bluuebunny/crossref_metadata_2025 huggingface dataset:
Diamagnetism in a plasma
Authors: Cole K. D.
Published: July 2002

This is an extension of a study of electric currents and force balance orthogonal to a unidirectional magnetic field of induction (B) in a collisionless plasma [K. D. Cole, Phys. Plasmas 3, 2717 (1996)]. All quantities are assumed to vary only in one direction orthogonal to B. Two new concepts are introduced, which are not in conventional plasma physics. First a magnetic polarization effect corresponding to electric polarization is found. This defines a contribution to the magnetic moment per unit volume of −ρmc2E2/B3. Second the conduction current is defined in terms of the curl of the magnetic field intensity H, in accordance with Maxwell. In this scheme, when there is no electric field, (1/4π)(∂H/∂z)B+(∂p⊥/∂z)=0, instead of the conventional equation with B/μ0 in place of H. In both schemes, j×B−∇p⊥=0. Though this makes no difference to the electric current intensity calculated in the two schemes, and no difference to the force balance equation, it makes a significant difference to the relationship between B and p⊥ in many plasma conditions of interest. When the electric field is nonzero, the situation is more complex. It is shown that the new diamagnetism of this paper and the dielectric current of the earlier one are consistent with special relativity. Some applications of the theory are discussed.
Forbidden regions in ion temperatures for ion acoustic compressive solitary waves for a plasma with two electron temperatures
Authors: Ghosh S. S. , Sekar Iyengar A. N.
Published: July 2002

It is observed that, there exist certain forbidden regions in ion temperatures, for a compressive solitary wave, which do not have any solitary wave solution. These regions exist for a two- as well as a single-electron temperature plasma. A physical model is proposed to explain this phenomenon on the basis of the energy relation. It is revealed that a one-dimensional single warm ion fluid consists of two distinct energy components. A simple algebraic condition has been derived which expresses the balance between nonlinearity and dispersion for a fully nonlinear solitary wave. It shows that both the nonlinearity and dispersion decrease with increasing ion temperature and that the balance condition is well maintained for the whole spectrum of solitary wave solutions.
Thermal equilibrium of warm clouds of charge with small aspect ratio
Authors: Paulson Deborah L. , Spencer Ross L.
Published: July 2002

Global thermal equilibrium computations are presented for non-neutral plasmas whose radial size is much larger than their axial thickness. Axial and radial density profiles are computed for both ideal and nonideal Penning trap fields. Simple results are obtained in the limits of both low and high central density. Comparison is made to the grid calculations of Mason et al. [Phys. Plasmas 3 (5), 1502 (1996)].
Equations describing strong laser field in interaction with matter within a plasma in the hydrodynamic limit
Authors: Lehner Thierry
Published: July 2002

General equations proving useful in the context of strong electron photon interactions are derived. Such interactions are driven by launching short and powerful laser pulses on a material medium. Relativistic nonlinear equations are obtained from the system of fluid and Maxwell equations through a suitable perturbative approach into three main parameters: q (energy parameter), δ (time parameter), and α (spatial parameter). Numerous phenomena are recovered—such as relativistic self-focusing, low-frequency electric and magnetic wake fields generation, the creation of pulse frequency harmonics, nonlinear Compton scattering—while their generating sources are specified. But also new processes are identified, arising from possible couplings among the above quoted effects. Particular emphasis is put on the coupling of self-induced magnetic field with its generating pump, leading itself to the possible magnetic relativistic guiding of light.
Extended theory of interface localized modes
Authors: Lortz D. , Spies G. O.
Published: July 2002

The theory of resistive free boundary modes localized at the plasma–vacuum interface in a plane slab equilibrium is improved and extended. If one proceeds to sufficiently small wavelengths, then the stability criterion (the current density vector must not have a component along the magnetic field vector in the interface) remains unchanged, but the unstable eigenmodes become independent of resistivity, and their growth rates diverge like the inverse square root of the wavelength.
Properties of the tearing mode in periodic current sheets
Authors: Shen C. , Liu Z. X.
Published: July 2002

The tearing mode in multiple periodic current sheet systems with a hyperbolic tangent profile of magnetic shear has been investigated. It is found that the linear growth rate of the antisymmetric and symmetric mode tearing instability increases and decreases, respectively, with the current sheet separation decreasing, which is similar to the situation with the step function profile of current density [A. Otto and G. T. Birk, Phys Fluids B 4, 3811 (1992)]. But, the results here also show that the linear growth rate of the antisymmetric mode tearing instability maximizes at certain current sheet separation. The linear growth of the tearing instability in multiple current sheet systems is not enhanced as greatly as has usually been expected.
Analytic model for the dynamics of fast ignition
Authors: Piriz A. R. , Sánchez M. M.
Published: July 2002

A relatively simple model for the dynamics of fast ignitors is presented. The model describes the processes taking place when a precompressed fuel is heated by a particle beam (ions or electrons) characterized by its range R, and the focal spot and intensity of the beam. As a result, the ignition conditions are calculated in terms of such parameters and of the fuel density. The model provides scaling laws for the ignition energy and for the beam intensity in which the dependence on the particles range is included. Besides, it gives a simple interpretation of reported simulations and explains the apparent discrepancies between them and previous analytic estimations. The importance of using particles with R⩽0.3 g/cm2 turns out to be from the dependence of the ignition energy and the target gain on the range.
Studies of global stability of field-reversed configuration plasmas using a rigid body model
Authors: Ji H. , Yamada M. , Kulsrud R. , Pomphrey N. , Himura H.
Published: July 2002

