bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical simulations of annular wire-array z-pinches in (x,y), (r,θ), and (r,z) geometries
Authors: Marder B. M. , Sanford T. W. L. , Allshouse G. O.
Published: July 2002

The Total Immersion Particle [B. M. Marder, Math. Comput. 29, 434 (1973)] code has been used in several two-dimensional geometries to understand better the measured dynamics of annular, aluminum wire-array z-pinches. The areas investigated include the formation of the plasma sheath from current-induced individual wire explosions, the effects of wire number and symmetry on the implosion dynamics, and the dependence of the Rayleigh–Taylor instability growth on initial sheath thickness. A qualitative change in the dynamics with increasing wire number was observed, corresponding to a transition between a z-pinch composed of nonmerging, self-pinching individual wires, and one characterized by the rapid formation and subsequent implosion of a continuous plasma sheath. A sharp increase in radiated power with increasing wire number has been observed experimentally near this calculated transition. Although two-dimensional codes have correctly simulated observed power pulse durations, there are indications that three-dimensional effects are important in understanding the actual mechanism by which these pulse lengths are produced.
Waves and instabilities in streaming magnetized plasma
Authors: Mohanty J. N. , Naik Antaryami
Published: July 2002

The evolution of electrostatic waves and excitation of instabilities in two-component streaming plasmas in a magnetic field are analytically studied. The modified spectrum and growth term of instability are derived to explain the physical situations like cold plasma, warm plasma, ion fusion aspect along with the uncoupled mode. The striking features of the analysis are displayed in the form of profiles of the refractive index (η) varying with particle thermal energy (KT) in different streaming regimes (V0). The effect of streaming enhances the frequency propagation of waves or dispersion, including phase velocity and refractive indices. However, the damping or growth term of instability is diminished, appreciably owing to the streaming. The findings may have important implications to space and magnetospheric settings where a beam–plasma interaction exists.
Plasma oscillons in spherically bounded magnetized plasmas
Authors: Stenflo L. , Yu M. Y.
Published: July 2002

Oscillating patterns of nonlinear waves in a cold non-neutral magnetized plasma bounded by a spherical dielectric are obtained. These patterns, or oscillons, are composed of bulk and surface waves that are strongly but not resonantly coupled.
Numerical simulations and theoretical analysis of proposed heavy-ion-matter experiments at the GSI Darmstadt accelerator facility
Authors: Tahir N. A. , Hoffmann D. H. H. , Maruhn J. A. , Lutz K.-J. , Bock R.
Published: July 2002

This paper presents one- and two-dimensional computer simulations of the hydrodynamic response of solid cylindrical targets made of different materials that are irradiated by intense beams of energetic ions. The beam parameters considered in this study correspond to the design parameters of the heavy ion beam that will be produced at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt heavy ion synchrotron facility (SIS) in 1999. A few calculations, however, were also done using the beam parameters that are currently available at the SIS. Different values for specific energy deposition including 1, 10, 50, and 100 kJ/g, respectively, have been considered, whereas a number of different pulse lengths, namely, 10, 50, 100, and 200 ns, have been assumed. Various target materials, for example, solid lead, solid neon, and solid hydrogen, have been used. It is expected that this simulation study will be very helpful in the design of efficient targets for the future experiments at the GSI. These experiments will hopefully provide very useful information about many important basic physics phenomena, such as enhanced energy loss of heavy ions in hot dense plasmas, equation-of state (EOS) of matter under extreme conditions, material opacity and shock wave propagation. Another very interesting experiment with important practical implications that could be done at this facility may be the creation of metallic hydrogen by imploding appropriately designed multilayered targets containing a layer of frozen hydrogen. This paper presents the design of such a target, together with implosion simulations of this target using a hydrodynamic simulation model. These simulations show that it may be possible to compress the frozen hydrogen to achieve the theoretically predicted physical conditions necessary for hydrogen metallization (a density of the order of 1 to 2 g/cm3, a temperature of a few 0.1 eV and a pressure of about 2–5 megabar). In some cases, compression of frozen deuterium was also studied.
Measurements and code comparison of wave dispersion and antenna radiation resistance for helicon waves in a high density cylindrical plasma source
Authors: Schneider D. A. , Borg G. G. , Kamenski I. V.
Published: July 2002

Helicon wave dispersion and radiation resistance measurements in a high density (ne≈1019−1020 m−3) and magnetic field (B<0.2 T) cylindrical plasma source are compared to the results of a recently developed numerical plasma wave code [I. V. Kamenski and G. G. Borg, Phys. Plasmas 3, 4396 (1996)]. Results are compared for plasmas formed by a double saddle coil antenna and a helical antenna. In both cases, measurements reveal a dominance of the m=+1 azimuthal mode to the exclusion of most other modes; in particular, no significant m=−1 mode was observed. The helical antenna, designed to launch m<0 and m>0 modes in opposite directions along the field, resulted in an axially asymmetric discharge with very little plasma on the m<0 side of the antenna. For both antennas, good agreement of the antenna radiation resistance and wave dispersion with the model was obtained. It is concluded that unshielded antennas formed from current loops with an important |m|=1 component for the conditions of our experiment, couple most of their power to the m=+1 helicon mode and thus have negligible parasitic, nonhelicon plasma loading. This result greatly simplifies calculations of power balance in these sources by identifying the helicon as the mode by which energy is transferred to the plasma.
Perturbed plasma equilibria
Authors: Boozer Allen H.
Published: July 2002

