bluuebunny/crossref_metadata_2025 huggingface dataset:
Γ
-conformal algebras
Authors: Golenishcheva-Kutuzova Maria I. , Kac Victor G.
Published: July 2002

Γ -conformal algebra is an axiomatic description of the operator product expansion of chiral fields with simple poles at finitely many points. We classify these algebras and their representations in terms of Lie algebras and their representations with an action of the group Γ. To every Γ-conformal algebra and a character of Γ we associate a Lie algebra generated by fields with the OPE with simple poles. Examples include twisted affine Kac–Moody algebras, the sin algebra (which is a “Γ-conformal” analogue of the general linear algebra) and its analogues, the algebra of pseudodifferential operators on the circle, etc.
Return to equilibrium
Authors: Bach Volker , Fröhlich Jürg , Sigal Israel Michael
Published: July 2002

We study an atom with finitely many energy levels in contact with a heat bath consisting of photons (blackbody radiation) at a temperature T>0. The dynamics of this system is described by a Liouville operator, or thermal Hamiltonian, which is the sum of an atomic Liouville operator, of a Liouville operator describing the dynamics of a free, massless Bose field, and a local operator describing the interactions between the atom and the heat bath. We show that an arbitrary initial state that is normal with respect to the equilibrium state of the uncoupled system at temperature T converges to an equilibrium state of the coupled system at the same temperature, as time tends to +∞ (return to equilibrium).
The generation of vortices in high-resolution, two-dimensional decaying turbulence and the influence of initial conditions on the breaking of self-similarity
Authors: Santangelo P. , Benzi R. , Legras B.
Published: July 2002

The results of a very-high-resolution numerical integration of a two-dimensional turbulent flow is presented. The very long integration time allows a discussion of both the initial and the ultimate stage of the decay. It is established that the k−3 enstrophy inertial range is only obtained as a transient state of the system. For very long time the emergence of coherent vortices destroys the scale invariance and produces a rather steep spectrum. In addition, the long term behavior strongly depends on the initial conditions. For steep enough initial spectrum, a large-scale energy containing a range in which vortices are directly linked to the initial conditions separates from a small-scale range in which many coherent structures originate from inviscid instabilities. The results are compared with previous studies and the origin of self-similarity breaking in turbulent decay is discussed.
Taylor vortices in short fluid columns with large radius ratio
Authors: Bielek C. A. , Koschmieder E. L.
Published: July 2002

The formation of Taylor vortices after sudden starts of the inner cylinder has been studied experimentally in fluid columns with aspect ratios Γ=3.0, 3.25, 3.5, and 3.75, with radius ratio η=0.605, bounded by resting top and bottom boundaries. It has been found that the aspect ratio of the column has a profound effect on the number of Taylor vortices formed in the column. Anomalous three-vortex flows have been found with Γ=3.0, 3.25, and 3.75, but not with Γ=3.5. Anomalous three-vortex flows do not form directly after a sudden start, but originate from the decay of anomalous four-vortex patterns with Γ=3.0 and Γ=3.25, and from six-vortex patterns with Γ=3.75.
Nonlinear interfacial stability of core-annular film flows
Authors: Papageorgiou D. T. , Maldarelli C. , Rumschitzki D. S.
Published: July 2002

In this paper the weakly nonlinear stability of two-phase core-annular film flows in the limit of small film thickness and in the presence of both viscosity stratification and interfacial tension is examined. Rational asymptotic expansions are used to derive some novel nonlinear evolution equations for the interface between the phases. The novel feature of the equations is that they include a coupling between core and film dynamics thus enabling a study of its effect on the nonlinear evolution of the interface. The nonlinear interfacial evolution is governed by modified Kuramoto–Sivashinsky equations in the cases of slow and moderate flow [the former also developed by Frenkel, Sixth Symposium on Energy Engineering Sciences (Argonne Lab. Pub. CONF-8805106, 1988), p.100, using different asymptotic methods], which include new nonlocal terms that reflect core dynamics. These equations are solved numerically for given initial conditions and a range of parameters. Some interesting behavior results, such as transition (in parameter space) of chaotic solutions into traveling-wave pulses with more than one characteristic length scale.
The response of a nearly supercritical pure fluid to a thermal disturbance
Authors: Zappoli Bernard
Published: July 2002