Global stability of field-reversed configuration (FRC) plasmas has been studied using a simple rigid body model in the parameter space of s (the ratio of the separatrix radius to the average ion gyro-radius) and plasma elongation E (the ratio of the separatrix length to the separatrix diameter). Tilt stability is predicted, independent of s, for FRC’s with low E (oblate), while the tilt stability of FRC’s with large E (prolate) depends on s/E. It is found that plasma rotation due to ion diamagnetic drift can stabilize the tilt mode when s/E≲1.7. The so-called collisionless ion gyro-viscosity also is identified to stabilize tilt when s/E≲2.2. Combining these two effects, the stability regime broadens to s/E≲2.8, consistent with previously developed theories. A small additional rotation (e.g., a Mach number of 0.2) can improve tilt stability significantly at large E. A similar approach is taken to study the physics of the shift stability. It is found that radial shift is unstable when E<1 while axial shift is unstable when E>1. However, unlike tilt stability, gyro-viscosity has little effect on shift stability.
A self-consistent theory of the folded waveguide klystron
Authors: Uhm Han S.
Published: July 2002

A self-consistent nonlinear theory of the folded-waveguide klystron is developed based on nonlinear current modulation of an electron beam propagating through a drift tube that is periodically connected to bends of the waveguide. The theory developed here can also apply to multiple-cavity klystrons. A closed integrodifferential equation for the current modulation is obtained in terms of time and propagation distance. This equation includes information of all the energy modulations initiated at the gaps by electric fields of microwaves propagating in the waveguide. Properties of the folded-waveguide klystron are investigated, assuming that the energy modulations introduced at each gap are sinusoidal functions with the same amplitude. The power transfer from the beam to microwave is described in terms of the propagation distance and phase shift αn of the energy modulations launched at gaps. It is shown that the phase difference between the current modulation and the induced voltage at gap is essential for efficient power transfer from the beam’s kinetic energy to microwave field energy. The optimum value of the phase shift for maximum power transfer from the beam to microwaves occurs at αn=−2.07, which may be useful for design of the folded-waveguide klystron.
Theory of asymmetry-induced transport in a non-neutral plasma
Authors: Eggleston D. L. , O’Neil T. M.
Published: July 2002

Radial transport produced by static nonaxisymmetric fields is thought to limit the confinement of non-neutral plasmas and experiments with applied asymmetries have verified that such fields do produce transport. A theoretical model of such transport is presented which is appropriate for long, thin plasmas. The theory allows for asymmetries with nonzero frequency and includes the linear collective response to applied wall voltages. For the regime where the effective collision frequency is large, the asymmetry-induced radial particle flux is derived from the drift kinetic/Poisson equations including collisions. For low collision frequencies a heuristic derivation is given. In both regimes the resulting transport is dominated by particles that move in resonance with the asymmetry. Possible applications of the theory to several experiments are discussed.
Nonlinear dynamics of Kelvin–Helmholtz unstable magnetized jets: Three-dimensional effects
Authors: Keppens R. , Tóth G.
Published: July 2002

A numerical study of the Kelvin–Helmholtz instability in compressible magnetohydrodynamics is presented. The three-dimensional simulations consider shear flow in a cylindrical jet configuration, embedded in a uniform magnetic field directed along the jet axis. The growth of linear perturbations at specified poloidal and axial mode numbers demonstrate intricate nonlinear coupling effects. The physical mechanisms leading to induced secondary Kelvin–Helmholtz instabilities at higher mode numbers are identified. The initially weak magnetic field becomes locally dominant in the nonlinear dynamics before and during saturation. Thereby, it controls the jet deformation and eventual breakup. The results are obtained using the Versatile Advection Code [G. Tóth, Astrophys. Lett. Commun. 34, 245 (1996)], a software package designed to solve general systems of conservation laws. An independent calculation of the same Kelvin–Helmholtz unstable jet configuration using a three-dimensional pseudospectral code gives important insights into the coupling and excitation events of the various linear mode numbers.
One-dimensional plasma expansion into a vacuum in the field of an electromagnetic wave
Authors: Ivlev A. V. , Fortov V. E.
Published: July 2002

The exact analytical solution is obtained for the one-dimensional problem of an underdense collisionless plasma expansion into a vacuum in the field of an electromagnetic (EM) wave. It is shown that the high-frequency pressure of the EM wave results in significant acceleration of the plasma expansion. The problem of an overdense plasma expansion is also solved using the perturbation method.
Investigation of tritium pathways in the Joint European Torus (JET) tokamak
Authors: Hillis D. L. , Hogan J. T. , Andrew P. , Coad J. P. , Ehrenberg J. K. , Groth M. , Houlberg W. A. , von Hellermann M. , Horton L. D. , Matthews G. F. , Monk R. D. , Morgan P. D. , Stamp M. F. , Zastrow K-D.
Published: July 2002