The computation of perturbed plasma equilibria is required to assess the properties of resistive wall modes and to investigate the driving of magnetic islands in toroidal plasmas by external field errors. The computation of perturbed equilibria using the so-called δW techniques of magnetohydrodynamic (MHD) stability codes is explained. The theory is developed that is required to use the results of a perturbed equilibrium calculation for either a study of resistive wall modes or of the elimination of magnetic islands by the use of trim coils.
General Taylor configuration expansion revisited
Authors: Chu M. S. , Jensen T. H. , Bellan P. M.
Published: July 2002

The validity of a series expansion proposed previously [T. H. Jensen and M. S. Chu, Phys. Fluids 27, 2881 (1984)] for describing general Taylor configurations of magnetized plasmas has been reexamined because an apparent paradox was realized. From analyses of simple cases which can be dealt with mostly analytically, it is concluded that the paradox is a Gibbs phenomenon, and that the series expansion is valid.
Wire array Z-pinch insights for enhanced x-ray production
Authors: Sanford T. W. L. , Mock R. C. , Spielman R. B. , Haines M. G. , Chittenden J. P. , Whitney K. G. , Apruzese J. P. , Peterson D. L. , Greenly J. B. , Sinars D. B. , Reisman D. B. , Mosher D.
Published: July 2002

Comparisons of measured total radiated x-ray power from annular wire-array z-pinches with a variety of models as a function of wire number, array mass, and load radius are reviewed. The data, which are comprehensive, have provided important insights into the features of wire-array dynamics that are critical for high x-ray power generation. Collectively, the comparisons of the data with the model calculations suggest that a number of underlying dynamical mechanisms involving cylindrical asymmetries and plasma instabilities contribute to the measured characteristics. For example, under the general assumption that the measured risetime of the total-radiated-power pulse is related to the thickness of the plasma shell formed on axis, the Heuristic Model [IEEE Trans. Plasma Sci. 26, 1275 (1998)] agrees with the measured risetime under a number of specific assumptions about the way the breakdown of the wires, the wire-plasma expansion, and the Rayleigh–Taylor instability in the r–z plane, develop. Likewise, in the high wire-number regime (where the wires are calculated to form a plasma shell prior to significant radial motion of the shell) the comparisons show that the variation in the power of the radiation generated as a function of load mass and array radius can be simulated by the two-dimensional Eulerian-radiation- magnetohydrodynamics code (E-RMHC) [Phys. Plasmas 3, 368 (1996)], using a single random-density perturbation that seeds the Rayleigh–Taylor instability in the r–z plane. For a given pulse-power generator, the comparisons suggest that (1) the smallest interwire gaps compatible with practical load construction and (2) the minimum implosion time consistent with the optimum required energy coupling of the generator to the load should produce the highest total-radiated-power levels.
Direct drive acceleration of planar liquid deuterium targets
Authors: Sethian J. D. , Bodner S. E. , Colombant D. G. , Dahlburg J. P. , Obenschain S. P. , Pawley C. J. , Serlin V. , Gardner J. H. , Aglitskiy Y. , Chan Y. , Deniz A. V. , Lehecka T. , Klapisch M.
Published: July 2002

The Nike laser (∼2–3 kJ, ∼1014 W/cm2) has been used to ablatively accelerate planar liquid deuterium targets. These experiments are designed to test some aspects of a high gain direct drive target design. The target consists of a low-density foam that is filled with liquid deuterium and covered with a thin polyimide membrane. The measured target trajectory agrees well with one-dimensional (1D) simulations. The growth of the areal mass modulations were measured with a new, 1.26 keV x-ray backlighter. The modulations appear later and grow to a smaller amplitude when the foot of the laser pulse is made spatially smoother. A thin layer of gold on the front of the target reduces the modulations. The results are compared with 2D modeling.
Nonlinear dynamics and anomalous energy transport in an electrostatic ion-temperature-gradient driven drift-dissipative mode
Authors: Rafiq Tariq , Mirza Arshad M. , Qamar Anisa , Murtaza G. , Shukla P. K.
Published: July 2002

By using Braginskii transport equations for ions and Boltzmann distribution for electrons, a new system of nonlinear equations governing the dynamics of low-frequency, short-wavelength electrostatic waves in the presence of equilibrium density, temperature, magnetic field, and electrostatic potential gradients has been derived. New ion-temperature-gradient (ITG) driven drift-dissipative modes are shown to exist. An expression for anomalous ion energy transport caused by nonthermal electrostatic fluctuations is also derived. Furthermore, possible stationary solutions of the nonlinear system are obtained in the form of double vortex. On the other hand, the temporal behavior of newly derived nonlinear dissipative systems can be described by the generalized Lorenz–Stenflo equations which admit chaotic solutions. The results of the present investigation should be helpful for understanding the wave phenomena in space and tokamak plasmas.
Theory of the thermal filamentation instability in the collisionless transport limit
Authors: Bendib A. , Bouzid F. , Bendib K. , Matthieussent G.
Published: July 2002

