bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling low pressure collisional plasma sheath with space-charge effect
Authors: Roy Subrata , Pandey B. P. , Poggie Jonathan , Gaitonde Datta V.
Published: May 2003

The present work develops a computationally efficient one-dimensional subgrid embedded finite element formulation for plasma-sheath dynamics. The model incorporates space-charge effect throughout the whole plasma and the sheath region using multifluid equations. Secondary electron emission is not considered. A third-order temperature dependent polynomial is used to self-consistently calculate the rate of ionization in the plasma dynamic equations. The applications include dc and rf sheath inside a glow discharge tube where the noble gas is immobile, and a partially ionized plasma sheath inside an electric propulsion thruster channel in which the gas flows. The electron and ion number densities of the numerical solution decrease in the sheath region as expected. The ion velocity and electron temperature profiles also exhibit the expected behavior. The computed sheath potential compares well with the available experimental data.
Effect of capillary-condensed water on the dynamic friction force at nanoasperity contacts
Authors: Sirghi L.
Published: May 2003

A single nanoasperity contact in ambient air is usually wetted by capillary condensation of water vapor and is surrounded by a water meniscus. This phenomenon strongly affects the contact friction, not only by the effect of meniscus loading force (superficial tension and capillary forces), but also by a friction force that accounts for the energy loss in the meniscus movement along with the sliding contact. Occurrence of the water-meniscus-generated friction is experimentally proved by atomic force microscopy measurements of the tip–sample friction force at minimum possible external load (before pull-off). A qualitative explanation for the observed dependence of the friction force on air humidity and solid surface wettability is proposed.
Lagrangian simulation of explosively driven magnetohydrodynamic generator
Authors: Kim Deok-Kyu , Seo Min Su , Kim Inho
Published: May 2003

A series of time-dependent one-dimensional simulations has been carried out on the hydrodynamic behavior of argon and air plasmas in an explosively driven magnetohydrodynamic power generator. The thermodynamic properties of plasma gases are computed using equation-of-state data obtained from a detailed theoretical model. The plasma conductivities are given by a mixture rule, which comprises the fully and weakly ionized plasma approximations. The effects of the initial pressure and the magnetic field strength on the plasma behavior in the flow channel are examined over a moderate range of operating conditions, and then the computed results are compared with the experimental measurements, showing good agreement for the case of low magnetic Reynolds number.
On noncommutative orbifolds of K3 surfaces
Authors: Belhaj A. , Manjarı́n J. J. , Resco P.
Published: May 2003

Using the algebraic geometry method of Berenstein and Leigh for the construction of the toroidal orbifold (T2×T2×T2)/(Z2×Z2) with discrete torsion and considering local K3 surfaces, we present noncommutative aspects of the orbifolds of product of K3 surfaces. In this way, the ordinary complex deformation of K3 can be identified with the resolution of stringy singularities by noncommutative algebras using crossed products. We give representations and make some comments regarding the fractionation of branes. Illustrating examples are presented.
Geometrical phases for the G(4,2) Grassmannian manifold
Authors: Karle Regina , Pachos Jiannis
Published: May 2003

We generalize the usual Abelian Berry phase generated for example in a system with two nondegenerate states to the case of a system with two doubly degenerate energy eigenspaces. The parametric manifold describing the space of states of the first case is formally given by the G(2,1) Grassmannian manifold, while for the generalized system it is given by the G(4,2) one. For the latter manifold which exhibits a much richer structure than its Abelian counterpart we calculate the connection components, the field strength and the associated geometrical phases that evolve nontrivially both of the degenerate eigenspaces. A simple atomic model is proposed for their physical implementation.
Phase diagram of solutions of stiff-chain macromolecules: A Monte Carlo simulation
Authors: Ivanov V. A. , Stukan M. R. , Müller M. , Paul W. , Binder K.
Published: May 2003

The phase diagram of solutions of semiflexible macromolecules consisting of N=20 effective monomer units has been studied by means of grand canonical Monte Carlo computer simulation using the bond fluctuation model and the configurational bias scheme. Two different types of interactions were taken into account: an intramolecular potential controlling the chain stiffness and an attractive interaction between effective monomer units to model variable solvent quality. A very broad region of density from dilute solution to melt has been covered in the simulation. The phase diagram shows a strong increase of the density difference between isotropic dilute and nematic dense phases upon increasing attraction between monomer units (i.e., decreasing the temperature). We discuss methods of locating the isotropic–nematic transition point and the effects of the vicinity of the polymer–solvent critical point on the properties of the liquid crystalline transition.
Magnetic fluctuations of a large nonuniform plasma column
Authors: Maggs J. E. , Morales G. J.
Published: May 2003

A broad experimental survey is made of the properties of spontaneously generated magnetic fluctuations in a large linear device in which the plasma density has different cross-field gradient scales. It includes steep gradients at the plasma–wall edge as well as at an interior plasma–plasma interface, thus phenomena of interest to magnetic fusion research as well as to space plasma physics is illustrated. Fluctuations in uniformly magnetized columns and plasmas with an axial gradient in the confining magnetic field are studied. Some of the highlights include the identification of a universal spectrum for drift-Alfvén turbulence and the role of partial reflections in shaping the spectra.
Effect of large magnetocrystalline anisotropy on the spin wave linewidth in Zn–Y hexagonal ferrite
Authors: Nazarov Alexey V. , Patton Carl E.
Published: May 2003