The response of a nearly supercritical pure fluid confined in a slablike container to a slowly varying temperature disturbance at the boundary is studied. The matched asymptotic expansion technique is used to describe the core and boundary-layer flows on the short acoustic time scale. The multiple scale expansion technique is then used to obtain the solution of the equations for longer times. It is found (i) that the acoustic field generated in the bulk is stronger, the closer the initial conditions are to the critical point; (ii) that the heat transport is much faster than the conductive transport owing to the very high compressibility of the fluid; (iii) the scaling laws and the equations describing the flow field on the long time scale confirm both the numerical results obtained previously as well as their interpretation in terms of the piston effect which provokes the enhancement of the heat transport by adiabatic compressive heating of the bulk phase.
On streamwise vortices in high Reynolds number supersonic axisymmetric jets
Authors: Arnette S. A. , Samimy M. , Elliott G. S.
Published: July 2002

Pitot pressure measurements and flow visualizations were used to investigate streamwise vortices previously observed in underexpanded jets. A simple model was developed, which gives reasonable agreement with the pressure measurements. A converging nozzle and converging–diverging nozzle of design Mach number 1.5 were used to generate jet flows of equivalent Mach numbers up to 2.5 (stagnation to ambient pressure ratios up to 17.1). By operating the nozzles fully expanded, overexpanded, and underexpanded, insight was gained into both the occurrence and cause for formation of the vortices. Spatially stationary streamwise vortices were found to exist in the near-field region around the circumference of underexpanded jets in the vicinity of the jet boundary. Short exposure visualizations show the vortices persist much farther downstream with a loss of spatial organization. Visualizations suggest adjacent vortices have streamwise vorticity of opposite sign, so the action of adjacent vortices is to either pump jet fluid radially outward or entrain ambient fluid radially inward toward the jet. The downstream extent, strength, and number of vortices around the jet circumference increase with degree of underexpansion. A large number of vortices is found near the nozzle exit. Fewer vortices of larger scale are found farther downstream, indicative of a merging process. The absence of the vortices in fully expanded and overexpanded jets suggests the vortices are a consequence of a Taylor–Goertler-type instability.
High-frequency noise on antennas in plasmas
Authors: Stenzel R. L.
Published: July 2002

Fluctuations in plasmas at frequencies near the electron plasma frequency (ωp ) have been measured with in situ wire antennas. Such observations are important to the understanding of basic plasma properties (discrete versus collective effects) and the use of antennas in plasmas. The experiments are performed in a large (1 m diameter ×2 m) pulsed dc discharge and afterglow plasma (ne<1012 cm−3, kTe<5 eV) with a weak axial magnetic field (B0=5 G). The fluctuations are detected from wire antennas (length L≫radius a≳Debye length λD) with a low-noise microwave receiver. The observations reveal three different physical processes that determine the noise spectra: (i) single particle shot noise, (ii) collective oscillations by bounded sheath–plasma resonances, and (iii) noise enhancements by longitudinal plasma waves. The first phenomenon (shot noise) gives rise to broadband noise (ωωp) on both electric and magnetic antennas. In the evanescent regime (ω<ωp) , the shot noise is induced by random electron motions through distances of, at most, a collisionless skin layer (c/ωp) around the antenna. Recalling that electron transit-time effects cause absorption of waves in a collisionless skin layer (known as ‘‘anomalous’’ skin absorption) the present observation of a collisionless skin emission effect can also be understood by the equivalence of blackbody absorption and emission coefficients. The second phenomenon (sheath–plasma resonance) is observed as a narrow resonant enhancement in the shot noise below cutoff (ω<ωp) on electric antennas surrounded by sheaths or dielectrics. The series sheath–plasma resonance, usually identified from reflection/absorption measurements with incident waves, is established here, for the first time, as a feature of thermal emission spectra from antennas. The third phenomenon (plasma dielectric ε(ω,k)→0) produces a broad noise enhancement at ω≳ωp on electric antennas.