The neutral tritium concentration in the subdivertor region of JET [P.-H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 (1985)] is measured during deuterium-to-tritium changeover experiments with a novel species-selective Penning gauge coupled to a high-resolution spectrometer. The subdivertor measurements, when compared with edge and strike point values, are a sensitive characterization of the status of the wall saturation. The neutral transport code (EIRENE) [D. Reiter, Forschungszentrum, Juelich: Report Juel-2599 (1992)] and a wall hydrogen trapping and diffusion code (WDIFFUSE) [J. Hogan, R. Maingi, P. Mioduszewski et al., J. Nucl. Mater. 241, 612 (1997)] evaluate the wall tritium recycling coefficients (RT) and compare them with quantitative, testable models for the dynamic exchange between recycling surfaces and the edge/pedestal region. Using the dynamic inventory model, tritium transport following the injection of trace amounts of tritium has been investigated for JET high-confinement mode (H-mode) discharges with Edge Localized Modes (ELMs). Upon explicitly treating the ELMs as instantaneous magnetohydrodynamic events, residual (intra-ELM) radial diffusivities are reduced and can be compared with neoclassical levels.
Intense laser pulse propagation and channel formation through plasmas relevant for the fast ignitor scheme
Authors: Mackinnon A. J. , Borghesi M. , Gaillard R. , Malka G. , Willi O. , Offenberger A. A. , Pukhov A. , Meyer-ter-Vehn J. , Canaud B. , Miquel J. L. , Blanchot N.
Published: July 2002

Measurements of self-channeling of picosecond laser pulses due to relativistic and ponderomotive expulsion effects have been obtained in preformed plasmas at laser irradiances between 5–9×1018 Wcm−2. The self-focused channel was surrounded by a multi-megagauss magnetic field. The orientation of the field was consistent with a forward going relativistic electron beam propagating along the laser pulse. Self-channeling and magnetic field generation mechanisms were modeled by multidimensional particle-in-cell (PIC) simulations and good agreement was obtained with the experimental observations. Measurements of the channel expansion after the interaction were obtained and the rate of expansion was consistent with a blast wave solution. The level of transmission of an intense laser pulse through such performed density channels was observed to increase significantly compared to the case without a channel. High levels of transmission of an intense laser pulse through microtubes were also observed. The relevance of these results to the fast ignitor is discussed.
Recent progress in the simulation of heavy ion beams
Authors: Haber I. , Friedman A. , Grote D. P. , Lund S. M. , Kishek R. A.
Published: July 2002

Production of electric power by using a beam of heavy ions to ignite an inertially-confined fusion target requires the focusing of high-power beams onto a small spot several meters distant from the final lens system. Beams with the necessary intensity generally behave like warm nonneutral bounded plasmas where beam kinetic temperatures are sufficiently high that a cold-plasma description can be inadequate for describing the collective space-charge modes. In view of the complexity of the self-consistent nonlinear dynamics, analytic study has largely been limited to the singular Kapchinskij–Vladimirskij (K–V) distribution. Numerical simulations, primarily using the WARP [D. P. Grote, A. Friedman, I. Haber, W. Fawley, and J. L. Vay, Nucl. Instrum. Methods Phys. Res. A 415, 428 (1998)] particle-in-cell (PIC)/accelerator code, have been employed to identify the degree to which the analytic results, especially the predictions of unstable modes, are applicable to realistic beam distributions. During extensive benchmarking of the code against experiments at Lawrence Berkeley National Laboratory, Lawrence Livermore National Laboratory, and the University of Maryland, a particular feature of the beam which has been seen in both experiment and simulation is the launching, in the source region, of collective warm-plasma oscillations similar to those predicted on the basis of the K–V analysis.
Effect of impurity ions on the charge separation and velocity shear at a plasma edge
Authors: Shoucri M. , Pohn E. , Bertrand P. , Knorr G. , Kamelander G. , Manfredi G. , Ghizzo A.
Published: July 2002

The problem of the formation of charge separation in a plasma in the presence of a steep density gradient, the self-consistent electric field and the associated E×B velocity, are studied using a two-dimensional (2D) gyro-kinetic Vlasov code for the ions, with electrons following an adiabatic law. The code shows the formation of a one-dimensional (1D) equilibrium charge at the plasma edge. It is also shown that the presence of a small fraction of impurity ions at the plasma edge can have a significant effect in increasing the effective charge separation and the associated electric field. The present results show that only a kinetic code can solve the problem of the equilibrium electric field in the presence of a density gradient, and point to the important role played by the ions’ gyro-radius in establishing a charge separation at a plasma edge.
Temporal behavior of backscattering instabilities in a three-dimensional cylindrical hot spot: II. Modified decay regime and user guide
Authors: Divol Laurent , Mounaix Philippe
Published: July 2002

The space and time behavior of backscattering instabilities in a three-dimensional cylindrical hot spot of finite lifetime is computed analytically in the modified decay regime. A uniform expression for the instantaneous backscattered power, valid for any time and any hot spot intensity in the modified decay regime, is proposed. It is shown that the finite transverse size of the hot spot leads to important diffraction effects that can significantly reduce the hot spot backscattered energy. A “user guide” is provided for an easy use of this model whatever the instantaneous regime of the instability: Standard decay, modified decay, and nonlinearly saturated (the hot spot depletion being taken into account heuristically). All the parameters needed in this model are given in physical units for both stimulated Brillouin scattering and stimulated Raman scattering.
Photon gas in a relativistic magnetoplasma
Authors: Tsintsadze N. L. , Hakimi Pajouh H. , Tsintsadze L. N. , Mendonca J. T. , Shukla P. K.
Published: July 2002