The kinetic theory of the stationary and nonstationary thermal filamentation instability in fully ionized plasmas is presented. The collisionless transport limit is modeled by new transport coefficients corresponding to the convective heat flux and the temperature anisotropy. It is shown, in particular, that these effects decrease the instability growth rates by about a factor of 2, in conditions typical of current experiments. These two effects, combined with the collisional absorption of the light energy by the plasma electrons, lead to a thermal growth rate comparable to the one due to the ponderomotive drive. The comparison of the results obtained in this work with the principal results established in the literature, is also presented.
Long-time tails do not necessarily imply self-organized criticality or the breakdown of the standard transport paradigm
Authors: Krommes John A. , Ottaviani Maurizio
Published: July 2002

Numerical measurements and analytical studies are performed on a stochastic model with features relevant to plasma confinement. Although the model lacks crucial features of self-organized criticality (SOC) and its transport can be computed by standard techniques, it nevertheless exhibits intermittency and algebraic time correlations. This suggests that SOC need not be the explanation for observed long-time tails in experimental fluctuation data. Arguments based on the renormalized spectral balance equation, and simulation of a standard nonlinear paradigm, predict a range of Hurst exponents in reasonable agreement with the observations without invoking submarginal dynamics.
Stationary laser beam filaments in a semicollisional plasma
Authors: Bychenkov V. Yu. , Mima K. , Tikhonchuk V. T.
Published: July 2002

The cumulative effect of the ponderomotive force, inverse Bremsstrahlung heating and nonlocal electron thermal transport on the formation of steady state laser channels in an underdense weakly collisional plasma is studied for the first time. The nonlocal effects change both the distribution of the laser intensity and the density profile in the channel. Compared to the ponderomotive case, the addition of the nonlocal thermal effects results in density channels that are wider and deeper, and the power required for their formation is significantly decreased. The wide-spread density depletion wings of filaments created by nonlocal thermal effects can be important in the interaction of speckled laser beams with a plasma.
Regularized magnetic islands. I. Hyperviscosity and profile functions
Authors: Mikhailovskii A. B. , Pustovitov V. D. , Tsypin V. S. , Smolyakov A. I.
Published: July 2002

The problem of the magnetic island profile functions is analyzed. An idea is suggested that discontinuities of the profile function derivatives at the island separatrix, which appear in simplified plasma models, can be eliminated by means of more complete description, i.e., the profile functions can be regularized. As an example, the regularization of the electrostatic potential profile function is considered for the case of a slab geometry when its formation is governed by the perpendicular viscosity. It is shown that for regularization of such a profile function one should keep in the viscosity expression the terms with higher derivatives of the velocity, usually referred to as hyperviscosity terms. A model expression for the hyperviscosity is derived by the Grad-type method. Physically, the regularization is related to the appearance of a part of the profile function localized in the Larmor transition layer near the island separatrix.
A theoretical model for the generation of co-current rotation by radio frequency heating observed on Alcator C-Mod
Authors: Chang C. S. , Bonoli P. T. , Rice J. E. , Greenwald M. J.
Published: July 2002

A theoretical model is presented which shows generation of a co-current directional central toroidal rotation by a nondirectional on-axis ion cyclotron heating in the Low-mode plasmas of Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. It is shown that a net-inward nonambipolar transport of resonant ions can induce such a rotation. The net-inward transport is a result of the passing ion dominant transport process in Alcator C-Mod. The opposite horizontal orbit shift between co- and counter-passing ions makes the wave absorption asymmetric in k∥.
Self-organized criticality, long-time correlations, and the standard transport paradigm
Authors: Krommes John A.
Published: July 2002

Some aspects of low-frequency, long-wavelength fluctuations are considered. A stochastic model is used to show that power-law time correlations need not arise from self-organized criticality. A formula for the frequency spectrum of uncorrelated, overlapping avalanches is shown to be a special case of the spectral balance equation of renormalized statistical turbulence theory. It is argued that there need be no contradiction between the presence of long-time correlations and the existence of local transport coefficients.
Ballooning stability of a point dipole equilibrium
Authors: Simakov Andrei N. , Catto Peter J. , Krasheninnikov S. I. , Ramos Jesus J.
Published: July 2002

The energy principle is employed to show that the equilibrium confined by the magnetic field of a point dipole is stable to ballooning modes.
Multiple eigenmode analysis and density jumps in planar surface-wave plasmas with slot-antenna excitation
Authors: Ghanashev I. , Sugai H.
Published: July 2002

Electromagnetic surface waves excitation in cold overdense uniform planar plasma is modeled and numerically analyzed. The microwave is injected through a slot antenna cut in the wall of the metal vessel and the forced electromagnetic oscillation localized in the slot is expanded in a series of eigenmodes, which propagate through a dielectric plate into the plasma. A general formulation for arbitrary plasma cross section and slot aperture geometry is given. It provides the basis for optimizing the slot-antenna design. The general approach is analytically applied to a large-area planar cylindrical plasma excited by an annular slot. Numerical results on the relative intensities of the individual eigenmodes and the overall field distribution at the plasma-dielectric interface for various electron densities are reported. The computed density dependence of the power reflection coefficient is shown to lead to density jumps and power-density hysteresis loops corresponding to the experimentally observed behavior.
New quantitative line scanning technique for homogeneity assessment of semi-insulating GaAs wafers
Authors: Windscheif J. , Baeumler M. , Kaufmann U.
Published: July 2002