The oblique pumping spin wave instability threshold microwave field amplitude hcrit was measured as a function of the static field for both in-plane and out-of-plane static fields for a thin disk of Mn substituted Zn–Y (Ba2Zn2Fe12O22) easy plane hexagonal ferrite at 16.7 GHz. Theoretical fits to the in-plane static field data gave a single value of 12.8 Oe for the spin wave linewidth ΔHk. The best overall fits were for a critical spin wave mode wave number k close to zero and there was no correlation with the critical mode propagation directions. The fitted ΔHk for the out-of-plane static field geometry was found to increase with the out-of-plane magnetization angle, but showed no correlation with the critical mode propagation directions. These critical modes had k≈0 as well. The fitted ΔHk increased from the in-plane field value of 12.8 Oe to a value of 21 Oe at the maximum accessible magnetization angle of 70° relative to the disk plane. This increase in spin wave damping with the out-of-plane angle is attributed to spin orbit coupling.
Ion irradiation effects in platinum oxide films
Authors: Machalett F. , Gärtner K. , Edinger K. , Diegel M.
Published: May 2003

Ion irradiation reduces the resistance of semi-insulating platinum oxide thin films by several orders of magnitude. The influence of ion dose and ion energy on the change in sheet resistance has been investigated. Ion sputtering and volume reduction, which is caused by oxygen expulsion, have been measured using an atomic force microscope. For information about film composition and thickness Rutherford backscattering analysis has been carried out. The experimental results can be understood by a simple model of the ion irradiation effects in noble metal oxide films.
Microstructural, optical, and magnetic properties of Mn-implanted p-type GaN
Authors: Baik Jeong Min , Lee Jong-Lam , Shon Yoon , Kang Tae Won
Published: May 2003

The effect of microstructural change on both magnetic and optical properties of Mn-implanted p-type GaN was studied. A dilute magnetic semiconductor was achieved by implanting Mn ions into p-type GaN and subsequently annealing. The magnetization measurement showed that the Curie temperature was the highest in the 800 °C annealed sample due to the formation of Ga–Mn magnetic phases. The annealing at a higher temperature of 900 °C produced antiferromagnetic Mn–N compounds such as Mn6N2.58 and Mn3N2, leaving N vacancies. This provides evidence that N vacancies played a critical role in weakening the ferromagnetic property in the Mn-implanted GaN. The photoluminescence peak at 2.92 eV became strong after annealing at 800 °C, indicating an increase in hole concentration due to an enhanced activation of Mn impurities in p-type GaN. The intensity of Raman modes at 290 and 670 cm−1 decreased drastically as annealing temperature increased (>800 °C), due to the reduction in Mn-implantation-induced lattice imperfections. From this, it is proposed that the increase in magnetic properties of Mn-implanted GaN originated from the enhancement in the crystallinity as well as the production of Ga–Mn magnetic phases.
Nonlinear temperature dependence of switching fields in relief-structured Co/Ni80Fe20 particles
Authors: Nozaki Y. , Matsuyama K. , Ishii S.
Published: May 2003

The temperature dependence of the switching properties in low sub-μm scale Co/Ni80Fe20 particles with different aspect ratios L/w (=1–6, w=0.4 μm) and thicknesses t (=1–5 nm) are investigated using magnetotransport measurements of relief-structured films with giant magnetoresistance. The shape of each particle is a parallelogram with slope angle θ(=0°–60°). The switching field of the particles increased with θ due to the variation of the end-domain configuration. Nonlinear temperature dependence of the switching field was observed, which could be explained by the thermal nucleation theory of the magnetization reversal at below 150 K. The amplitude of the energy barrier height ΔE for the reversal was strongly suppressed by decreasing the thickness of the particles. This degradation of the thermal stability may be caused by the variation of the film morphology by decreasing the thickness.
Theoretical study of the rovibrationally resolved transitions of CaH
Authors: Weck P. F. , Stancil P. C. , Kirby K.
Published: May 2003

Comprehensive absorption line lists for CaH40 have been calculated for electronic transitions from the X 2Σ+ ground state to the A 2Π, B/B′ 2Σ+, and E 2Π low-lying excited states. The lists include transition energies and oscillator strengths for all possible allowed transitions and were computed using the most recent set of theoretical potential energy curves and dipole transition moment functions, with adjustments to account for experimental dissociation energies and asymptotic limits. Good agreement with previous calculations and available experimental data has been obtained. Oscillator strengths for the transition from the X 2Σ+ state to the D 2Σ+ state are also given, but due to the large uncertainty of the available dipole transition moment function the line list is highly uncertain. For the C 2Σ+←X 2Σ+ transition, a Franck–Condon approximation has been used.
Thermal decomposition of ethanol. II. A computational study of the kinetics and mechanism for the H+C2H5OH reaction
Authors: Park J. , Xu Z. F. , Lin M. C.
Published: May 2003