A novel kinetic equation for the spectral function of high-frequency relativistically intense electromagnetic (EM) waves (photon gas) in the presence of a strong magnetic field in a plasma is derived. The spectral function is defined in eight-dimensional coordinates comprising the wave vector and frequency spaces. The nonlinear dynamics of the photon gas for the fully relativistic case is studied by means of the newly derived kinetic equation, and the equations describing the slow motion of plasma under the action of the photon ponderomotive force are obtained. A dispersion relation for relativistic self-modulation of the photon gas with an arbitrary amplitude of the EM waves is derived and new branches for the modulational instability are found. The kinetic equation also yields the fluid equations for the photon gas.
Effect of edge turbulence on threshold of multifaceted asymmetric radiation from the edge instability in tokamaks
Authors: Goswami Rajiv , Singh Raghvendra , Kaw Predhiman
Published: July 2002

The radiative condensation instability believed to be responsible for the occurrence of multifaceted asymmetric radiation from the edge (MARFE) in tokamaks has been investigated in the presence of a background turbulence of saturated drift resistive ballooning modes (DRBM). These modes are a natural source for edge fluctuations in tokamaks. For the m=1 MARFE, it has been shown that a modest level of DRBM turbulence can significantly lower the threshold radiation rate required for excitation of MARFEs, enhance its growth rate, and introduce a real part to the mode frequency. Some of these results are consistent with observations of onset of MARFEs in recent experiments on Joint European Torus (JET) [Proceedings of the 17th European Conference on Controlled Fusion and Plasma Heating, Amsterdam (European Physical Society, Petit-Lancy, 1990), Vol. 14b-1, p. 339] and Tokamak Experiment for Technology Oriented Research (TEXTOR) [Phys. Rev. Lett. 71, 1549 (1993)].
Collimation mechanism of magnetohydrodynamic jets based on helicity and cross-helicity dynamos, with reference to astronomical jets
Authors: Yoshizawa Akira , Yokoi Nobumitsu , Kato Hirofumi
Published: July 2002

A collimation mechanism of magnetohydrodynamic jets is sought on the basis of the turbulent-dynamo theory within the framework of fluid incompressibility. The momentum diffusion giving rise to the broadening of the jets is suppressed through the cross-helicity effect, whereas the diffusion of the magnetic field is hampered owing to the helicity effect. These two effects lead to the collimation of electrically conducting jets in the presence of turbulent fluctuations of the velocity and magnetic field. The relevance to collimated astronomical jets is discussed with the reservation about effects of fluid compressibility.
Direct modulation and active mode locking of ultrahigh speed GaAlAs lasers at frequencies up to 18 GHz
Authors: Lau K. Y. , Yariv A.
Published: July 2002

It is demonstrated that an ultrahigh speed window buried heterostructure GaAlAs laser fabricated on a semi-insulating substrate can be used as a narrowband signal transmitter in the Ku band frequency range (12–20 GHz). The modulation efficiency can be increased over a limited bandwidth by a weak optical feedback. A stronger optical feedback enables one to actively mode lock the laser diode at a very high repetition rate up to 17.5 GHz, producing pulses ∼12 ps long.
Photoreflectance characterization of interband transitions in GaAs/AlGaAs multiple quantum wells and modulation-doped heterojunctions
Authors: Glembocki O. J. , Shanabrook B. V. , Bottka N. , Beard W. T. , Comas J.
Published: July 2002

The optical modulation technique of photoreflectance (PR) has been applied to characterize the interband transitions in GaAs/AlGaAs multiple quantum wells (MQW) and modulation-doped heterojunctions at room temperature. The spectra of the MQW show ‘‘derivativelike’’ reflectance features due to allowed interband transitions from heavy and light hole subbands to conduction subbands, and the E0(Γ8,v→Γ6,c) transitions of the AlGaAs layers. Our data are consistent with a square well calculation using a conduction-band offset of 60% of the band-gap discontinuity. For modulation-doped heterojunctions, a correlation is observed between a PR feature approximately 18 meV above the GaAs fundamental gap and the presence of a two-dimensional electron gas.
Gain and absorption measurements of electron beam pumped, high Kr concentration KrF gas mixtures
Authors: Salesky E. T. , Kimura Wayne D.
Published: July 2002

Gain and out-of-band absorption measurements are made of electron beam pumped KrF laser mixtures (Ar diluent) with 4–99.6% Kr concentrations. The average pump rate is maintained at ≊120 kW/cm3 by adjusting the total pressure of the mixtures to compensate for the different electron stopping powers of Ar and Kr. The F2 number density is also kept constant at 8.9×1016 cm−3. Measured small-signal gains (g0−α) are 3.8%/cm at 4% Kr, 3.3%/cm at 50% Kr, and 3.2%/cm at 99.6% Kr. The out-of-band absorptions (261 nm) are 0.43%/cm at 4% Kr, 0.74%/cm at 50% Kr, and 0.87%/cm at 99.6% Kr. Our computer model agrees with the results and predicts in-band absorptions (248 nm) of 0.29%/cm at 4% Kr, 0.44%/cm at 50% Kr, and 0.52%/cm at 99.6% Kr.
CO2 laser-induced chemical vapor deposition of titanium silicide films
Authors: West Gary A. , Gupta Arunava , Beeson Karl W.
Published: July 2002

