bluuebunny/crossref_metadata_2025 huggingface dataset:
Solutions of the plasma equilibrium within the magnetic field of a point dipole
Authors: Tur A. V. , Maurice S. , Blanc M. , Yanovsky V. V.
Published: July 2002

For axial symmetry, in this study we report on analytical solutions of the magnetohydrodynamics (MHD) equilibrium of a plasma within the magnetic environment of a point dipole. Inside the plasma domain, the magnetic flux function is calculated for a given shape of the pressure profile. Another magnetic flux function is derived outside the plasma domain, where the pressure vanishes. Both functions and their normal derivatives are continuous through the boundary between the two domains; together they constitute the solution of our problem. A pressure profile that is a quadratic function of the magnetic flux function is used. It leads to an exact solution which is a simple function of the spherical coordinates. It is shown that, in this situation, the plasma energy is distributed over the entire plasma domain. This solution is well suited as a magnetospheric model. The magnetic flux function determined outside the plasma domain is expressed by a very general formula that is valid for wide classes of both the pressure profiles and the shape of a plasma exterior boundary. Far away from this boundary, as near the coordinate center, the magnetic field tends to be dipolar.
Neutral diffusion and anomalous effects on ion flow shear
Authors: Fülöp T. , Catto Peter J. , Helander P.
Published: July 2002

Bulk ion flow can be modified by ion–neutral interactions in the edge region of tokamaks where neutral atoms are abundant. In this region, the standard neoclassical expression for the ion flow is not consistent with the experimental observations. Previous work in the plateau regime [Valanju et al., Phys. Fluids B 4, 2675 (1992)] suggests that the discrepancy is caused mainly by momentum loss due to ion–neutral charge exchange. In the present work we show that in the absence of neutral diffusion, charge exchange cannot affect the neoclassical transport of the ions significantly if the neutral mean-free path is short and the neutral density is low compared to the plasma density. However, the inclusion of neutral diffusion and anomalous processes in the plateau regime can substantially modify parallel ion flow inside the separatrix as determined by the flux surface averaged momentum constraint. Moreover, the local shear in the flow may be determined by a balance between neutral diffusion and anomalous effects which may then modify the turbulent transport.
The broad bandwidth paradigm for stimulated Raman scattering in laser–plasma interactions
Authors: Rose Harvey A.
Published: July 2002

The experimentally observed bandwidth of backscattered stimulated Raman scattered (SRS) light may be large enough to overwhelm the bandwidth of the induced Langmuir wave turbulence. In this broad bandwidth regime it is possible to simply relate the spatial variation of the reflectivity to its spectral density at the SRS matching frequency, without further assumption as to the nature of the turbulence. It is suggested that this regime is a useful starting point for modeling the self consistent nonlinear plasma response to SRS, and towards this end a novel stochastic quasi-linear Markovian model is introduced.
Model simulations of Langmuir wave turbulence induced by the Raman instability
Authors: Rose Harvey A.
Published: July 2002

One and two dimensional simulations of a variant of Zakhavov’s equations of nonlinear Langmuir wave dynamics, driven by a model source owing to Stimulated Raman backscatter (SRS) at wavenumber k1, yield constitutive relations between the energy injection rate, 〈ε〉, from SRS, and various local Langmuir wave turbulence properties, such as the mean spatial and spectral energy densities and effective damping rate, νeff. In the inertial regime, where the Langmuir wave damping is significant only for k>k1, it is found that these relations are insensitive to the source bandwidth, ν, if ν⩾νeff, and are well represented by power laws in 〈ε〉. Approximate scaling relations of νeff with the ion acoustic damping rate coefficient, νia, show that the turbulence is over-stable with respect to the Langmuir decay instability, as determined by νia and νeff, by a factor of order unity.
Wavelength scaling of high-intensity illumination of an exploded foil
Authors: Cobble J. A. , Johnson R. P. , Mason R. J.
Published: July 2002

A preformed plasma simulating the corona of a fast ignitor target has been probed with 527 nm and 1054 nm lasers exceeding the critical power for ponderomotive electron cavitation. For both colors, the f number of the probe beam is increased as it propagates through the plasma. Transmission of the diffraction-limited beams is higher for the green than for the 1 μm case.
Tertiary proton diagnostics in future inertial confinement fusion experiments
Authors: Cremer S. , Verdon C. P. , Petrasso R. D.
Published: July 2002

Recently, it was proposed to use energetic (up to 31 MeV) tertiary protons produced during the final stage of inertial confinement fusion implosions to measure the fuel areal density of compressed deuterium–tritium (DT). The method is based on seeding the fuel with He3. The reaction of He3 ions with the energetic knock-on deuterons, produced via the elastic scattering of 14.1 MeV neutrons, is a source of very energetic protons capable of escaping from very large areal density targets. This work presents results of detailed time-dependent Monte Carlo simulations of the nuclear processes involved in producing and transporting these protons through imploding targets proposed for direct-drive experiments on OMEGA [D. K. Bradley et al., Phys. Plasmas 5, 1870 (1998)] and the National Ignition Facility [S. W. Haan et al., Phys. Plasmas 2, 2480 (1995)].
Geomagnetic pulsation generation as a result of whistler wave scattering
Authors: Yukhimuk Vyacheslav , Roussel-Dupre Robert , Symbalisty Eugene
Published: July 2002