The kinetics and mechanism for the H+C2H5OH reaction, a key chain-propagation step in the high temperature decomposition and combustion of ethanol, have been investigated with the modified GAUSSIAN -2 (G2M) method using the structures of the reactants, transition states and products optimized at the B3LYP/6-311+G(d,p) level of theory. Four transition states have been identified for the production of H2+CH3CHOH (TS1), H2+CH2CH2OH (TS2), H2+C2H5O (TS3), and H2O+C2H5 (TS4) with the corresponding barriers, 7.18, 13.30, 14.95, and 27.10 kcal/mol. The predicted rate constants and branching ratios for the three H-abstraction reactions have been calculated over the temperature range 300–3000 K using the conventional and variational transition state theory with quantum-mechanical tunneling corrections. The predicted total rate constant, kt=3.15×103T3.12 exp(−1508/T) cm3 mol−1 s−1, agrees reasonably with existing experimental data; in particular, the result at 423 K was found to agree quantitatively with an available experimental value. The small deviation between the predicted kt and another set of experimental data measured at 295–700 K has been examined by kinetic modeling; the deviation is attributable to insufficient corrections for contributions from secondary reactions.
Collisional intersystem crossing of N2(a′ 1Σu−) to produce triplet-state molecular nitrogen
Authors: Umemoto Hironobu , Oku Mizuki , Iwai Tetsuya
Published: May 2003

The production of N2(B 3Πg, v=0) was identified in the collisional deactivation of N2(a′ 1Σu−, v=0) by Xe, Kr, Ar, O2, and NO. N2(B, v=0) was probed by laser-induced fluorescence via the C 3Πu state. N2(a′, v=0) was produced by energy transfer from N2(a 1Πg, v=0) which was produced by two-photon excitation of N2(X 1Σg+). The rate constant for the intersystem crossing was the largest for Xe and the smallest for Ar. The rate constants relative to that for O2 were 11(Xe), 1.1(Kr), ≈0.001(Ar), and 5.8(NO). Except for NO, the difference in these rate constants is mainly attributable to that in the overall rate constant for the deactivation and the quantum yields are comparable. As for NO, the overall rate constant is one order of magnitude larger than that for O2 and comparable to that for Xe, while the yield for the intersystem crossing is around half of that for O2 or Xe. The rate constants for the intersystem crossing by H2 and CH4 are less than 1% of O2. This is consistent with the high yields for the production of H atoms. The rate constant for N2 is four orders of magnitude smaller than that for O2 and the yield for the intersystem crossing is less than 0.02.
Theoretical and experimental studies on the hydroquinone–argon 1:1 complex: A blueshifted O–H–Ar bond
Authors: Meenakshi P. S. , Biswas N. , Wategaonkar S.
Published: May 2003

Resonantly enhanced multiphoton ionization (REMPI), hole burning studies, as well as ab initio calculations on the hydroquinone–argon 1:1 complex, are presented in this paper. The REMPI spectrum displayed features due to the complex up to ∼1250 cm−1, which is unusually high for such complexes. The complex, like the monomer, was found to exist in both cis- and trans- conformational forms. Ab initio calculations carried out at the MP2/6-31+G* level for the S0 state of the complex yielded pi-bonded as well as H-bonded structures. The experimentally observed conformers are assigned pi-bonded structures. The high stability of the complex was attributed to slow vibrational predissociation rate constants due to the existence of multiple minima in the potential energy surface. Calculations on the S0 state of the H-bonded aminophenol–argon 1:1 complex are also reported and compared with the corresponding hydroquinone–argon structures. The H-bonded structures were found to possess blueshifted H bonds.
Destabilizing effect of plane Couette flow
Authors: Tatsuno T. , Yoshida Z. , Mahajan S. M.
Published: May 2003

In contrast to its well-known stabilization of the low-frequency plasma motions, a shear flow may equally effectively destabilize a class of plasma modes. The latter quality of the flow is illustrated by studying an incompressible ideal plasma with a simple velocity profile (Couette flow in a finite interval); it is found that interchange modes are driven more unstable through their interactions with the shear flow. In the presence of the flow shear, the growth rate of the perturbation increases due to the coupling of the Alfvén wave with a Rayleigh–Taylor-type instability drive. Marginally stable modes in the flowless equilibrium achieve their maximum growth rate when the maximum flow velocity becomes comparable to the Alfvén velocity. At larger shear flow velocities, however, the stabilizing “stretching” effect becomes dominant and the instability is quenched.
Shear Alfvén wave density dips in electron-positron-ion plasmas
Authors: Saleem H. , Mahmood S.
Published: May 2003

Nonlinear kinetic Alfvén waves are studied in an electron-positron-ion plasma. It is noticed that the nonlinearity and Larmor radius effects can compromise to give rise to density dips in such plasmas in the super Alfvénic regime. A comparison is presented with previous results, which predict density humps in such plasmas if ion inertial effects parallel to the external magnetic field are taken into account.
Simulated expansion of an ultra-cold, neutral plasma
Authors: Robicheaux F. , Hanson James D.
Published: May 2003