We have developed a CO2 laser-induced chemical vapor deposition (CVD) process to deposit films of titanium silicide from a gaseous mixture of SiH4 and TiCl4. Such films are suitable for gate electrodes, interconnects, and contacts on present and future generations of very large scale integrated circuits. Films deposited at a 400 °C substrate temperature are amorphous and have a resistivity of 300 μΩ cm. Annealing at 800 °C converts the films to polycrystalline TiSi2 with a resistivity of 20 μΩ cm. The initial film composition can be varied by changing the SiH4/TiCl4 gas ratio. The CO2 laser induces thermal chemical reactions in the CVD reactor. Observed gas phase reaction products are those predicted by thermodynamics.
X-ray determination of dislocation density in epitaxial ZnCdTe
Authors: Qadri Syed B. , Dinan J. H.
Published: July 2002

Widths of double-crystal x-ray rocking curves for epitaxial layers of the ternary alloy ZnCdTe are found to correlate with lattice parameter misfit between layer and substrate. Estimates of dislocation density in the layers are made using these widths. Values thus obtained are found to be in good agreement with values reported for other materials with similar misfit.
Low-resistance nonalloyed ohmic contacts to Si-doped molecular beam epitaxial GaAs
Authors: Kirchner P. D. , Jackson T. N. , Pettit G. D. , Woodall J. M.
Published: July 2002

We have found evidence that the surface depletion charge density in molecular beam epitaxial n-GaAs doped heavily with Si approaches the Si concentration. In situ metallization of the as-grown surface of GaAs uniformly doped with Si at 1×1020 cm−3 yields a specific contact resistivity of 1.3 μΩ cm2, indicating a space-charge density about equal to the silicon density despite a measured bulk electron density of 4×1018 cm−3. This contact resistivity is among the lowest for nonalloyed ohmic contacts to n-GaAs. We attribute the large discrepancy between surface space-charge density and bulk electron density to the amphoteric behavior of silicon in GaAs. Surface Fermi-level pinning and arsenic stabilization create a surface depletion region where donor site selection predominates, whereas the extrinsic electron density in the bulk causes self-compensation.
Optical behavior of the U band in relation to EL2 and EL6 levels in boron-implanted GaAs
Authors: Samitier J. , Morante J. R. , Giraudet L. , Gourrier S.
Published: July 2002

The majority-carrier optical emission cross section and the concentrations of the levels induced by boron implantation at 1010 ions/cm2 in GaAs have been obtained usng a new optical isothermal capacitance transient spectroscopy in the photon energy range 0.8–1.4 eV. The experimental optical data obtained at different annealing temperatures show only the presence of the EL2 and EL6 electron trap levels. Moreover, they are in complete agreement with the existence of an interaction between these two deep trap levels. Besides, this interaction has been found to be at the origin of U band in the deep level transient spectroscopy (DLTS) spectra and of the anomalous dependence on the annealing temperature of the level concentrations deduced from the optical technique and DLTS spectra. off
Excitonic photoluminescence linewidths in AlGaAs grown by molecular beam epitaxy
Authors: Reynolds D. C. , Bajaj K. K. , Litton C. W. , Yu P. W. , Klem J. , Peng C. K. , Morkoç H. , Singh Jasprit
Published: July 2002

The linewidths of excitonic transitions were measured in AlxGa1−xAs, grown by molecular beam epitaxy as a function of alloy composition x for values of x≲0.43 using high resolution photoluminescence spectroscopy at liquid helium temperature. The values of the linewidths thus measured are compared with the results of several theoretical calculations in which the dominant broadening mechanism is assumed to be the statistical potential fluctuations caused by the components of the alloy. An increase in the linewidth as a function of x is observed which is in essential agreement with the prediction of the various theoretical calculations. The linewidths of the excitonic transitions in AlxGa1−xAs observed in the present work are the narrowest ever reported in the literature, for example σ=2.1 meV for x=0.36, thus indicating very high quality material.
Effect of organic contaminants on the oxidation kinetics of silicon at room temperature
Authors: Licciardello Antonino , Puglisi Orazio , Pignataro Salvatore
Published: July 2002

The oxidation kinetics of HF-etched n- and p-doped silicon in air at room temperature have been studied by electron spectroscopy for chemical analysis. No great differences have been found between the n- and p-type oxidation kinetics at the low doping level of the studied samples. The rate of oxide growth on the HF-etched surface is much lower than that on a silicon surface obtained by fracture in air of a silicon monocrystal. The behavior of a silicon sample fractured in de-ionized water and then oxidized in air at room temperature is intermediate. The above findings have been interpreted on the basis of surface reactions involving the plasticizers of the HF and water containers. These reactions produce carbon-rich hydrophobic surfaces which retard the silicon oxide growth. A mechanism for the involved surface reactions is proposed.
Effect of surface preparation on elastic precursor decay in shocked pure lithium fluoride
Authors: Tunison K. S. , Gupta Y. M.
Published: July 2002

To understand the mechanisms for elastic precursor decay in pure 〈100〉 lithium fluoride (LiF) crystals under shock loading we have examined the role of large dislocation densities produced near the crystal surfaces by sample preparation. After unsuccessfully trying various methods to produce flat and undamaged samples, we chose to harden the surfaces by inward diffusion of magnesium fluoride (MgF2). A combination of heat treatments and other procedures was developed to ensure hardened or doped surfaces with a maximum depth of 600 μm and an undoped interior. Sample characterization was carried out using dislocation etch pit, hardness, and electron microprobe measurements. A 0.24-mm sample, with MgF2 completely diffused through, gave a 16.7-kbar precursor amplitude. A 2.93-mm sample with a front diffusion layer of 0.14–0.39 mm and a back diffusion layer of 0.00–0.32 mm and undoped interior gave a precursor amplitude of 2.1 kbar. These experiments indicate that the surface damage layer does not control precursor decay in pure LiF crystals.
Zone refining and enhancement of solid phase epitaxial growth rates in Au-implanted amorphous Si
Authors: Jacobson D. C. , Poate J. M. , Olson G. L.
Published: July 2002