Nonlinear scattering of whistler mode waves by kinetic Alfvén waves (KAWs) is considered. The evolution of whistler mode wave decay instability in time and two spatial dimensions is studied under the approximation of two-fluid magnetohydrodynamics. It is shown that efficient coupling between whistler waves and KAWs is possible due to the finite Larmor radius effect for Alfvén waves. The considered process can lead to significant enhancement of KAWs. The nonlinear scattering of whistler waves by KAWs is proposed as a possible mechanism of enhancement and generation of geomagnetic pulsations in the magnetosphere and ionosphere of the Earth.
Probe measurements of electron energy distributions in a strongly magnetized low-pressure helium plasma
Authors: Demidov V. I. , Ratynskaia S. V. , Armstrong R. J. , Rypdal K.
Published: July 2002

Electron energy distributions (EED) in strongly magnetized (∼0.3 T), low-pressure (∼0.2 Pa) helium plasmas of the toroidal device “Blaamann” [K. Rypdal et al., Plasma Phys. Controlled Fusion 36, 1099 (1994)] have been measured. In the analysis of measurements, one applies a simplified expression for the limit of a strongly magnetized plasma relating the electron energy distribution to the first derivative of electron probe current with respect to the probe potential. It is shown that for the conditions investigated this approximation gives the same electron densities and slightly lower temperatures (up to 10%) as the kinetic theory for arbitrary magnetic field strength. Cylindrical probes, which are oriented along and perpendicular to the magnetic field, are used in the measurements. It is shown that these probes give nearly identical results. However, the probe that is oriented perpendicular to the magnetic field can give the electron energy distribution in a wider energy range and with better accuracy. The cross-field diffusion coefficient of electrons near the probe is estimated and shown to be classical.
One-dimensional full wave treatment of mode conversion process at the ion–ion hybrid resonance in a bounded tokamak plasma
Authors: Monakhov I. , Bécoulet A. , Fraboulet D. , Nguyen F.
Published: July 2002

A consistent picture of the mode conversion (MC) process at the ion–ion hybrid resonance in a bounded plasma of a tokamak is discussed, which clarifies the role of the global fast wave interference and cavity effects in the determination of the MC efficiency. This picture is supported by simulations with the kinetic code “VICE” [Fraboulet et al., “One-D full-wave description of plasma emission and absorption in the Ion Cyclotron Range of Frequency in tokamaks,” submitted to Phys. Plasmas]. The concept of the “global resonator,” formed by the R=n∥2 boundary cutoffs [Saoutic et al., Phys. Rev. Lett. 76, 1647 (1996)], is justified, as well as the importance of a proper tunneling factor choice ηcr=0.22 [Ram et al., Phys. Plasmas 3, 1976 (1996)]. The MC scheme behavior appears to be very sensitive to the MC layer position relative to the global wave field pattern. Optimal MC regimes are found to be attainable without requirement of a particular parallel wave number choice.
Finite-beta equilibria for Wendelstein 7-X configurations using the Princeton Iterative Equilibrium Solver code
Authors: Arndt S. , Merkel P. , Monticello D. A. , Reiman A. H.
Published: July 2002

Fixed- and free-boundary equilibria for Wendelstein 7-X (W7-X) [W. Lotz et al., Plasma Physics and Controlled Nuclear Fusion Research 1990 (Proc. 13th Int. Conf. Washington, DC, 1990), (International Atomic Energy Agency, Vienna, 1991), Vol. 2, p. 603] configurations are calculated using the Princeton Iterative Equilibrium Solver (PIES) [A. H. Reiman et al., Comput. Phys. Commun., 43, 157 (1986)] to deal with magnetic islands and stochastic regions. Usually, these W7-X configurations require a large number of iterations for PIES convergence. Here, two methods have been successfully tested in an attempt to decrease the number of iterations needed for convergence. First, periodic sequences of different blending parameters are used. Second, the initial guess is vastly improved by using results of the Variational Moments Equilibrium Code (VMEC) [S. P. Hirshmann et al., Phys. Fluids 26, 3553 (1983)]. Use of these two methods have allowed verification of the Hamada condition and tendency of “self-healing” of islands has been observed.
Lagrangian formulation for neutrino–plasma interactions
Authors: Brizard Alain J. , Wurtele Jonathan S.
Published: July 2002

A Lagrangian formalism is used to derive coupled nonlinear equations for collective interactions between an intense neutrino flux and a relativistic cold plasma fluid with multiple particle species. In order to focus on the self-consistent (collective) treatment of neutrino–plasma interactions, quantum effects are ignored throughout and the (spinless) neutrinos are represented by a complex-valued Klein–Gordon scalar field. Through the application of Noether’s method, the conservation laws for energy, momentum and wave action are derived explicitly. The transfer of energy, momentum and wave action between the neutrinos and the electromagnetic-plasma is discussed in the context of astrophysical applications (e.g., type II supernova explosions).
On the nature of chaotic regions in dissipative hydrodynamics and magnetohydrodynamics
Authors: Titov Vyacheslav S. , Priest Eric R. , Lonie David P.
Published: July 2002