The details of recent calculations of the expansion of ultra-cold, neutral plasmas are given. The calculations are performed at several levels. The simplest level assumes an ansatz for the form of the electron and ion density; the result is a simple, ordinary differential equation which can be easily solved. The medium level of sophistication assumes the electrons are in thermal equilibrium but does not assume a particular form for the ion density; the result is a partial differential equation which is solved numerically. For the highest level of sophistication, a Monte Carlo technique is used to solve for the electron phase space distribution and solve for the ion motion in the resulting mean field. All levels of simulation include three body recombination and electron-Rydberg scattering. This paper contains the results of our simulations and compares them to measurements made on ultra-cold plasmas. Three body recombination is found to be important at very low temperatures since it is a heating mechanism for the electron gas. The collisions between cold electrons and Rydberg atoms are another important source of electron heating and de-excitation of atoms formed in the plasma. The evolution of the distribution of atoms in the plasma is simulated and several counter-intuitive effects that have been observed can be explained. Our simulations show that the plasma coupling constant does not become larger than ∼1/5 for the reported experiments. The behavior of plasma processes are investigated, e.g., ion acoustic waves, spike formation, and electron evaporation. The evolution of a cold Rydberg gas into a plasma is also simulated but certain properties of our simulation do not agree with measurements.
Two-dimensional photonic crystal at THz frequencies constructed by metal-coated cylinders
Authors: Wang Shao-Wei , Lu Wei , Chen Xiao-Shuang , Li Zhi-Feng , Shen Xue-Chu , Wen Weijia
Published: May 2003

Two-dimensional photonic crystals with photonic bandgaps (PBGs) in the terahertz (THz) frequency regime have been constructed by arraying metal-coated cylinders. PBGs were observed at 2.2 and 4.7 THz for the samples with lattice constant of 140 and 70 μm, respectively. Experimental results show that the PBGs were realized with only a small number of layers (⩽10 periods) and they are robust against positional disorder (about 14%). Thus, metal-coated cylinders provide us with a promising approach to create robust PBGs in the THz frequency range.
Magnetic properties and magneto-optical effect of Co0.5Fe2.5O4 nanostructured films
Authors: Gu B. X.
Published: May 2003

Co 0.5 Fe 2.5 O 4 ferrite films with nanostructure have been prepared by rf sputtering on a quartz substrate and annealed at temperatures from 400 to 600 °C in air. The magnetic properties and magneto-optical effect have been studied. It has been found that nanocrystalline Co0.5Fe2.5O4 films exhibit good magnetic properties and a large magneto-optical effect. The magnetization, coercivity, remanence ratio, and maximum energy product are 455 emu/cm3, 2.8 kOe, 0.72 MGOe, and 2.4 MGOe, respectively. The magneto-optical Faraday rotation is <−3.4°/μm at short wavelength and 4°/μm at the wavelength of 740 nm. These characteristics make them very promising to be used for magneto-optical recording and for permanent magnets. The influence of the distribution of Co2+ ions on magnetic properties and magneto-optical effect has been discussed.
Improved photoluminescence of pulsed-laser-ablated Y2O3:Eu3+ thin-film phosphors by Gd substitution
Authors: Bae Jong Seong , Jeong Jung Hyun , Yi Soung-soo , Park Jung-Chul
Published: May 2003

Gd-substituted Y2−xGdxO3:Eu3+ luminescent thin films have been grown on Al2O3 (0001) substrates using pulsed-laser deposition. The films grown under different deposition conditions have been characterized using microstructural and luminescent measurements. The crystallinity, surface morphology, and photoluminescence (PL) of the films are highly dependent on the amount of Gd. The PL brightness data obtained from Y2−xGdxO3:Eu3+ films grown under optimized conditions have indicated that the PL brightness is more dependent on the surface roughness than on the crystallinity of the films. In particular, the incorporation of Gd into Y2O3 lattice could induce a remarkable increase of PL. The highest emission intensity was observed with Y1.35Gd0.60Eu0.05O3, thin film whose brightness was increased by a factor of 3.1 in comparison with that of Y2O3:Eu3+ films. This phosphor has promise for application to the flat panel displays.
Exact nonlinear analytic Vlasov–Maxwell tangential equilibria with arbitrary density and temperature profiles
Authors: Mottez F.
Published: May 2003

The tangential layers are characterized by a bulk plasma velocity and a magnetic field that are perpendicular to the gradient direction. They have been extensively described in the frame of the magnetohydrodynamic (MHD) theory. But the MHD theory does not look inside the transition region if the transition has a size of a few ion gyroradii. A series of kinetic tangential equilibria, valid for a collisionless plasma is presented. These equilibria are exact analytical solutions of the Maxwell–Vlasov equations. The particle distribution functions are sums of an infinite number of elementary functions parametrized by a vector potential. Examples of equilibria relevant to space plasmas are shown. A model for the deep and sharp density depletions observed in the auroral zone of the Earth is proposed. Tangential equilibria are also relevant for the study of planetary environments and of remote astrophysical plasmas.
Transient ultrasonic guided waves in layered plates with rectangular cross section
Authors: Mukdadi Osama M. , Datta Subhendu K.
Published: May 2003

Transient ultrasonic guided waves in anisotropic layered plates with finite and infinite width are presented in this article. A semianalytical finite-element method is adopted to study the guided waves in both infinite- and finite-width elastic plates. Three-noded beam elements in the thickness direction are used in infinite plate model, whereas the cross section of the finite-width plate is represented by nine-noded quadrilateral elements. Propagation in the axial direction is modeled by analytical wave functions. Elastodynamic Green’s functions are derived using modal summation in the frequency–wave number and time–space domains. Results for dispersion and transient analysis of guided waves in infinite nickel plates are presented and compared with those of finite-width plates. Group velocities are calculated and wave arrival times are computed for different plate cross sections. Numerical results show a significant influence of the plate aspect ratio on the dispersion and transient wave response. The complex natures of mode dispersion and propagation due to several mode excitation in finite-width plates require such quantitative analysis to afford easy interpretation. These results play a role of guidance for nondestructive material evaluation.
Spectral properties of Pauli operators on the Poincaré upper-half plane
Authors: Inahama Yuzuru , Shirai Shin-ichi
Published: May 2003