Epitaxial crystallization of Au-implanted amorphous Si layers has been studied over the temperature range 515–735 °C. During crystallization, Au is zone refined into the remaining amorphous layer, resulting in an Au concentration that increases as the layer becomes thinner. The rate of solid phase epitaxy increases rapidly with Au concentration over the range from 0.15 to approximately 0.50 at. %. At higher concentrations the rate enhancement diminishes and above 0.70 at. % severe retardation of epitaxy is observed to occur.
Photoreflectance of GaAs/GaAlAs multiple quantum wells: Topographical variations in barrier height and well width
Authors: Parayanthal P. , Shen H. , Pollak Fred H. , Glembocki O. J. , Shanabrook B. V. , Beard W. T.
Published: July 2002

Using photoreflectance at room temperature we have evaluated the topographical variations in quantum level transitions of a GaAs/GaAlAs multiple quantum well (220 Å/150 Å) due to changes in barrier height and quantum well width. The spatial resolution of the measurement was about 100 μm. A key feature of our analysis is the ability to fit the electromodulation spectra by a third-derivative functional form line shape factor. We can detect barrier height changes of several millielectron volts and variations in well width as small as 2 Å.
Optically induced switching in a p-channel double heterostructure optoelectronic switch
Authors: Taylor G. W. , Mand R. S. , Simmons J. G. , Cho A. Y.
Published: July 2002

The switching time of a double heterostructure optoelectronic switch is investigated using an optical input. A switch-on time of 5–10 ps is obtained using a discrete microwave package. The fall time of 10 ns is limited almost totally by the parasitic capacitance of the package. The switching operation shows the unique ability to turn on and off with the incident optical signal.
Steady-state photocarrier grating technique for diffusion length measurement in photoconductive insulators
Authors: Ritter D. , Zeldov E. , Weiser K.
Published: July 2002

A new simple technique for the determination of the diffusion length in photoconductive insulators is presented. A steady-state photocarrier grating is created by two interfering laser beams, and the magnitude of the secondary photocurrent perpendicular to the grating fringes is measured. The measurement is then repeated when the two beams are incoherent. From a determination of the two photocurrents as a function of grating period the diffusion length of the photocarriers can be obtained. The method can yield accurate results to 5% of the laser wavelength.
InGaAsP/InP quantum well modulators grown by gas source molecular beam epitaxy
Authors: Temkin H. , Gershoni D. , Panish M. B.
Published: July 2002

The quantum confined Stark effect has been studied in InGaAsP/InP rib waveguide structures grown by gas source molecular beam epitaxy. Using 100-Å-wide wells of InGaAs, a room-temperature exciton shift of about 250 Å has been observed for a bias voltage of 6 V. At a wavelength of 1.64 μm a modulation depth of 35% has been achieved at a frequency of 500 MHz. We show that efficient modulation at shorter wavelength cannot be obtained in structures with thinner ternary wells. Instead, we propose and demonstrate InGaAsP modulators operating at a wavelength as short as 1.3 μm.
Shock-wave-induced collision broadening of the photoluminescence spectra in GaSe
Authors: Lu X. Z. , Lee S. , Garuthara R. , Alfano R. R.
Published: July 2002

Significant spectral broadening of the photoluminescence in GaSe under the picosecond-laser-driven shock pressure has been observed for the first time. The broadening of the spontaneous emission was found to be proportional to the shock pressure and attributed to a shock-wave-induced exciton collision mechanism due to the directional motion of particles in the shocked region.
Impact ionization coefficient ratio in InGaAs/InAlAs superlattice avalanche photodiodes determined from noise measurements
Authors: Yu Young-June , Bosman Gijs , Bhattacharya P. K.
Published: July 2002

The noise of an In0.53Ga0.47As/In0.52Al0.48As superlattice avalanche photodiode is measured at 700 MHz as a function of applied reverse bias voltage. From the measured data the ratio k of the hole to electron impact ionization coefficients is determined. This ratio is equal to 6 in the field range (0.8–2.3)×105 V/cm; beyond this range k decreases with increasing field. The field dependence of k is attributed to a transition from ionization across the valence-band-edge discontinuity to band-to-band ionization.
Optical properties of transparent and heat reflecting ZnO:Al films made by reactive sputtering
Authors: Jin Z.-C. , Hamberg I. , Granqvist C. G.
Published: July 2002

Heavily doped ZnO:Al films were made by dual-target reactive magnetron sputtering. Spectrophotometry in the 0.3–50 μm range proved that the films were suitable for energy-efficient windows. The optical data could be reconciled with a theory involving a free-electron gas damped by ionized impurity scattering.
rf properties of an oxide-superconductor half-wave resonant line
Authors: Delayen J. R. , Goretta K. C. , Poeppel R. B. , Shepard K. W.
Published: July 2002

Cylindrical rods of YBa2Cu3O7−δ from 0.6 to 1.9 mm in diameter and up to 565 mm long have been used as the center conductor of a half-wave resonant coaxial line. The resonant line can be immersed in liquid nitrogen, which provides excellent cooling of the current-carrying superconducting surface, even in the presence of substantial rf losses. Such superconducting resonant lines have been operated at frequencies of a few hundred MHz and at rf surface magnetic fields up to 150 G at 77 K.
Magnetically insulated transmission line oscillator
Authors: Clark M. Collins , Marder Barry M. , Bacon Larry D.
Published: July 2002