A new technique for recording spatial fluctuations of the optical absorption in GaAs slices is reported. It is based on a commercially available linear silicon diode array which monitors the optical absorption attributed to the omnipresent deep trap named EL2. Because this technique gives quantitative results, displays excellent spatial resolution (200 μm for 2-in. wafers), and is fast, it appears to be highly useful for wafer characterization and quality control.
Direct variation of metal-GaAs Schottky barrier height by the influence of interface S, Se, and Te
Authors: Waldrop J. R.
Published: July 2002

The electrical properties and interface chemistry of Schottky barrier contacts (Ag, Al, Au, Mn, Pd, and Ti) formed on n-type GaAs (100) surfaces that had prior exposure to elemental S, Se, and Te are compared to those of ideal (metal deposited onto a clean surface) contacts. The interface Fermi energy EiF and interface chemistry during contact formation were investigated by x-ray photoemission spectroscopy; the accompanying Schottky barrier height φB was measured by current-voltage and capacitance-voltage techniques. A substantial decrease in φB (and correlated change in EiF ) for Al and Mn contacts is associated with a contact metal-chalcogen chemical reaction at the Schottky barrier interface, while some nonreactive noble metal-chalcogen interfaces have a φB increase. By choice of contact metal and interface chalcogen a φB range of >0.6 eV is obtained (∼0.35 to 1.0 eV, which is >40% of the GaAs band gap) via corresponding changes in EiF.
Phase sensitive detection of persistent spectral holes using synchronous ultrasonic modulation
Authors: Moerner W. E. , Huston A. L.
Published: July 2002

We present a new technique for the detection of spectral holes or other strain-sensitive spectral features using phase-sensitive ultrasonic modulation. The spectral feature is directly modulated at MHz rates in synchrony with an applied rf ultrasonic field, hence the method offers zero background and the potential for high-speed detection with sensitivity near the quantum limit. The technique is demonstrated for a model color center system, and the characteristic line shape is analyzed in detail. The method is more sensitive than laser FM (frequency modulation) spectroscopy for low modulation frequencies, and should be generally useful for the detection of strain-sensitive spectral features.
Picosecond recombination of charged carriers in GaAs
Authors: McLean D. G. , Roe M. G. , D’Souza A. I. , Wigen P. E.
Published: July 2002

The recombination kinetics of charged carriers in GaAs have been investigated on a picosecond time scale. A pump-probe technique was used to measure reflectivity changes as a function of time up to 900 ps. Initially, (1.2±0.2)×1020 carriers/cm3 were excited into the conduction band. The decay curve indicated dominance of a three-body (Auger) recombination process up to about 120 ps, with two-body recombination processes dominating after 120 ps, the switch occurring at a carrier concentration of (5±2)×1019 carriers/cm3. Values for the Auger recombination coefficient and the two-body recombination coefficient were determined to be (7±4)×10−31 cm6 s−1 and (3.4±1.7)×10−11 cm3 s−1, respectively. The change in reflectance was observed to have essentially returned to zero within 900 ps.
Generation of soft x rays using a rare gas-hydrogen plasma focus and its application to x-ray lithography
Authors: Kato Y. , Be S. H.
Published: July 2002

A conventional plasma focus device was used to produce a hot, dense plasma which emits radiation strongly in the soft x-ray region. We showed experimentally that a plasma produced from a mixture of hydrogen and a rare gas such as neon, argon, or krypton is an effective source of a characteristic x ray of the rare gas. Resist exposures and pattern printings were made to evaluate the feasibility of this device as a source for soft x-ray lithography.
Enhancement of open circuit voltage in high efficiency amorphous silicon alloy solar cells
Authors: Guha S. , Yang J. , Nath P. , Hack M.
Published: July 2002

We have developed a microcrystalline fluorinated p+ silicon alloy which has high dark conductivity and low optical loss. Incorporation of this material in single and tandem amorphous silicon alloy based solar cells has resulted in increased open circuit voltage and conversion efficiency.
Elimination of end-of-range and mask edge lateral damage in Ge+ preamorphized, B+ implanted Si
Authors: Ajmera A. C. , Rozgonyi G. A.
Published: July 2002

The problems of residual extended defects due to end-of-range ion implantation damage and mask edge lateral damage have been solved in this study for shallow boron junctions preamorphized via germanium ion implantation. Defect elimination has been achieved by adjusting the germanium ion energy, dose, and annealing temperature and ambient to minimize the local interstitial point defect concentration and optimize the role of the free surface in defect annihilation. For the combination of shallow, low dose 40 or 60 keV/2×1014 cm−2 Ge+ and 8 keV/1×1015 cm−2 B+ implants, a defect-free structure was obtained following a 1050 °C, 10 s rapid thermal anneal (RTA) in a nonoxidizing Ar ambient. For these samples, boron profiles determined using secondary ion mass spectroscopy (SIMS) showed the junction depth to be approximately 0.17 μm at a background doping of 1×1017 cm−3 with a sheet resistance of 136 Ω/⧠.
Simulation studies of the dynamic behavior of semiconductor lasers with Auger recombination
Authors: Tang Ming , Wang Shyh
Published: July 2002