A region with chaotic magnetic field lines where the magnetic field (B) and plasma velocity (v) are continuous and differentiable and satisfy the dissipative incompressible magnetohydrodynamic equations with magnetic diffusivity η and kinematic viscosity ν is considered. It is proved then that if v×B and (∇×v)×v are potential, the structurally stable solutions describing such chaotic regions are characterized by a decaying linear magnetic force-free field and Beltrami flow of the form B=B0 exp(−α2ηt)b, v=v0 exp(−α2νt)b, where b=b(r) such that ∇×b=αb, ∇⋅b=0 and B0, v0, and α are constants. Purely hydrodynamic flows are a particular case with B0=0. A simple example of a chaotic force-free field is also constructed.
Nonlinear evolution of broad-bandwidth, laser-imprinted nonuniformities in planar targets accelerated by 351-nm laser light
Authors: Smalyuk V. A. , Boehly T. R. , Bradley D. K. , Goncharov V. N. , Delettrez J. A. , Knauer J. P. , Meyerhofer D. D. , Oron D. , Shvarts D. , Srebro Y. , Town R. P. J.
Published: July 2002

Planar, 20 and 40 μm thick CH targets have been accelerated by 351 nm laser beams of the OMEGA laser system [Opt. Commun. 133, 495 (1997)]. Different beam-smoothing techniques were employed including distributed phase plates, smoothing by spectral dispersion, and distributed polarization rotators. The Rayleigh–Taylor evolution of three-dimensional (3D) broadband planar-target perturbations seeded by laser nonuniformities was measured using x-ray radiography at ∼1.3 keV. Fourier analysis shows that the perturbations evolve to longer wavelengths and the shorter wavelengths saturate. The saturation amplitudes and rates of growth of these features are consistent with the predictions of Haan [Phys. Rev. A 39, 5812 (1989)].
Longitudinal collective modes of strongly coupled dusty plasmas at finite frequencies and wavevectors
Authors: Murillo M. S.
Published: July 2002

Dusty plasmas offer a unique method for testing dynamical wave theories in the strong Coulomb coupling regime. Recently, there have been many theoretical models, based on either a generalized hydrodynamic or on kinetic descriptions, developed to describe dust acoustic waves under conditions of strong coupling. These theories attempt to extend the usual acoustic wave dispersion relation to the strong coupling regime and, in some cases, to finite frequencies and wave vectors beyond the hydrodynamic limit. Here a multicomponent kinetic theory is used to obtain the dust acoustic wave dispersion relation in terms of an approximate dynamic local field correction, and comparison is made with the viscoelastic Navier–Stokes description.
Eikonal scattering cross section for electron–ion elastic collision in uniformly magnetized dense plasmas
Authors: Jung Young-Dae
Published: July 2002

Eikonal approximation is applied to investigate elastic electron–ion collisions in uniformly magnetized dense plasmas. Plasma screening effects on eikonal scattering cross sections for electron–ion elastic collisions in uniformly magnetized plasmas are also investigated. The electron–ion interaction potential in uniformly magnetized plasmas has been obtained by the introduction of the longitudinal plasma dielectric function. The straight-line trajectory method is applied to the path of the projectile electron in order to investigate the variation of the eikonal phase as a function of the impact parameter and plasma parameters. The eikonal differential elastic cross section substantially decreases with the decrease of the ratio of the projectile velocity (v) to the electron thermal velocity (vT). For a given velocity ratio, the eikonal cross section is decreased with the increase of the ratio of the electron thermal frequency (ωT) to the Larmor frequency (ωB). It is also found that the position of the maximum eikonal cross section is shifted to the ion core with the decrease in the velocity ratio (v/vT).
Legendre polynomial expansion method for evaluating multipole moments of current density in toroidal devices
Authors: Seo Seong-Heon , Kim Jay , Huh S. H. , Choe W. , Chang Hong-Young , Jeong Seung-Ho
Published: July 2002

It is found that multipole moments of toroidal current are given by the coefficients of the Legendre polynomial expansion of the magnetic field on a meridian contour. Using this fact and the orthogonality of the Legendre polynomials, a method is proposed for evaluating the moments from magnetic field measurements through an open-contour. As an application, exact expressions of the first few order moments and the current center position are formulated. Results show that this method is applicable to any aspect ratio tokamaks without the limitation of small displacement of the current center.
A novel charged-particle diagnostic for compression in inertial confinement fusion targets
Authors: Radha P. B. , Skupsky S. , Petrasso R. D. , Soures J. M.
Published: July 2002

A new technique for diagnosing compression in multiple regions of inertial confinement fusion targets is discussed. This diagnostic uses knock-on deuterons and protons that have been elastically scattered by 14.1 MeV deuterium–tritium (DT) fusion neutrons. The target is composed of three different materials: DT gas contained in a plastic shell overcoated by deuterated plastic. The effect on the knock-on deuteron spectrum of mixing of these layers from hydrodynamic instabilities is also discussed.
Mean square displacement in small-scale nonlinear dynamos
Authors: Kim Eun-jin
Published: July 2002