The magnetically insulated transmission line oscillator (MILO) is a concept for producing very high-power microwave pulses. It is based on the relativistic flow of electrons in a magnetically insulated transmission line on which a slow wave structure, typically thin metal vanes, has been imposed. Its geometry is similar to that of a linear magnetron or crossed-field amplifier. In the MILO, however, the magnetic field which insulates the diode is produced by the electron flow itself rather than being imposed by external coils. This eliminates the need to match the applied voltage to the magnetic field. Because both the insulating magnetic field and accelerating voltage are produced by the same source, the device can be run at high voltages without electrical breakdown.
Partial purification and identification of GDP-mannose 3",5"-epimerase of
Arabidopsis thaliana
, a key enzyme of the plant vitamin C pathway
Authors: Wolucka Beata A. , Persiau Geert , Van Doorsselaere Jan , Davey Mark W. , Demol Hans , Vandekerckhove Joël , Van Montagu Marc , Zabeau Marc , Boerjan Wout
Published: July 2002

The first step in the biosynthetic pathway of vitamin C in
plants is the formation, at the level of sugar nucleotide, of
l
-galactosyl residues, catalyzed by a largely unknown
GDP-
d
-mannose 3",5"-epimerase. By using combined
conventional biochemical and mass spectrometry methods, we obtained a
highly purified preparation of GDP-
d
-mannose
3",5"-epimerase from an
Arabidopsis thaliana
cell
suspension. The native enzyme is an 84-kDa dimer, composed of two
apparently identical subunits. In-gel tryptic digestion of the enzyme
subunit, followed by peptide sequencing and a
blast
search,
led to the identification of the epimerase gene. The closest homolog of
the plant epimerase is the
BlmG
gene product of
Streptomyces
sp., a putative NDP-
d
-mannose
5"-epimerase. The plant GDP-
d
-mannose 3",5"-epimerase is,
to our knowledge, a novel member of the extended short-chain
dehydrogenase/reductase family. The enzyme was cloned and expressed
in
Escherichia coli
cells.
Identification of a gene at 11q23 encoding a guanine nucleotide exchange factor: Evidence for its fusion with
MLL
in acute myeloid leukemia
Authors: Kourlas Peter J. , Strout Matthew P. , Becknell Brian , Veronese Maria Luisa , Croce Carlo M. , Theil Karl S. , Krahe Ralf , Ruutu Tapani , Knuutila Sakari , Bloomfield Clara D. , Caligiuri Michael A.
Published: July 2002

We have identified a gene at 11q23, telomeric to
MLL
, that encodes a guanine nucleotide exchange factor (GEF). This gene is transcribed into a 9.5-kb mRNA containing a 4.6-kb ORF. By Northern analysis, it was found to be expressed in all human tissues examined including peripheral blood leukocytes, spleen, prostate, testis, ovary, small intestine, colon, and minimally in thymus. Analysis of the predicted protein sequence indicates that it has strong homology to several members of the family of Rho GEFs that includes such oncogenes as
Dbl
,
Vav
,
Tiam
, and
Bcr
. A patient with primary acute myeloid leukemia (AML) and a karyotype of 51,XY,+8,+19,+3mar was found to have the 5′ end of
MLL
at exon 6 fused in-frame with the 3′ end of almost the entire ORF of this gene, which we named
LARG
for leukemia-associated Rho GEF. Transcriptional orientation of both genes at 11q23 is from centromere to telomere, consistent with other data that suggest the
MLL-LARG
fusion resulted from an interstitial deletion rather than a balanced translocation.
LARG
does not appear to have any homology with other
MLL
partner genes reported thus far. Thus,
LARG
represents an additional member of the GEF family and a novel
MLL
fusion partner in acute myeloid leukemia.
High-sensitivity array analysis of gene expression for the early detection of disseminated breast tumor cells in peripheral blood
Authors: Martin Katherine J. , Graner Edgard , Li Yi , Price Laura M. , Kritzman Brian M. , Fournier Marcia V. , Rhei Esther , Pardee Arthur B.
Published: July 2002

Early detection is an effective means of reducing cancer mortality.
Here, we describe a highly sensitive high-throughput screen that can
identify panels of markers for the early detection of solid tumor cells
disseminated in peripheral blood. The method is a two-step combination
of differential display and high-sensitivity cDNA arrays. In a primary
screen, differential display identified 170 candidate marker genes
differentially expressed between breast tumor cells and normal breast
epithelial cells. In a secondary screen, high-sensitivity arrays
assessed expression levels of these genes in 48 blood samples, 22 from
healthy volunteers and 26 from breast cancer patients. Cluster analysis
identified a group of 12 genes that were elevated in the blood of
cancer patients. Permutation analysis of individual genes defined five
core genes (
P
≤ 0.05,
permax
test). As
a group, the 12 genes generally distinguished accurately between
healthy volunteers and patients with breast cancer. Mean expression
levels of the 12 genes were elevated in 77% (10 of 13) untreated
invasive cancer patients, whereas cluster analysis correctly classified
volunteers and patients (
P
= 0.0022, Fisher's
exact test). Quantitative real-time PCR confirmed array results and
indicated that the sensitivity of the assay (1:2 ×
10
8
transcripts) was sufficient to detect disseminated
solid tumor cells in blood. Expression-based blood assays developed
with the screening approach described here have the potential to detect
and classify solid tumor cells originating from virtually any primary
site in the body.
Kinetics of protein import into isolated
Xenopus
oocyte nuclei
Authors: Radtke Thomas , Schmalz Dirk , Coutavas Elias , Soliman Tarik M. , Peters Reiner
Published: July 2002