We apply the phase portrait analysis to semiconductor lasers with Auger recombination and extend the analysis to high modulation frequency ωm. On the two-dimensional bifurcation diagram of modulation depth and modulation frequency, there are seven regions: digital pulsing regions, analog modulation region, period doubling regions, chaos regions, and one multiloop region. It is found that Auger recombination tends to suppress chaos for ωm<ωr, the relaxation frequency. However, for ωm>ωr, chaotic behavior becomes prominent. Furthermore, in the pulsing region, the maximum pulsation frequency is limited to a value around ωr even though ωm may be twice or three times ωr. A normalized two-dimensional bifurcation diagram defining the digital pulse region and the analog modulation region is presented for the purpose of locating the suitable region for analog and digital operation of semiconductor lasers.
Measurements of axial nonuniformities in gas-puff implosions
Authors: Hsing W. W. , Porter J. L.
Published: July 2002

Large-scale-length axial nonuniformities in imploding z pinches have been measured with a time-resolved framing pinhole camera and a filtered x-ray diode. The nonuniform initial conditions of the annular gas column result in a nonsimultaneous stagnation along the pinch axis. By tailoring the initial gas-puff density distribution we have reduced the time interval of stagnation from 10 ns to below 4 ns for a 2-cm-long pinch.
Ultrasensitive coherent Raman technique with picosecond lasers
Authors: Schauer M. W. , Pellin M. J. , Biwer B. M. , Gruen D. M.
Published: July 2002

The use of picosecond, Q-switched lasers and advanced polarization schemes has led to the development of a coherent Raman technique with the sensitivity of coherent anti-Stokes Raman spectroscopy experiments but without the troublesome phase-matching requirements. Experiments in dilute solutions of benzene indicate a limit of sensitivity for the current apparatus of 2.5×10−4 M in two minutes of signal averaging over 150 cm−1. Possible applications to the in situ study of passive films and thin films on transparent media are discussed.
Green infrared-pumped erbium upconversion laser
Authors: Silversmith A. J. , Lenth W. , Macfarlane R. M.
Published: July 2002

An upconversion pumping scheme was used to produce cw laser action at 0.55 μm in YAlO3:Er3+ at temperatures up to 77 K on the 4S3/2→4I15/2 transition. Two infrared dye lasers at 792.1 and 839.8 nm were used as the pump sources for stepwise two-photon excitation of the 4S3/2 upper laser level at 18 406 cm−1. The laser operates in the fundamental TEM00 mode, and a cw output power of ∼1 mW was achieved with ∼200 mW pump power from each infrared dye laser.
New effects of structure in momentum and real space on nonlinear transport across heterojunction band discontinuities
Authors: Kim Kiwook , Hess Karl , Capasso Federico
Published: July 2002

Nonlinear transport across heterojunction band discontinuities in strong electric fields is theoretically investigated. New phenomena which depend on the combined effects of the structure in real and momentum space are found. Reflections at the heterointerfaces, carrier overheating at the potential step, and the L→Γ, X→Γ backscattering bottlenecks produce pronounced peaks and anisotropies in the energy and momentum distributions, respectively. Finally, a new ionization rate overshoot effect due to the band structure difference between the two materials is discussed.
Photon detection with cooled avalanche photodiodes
Authors: Robinson D. L. , Metscher B. D.
Published: July 2002

Commercial avalanche photodiodes have been operated as single-photon detectors at an optimum operating temperature and bias voltage. These detectors were found to be 1.5–3 times more sensitive than presently available photomultiplier tubes (PMT’s). Both single-photon detection probability and detector noise increase with bias voltage; detection probabilities greater than twice that of a PMT were obtained with detector noise levels below 100 counts per second. Higher probabilities were measured at higher noise levels. The sources of noise and their dependence on temperature and bias voltage are discussed.
Reduction in the concentration of D
X centers in Si-doped GaAlAs using the planar doping technique
Authors: Etienne B. , Thierry-Mieg V.
Published: July 2002

Whereas the concentration of DX centers in Si-doped GaAlAs is nearly independent of the molecular beam epitaxy growth conditions when conventional bulk doping is used, we have discovered that it can be strongly reduced in the case of planar doping. Most interesting is the fact that this reduction is more effective at a high doping level. At an electron density of 2×1013 cm−2 per doping plane [3% of the atomic density of a (100) oriented GaAlAs surface] we observe a nearly complete disappearance of these centers. A possible origin of this unexpected effect is proposed.
Linewidth broadening due to longitudinal spatial hole burning in a long distributed feedback laser
Authors: Wu Ming-Chiang , Lo Yu-hwa , Wang Shyh
Published: July 2002

The spectral linewidth of a long distributed feedback (DFB) laser above threshold at high output power is investigated theoretically. Longitudinal spatial hole burning (LSHB) is shown to have important effects on the lasing characteristics and the broadening of linewidth. A four-wave coupled mode equation is developed to describe the LSHB phenomenon. The threshold gain and the carrier distribution are solved self-consistently above threshold at various output powers. The simulation results show that the power-linewidth product can increase by a factor greater than 2 due to LSHB for long DFB lasers. This theory has important implications on the limits of linewidth reduction by increasing the length of DFB lasers and on possible future device design for coherent optical system.
Direct force measurement in scanning tunneling microscopy
Authors: Tang S. L. , Bokor J. , Storz R. H.
Published: July 2002