The stretching property of an underlying fluid is studied in a simplified statistical model of small-scale nonlinear dynamos, via the evolution of the mean square displacement. The nonlinear effect of small-scale magnetic fields is consistently incorporated in the limit of low kinetic Reynolds number. The mean square displacement between two neighboring particles, as well as that of a single particle, is shown to be reduced due to the Lorentz force associated with small-scale magnetic fields. This reduction is suggested to lead to the suppression of stretching of magnetic field lines, and subsequently to the saturation of the growth of small-scale magnetic fields.
Thermal transport barrier in heliotron-type devices (Large Helical Device and Compact Helical System)
Authors: Ohyabu N. , Fujisawa A. , Ashikawa N. , deVries P. , Emoto M. , Funaba H. , Goto M. , Hamada Y. , Iguchi H. , Ida K. , Itoh K. , Isobe M. , Idei H. , Inagaki S. , Inoue N. , Ikeda K. , Kado S. , Kubo S. , Kumazawa R. , Khlopenkov K. , Kaneko O. , Kawahata K. , Komori A. , Kobuchi T. , Lee S. , Liang Y. , Minami T. , Miyazawa J. , Masuzaki S. , Matsuoka K. , Morisaki T. , Morita S. , Murakami S. , Mutoh T. , Muto S. , Narihara K. , Nagayama Y. , Nakamura Y. , Nakanishi H. , Nishimura K. , Nishimura S. , Noda N. , Nomura I. , Notake T. , Okamura S. , Ohdachi S. , Ohkubo K. , Oka Y. , Osakabe M. , Ozaki T. , Peterson B. J. , Pavlichenko R. O. , Sagara A. , Sakakibara S. , Sakamoto R. , Sasao M. , Sato K. , Sanuki H. , Saito K. , Sato M. , Seki T. , Shimizu A. , Shimozuma T. , Shoji M. , Suzuki H. , Sudo S. , Sasao H. , Takeiri Y. , Tanaka K. , Toi K. , Tokuzawa T. , Tsumori K. , Tsuzuki K. , Takechi M. , Tamura N. , Torii Y. , Yamada I. , Yamada H. , Yamaguchi S. , Yamazaki K. , Yokoyama M. , Yoshimura Y. , Yamamoto S. , Watanabe K. Y. , Watanabe T. , Watari T. , Motojima O. , Fujiwara M.
Published: July 2002

In the discharges of the Large Helical Device [O. Motojima et al., Proceedings of the 16th Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 3, p. 437], a significant enhancement of the energy confinement has been achieved with an edge thermal transport barrier, which exhibits a sharp gradient at the edge. Key features associated with the barrier are quite different from those seen in tokamaks (i) almost no change in particle (including impurity) transport, (ii) a gradual formation of the barrier, (iii) a very high ratio of the edge temperature to the average temperature, (iv) no edge relaxation phenomenon. In the electron cyclotron heating (ECH) heated discharges in the Compact Helical System [K. Matsuoka et al., in Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, France, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 411], the internal electron transport barrier has been observed, which enhances the central electron temperature significantly. High shear of the radial electric field appears to suppress the turbulence in the core region and enhance the electron confinement there.
Plasma equilibria in dipolar magnetic configurations
Authors: Krasheninnikov S. I. , Catto Peter J. , Hazeltine R. D.
Published: July 2002

The effects of finite plasma pressure and pressure anisotropy, toroidal rotation and gravity on the equilibrium, flow, and stability of plasma in dipolar magnetic configurations are considered. Dipolar equilibria are of interest for magnetic confinement experiments in the laboratory and understanding the physics of magnetospheric and astrophysical plasmas. It is demonstrated that realistic solutions of the appropriate ideal magnetohydrodynamics (MHD) equations can be found in a separable form which drastically simplifies the equations and even allows us to analytically obtain some limiting forms of the nonlinear solutions. The MHD stability of these equilibria is explicitly evaluated in some cases.
Production of enhanced pressure regions due to inhomogeneities in inertial confinement fusion targets
Authors: Goldman S. R. , Barnes Cris W. , Caldwell S. E. , Wilson D. C. , Batha S. H. , Grove J. W. , Gittings M. L. , Hsing W. W. , Kares R. J. , Klare K. A. , Kyrala G. A. , Margevicius R. W. , Weaver R. P. , Wilke M. D. , Dunne A. M. , Edwards M. J. , Graham P. , Thomas B. R.
Published: July 2002

It is shown that regions of enhanced pressure have been produced in targets with indirect radiation drive in planar and cylindrically convergent geometry through the interaction between the flows caused by target inhomogeneities and the main target drive. Design calculations for National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] targets with beryllium ablators formed by bonded hemi-shells [D. C. Wilson et al., Bull. Am. Phys. Soc. 43, 1667 (1998)] indicate that related behavior produces a seed perturbation in the ablator which can in some cases lead to the suppression of ignition. From simulation and analysis of the NIF problem in the planar geometry analog, a scaling for the perturbation, which should be useful for validation of the behavior with lower energy drive and smaller-scale geometries, is derived.
Collective neutrino–plasma interactions
Authors: Silva L. O. , Bingham R. , Dawson J. M. , Mendonça J. T. , Shukla P. K.
Published: July 2002

The theoretical framework for the study of nonlinear neutrino–plasma interactions is presented. The quasiclassical single neutrino dynamics is examined in detail, demonstrating the features giving rise to plasma instability scenarios, and establishing analogies with electron and photon dynamics in a plasma. Using the relativistic kinetic equations for the neutrinos coupled with the plasma, the classic nonlinear collisionless plasma processes are examined; the neutrino streaming instability, and neutrino Landau damping. The astrophysical consequences of collective neutrino–plasma interactions in supernovas and neutron stars will also be discussed.
Electron beam–target interaction and spot size stabilization in flash x-ray radiography
Authors: Kwan Thomas J. T. , Snell Charles M. , Christenson Peggy J.
Published: July 2002