We investigate the essential spectrum of the Pauli operators (and the Dirac and the Schrödinger operators) with magnetic fields on the Poincaré upper-half plane. The magnetic fields under consideration are asymptotically constant (which may be equal to zero), or diverge at infinity. Moreover, the Aharonov–Casher type result is also considered.
Richardson’s constant in inhomogeneous silicon carbide Schottky contacts
Authors: Roccaforte Fabrizio , La Via Francesco , Raineri Vito , Pierobon Roberto , Zanoni Enrico
Published: May 2003

The electrical characterization of nickel silicide Schottky contacts on silicon carbide (4H–SiC) is reported in this article. In spite of the nearly ideal behavior of the contact at room temperature (n=1.05), the electrical behavior monitored in a wide temperature range exhibited a deviation from the ideality at lower temperatures, thus suggesting that an inhomogeneous barrier has actually formed. A description of the experimental results by the Tung’s model, i.e., considering an effective area of the inhomogeneous contact, provided a procedure for a correct determination of the Richardson’s constant A**. An effective area lower than the geometric area of the diode is responsible for the commonly observed discrepancy in the experimental values of A** from its theoretical value in silicon carbide. The same method was applied to Ti/4H–SiC contacts.
Generalized magnetic helicity, large-scale magnetic field, and dynamo saturation
Authors: Milano L. J. , Matthaeus W. H. , Dmitruk P.
Published: May 2003

Dynamo effect allows the creation of large-scale magnetic fields through a purely mechanical driving. The efficiency of the process is much higher if the driving is helical. This effect has long been postulated to saturate when the self-generated large-scale magnetic field grows above a certain threshold. On the other hand, some numerical results show an apparent suppression of the dynamo effect in the presence of a strong, large-scale, externally supported magnetic field. Here, it is shown that the overall behavior in these extreme cases, as well as in intermediate cases, can be understood in terms of: (i) conservation of a generalized form of magnetic helicity; and (ii) a proper separation of scales. These results lend additional perspective to the sometimes difficult issue of the relationship between the physics of externally supported and locally supported mean magnetic fields. In particular, it is shown that: (a) the externally supported component has a predominant role in quenching dynamo action; (b) the electromotive force tends to be aligned to the sum of both components, that is to the total large-scale magnetic field.
Dynamics of high energy runaway electrons in the Frascati Tokamak Upgrade
Authors: Esposito B. , Martı́n-Solı́s J. R. , Poli F. M. , Mier J. A. , Sánchez R. , Panaccione L.
Published: May 2003

The dynamics of high energy (up to 20 MeV) runaway electrons in the Frascati Tokamak Upgrade (FTU) is investigated using a gamma-ray spectrometer system which detects photons produced when runaway electrons interact with the plasma facing components [B. Esposito et al., Nucl. Instrum. Methods 476, 522 (2002)]. Runaway electrons are usually generated during the plasma current ramp-up, accelerated to MeV energies, and contained stably during the whole discharge time, which lasts for more than one second. This time is long enough for them to reach the limiting energy that results from the balance between acceleration in the electric field, collisions with the plasma particles and synchrotron radiation losses. The maximum energy inferred from the gamma spectra is shown to be in agreement with the runaway limiting energy predicted by a test particle description of the runaway dynamics [J. R. Martı́n-Solı́s et al., Phys. Plasmas 5, 2370 (1998)]. It is found that the runaway energy behavior during the discharge is determined by the time evolution of the plasma parameters only (mainly E∥, ne and Zeff) and that the synchrotron radiation losses associated with the electron gyromotion around the magnetic field lines can explain the measured limiting runaway energy in FTU.
Magnetic field perturbations correlated with large amplitude lower-hybrid waves in a high-voltage linear plasma discharge
Authors: Popel S. I. , Elsässer K. , Takeda Y. , Inuzuka H.
Published: May 2003

The results of theoretical and experimental studies on excitation of magnetic field perturbations correlated with large amplitude lower-hybrid (LH) wave bursts in a high-voltage linear plasma discharge are presented. It is shown that the magnetic field perturbations, which are excited in the experiments mostly in the paramagnetic sense, are associated with the development of the magneto-modulational processes. The equations describing the relationship between the quasistationary magnetic field perturbations, plasma density perturbations, and the fields of LH waves are derived using the general nonlinear formalism developed for description of the modulational effects in arbitrary media. The results obtained on the basis of these equations are compared with the experimental data using the following parameters: the values of the magnitude of the magnetic field perturbations; the direction of the vector of the magnetic field perturbation; the correlation between the magnetic field perturbations and the electron density perturbations. It is shown that for LH waves propagating in one plane theoretical predictions are in a good agreement with the experimental results.
A numerical study of forced magnetic reconnection in the viscous Taylor problem
Authors: Fitzpatrick Richard
Published: May 2003