A novel force measurement using a scanning tunneling microscope as a forced oscillator is described. Results obtained from tunneling between a tungsten tip and a graphite substrate show that a maximum tip-sample force about 10−6 N exists during the constant current mode of operation. These results are in agreement with a previous model where large contact areas insulated by contaminants between tip and substrate were suggested as a cause of large tip-sample interaction forces. This method can achieve a force sensitivity of 10−8 N and for conductive substrates provide a simple, versatile alternative to existing methods of atomic force microscopy.
Functional requirement of aquaporin-5 in plasma membranes of sweat glands
Authors: Nejsum Lene N. , Kwon Tae-Hwan , Jensen Uffe B. , Fumagalli Ornella , Frøkiaer Jørgen , Krane Carissa M. , Menon Anil G. , King Landon S. , Agre Peter C. , Nielsen Søren
Published: July 2002

The distribution and function of aquaporins (AQPs) have not
previously been defined in sweat glands. In this study, AQP1, AQP3, and
AQP5 mRNA were demonstrated in rat paw by reverse transcription
(RT)–PCR, but AQP2 and AQP4 were not. AQP1, AQP3, and AQP5 protein
were confirmed in these tissues by immunoblotting. AQP1 was identified
in capillary endothelial cells by immunohistochemical labeling, but not
in sweat glands or epidermis. Abundant AQP3 expression was seen in
basal levels of epidermis, but not in sweat glands. AQP2 and AQP4 were
not observed in either skin or sweat glands. Immunohistochemical
labeling revealed abundant AQP5 in secretory parts of rat and mouse
sweat glands, where immunoelectron microscopy demonstrated abundant
AQP5 labeling in the apical plasma membrane. AQP5 immunolabeling of
human sweat glands yielded a similar pattern. To establish the role of
AQP5 in sweat secretion, we tested the response of adult mice to s.c.
injection of pilocarpine, as visualized by reaction of secreted amylase
with iodine/starch. The number of active sweat glands was
dramatically reduced in AQP5-null (−/−) mice compared with
heterozygous (+/−) and wild-type (+/+) mice. We conclude that the
presence of AQP5 in plasma membranes of sweat glands is essential for
secretion, providing potential insight into mechanisms underlying
mammalian thermoregulation, tactile sensitivity, and the
pathophysiology of hyperhidrosis.
Effect of HIV integrase inhibitors on the RAG1/2 recombinase
Authors: Melek Meni , Jones Jessica M. , O'Dea Mary H. , Pais Godwin , Burke Terrence R. , Pommier Yves , Neamati Nouri , Gellert Martin
Published: July 2002

Assembly of functional Ig and T cell receptor genes by V(D)J
recombination depends on site-specific cleavage of chromosomal DNA by
the RAG1/2 recombinase. As RAG1/2 action has mechanistic
similarities to DNA transposases and integrases such as HIV-1
integrase, we sought to determine how integrase inhibitors of the
diketo acid type would affect the various activities of RAG1/2. Both
of the inhibitors we tested interfered with DNA cleavage and
disintegration activities of RAG1/2, apparently by disrupting
interaction with the DNA motifs bound specifically by the recombinase.
The inhibitors did not ablate RAG1/2's transposition activity or
capture of nonspecific transpositional target DNA, suggesting this DNA
occupies a site on the recombinase different from that used for
specific binding. These results further underscore the similarities
between RAG1/2 and integrase and suggest that certain integrase
inhibitors may have the potential to interfere with aspects of B and T
cell development.
Examination of hypotheses for the Permo–Triassic boundary extinction by carbon cycle modeling
Authors: Berner Robert A.
Published: July 2002

The biological extinction that occurred at the Permian–Triassic boundary represents the most extensive loss of species of any known event of the past 550 million years. There have been a wide variety of explanations offered for this extinction. In the present paper, a number of the more popular recent hypotheses are evaluated in terms of predictions that they make, or that they imply, concerning the global carbon cycle. For this purpose, a mass balance model is used that calculates atmospheric CO
2
and oceanic δ
13
C as a function of time. Hypotheses considered include: (
i
) the release of massive amounts of CO
2
from the ocean to the atmosphere resulting in mass poisoning; (
ii
) the release of large amounts of CO
2
from volcanic degassing; (
iii
) the release of methane stored in methane hydrates; (
iv
) the decomposition and oxidation of dead organisms to CO
2
after sudden mass mortality; and (
v
) the long-term reorganization of the global carbon cycle. The modeling indicates that measured short-term changes in δ
13
C at the boundary are best explained by methane release with mass mortality and volcanic degassing contributing in secondary roles. None of the processes result in excessively high levels of atmospheric CO
2
if they occurred on time scales of more than about 1,000 years. The idea of poisoning by high levels of atmospheric CO
2
depends on the absence of subthermocline calcium carbonate deposition during the latest Permian. The most far-reaching effect was found to be reorganization of the carbon cycle with major sedimentary burial of organic matter shifting from the land to the sea, resulting in less burial overall, decreased atmospheric O
2
, and higher atmospheric CO
2
for the entire Triassic Period.
Vitamin E reduces chromosomal damage and inhibits hepatic tumor formation in a transgenic mouse model
Authors: Factor Valentina M. , Laskowska Danuta , Jensen Michael Rugaard , Woitach Joseph T. , Popescu Nicholas C. , Thorgeirsson Snorri S.
Published: July 2002