A high current relativistic electron beam incident on a high-Z target to produce bremsstrahlung photons for radiographic applications can be subjected to charge neutralization by target plasma ion production due to energy deposition by the electron beam. This partial charge neutralization can lead to premature focusing of the electron beam at a distance away from the target and subsequent radial divergence. Furthermore, as the ion column continues to expand, the focal point moves upstream along the path of the electron beam, causing the beam spot on the target to grow in time. The increase in radiation spot size is detrimental to the spatial resolution of radiographic images. The ion effects were confirmed via particle-in-cell simulations and analysis, and methods were investigated to suppress the growth of the electron beam spot size in single- and multiple-pulse radiographic applications. The concept of a self-biased target was proposed and validated by computer simulation showing that the electron beam can be used in a configuration to establish an electric potential between the target and the collimator. This potential can effectively trap the ions, limit the ion column length, and thereby maintain the electron beam spot size. Another approach is the placement of a thin metallic foil at 1–2 cm in front of the target, which serves as a barrier to the ions but is essentially transparent to the incoming electron beam. Our study also showed that optimized confinement of plasma ions with the electromagnetic or the mechanical method can provide an additional ion-focusing effect which leads to a desirable further reduction of the beam spot size.
Comment on “On the existence of Alfvénic solitary waves” [Phys. Plasmas 6, 4778 (1999)]
Authors: Shukla P. K. , Stenflo L.
Published: July 2002

It is argued that Seyler and Lysak [Phys. Plasmas 6, 4778 (1999)] have reached an ambiguous conclusion with regard to the existence of planar dispersive Alfvénic solitary waves in a uniform magnetoplasma. The properties of dispersive inertial and kinetic Alfvén waves are re-examined, and shortcomings of Seyler and Lysak’s work are pointed out.
Stellarator coil optimization by targeting the plasma configuration
Authors: Boozer Allen H.
Published: July 2002

Modern stellarators are designed by choosing the shape of the plasma to optimize its physics properties. Unfortunately practical magnetic-field coils cannot precisely support a plasma that has the shape prescribed by the optimization. A method will be given for optimizing plasmas so the only plasma shapes that are considered are the ones that are consistent with efficient, well-separated coils. The method provides a clear prescription for finding the minimal number of independent coils needed for the flexibility to produce many important plasma configurations. The number of free parameters that exist for optimizing stellarator configurations will be determined. About thirty parameters in stellarator design are of comparable difficulty for coils to produce as the four shape parameters that are used in defining tokamaks through squareness.
Measurement of the characteristic x ray of oxygen and other ultrasoft x rays using mercuric iodide detectors
Authors: Iwanczyk J. S. , Dabrowski A. J. , Huth G. C. , Economou T. E.
Published: July 2002

This letter reports the detection and resolution of the characteristic x ray of oxygen at 523 eV and other ultrasoft x rays (photons energy less than 1 keV) using radiation detectors fabricated from the compound semi-insulator mercuric iodide (HgI2). These detectors are capable of operation at room ambient but in these experiments were slightly cooled using a Peltier element to 0 °C. A pulsed light feedback preamplifier with a Peltier element cooled (to −30 °) first stage field-effect transistor was used to amplify signals from the detector. Overall system noise level was 185 eV (full width at half-maximum) limited by the temperature of the first stage field-effect transistor. With optimal cooling of this element the characteristic x ray of carbon at 282 eV should be measurable. These results would seem to be important in measurement of biological samples in electron column instruments.
Theoretical and experimental capacitance-voltage behavior of Al0.3Ga0.7As/GaAs modulation-doped heterojunctions: Relation of conduction-band discontinuity to donor energy
Authors: Norris George B. , Look D. C. , Kopp W. , Klem J. , Morkoç H.
Published: July 2002

For the first time, we show that the capacitance-voltage behavior of modulation-doped heterojunctions may be accurately described by a first-principles theory that includes self-consistent quantum two-dimensional (2-D) electron subbands in the GaAs, numerical solution of Poisson’s equation for band bending and space charge in the (Al,Ga) As, and series resistance in the 2-D channel and heterointerface. The excellent agreement found between the theory and measurements on selected high-quality Al0.3Ga0.7As/GaAs heterojunctions allows accurate determination of the maximum 2-D carrier concentration. From this, we find a strong relationship between the conduction-band discontinuity and donor binding energy, giving offsets of 76 and 66% of the direct gap discontinuity for binding energies of 66 and 30 meV, as derived from published data.
Monolithic integration of GaAs light-emitting diodes and Si metal-oxide-semiconductor field-effect transistors
Authors: Ghosh Ruby N. , Griffing Bruce , Ballantyne Joseph M.
Published: July 2002

A monolithic optoelectronic circuit, consisting of a GaAs light-emitting diode (LED) driven by a Si metal-oxide-semiconductor (MOS) transistor, has been fabricated. Light output as a function of applied gate voltage was measured. The LED’s were fabricated in GaAs layers on Ge-coated Si substrates containing MOS transistors. Normal transistor performance was observed after the GaAs LED fabrication, indicating that GaAs and Si processing technologies appear to be compatible.
Improvement of the bulk laser damage threshold of potassium dihydrogen phosphate crystals by ultraviolet irradiation
Authors: Yokotani A. , Sasaki T. , Yoshida K. , Yamanaka T. , Yamanaka C.
Published: July 2002

Potassium dihydrogen phosphate (KDP) crystals were grown under the irradiation of ultraviolet light. The bulk laser damage threshold was improved to two to three times (15–20 J/cm2) compared to the case of crystals grown by conventional methods. Microbes such as germs and bacteria are frequently generated in the KDP solution with the usual growth method. The ultraviolet light reduces or eliminates organic materials such as microbes or their carcasses incorporated into the crystal, which are the cause of low damage threshold.
High brightness red electroluminescence in CaS:Eu thin films
Authors: Tanaka K. , Mikami A. , Ogura T. , Taniguchi K. , Yoshida M. , Nakajima S.
Published: July 2002