Two-dimensional, nonlinear magnetohydrodynamical simulations are used to investigate the so-called Taylor problem, in which a small amplitude boundary perturbation is suddenly applied to a tearing stable, slab plasma equilibrium—the perturbation being such as to drive magnetic reconnection within the plasma. This type of reconnection, which is not due to an intrinsic plasma instability, is generally known as “forced reconnection.” For numerical reasons, the investigation is restricted to the large magnetic Prandtl number limit. The simulation results are highly consistent with the analysis of Hahm and Kulsrud [Phys. Fluids 28, 2412 (1985)] (modified by strong plasma viscosity). At high perturbation amplitudes, the system exhibits a phase of Sweet–Parker reconnection, as predicted by Wang and Bhattacharjee [Phys. Fluids B 4, 1795 (1992)]. An expression for the threshold perturbation amplitude required to trigger Sweet–Parker reconnection is derived, and successfully benchmarked against numerical simulations. This expression suggests that a Sweet–Parker phase is only likely to occur during forced reconnection in tokamaks when the plasma is extremely hot and perturbation amplitude relatively large.
Superparamagnetism and the future of magnetic random access memory
Authors: Cowburn R. P.
Published: May 2003

The role of superparamagnetism in determining the ultimate density of magnetic random access memory is investigated. Numerical calculations show that superparamagnetism can easily be stabilized even in elements of just a few nanometers in size by the shape anisotropy generated in elements with lateral elongation. The limitation to increasing density is shown to come not directly from superparamagnetism, but from the high current densities which will be required to write data to elements that have been stabilized against superparamagnetism. Experimental measurements on planar elliptical Permalloy nanoelements show the capability of shape anisotropy to generate large stabilization fields.
Model of the tokamak edge density pedestal including diffusive neutrals
Authors: Burrell K. H.
Published: May 2003

Several previous analytic models of the tokamak edge density pedestal have been based on diffusive transport of plasma plus free-streaming of neutrals. This latter neutral model includes only the effect of ionization and neglects charge exchange. The present work models the edge density pedestal using diffusive transport for both the plasma and the neutrals. In contrast to the free-streaming model, a diffusion model for the neutrals includes the effect of both charge exchange and ionization and is valid when charge exchange is the dominant interaction. Surprisingly, the functional forms for the electron and neutral density profiles from the present calculation are identical to the results of the previous analytic models. There are some differences in the detailed definition of various parameters in the solution. For experimentally relevant cases where ionization and charge exchange rate are comparable, both models predict approximately the same width for the edge density pedestal.
Fast magnetic field penetration into a magnetized plasma with nonuniform density: A new family of shock solutions
Authors: Gomberoff K.
Published: May 2003

Fast magnetic field-penetration into almost collisionless plasmas occurs through two previously studied mechanisms. The magnetic field can penetrate as a shock wave, into plasmas of nonuniform density or in cases with magnetic field curvature. While the shock velocity is defined by the large Hall resistivity, the shock width depends on the collisional resistivity. Fast magnetic field penetration as a whistler wave occurs also into magnetized plasmas with a longitudinal field. The present study combines both mechanisms by considering a nonuniform density plasma and requiring that it is previously magnetized with longitudinal as well as transverse magnetic fields. It is shown that a new family of shock solutions exists where the shock width is unaffected by the collisional resistivity and a whistler shock precursor appears. Below some threshold for the longitudinal field (which depends on the collisional resistivity), the solution is as the previously studied shocks with no precursor, however, above this threshold, the new shock solution appears. If an additional preexisting transverse component of magnetic field exists, there is no need for collisional resistivity in order to have such shock solutions. The problem is studied for the various cases and parameters.
Conversion of trapped upper hybrid oscillations and Z mode at a plasma density irregularity
Authors: Hall J. O. , Leyser T. B.
Published: May 2003

Analytic expressions are presented for the conversion of electromagnetically driven quasi-trapped upper hybrid oscillations (i.e., trapping predicted within electrostatic approximation) and electromagnetic Z mode at a density irregularity in a magnetized plasma. This problem is of importance for the theory of plasma density irregularities excited in the ionosphere during radio wave experiments. The governing equations are solved in a slab geometry using a scale length separation method. The results are valid for arbitrary low resonances of the quasi-trapped field. The theory predicts non-Lorentzian skewed shapes of the resonances for the Z mode radiation.
A framework for the development and testing of an edge pedestal model: Formulation and initial comparison with DIII-D data
Authors: Stacey W. M. , Groebner R. J.
Published: May 2003

A framework has been formulated for the further development and testing of a predictive edge pedestal model. This framework combines models for the interaction of the various physical phenomena acting in the edge pedestal—transport, neutral fueling penetration, atomic physics cooling, MHD (magnetohydrodynamic) stability limit, edge density limit—to determine the pedestal widths and gradient scale lengths. Predictive models for some of these specific phenomena have been compared with DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] measurements. It was found that a neutral penetration model for the density width and a MHD model for the maximum pedestal pressure for stability against ideal pressure-driven surface modes were roughly consistent with experimental observation, but that in both cases some refinements are needed. The major impediments to implementation of a predictive edge pedestal model within the framework of this paper are the lack of knowledge of transport coefficients in the pedestal and the unavailability of a usable characterization of the state-of-the-art MHD stability-limit surface in the space of edge parameters. Efforts to remedy these and other deficiencies and to establish a predictive model for the calculation of density, temperature and pressure widths and gradients in the edge pedestal are suggested.
The low-field density peak in helicon discharges
Authors: Chen Francis F.
Published: May 2003