It can only be observed in the open-loop voltage of dipoles measured with a high-impedance transformer between antenna and 50Ω transmission line. The enhanced noise is electrostatic, randomly polarized, but not significantly enhanced by the presence of energetic electron tails. Existing theories can describe the new observations qualitatively but not quantitatively.
The local–global analysis of the stimulated Brillouin scattering in the regime of nonlinear sound waves
Authors: Rozmus W. , Casanova M. , Pesme D. , Heron A. , Adam J.-C.
Published: July 2002

The effect of ion sound wave (ISW) nonlinearities on the stimulated Brillouin scattering (SBS) in long plasmas is investigated within the framework of the Korteweg–de Vries–Maxwell equations. The nonlinear evolution of the driven ISW results in the localization of the ion density on a scale shorter than the wavelength (λs) of the resonant ISW satisfying SBS three-wave matching conditions. Since the transverse wave amplitudes vary on a much longer scale, a local–global modeling of SBS is proposed in which this scale separation is exploited. The local part of the procedure includes a solution to the damped KdV equation with periodic boundary conditions and driven by a constant amplitude ponderomotive force. In the global part of the analysis approximate solutions for the transverse waves in long plasmas are constructed using the results from the local part. Particle-in-cell simulations have been performed in order to investigate the importance of kinetic effects for the local model. Numerical results obtained from the solutions to the KdV–Maxwell equations are well approximated by the local–global modeling. They are also compared with the results of a harmonic decomposition approximation.
Induced spatial incoherence effects on the convective Raman instability
Authors: Guzdar P. N. , Liu C. S. , Lehmberg R. H.
Published: July 2002

The effect of the induced spatial incoherence method of laser beam smoothing on the convective Raman instability in an underdense inhomogeneous plasma has been studied. It is found that only for bandwidths comparable to, or greater than, the time-averaged homogeneous growth rate, is there significant smoothing effect resulting in a reduction of the backscattered radiation. However, for narrow bandwidths the backscattered radiation computed in the presence of hot spots created by the echelons, turns out to be significantly less than the levels observed in the Naval Research Laboratory experiments. We argue that a combination of three-dimensional effects and filamentation may account for the observed level of enhanced scattering.
Negative viscosity effect in large-scale flows
Authors: Sivashinsky G. , Yakhot V.
Published: July 2002

This study considers a large-scale flow maintained by a small-scale periodic force field. If the field is essentially anisotropic, the system of small-scale eddies thereby generated turns out to be unstable to long-wave disturbances, i.e., the effective viscosity of the corresponding large-scale flow is negative. If the applied field is sufficiently isotropic, the long-wave instability disappears so that the effective viscosity is positive.
Spectrum and propagation of lower-hybrid waves in a tokamak plasma
Authors: Watterson R. L. , Takase Y. , Bonoli P. T. , Porkolab M. , Slusher R. E. , Surko C. M.
Published: July 2002

Waveguide-launched lower-hybrid waves are studied using CO2 laser scattering techniques. These experiments provide the first direct experimental confirmation that the spectrum of the wavelengths inside, but near the edge of, the tokamak plasma is similar to predictions of linear theory for the launched spectrum and varies with waveguide phasing. The spectrum observed farther into the plasma is dominated by longer wavelengths and is much less dependent on phasing. An explanation of these results is presented.
Generation of poloidally rotating spheromaks by the conical theta pinch
Authors: Kawai K. , Pietrzyk Z. A. , Hunter H. T.
Published: July 2002