We have previously shown that chronic activation of mitogenic signaling induced by over-expression of c-
myc
and transforming growth factor-α (TGFα) transgenes in mouse liver induces a state of oxidative stress. We therefore proposed that increased reactive oxygen species (ROS) generation might be responsible for the extensive chromosomal damage and acceleration of hepatocarcinogenesis characteristic for TGFα/c-
myc
mice. In this study, we show that vitamin E (VE), a potent free radical scavenging antioxidant, is able to protect liver tissue against oxidative stress and suppress tumorigenic potential of c-
myc
oncogene. Dietary supplementation with VE, starting from weaning, decreased ROS generation coincident with a marked inhibition of hepatocyte proliferation while increasing the chromosomal as well as mtDNA stability in the liver. Similarly, dietary VE reduced liver dysplasia and increased viability of hepatocytes. At 6 mo of age, VE treatment decreased the incidence of adenomas by 65% and prevented malignant conversion. These results indicate that ROS generated by over-expression of c-
myc
and TGFα in the liver are the primary carcinogenic agents in this animal model. Furthermore, the data demonstrate that dietary supplementation of VE can effectively inhibit liver cancer development.
Caspase-3: A vulnerability factor and final effector in apoptotic death of dopaminergic neurons in Parkinson's disease
Authors: Hartmann Andreas , Hunot Stéphane , Michel Patrick P. , Muriel Marie-Paule , Vyas Sheela , Faucheux Baptiste A. , Mouatt-Prigent Annick , Turmel Hélène , Srinivasan Anu , Ruberg Merle , Evan Gerard I. , Agid Yves , Hirsch Etienne C.
Published: July 2002

Caspase-3 is an effector of apoptosis in experimental
models of Parkinson's disease (PD). However, its potential role in the
human pathology remains to be demonstrated. Using caspase-3
immunohistochemistry on the postmortem human brain, we observed a
positive correlation between the degree of neuronal loss in
dopaminergic (DA) cell groups affected in the mesencephalon of PD
patients and the percentage of caspase-3-positive neurons in these cell
groups in control subjects and a significant decrease of
caspase-3-positive pigmented neurons in the substantia nigra pars
compacta of PD patients compared with controls that also could be
observed in an animal model of PD. This suggests that neurons
expressing caspase-3 are more sensitive to the pathological process
than those that do not express the protein. In addition, using an
antibody raised against activated caspase-3, the percentage of active
caspase-3-positive neurons among DA neurons was significantly higher in
PD patients than in controls. Finally, electron microscopy analysis in
the human brain and
in vitro
data suggest that caspase-3
activation precedes and is not a consequence of apoptotic cell
death in PD.
Constraints on nebular dynamics and chemistry based on observations of annealed magnesium silicate grains in comets and in disks surrounding Herbig Ae/Be stars
Authors: Hill Hugh G. M. , Grady Carol A. , Nuth Joseph A. , Hallenbeck Susan L. , Sitko Michael L.
Published: July 2002

Understanding dynamic conditions in the Solar Nebula is the
key to prediction of the material to be found in comets. We suggest
that a dynamic, large-scale circulation pattern brings processed dust
and gas from the inner nebula back out into the region of cometesimal
formation—extending possibly hundreds of astronomical units (AU) from
the sun—and that the composition of comets is determined by a chemical
reaction network closely coupled to the dynamic transport of dust and
gas in the system. This scenario is supported by laboratory studies of
Mg silicates and the astronomical data for comets and for
protoplanetary disks associated with young stars, which demonstrate
that annealing of nebular silicates must occur in conjunction with a
large-scale circulation. Mass recycling of dust should have a
significant effect on the chemical kinetics of the outer nebula by
introducing reduced, gas-phase species produced in the higher
temperature and pressure environment of the inner nebula, along with
freshly processed grains with “clean” catalytic surfaces to the
region of cometesimal formation. Because comets probably form
throughout the lifetime of the Solar Nebula and processed (crystalline)
grains are not immediately available for incorporation into the first
generation of comets, an increasing fraction of dust incorporated into
a growing comet should be crystalline olivine and this fraction can
serve as a crude chronometer of the relative ages of comets. The
formation and evolution of key organic and biogenic molecules in comets
are potentially of great consequence to astrobiology.
Solution
19
F nuclear Overhauser effects in structural studies of the cytoplasmic domain of mammalian rhodopsin
Authors: Loewen Michèle C. , Klein-Seetharaman Judith , Getmanova Elena V. , Reeves Philip J. , Schwalbe Harald , Khorana H. Gobind
Published: July 2002

19
F nuclear Overhauser effects (NOEs) between fluorine
labels on the cytoplasmic domain of rhodopsin solubilized in detergent
micelles are reported. Previously, high-resolution solution
19
F NMR spectra of fluorine-labeled rhodopsin in detergent
micelles were described, demonstrating the applicability of this
technique to studies of tertiary structure in the cytoplasmic domain.
To quantitate tertiary contacts we have applied a transient
one-dimensional difference NOE solution
19
F NMR experiment
to this system, permitting assessment of proximities between fluorine
labels specifically incorporated into different regions of the
cytoplasmic face. Three dicysteine substitution mutants
(Cys-140–Cys-316, Cys-65–Cys-316, and Cys-139–Cys-251) were labeled
by attachment of the trifluoroethylthio group through a disulfide
linkage. Each mutant rhodopsin was prepared (8–10 mg) in
dodecylmaltoside and analyzed at 20°C by solution
19
F
NMR. Distinct chemical shifts were observed for all of the rhodopsin
19
F labels in the dark. An up-field shift of the Cys-316
resonance in the Cys-65–Cys-316 mutant suggests a close proximity
between the two residues. When analyzed for
19
F-
19
F NOEs, a moderate negative enhancement
was observed for the Cys-65–Cys-316 pair and a strong negative
enhancement was observed for the Cys-139–Cys-251 pair, indicating
proximity between these sites. No NOE enhancement was observed for the
Cys-140–Cys-316 pair. These NOE effects demonstrate a solution
19
F NMR method for analysis of tertiary contacts in high
molecular weight proteins, including membrane proteins.
NQO1 stabilizes p53 through a distinct pathway
Authors: Asher Gad , Lotem Joseph , Kama Rachel , Sachs Leo , Shaul Yosef
Published: July 2002