Although the density n in helicon discharges increases monotonically with magnetic field B for B larger than a few hundred gauss, as expected from theory, a pronounced density peak is often observed at B∼50 G or below. A peak in antenna loading is indeed found in computations using a fluid code as long as reflections from an endplate are taken into account. Various tests show that this peak is caused by constructive interference from the reflected wave. This effect can be used in the design of compact helicon plasma injectors. In addition, it can be the cause of density enhancements previously observed using cusped magnetic fields or aperture limiters.
Grad-B drift transport and focusing of high-current electron beams
Authors: Strasburg S. , Hinshelwood D. D. , Ottinger P. F. , Schumer J. W. , Commisso R. , Stephanakis S. , Chorny V. , Rose D. V.
Published: May 2003

This paper studies, experimentally and theoretically, the grad-B drift transport and focusing of an intense electron beam. The Naval Research Laboratory’s Gamble II generator [J. D. Shipman, Jr., IEEE Trans. Nucl. Sci. NS-18, 243 (1971)] was used to drive a 900 kV, 850 kA beam. This beam drifted in the radial magnetic field gradient produced by an on-axis guide wire carrying a current of 40 kA. An additional axial magnetic field gradient generated by focusing spokes was used to radially compress the beam. Experimental diagnostics included radially resolved calorimetry and radiachromic film to study wall losses. Extensive simulations were performed to better understand the experiments. Energy transport efficiencies were slightly below 70%, and the focusing spokes were observed to compress the beam radius by approximately 10%.
Observation of inward and outward particle convection in the core of electron cyclotron heated and current driven plasmas in the Tokamak à Configuration Variable
Authors: Furno I. , Weisen H. ,   TCV team
Published: May 2003

In the Tokamak à Configuration Variable [F. Hofmann, J.B. Lister, M. Anton et al., Plasma Phys. Controlled Fusion 36, B277 (1994)], inward or outward convection in the core of electron cyclotron heated and current driven plasmas is observed, depending on discharge conditions. In sawtoothing discharges with central electron cyclotron heating, outward convection is observed when a quasicontinuous m=1 kink mode is present, resulting in inverted sawteeth on the central electron density, while in the absence thereof, inward convection between successive sawtooth crashes leads to “normal” sawteeth. The occurrence of a kink mode depends sensitively on plasma triangularity. When sawteeth are stabilized with central co- or counterelectron cyclotron current drive, stationary hollow electron density profiles are observed in the presence of m=1 modes, while peaked or flat profiles are observed in magnetohydrodynamic quiescent discharges. The observation of peaked density profiles in fully electron cyclotron driven plasmas demonstrates that pinch processes other than the Ware pinch must be responsible for these phenomena.
Dynamics of helicity transport and Taylor relaxation
Authors: Diamond P. H. , Malkov M.
Published: May 2003

A simple model of the dynamics of Taylor relaxation is derived using symmetry principles alone. No statistical closure approximations are invoked or detailed plasma model properties assumed. Notably, the model predicts several classes of nondiffusive helicity transport phenomena, including traveling nonlinear waves and superdiffusive turbulent pulses. A universal expression for the scaling of the effective magnetic Reynolds number of a system undergoing Taylor relaxation is derived. Some basic properties of intermittency in helicity transport are examined.
Chemical images generated by large area homogeneous illumination of metal–insulator–semiconductor structures
Authors: Filippini D. , Lundström I.
Published: May 2003

A controlled large area illumination, provided by a computer screen, is used to induce distinctive chemical images in field-effect devices upon gas exposure. The present measuring method uses the concurrent optical and chemical modulation of the semiconductor surface potential of a single metal–oxide–semiconductor capacitor to generate two-dimensional photocurrent images, displayed in a bias voltage-color space. Selective patterns for hydrogen, ammonia, and propene are demonstrated. The use of a computer screen as a programable light source allows to simplify a normally complex setup, making the technique more attractive for practical applications.
Orientation dependence of piezoelectric properties of single domain 0.67Pb(Mn1/3Nb2/3)O3–0.33PbTiO3 crystals
Authors: Zhang Rui , Jiang Bei , Cao Wenwu
Published: May 2003

The orientation dependence of piezoelectric properties has been calculated for 0.67Pb(Mn1/3Nb2/3)O3–0.33PbTiO3 (PMN–33%PT) single-domain crystals based on the measured full matrix properties. It is found that the maximum d33*=2411 pC/N and k33*=0.94 occur, respectively, in directions 63.0° and 70.8° from the spontaneous polarization direction. In [001] of the cubic coordinate, the rotated properties are d33*=2316 pC/N and k33*=0.93, respectively, comparable to the measured multidomain properties. The results revealed the origin of the superior electromechanical properties in [001] poled PMN–33%PT multidomain crystals as from the large d15 of the single-domain property.
Random density inhomogeneities and focusability of the output pulses for plasma-based powerful backward Raman amplifiers
Authors: Solodov A. A. , Malkin V. M. , Fisch N. J.
Published: May 2003