Spatially resolved spectroscopic measurements revealed the existence of poloidal vortex motion in spheromaks produced in a conical theta pinch. The poloidal velocity on the axis was estimated to be 5 cm/μsec. Large Doppler broadening, which previously was inferred as high ion temperature, is attributed to large internal fluid velocity. A possible effect of the vortex motion in a spheromak merging experiment is studied and found to enhance the fusion reaction rate as a finite temperature beam collider.
Local magnetohydrodynamic instabilities of cylindrical plasma with sheared equilibrium flows
Authors: Bondeson A. , Iacono R. , Bhattacharjee A.
Published: July 2002

The ideal magnetohydrodynamic stability of cylindrical equilibria with mass flows is investigated analytically and numerically. The flows modify the local (Suydam) criterion for instability at the resonant surfaces where k⋅B=0. Sheared flows below the propagation speed for the slow wave are found to be destabilizing for the Suydam modes. At a critical velocity, where the shear of the flow exactly balances the propagation of the slow wave along the sheared magnetic field, and the k⋅B=0 surface is at the edge of a slow wave continuum, there is instability regardless of the pressure gradient. Above the critical velocity, the k⋅B=0 surface is stable, but an infinite sequence of unstable modes still exists with frequencies accumulating toward the edge of the slow wave continuum at nonzero Doppler shifted frequency. The stability of the infinite sequences becomes a nonlocal problem whenever the accumulation frequency overlaps with a continuum at some other radial location.
Long waves at the interface between two viscous fluids
Authors: Charru François , Fabre Jean
Published: July 2002

Using a perturbation method up to the second order, the equation for long waves at the interface between two viscous fluids is derived for plane Couette–Poiseuille flow, and for moderate surface tension. The leading order equation is the Burgers equation. Higher order terms take into account linear dispersive effects and stabilizing effect of surface tension, and involve two more nonlinear terms. The exactness of the coefficients of this equation has been checked by using symmetry properties. For zero gravity, Poiseuille flow is shown to be stable if and only if the velocity profile is convex. Stable Couette flow can become unstable when the Reynolds number of one fluid is decreased, while keeping the other dimensionless parameters unchanged. Introducing characteristic length scales, the interface equation is put into ‘‘canonical’’ form.
Analysis of velocity data taken in Surface Tension Driven Convection Experiment in microgravity
Authors: Kamotani Y. , Ostrach S. , Pline A.
Published: July 2002

Some velocity field results from the Surface Tension Driven Convection Experiment (STDCE) that was conducted aboard the USML-1 Spacelab in 1992 are reported. 10 cSt silicone oil was placed in an open circular container (10 cm wide×5 cm deep) and heated either by a cylindrical heater (1.11 cm diam) placed along the centerline or by a CO2 laser to induce thermocapillary flow. Tests were conducted under varieties of powers, laser beam diameters, and free-surface shapes. The flow field was studied by flow visualization and the data were analyzed by a PIV technique. The results from the velocity measurement are presented and the effects of heating mode and free-surface shape on the flow are discussed. The results are also compared with a numerical analysis conducted in conjunction with the experiment. Good agreement is shown.
Binary fluid convection in a rotating cylinder
Authors: Net Marta , Mercader Isabel , Knobloch Edgar
Published: July 2002

The onset of convection in binary fluid mixtures in a rotating vertical cylinder is considered. Parameter values and boundary conditions relevant to experiments on 3He–4He mixtures with negative separation ratio are used. The eigenfunctions take the form of rigidly precessing spirals. The azimuthal wavenumber of the first unstable mode as the Rayleigh number increases is calculated as a function of the rotation rate and the separation ratio, as are the critical Rayleigh numbers and precession frequencies. Depending on the parameters the spirals may take the form of spatially extended body modes which fill the container, or of wall modes confined to its boundary. The former typically precess in the retrograde direction, while the latter are prograde. Under appropriate circumstances the binary system with a negative separation ratio becomes unstable for lower Rayleigh numbers than a pure fluid. This property of the system is enhanced by the wall modes.
On the transition to columnar convection
Authors: Alonso Arantxa , Net Marta , Knobloch Edgar
Published: July 2002