An
in vitro
assay for nucleocytoplasmic transport
was established in which signal-dependent protein import is reproduced
faithfully by isolated purified nuclei. The assay permits the precise
quantification of import kinetics and the discrimination between
translocation through the nuclear envelope and intranuclear transport.
Nuclei were manually isolated from
Xenopus
oocytes and
after manual purification incubated with a medium containing a green
fluorescent transport substrate, karyopherins α2 and β1, a red
fluorescent control substrate, an energy mix and, for keeping an
osmotic balance, 20% (wt/vol) BSA. Import of transport substrates
into the nucleus and exclusion of the control substrate were monitored
simultaneously by two-color confocal microscopy. Two widely differing
import substrates were used: the recombinant protein P4K [480 kDa,
four nuclear localization sequences (NLSs) per P4K tetramer], and
NLS-BSA (90 kDa, 15 NLSs). The measurements suggested that import, at
the specific conditions used in this study, consisted of two
consecutive processes: (
i
) the rapid equilibration of
the concentration difference across the nuclear envelope, a process
involving binding and translocation of substrate by the nuclear pore
complex , and (
ii
) the dissipation of the intranuclear
concentration difference by diffusion.
Comparing protein–ligand interactions in solution and single crystals by Raman spectroscopy
Authors: Altose Michael D. , Zheng Yuangang , Dong Jian , Palfey Bruce A. , Carey Paul R.
Published: July 2002

By using a Raman microscope, we show that it is possible to probe
the conformational states in protein crystals and crystal fragments
under growth conditions (in hanging drops). The flavin cofactor in the
enzyme
para
-hydroxybenzoate hydroxylase can assume two
conformations: buried in the protein matrix (“in”) or essentially
solvent-exposed (“out”). By using Raman difference spectroscopy,
we previously have identified characteristic flavin marker bands for
the in and out conformers in the solution phase. Now we show that the
flavin Raman bands can be used to probe these conformational states in
crystals, permitting a comparison between solution and crystal
environments. The in or out marker bands are similar for the respective
conformers in the crystal and in solution; however, significant
differences do exist, showing that the environments for the flavin's
isoalloxazine ring are not identical in the two phases. Moreover, the
Raman-band widths of the flavin modes are narrower for both in and out
conformers in the crystals, indicating that the flavin exists in a more
limited range of closely related conformational states in the crystal
than in solution. In general, the ability to compare detailed Raman
data for complexes in crystals and solution provides a means of
bridging crystallographic and solution studies.
Identification of protein oligomerization states by analysis of interface conservation
Authors: Elcock Adrian H. , McCammon J. Andrew
Published: July 2002

The discrimination of true oligomeric protein–protein
contacts from nonspecific crystal contacts remains problematic.
Criteria that have been used previously base the assignment of
oligomeric state on consideration of the area of the interface and/or
the results of scoring functions based on statistical potentials. Both
techniques have a high success rate but fail in more than 10% of
cases. More importantly, the oligomeric states of several proteins are
incorrectly assigned by both methods. Here we test the hypothesis that
true oligomeric contacts should be identifiable on the basis of an
increased degree of conservation of the residues involved in the
interface. By quantifying the degree of conservation of the interface
and comparing it with that of the remainder of the protein surface, we
develop a new criterion that provides a highly effective complement to
existing methods.
Organic protomolecule assembly in igneous minerals
Authors: Freund Friedemann , Staple Aaron , Scoville John
Published: July 2002

C—H stretching bands, ν
CH
, in the infrared
spectrum of single crystals of nominally high purity, of
laboratory-grown MgO, and of natural upper mantle olivine, provide an
“organic” signature that closely resembles the symmetrical and
asymmetrical C—H stretching modes of aliphatic —CH
2
units. The ν
CH
bands indicate that H
2
O and
CO
2
, dissolved in the matrix of these minerals, converted
to form H
2
and chemically reduced C, which in turn formed
C—H entities, probably through segregation into defects such as
dislocations. Heating causes the C—H bonds to pyrolyze and the
ν
CH
bands to disappear, but annealing at 70°C causes
them to reappear within a few days or weeks. Modeling dislocations in
MgO suggests that the segregation of C can lead to C

x

chains,
x
= 4, with the terminal C atoms anchored to the
MgO matrix by bonding to two O
−
. Allowing H
2
to react with such C

x

chains leads to
[O
2
C(CH
2
)
2
CO
2
] or
similar precipitates. It is suggested that such
C

x

—H

y

—O

z

entities represent protomolecules
from which derive the short-chain carboxylic and dicarboxylic and the
medium-chain fatty acids that have been solvent-extracted from crushed
MgO and olivine single crystals, respectively. Thus, it appears that
the hard, dense matrix of igneous minerals represents a medium in which
protomolecular units can be assembled. During weathering of rocks, the
protomolecular units turn into complex organic molecules. These
processes may have provided stereochemically constrained organics to
the early Earth that were crucial to the emergence of life.