
a-band gyrotron oscillator experiment
Authors: Gold S. H. , Fliflet A. W. , Manheimer W. M. , McCowan R. B. , Black W. M. , Lee R. C. , Granatstein V. L. , Kinkead A. K. , Hardesty D. L. , Sucy M.
Published: July 2002

A Ka- band gyrotron oscillator powered by a 600 kV pulse-line accelerator has produced approximately 100 MW at 35 GHz in a circular TE62 mode. It has also demonstrated frequency tuning over the range 28 to 49 GHz by operating in a family of TEm2 modes, with the azimuthal index m ranging from 4 to 10, by variation of the guide magnetic field. Operation is in general agreement with the predictions of theory.
Nonlinear gyrokinetic theory for finite-beta plasmas
Authors: Hahm T. S. , Lee W. W. , Brizard A.
Published: July 2002

A self-consistent and energy-conserving set of nonlinear gyrokinetic equations, consisting of the averaged Vlasov and Maxwell’s equations for finite-beta plasmas, is derived. The method utilized in the present investigation is based on the Hamiltonian formalism and Lie transformation. The resulting formulation is valid for arbitrary values of k⊥ρi and, therefore, is most suitable for studying linear and nonlinear evolution of microinstabilities in tokamak plasmas as well as other areas of plasma physics where the finite Larmor radius effects are important. Because the underlying Hamiltonian structure is preserved in the present formalism, these equations are directly applicable to numerical studies based on the existing gyrokinetic particle simulation techniques.