Convection in a rotating annulus with no-slip sidewalls, stress-free ends, radial gravity, and sideways heating is considered. The transition from fully three-dimensional convection cells to Taylor columns with increasing rotation rate is studied and its dependence on the annulus parameters is established.
Effect of a rapid expansion on the development of compressible free shear layers
Authors: Herrin J. L. , Dutton J. C.
Published: July 2002

Detailed mean velocity and turbulence data have been obtained with a laser Doppler velocimeter for two axisymmetric shear layers downstream of rapid expansions of different strengths. A comparison of the data in the near field (immediately downstream of separation) and far field (shear layer approaching self-similarity) is presented, and the fluid dynamic effects of the rapid expansion are ascertained for each regime. In general, the rapid expansion was found to distort the initial mean velocity and turbulence fields in the shear layer, in a manner similar to that in rapidly expanded, attached supersonic boundary layers; namely, two distinct regions were found in the initial shear layer: an outer region, where the turbulent fluctuations are quenched primarily due to mean compressibility effects (bulk dilatation), and an inner region, where turbulence activity is magnified due to the interaction of organized large-scale structures in the shear layer with low-speed fluid at the inner edge. With increasing strength of the rapid expansion, the effects in both regions become more pronounced, especially in the inner region, where turbulent fluctuations and mass entrainment rates are greatly magnified. Farther downstream, the turbulence activity of the large-scale eddies remains elevated, due to the rapid expansion, even though the relative distribution of the turbulence energy between the Reynolds stress components (structure of the turbulence) is independent of expansion strength.
Head-on collision of drops—A numerical investigation
Authors: Nobari M. R. , Jan Y.-J. , Tryggvason G.
Published: July 2002

The head-on collision of equal sized drops is studied by full numerical simulations. The Navier–Stokes equations are solved for the fluid motion both inside and outside the drops using a front tracking/finite difference technique. The drops are accelerated toward each other by a body force that is turned off before the drops collide. When the drops collide, the fluid between them is pushed outward leaving a thin layer bounded by the drop surface. This layer gets progressively thinner as the drops continue to deform, and in several of our calculations we artificially remove this double layer at prescribed times, thus modeling rupture. If no rupture takes place, the drops always rebound, but if the film is ruptured the drops may coalesce permanently or coalesce temporarily and then split again. Although the numerically predicted boundaries between permanent and temporary coalescence are found to be consistent with experimental observations, the exact location of these boundaries in parameter space is found to depend on the time of rupture.
Streamwise and spanwise vortex interaction in an axisymmetric jet. A computational and experimental study
Authors: Grinstein F. F. , Gutmark E. J. , Parr T. P. , Hanson-Parr D. M. , Obeysekare U.
Published: July 2002