The crystallinity and luminescent property were studied on evaporated CaS:Eu thin films. The orientation of the film strongly depends on substrate temperature. Distinct orientation was not observed in films deposited below 600 °C. Above 600 °C, the films tend to have a (111) orientation. The red emitting electroluminescent device with CaS:Eu active layer prepared at 680 °C which is strongly oriented to the [111] direction shows the brightness of 180 cd/m2 under 1 kHz alternate excitation.
Improved tribological properties of sputtered MoSx films by ion beam mixing
Authors: Kobs K. , Dimigen H. , Hübsch H. , Tolle H. J. , Leutenecker R. , Ryssel H.
Published: July 2002

Ion beam mixing yielded a distinct enhancement in the sliding life of sputtered MoSx films without any deterioration of the excellent lubrication properties. This effect occurs only at higher ion energies indicating an improved film-substrate adherence caused by a mixing of the interface, which was confirmed by secondary ion mass spectrometry and topographical investigations. In addition to that a considerable enhancement of the film density was found due to a reorientation of the MoSx platelets which led also to an improvement of the effective film thickness.
Plasma sprayed Fe76Nd16B8 permanent magnets
Authors: Overfelt R. A. , Anderson C. D. , Flanagan W. F.
Published: July 2002

Thin coatings (0.16 mm) and thick coatings (0.50 mm) of Fe76Nd16B8 were deposited on stainless-steel substrates by low pressure plasma spraying. Microscopic examination of the coatings in a light microscope revealed excessive porosity, but good bonding to the substrate. Fracture cross sections examined in a scanning electron microscope showed the grains to be equiaxed and approximately 1 μm or less in diameter in the as-sprayed condition. The intrinsic coercivities of the as-sprayed coatings varied from 5.8 to 10.9 kOe. The effects of post-spray heat treatments on the intrinsic coercivity are also given.
Hydrogen abstraction from hydrogenated amorphous silicon surface by hydrogen atoms
Authors: Muramatsu Yasuji , Yabumoto Norikuni
Published: July 2002

HD formation reaction was investigated in H2 plasma over deuterium-incorporating hydrogenated amorphous silicon (a-Si:H:D). The activation energy of the reaction was determined to be 2.4±0.7 kcal mol−1, which was consistent with the abstraction reaction of hydrogen from some silane derivatives by hydrogen atoms. The HD formation reaction could be explained by the hydrogen abstraction reaction. The hydrogen abstraction from a-Si:H surface by H atoms is thought to be one of the elementary growing processes of an a-Si:H in a SiH4-H2 plasma.
Low-energy ion extraction with small dispersion from an electron cyclotron resonance microwave plasma stream
Authors: Matsuoka Morito , Ono Ken’ichi
Published: July 2002

The effects of a magnetic field gradient on the energy profiles of ions extracted from an electron cyclotron resonance (ECR) microwave plasma stream are studied. The mean energy and energy dispersion of the ions strongly depend on the spatial distribution of the magnetic field strength. Ions with an energy dispersion of several eV are extracted from the plasma by the mirror field gradient application. Consequently, the magnetic field gradient, i.e., the magnetic flux profile, is one of the most important parameters in ECR microwave plasma processes.
Fundamental processes affecting recovery in hydrogen thyratrons
Authors: Braun C. G. , Erwin D. A. , Gundersen M. A.
Published: July 2002

Experimental measurements in the positive column of wall-confined high-current hydrogen thyratron discharges show a pronounced increase in atomic hydrogen excited state populations after the end of the current pulse. The decay rate of the electron and excited state population is observed to decrease as the energy flux increases. A time-dependent collisional-radiative model is used to calculate electron and excited state densities. This model is in reasonable agreement with experimental measurements and explains the afterpulse behavior. The analysis shows that the coupling between electron and atom temperatures is an important mechanism in high-power thyratron recovery. A new method using laser-induced fluorescence to obtain time-resolved Stark broadening data for electron density measurements is presented.
Direct measurement of evaporation during rapid thermal processing of capped GaAs
Authors: Haynes T. E. , Chu W. K. , Picraux S. T.
Published: July 2002

We have directly measured the amounts of arsenic evaporated through very thin encapsulating films during rapid thermal processing (RTP). The evaporated molecules are condensed onto a copper film on a sapphire substrate and subsequently counted using 5 MeV Rutherford backscattering analysis. For 20-nm-thick Si, SiO2, and Si3N4 caps, we have determined the temperature ranges for which a 10-s RTP cycle produces less than 1×1015/cm2 of evaporated As. Preliminary measurements at higher temperatures of the thickness and time dependence of evaporation through SiO2 caps of 20 to 60 nm thicknesses are also presented and discussed in terms of the mechanism of As loss.
Importance of heavy ion bombardment for H− formation in surface conversion sources
Authors: van Os C. F. A. , van Amersfoort P. W.
Published: July 2002

The interaction between the plasma and the converter in a surface conversion negative ion source is studied. The converter consists of a porous tungsten button, through which liquid cesium diffuses towards the surface. This scheme results in a low cesium density in the discharge. Xe+ and Ar+ ions are used to simulate the sputtering effect of Cs+ ion bombardment of the converter. The extracted H− beam energy profile and the conversion efficiency appeared to depend strongly on the heavy ion flux to the converter surface. The total H− current increases with an order of magnitude with increasing heavy ion flux. A maximum current density of 8 mA/cm2 of H− ions has been extracted.
Ledistor—a three-terminal double heterostructure optoelectronic switch
Authors: Taylor G. W. , Mand R. S. , Simmons J. G. , Cho A. Y.
Published: July 2002