Random plasma density inhomogeneities may defocus the output pulses of powerful backward Raman amplifiers (BRA). Because of ultrahigh intensities of even nonfocused BRA outputs, such distortions, if occur, are then difficult to correct. A simple expression is derived for the largest BRA length for which the output pulse focusability is not yet spoiled. Interestingly, this limitation does not depend on the pump laser intensity. A useful effect of plasma inhomogeneities that might help to suppress premature pump backscattering by thermal noise is also pointed out.
Role of laser beam geometry in improving implosion symmetry and performance for indirect-drive inertial confinement fusion
Authors: Turner R. E. , Amendt P. A. , Landen O. L. , Suter L. J. , Wallace R. J. , Hammel B. A.
Published: May 2003

The role of a high-Z radiation cavity or hohlraum in inertial confinement fusion is to convert laser energy into soft x-ray energy, in a highly spatially symmetric manner, so that a centrally located capsule containing deuterium and tritium can be uniformly imploded. In practice, however, the asymmetry introduced by the small number of high intensity laser beams can introduce significant perturbations in the drive uniformity. Experiments performed on Nova (10 beams) [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)] and Omega (using 40 beams) [J. M. Sources, R. L. McCrory, C. P. Verdon et al., Phys. Plasmas 3, 2108 (1996)] demonstrate a significant improvement in symmetry and target performance from a fourfold increase in the number of laser beams.
Optimized spectral collection efficiency obtained in diamond-based ultraviolet detectors using a three-electrode structure
Authors: Spaziani F. , Rossi M. C. , Salvatori S. , Conte G. , Ascarelli P.
Published: May 2003

A diamond detector structure for uniform and efficient collection of photogenerated carriers in the ultraviolet spectral range is presented. The device operating principle is based on the contemporary collection of both photogenerated carriers in the bulk, and photoelectrons emitted from the diamond surface. A three-electrode device structure is used, having dual side contacts on the diamond surfaces, and a separated third electrode in the vacuum. A large improvement of the detector responsivity, as well as a compensation of the efficiency losses usually occurring for highly absorbed radiation is achieved, while keeping comparable wavelength selectivity.
Fluorene-based polymer gain media for solid-state laser emission across the full visible spectrum
Authors: Xia Ruidong , Heliotis George , Bradley Donal D. C.
Published: May 2003

We report a study of the optical gain properties of three polyfluorenes with chemically tuned emission characteristics that span 400–800 nm. We demonstrate low threshold light amplification in the blue, green, and red spectral ranges via amplified spontaneous emission (ASE) in optically pumped planar asymmetric waveguides. Gain and loss measurements at the peak ASE wavelengths show large net gains, 22⩽g⩽66 cm−1, and low losses, 15⩾α⩾3 cm−1. Our findings establish fluorene-based polymers as an attractive family of materials for use in tuneable solid-state lasers that emit at wavelengths across the whole visible spectrum.
Supra-nonlinear photorefractive response of single-walled carbon nanotube- and C60-doped nematic liquid crystal
Authors: Khoo I. C. , Ding J. , Zhang Y. , Chen K. , Diaz A.
Published: May 2003

We have observed an extremely large electro-optically induced photorefractive effect in nematic liquid crystal doped with single-walled carbon nanotubes and fullerene C60. The effective refractive index change coefficient can be as large as 7 cm2/W, which is >1000 times larger than previous observations. We describe the basic mechanisms and conditions necessary for occurrence of such nonlinearities.
Noncovalent functionalization of carbon nanotubes by aromatic organic molecules
Authors: Zhao Jijun , Lu Jian Ping , Han Jie , Yang Chih-Kai
Published: May 2003

The interaction between carbon nanotubes and organic molecules including benzene (C6H6), cyclohexane (C6H12), and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ: C8N2O2Cl2) have been studied using first principles calculations. The equilibrium tube-molecule distance, adsorption energy, and charge transfer are obtained. The hybridization between the DDQ molecular level and nanotube valence bands transforms the semiconducting tube into a metallic one. Coupling of π electrons between tubes and aromatic molecules are observed. Our results show that noncovalent functionalization of carbon nanotubes by aromatic molecules is an efficient way to control the electronic properties of carbon nanotubes.
High magnetic field sensor using LaSb2
Authors: Young D. P. , Goodrich R. G. , DiTusa J. F. , Guo S. , Adams P. W. , Chan Julia Y. , Hall Donavan
Published: May 2003

The magnetotransport properties of single crystals of the highly anisotropic layered metal LaSb2 are reported in magnetic fields up to 45 T with fields oriented both parallel and perpendicular to the layers. Below 10 K the perpendicular magnetoresistance of LaSb2 becomes temperature independent and is characterized by a 100-fold linear increase in resistance between 0 and 45 T with no evidence of quantum oscillations down to 50 mK. The Hall resistivity is hole-like and gives a high field carrier density of n∼3×1020 cm−3. The feasibility of using LaSb2 for magnetic field sensors is discussed.
Transient-enhanced Si diffusion on native-oxide-covered Si(001) nanostructures during vacuum annealing
Authors: Lichtenberger H. , Mühlberger M. , Schäffler F.
Published: May 2003

We report on the transient-enhanced shape transformation of nanostructured Si(001) surfaces upon in vacuo annealing at relatively low temperatures of 900–950 °C for a few minutes. We find dramatic surface mass transport concomitant with the development of low-energy facets on surfaces that are covered by native oxide. The enhanced surface mass transport ceases after the oxide is completely desorbed, and it is also not observed on surfaces where the native oxide had been removed by HF before annealing.