The near-field of an azimuthally excited round jet was investigated in a combined computational/experimental study. The reaction zones in the jet were visualized using OH Planar-Laser- Induced-Fluorescence (PLIF) diagnostics. Both axisymmetric and azimuthal modes of the jet were excited to stabilize its spatial structure. Three-dimensional flame visualization of the laboratory jet reconstructed from multiple two-dimensional images acquired at constant phase angle, reveal a complex structure of the reaction zone. Time-dependent numerical simulations provided insight into the underlying fluid-dynamical processes leading to this flame structure. Simulations of reactive and non-reactive free jets used a Monotonically Integrated Large-Eddy-Simulation (MILES) approach, multi-species diffusive transport, global finite-rate chemistry and appropriate inflow/outflow boundary conditions. The flow visualizations of the experimental and computational jets strongly resemble each other, revealing tight coupling between axisymmetric vortex rings and braid (rib) vortices. The jet vorticity evolution is dominated by the dynamics of vortex-ring self-deformation induced by the azimuthal excitation imposed at the jet exit, the dynamics of rib vortices forming in the braid regions between undulating vortex rings, and strong interactions between rings and ribs. The observed topological features of the flow are directly related to the nearly-inviscid jet vorticity dynamics. These processes affect the mixing pattern of the jet, resulting in localized regions of high fuel concentration leading to combustion inactive regions in the flame, and other regions with enhanced mixing and a proper air-to-fuel ratio in the flame where the combustion process is intense. The vorticity dynamics and ensuing mixing processes determine the regions of combustion within the flame and thus the overall heat release pattern.
Accuracy of the lattice Boltzmann method for small Knudsen number with finite Reynolds number
Authors: Inamuro Takaji , Yoshino Masato , Ogino Fumimaru
Published: July 2002

The asymptotic theory proposed by Sone [in Rarefied Gas Dynamics, edited by D. Dini (Editrice Tecnico Scientifica, Pisa, 1971), p. 737] is applied to the investigation of the accuracy of the lattice Boltzmann method (LBM) for small Knudsen number with finite Reynolds number. The S-expansion procedure of the asymptotic theory is applied to LBM with the nine-velocity model and fluid-dynamic type equations are obtained. From the fluid-dynamic type equations it is found that by using the LBM we can obtain the macroscopic flow velocities and the pressure gradient for incompressible fluid with relative errors of O(ε′2) where ε′ is a modified Knudsen number which is of the same order as the lattice spacing and is related to a dimensionless relaxation time. In two problems, the Couette flow with flow injection and suction through porous walls and a three-dimensional flow through a square duct, the accuracy of LBM is examined for relaxation times between 0.8 and 1.7 and the validity of the asymptotic theory for LBM is shown.
The turbulence structure of a reattaching axisymmetric compressible free shear layer
Authors: Herrin J. L. , Dutton J. C.
Published: July 2002

The reattachment of a supersonic, axisymmetric shear layer downstream of a blunt-based afterbody is studied. Of primary interest are the effects of the “extra” strain rates, such as bulk compression, concave streamline curvature, and lateral streamline convergence associated with shear layer reattachment on the structure of the turbulence field. Experimental turbulence data obtained throughout the reattachment region with a two-component laser Doppler velocimeter are presented. In general, the axisymmetric compliant boundary reattachment process is shown to be different in character compared to the planar solid wall case. Most notably, significant reductions in the Reynolds stresses occur through the reattachment region due to the dominating effect of lateral streamline convergence as the flow approaches the axis. Similar to the planar solid wall case, however, a reduction in the mean turbulent transport toward the axis in the reattachment region was found, which suggests a radial containment of the large-scale eddies near the axis of symmetry. The reattachment process was also seen to have profound effects on the large-scale structures in the shear layer, primarily through reduced structural organization as indicated by instantaneous shear angle histograms.
Red sprites and blue jets: Thunderstorm-excited optical emissions in the stratosphere, mesosphere, and ionosphere
Authors: Sentman D. D. , Wescott E. M.
Published: July 2002

Recent low light level monochrome television observations obtained from the ground and from the space shuttle, and low light level color and monochrome television images obtained from aboard jet aircraft, have shown that intense lightning in mesoscale thunderstorm systems may excite at least two distinct types of optical emissions that together span the space between the tops of some thunderstorms and the ionosphere. The first of these emissions, dubbed ‘‘sprites,’’ are luminous red structures that typically span the altitude range 60–90 km, often with faint bluish tendrils dangling below. A second, rarer, type of luminous emission are ‘‘blue jets’’ that appear to spurt upward out of the anvil top in narrow cones to altitudes of 40–50 km at speeds of ∼100 km/s. In this paper the principal observational characteristics of sprites and jets are presented, and several proposed production mechanisms are reviewed.
Ion transport process around magnetic axis in tokamaks
Authors: Shaing K. C. , Hazeltine R. D. , Zarnstorff M. C.
Published: July 2002