A three-terminal double heterostructure optoelectronic switching (DOES) device is demonstrated. By making ohmic contact to the active region of the DOES device the switching characteristic may be controlled up to the punchthrough limit. The device emits light in the on state only and various combinations of voltage and optical input power can be used to switch the device.
Time-of-flight study on the thermal etching of Al with Cl2
Authors: Janssen R. J. A. A. , Kolfschoten A. W. , van Veen G. N. A.
Published: July 2002

A pseudorandom cross-correlation technique has been used to study the thermal etching of aluminum with molecular chlorine. Time-of-flight (TOF) distributions of particles desorbing from the substrate are measured by modulating the ejected product beam. Modulation is achieved by a rotating chopping disk with slots in a pseudorandom sequence on its periphery. TOF distributions and temperature-dependent reaction product yields are measured for the products desorbing from the chlorinated aluminum substrate in the temperature range from 300 to 900 K. A reaction mechanism will be discussed in some detail.
Light-induced degradation at the silicon/silicon dioxide interface
Authors: Gruenbaum P. E. , Sinton R. A. , Swanson R. M.
Published: July 2002

Single-crystal silicon point-contact solar cells show a degradation in their efficiency after being exposed to concentrated sunlight. This change has been linked to an increase in the surface recombination velocity. A similar effect is produced by carrier injection under forward bias. The annealing kinetics, the role of ultraviolet light, and possible causes for the creation of surface states are discussed.
Laser evaporation deposition of superconducting and dielectric thin films
Authors: Kwok H. S. , Mattocks P. , Shi L. , Wang X. W. , Witanachchi S. , Ying Q. Y. , Zheng J. P. , Shaw D. T.
Published: July 2002

We describe here the deposition of superconducting and dielectric thin films by the laser evaporation technique. The characterization of this process, and possible optimization with regards to wavelength and pulse duration of the laser will be discussed.
Gel behavior of keV ion irradiated polystyrene
Authors: Calcagno L. , Foti G. , Licciardello A. , Puglisi O.
Published: July 2002

Among the chemical and physical modifications induced by ion bombardment of polymers, the solubility changes are very important because of technological application for lithography in microelectronic devices. Solubility changes due to the occurrence of crosslinkings have been followed on monodisperse and polydisperse polystyrene after ion irradiations (1011–1014 ions/cm2, keV energy). By using the Inokuty gel theory [M. Inokuti J. Appl. Phys. 38, 2999 (1963)], the chemical yield (crosslinking/eV) has been determined for different molecular weights and molecular weight distributions.
Golden eagles, feral pigs, and insular carnivores: How exotic species turn native predators into prey
Authors: Roemer Gary W. , Donlan C. Josh , Courchamp Franck
Published: July 2002

Island ecosystems are particularly vulnerable to exotic species. Here we show how an introduced prey has led to the wholesale restructuring of an island food web, including the near extinction of an endemic carnivore. Introduced pigs, by providing abundant food, enabled golden eagles to colonize the California Channel Islands. Eagles preyed heavily on the island fox, whose resulting decline toward extinction released populations of the competitively inferior island skunk. The presence of exotic pigs led to major ecosystem shifts by indirectly causing predation to replace competition as the dominant force shaping these island communities.
Alternatively spliced
N
resistance gene transcripts: Their possible role in tobacco mosaic virus resistance
Authors: Dinesh-Kumar S. P. , Baker Barbara J.
Published: July 2002

The
N
gene, a member of the Toll-IL-1 homology region–nucleotide binding site–leucine-rich repeat region (LRR) class of plant resistance genes, encodes two transcripts,
N
S
and
N
L
, via alternative splicing of the alternative exon present in the intron III. The
N
S
transcript, predicted to encode the full-length N protein containing the Toll-IL-1 homology region, nucleotide binding site, and LRR, is more prevalent before and for 3 hr after tobacco mosaic virus (TMV) infection. The
N
L
transcript, predicted to encode a truncated N protein (N
tr
) lacking 13 of the 14 repeats of the LRR, is more prevalent 4–8 hr after TMV infection. Plants harboring a cDNA-
N
S
transgene, capable of encoding an N protein but not an N
tr
protein, fail to exhibit complete resistance to TMV. Transgenic plants containing a cDNA-
N
S
-bearing intron III and containing 3′
N
-genomic sequences, encoding both
N
S
and
N
L
transcripts, exhibit complete resistance to TMV. These results suggest that both
N
transcripts and presumably their encoded protein products are necessary to confer complete resistance to TMV.
Recombinatoric exploration of novel folded structures: A heteropolymer-based model of protein evolutionary landscapes
Authors: Cui Yan , Wong Wing Hung , Bornberg-Bauer Erich , Chan Hue Sun
Published: July 2002