Wild-type p53 is a tumor-suppressor gene that encodes a short-lived protein that, upon accumulation, induces growth arrest or apoptosis. Accumulation of p53 occurs mainly by posttranslational events that inhibit its proteosomal degradation. We have reported previously that inhibition of NAD(P)H: quinone oxidoreductase 1 (NQO1) activity by dicoumarol induces degradation of p53, indicating that NQO1 plays a role in p53 stabilization. We now have found that wild-type NQO1, but not the inactive polymorphic NQO1, can stabilize endogenous as well as transfected wild-type p53. NQO1-mediated p53 stabilization was especially prominent under induction of oxidative stress. NQO1 also partially inhibited p53 degradation mediated by the human papilloma virus E6 protein, but not when mediated by Mdm-2. Inhibitors of heat shock protein 90 (hsp90), radicicol and geldanamycin, induced degradation of p53 and suppressed p53-induced apoptosis in normal thymocytes and myeloid leukemic cells. Differences in the effectiveness of dicoumarol and hsp90 inhibitors to induce p53 degradation and suppress apoptosis in these cell types indicate that NQO1 and hsp90 stabilize p53 through different mechanisms. Our results indicate that NQO1 has a distinct role in the regulation of p53 stability, especially in response to oxidative stress. The present data on the genetic and pharmacologic regulation of the level of p53 have clinical implications for tumor development and therapy.
p63 identifies keratinocyte stem cells
Authors: Pellegrini Graziella , Dellambra Elena , Golisano Osvaldo , Martinelli Enrica , Fantozzi Ivana , Bondanza Sergio , Ponzin Diego , McKeon Frank , De Luca Michele
Published: July 2002

The proliferative compartment of stratified squamous epithelia
consists of stem and transient amplifying (TA) keratinocytes. Some
polypeptides are more abundant in putative epidermal stem cells than in
TA cells, but no polypeptide confined to the stem cells has yet been
identified. Here we show that the p63 transcription factor, a p53
homologue essential for regenerative proliferation in epithelial
development, distinguishes human keratinocyte stem cells from their TA
progeny. Within the cornea, nuclear p63 is expressed by the basal cells
of the limbal epithelium, but not by TA cells covering the corneal
surface. Human keratinocyte stem and TA cells when isolated in culture
give rise to holoclones and paraclones, respectively. We show by clonal
analysis that p63 is abundantly expressed by epidermal and limbal
holoclones, but is undetectable in paraclones. TA keratinocytes,
immediately after their withdrawal from the stem cell compartment
(meroclones), have greatly reduced p63, even though they possess very
appreciable proliferative capacity. Clonal evolution (i.e., generation
of TA cells from precursor stem cells) is promoted by the sigma isoform
of the 14-3-3 family of proteins. Keratinocytes whose 14-3-3σ has
been down-regulated remain in the stem cell compartment and maintain
p63 during serial cultivation. The identification of p63 as a
keratinocyte stem cell marker will be of practical importance for the
clinical application of epithelial cultures in cell therapy as well as
for studies on epithelial tumorigenesis.
Medial prefrontal cortex and self-referential mental activity: Relation to a default mode of brain function
Authors: Gusnard Debra A. , Akbudak Erbil , Shulman Gordon L. , Raichle Marcus E.
Published: July 2002

Medial prefrontal cortex (MPFC) is among those brain regions having
the highest baseline metabolic activity at rest and one that exhibits
decreases from this baseline across a wide variety of goal-directed
behaviors in functional imaging studies. This high metabolic rate and
this behavior suggest the existence of an organized mode of default
brain function, elements of which may be either attenuated or enhanced.
Extant data suggest that these MPFC regions may contribute to the
neural instantiation of aspects of the multifaceted “self.” We
explore this important concept by targeting and manipulating elements
of MPFC default state activity. In this functional magnetic resonance
imaging (fMRI) study, subjects made two judgments, one
self-referential, the other not, in response to affectively normed
pictures: pleasant vs. unpleasant (an internally cued condition, ICC)
and indoors vs. outdoors (an externally cued condition, ECC). The ICC
was preferentially associated with activity increases along the dorsal
MPFC. These increases were accompanied by decreases in both active task
conditions in ventral MPFC. These results support the view that dorsal
and ventral MPFC are differentially influenced by
attentiondemanding tasks and explicitly self-referential tasks.
The presence of self-referential
mental activity
appears
to be associated with increases from the baseline in dorsal MPFC.
Reductions in ventral MPFC occurred consistent with the fact that
attention-demanding tasks attenuate emotional processing. We posit that
both self-referential mental activity and emotional processing
represent elements of the default state as represented by activity in
MPFC. We suggest that a useful way to explore the neurobiology of the
self is to explore the nature of default state activity.