The role of recombination in evolution is compared with that of point mutations (substitutions) in the context of a simple, polymer physics-based model mapping between sequence (genotype) and conformational (phenotype) spaces. Crossovers and point mutations of lattice chains with a hydrophobic polar code are investigated. Sequences encoding for a single ground-state conformation are considered viable and used as model proteins. Point mutations lead to diffusive walks on the evolutionary landscape, whereas crossovers can “tunnel” through barriers of diminished fitness. The degree to which crossovers allow for more efficient sequence and structural exploration depends on the relative rates of point mutations versus that of crossovers and the dispersion in fitness that characterizes the ruggedness of the evolutionary landscape. The probability that a crossover between a pair of viable sequences results in viable sequences is an order of magnitude higher than random, implying that a sequence's overall propensity to encode uniquely is embodied partially in local signals. Consistent with this observation, certain hydrophobicity patterns are significantly more favored than others among fragments (i.e., subsequences) of sequences that encode uniquely, and examples reminiscent of autonomous folding units in real proteins are found. The number of structures explored by both crossovers and point mutations is always substantially larger than that via point mutations alone, but the corresponding numbers of sequences explored can be comparable when the evolutionary landscape is rugged. Efficient structural exploration requires intermediate nonextreme ratios between point-mutation and crossover rates.
The mechanism of catalysis of the chorismate to prephenate reaction by the
Escherichia coli
mutase enzyme
Authors: Hur Sun , Bruice Thomas C.
Published: July 2002

Molecular dynamics studies of the
Escherichia coli
chorismate mutase (EcCM), containing at the active site chorismate and in turn the transition state (TS), have been performed. The simulations show that TS is not bound any tighter than chorismate. Comparison of average polar interactions show they are virtually identical for interactions of EcCM with chorismate and the TS, whereas hydrophobic interactions with TS are much weaker than with chorismate. Interactions and the mechanism of catalysis of chorismate → prephenate by the EcCM enzyme are discussed.
Real-time and equilibrium19F-NMR studies reveal the role of domain–domain interactions in the folding of the chaperone PapD
Authors: Bann James G. , Pinkner Jerome , Hultgren Scott J. , Frieden Carl
Published: July 2002

PapD is a periplasmic chaperone essential for P pilus formation in pyelonephritic strains ofE. coli. It is composed of two domains, each of which contains a tryptophan residue (Trp-36 and Trp-128, in the N- and C-terminal domains, respectively). To explore the role of domain–domain interactions during folding, the protein was labeled with 6-fluorotryptophan for use in19F-NMR experiments.19F-NMR data collected as a function of urea concentration revealed the presence of a resonance caused by Trp-128 that was distinct from either the folded or unfolded resonances. The time course of refolding from urea was monitored by stopped-flow fluorescence, CD, and19F-NMR, each method showing multiple kinetic phases. The19F-NMR stopped-flow spectra, collected at 70 μM of protein with a fluorine cryoprobe, demonstrated that the intermediate was populated early in the folding process (<5 s). The slow disappearance of the intermediate and unfolded resonance occurred at the same rate as the appearance of the native resonances of both domains. The data are consistent with a model in which the C-terminal domain collapses rapidly to an intermediate, whereas the stabilization of the final structure is slow and requires folding of the N-terminal domain with concomitant readjustment of the C-terminal domain structure.
Overexpression of the N-terminal domain of TSG101 inhibits HIV-1 budding by blocking late domain function
Authors: Demirov Dimiter G. , Ono Akira , Orenstein Jan M. , Freed Eric O.
Published: July 2002

Efficient budding of HIV-1 from the plasma membrane of infected cells requires the function of a 6-kDa protein known as p6. A highly conserved Pro-Thr-Ala-Pro (PTAP) motif (the “late” or “L” domain), is critical for the virus-budding activity of p6. Recently, it was demonstrated that the product of tumor susceptibility gene 101 (TSG101), which contains at its N terminus a domain highly related to ubiquitin-conjugating (E2) enzymes, binds HIV-1 Gag in a p6-dependent fashion. We examined the impact of overexpressing the N-terminal region of TSG101 on HIV-1 particle assembly and release. We observed that this domain (referred to as TSG-5′) potently inhibits virus production. Examination of cells coexpressing HIV-1 Gag and TSG-5′ by electron microscopy reveals a defect in virus budding reminiscent of that observed with p6 L domain mutants. In addition, the effect of TSG-5′ depends on an intact p6 L domain; the assembly and release of virus-like particles produced by Gag mutants lacking a functional p6 PTAP motif is not significantly affected by TSG-5′. Furthermore, assembly and release of murine leukemia virus and Mason–Pfizer monkey virus are insensitive to TSG-5′. TSG-5′ is incorporated into virions, confirming the Gag/TSG101 interaction in virus-producing cells. Mutations that inactivate the p6 L domain block TSG-5′ incorporation. These data demonstrate a link between the E2-like domain of TSG101 and HIV-1 L domain function, and indicate that TSG101 derivatives can act as potent and specific inhibitors of HIV-1 replication by blocking virus budding.
Revised estimates of human cochlear tuning from otoacoustic and behavioral measurements
Authors: Shera Christopher A. , Guinan John J. , Oxenham Andrew J.
Published: July 2002

We develop an objective, noninvasive method for determining the frequency selectivity of cochlear tuning at low and moderate sound levels. Applicable in humans at frequencies of 1 kHz and above, the method is based on the measurement of stimulus-frequency otoacoustic emissions and, unlike previous noninvasive physiological methods, does not depend on the frequency selectivity of masking or suppression. The otoacoustic measurements indicate that at low sound levels human cochlear tuning is more than twice as sharp as implied by standard behavioral studies and has a different dependence on frequency. New behavioral measurements designed to minimize the influence of nonlinear effects such as suppression agree with the emission-based values. A comparison of cochlear tuning in cat, guinea pig, and human indicates that, contrary to common belief, tuning in the human cochlea is considerably sharper than that found in the other mammals. The sharper tuning may facilitate human speech communication.