Ion transport in the region close to the magnetic axis in tokamaks is investigated by solving the drift kinetic equation with the proper orbit topology taken into account. It is found that ion thermal conductivity remains finite in that region. The scaling of the thermal conductivity can be understood in terms of a random walk process in units of the poloidal flux.
Collisional δf method
Authors: Chen Yang , White Roscoe B.
Published: July 2002

A general method for including various collisional effects, such as the drag and diffusion of test particles due to background plasmas, the effect of particle source and sink, and the like-particle Coloumb collisions, is presented. The marker density g is generally unknown along the particle trajectory, and its evaluation depends on the way particles are initially loaded and new particles are injected into the simulation. The method is demonstrated for the problem of the nonlinear evolution of the toroidicity induced Alfvén eigenmode, driven by energetic α particles. The saturation amplitude is found to scale with the collision rate in a way as predicted by theory.
Signatures of deuterium recombination in the DIII-D divertor
Authors: Isler R. C. , McKee G. R. , Brooks N. H. , West W. P. , Fenstermacher M. E. , Wood R. D.
Published: July 2002

Thomson scattering measurements performed in the divertor of the DIII-D tokamak [Plasma Physics Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] during detached operation show that the electron temperatures are typically between 0.8 and 2.0 eV throughout a region which may extend several centimeters above the target plate. At such low temperatures the excitation of recycling deuterium atoms or impurities should be weak. Nevertheless, significant radiation is frequently detected in these locations. It has been suggested that recombination processes, which become important only below about 1.5 eV for deuterium, are responsible for the observed emission. This hypothesis has been investigated by comparing ratios of deuterium lines from attached and detached plasmas with theoretical ratios expected for ionizing or recombining conditions. The analysis of several discharges indicates that the mechanism for production of the emission changes from being collisional excitation of atomic deuterium to a mixture of collisional-radiative recombination and collisional excitation as plasmas evolve from attached to detached states. Localization of D-α emission to low-temperature regions using tangentially viewing camera data together with Thomson scattering results and measurements of deuterium atom temperatures are consistent with these conclusions.
Results from current drive experiments on the Helicity Injected Torus
Authors: Jarboe T. R. , Bohnet M. A. , Mattick A. T. , Nelson B. A. , Orvis D. J.
Published: July 2002

The Helicity Injected Torus [T. R. Jarboe, Fusion Technol. 15, 7 (1989)] is a low aspect ratio tokamak that is formed and sustained by coaxial helicity injection with no transformer. Toroidal plasma currents of over 200 kA have been achieved with electron temperatures in the 100 eV range and electron density between 1019 and 1020 m−3. The major radius is 0.3 m and the minor radius is 0.2 m. New results from equilibrium and stability analysis of the external magnetic diagnostics and new results from the Transient Internal Probe (TIP), an internal magnetic field diagnostic, are presented. A mechanism for the transfer of current drive on the open to the closed flux regions is presented.
Insights and applications of two-dimensional simulations to Z-pinch experiments
Authors: Peterson D. L. , Bowers R. L. , Matuska W. , McLenithan K. D. , Chandler G. A. , Deeney C. , Derzon M. S. , Douglas M. , Matzen M. K. , Nash T. J. , Spielman R. B. , Struve K. W. , Stygar W. A. , Roderick N. F.
Published: July 2002

A two-dimensional (2D) Eulerian radiation-magnetohydrodynamic code has been used to successfully simulate hollow metallic z-pinch experiments fielded on several facilities with a wide variety of drive conditions, time scales, and loads. The 2D simulations of these experiments reproduce important quantities of interest including the radiation pulse energy, power, and pulse width. This match is obtained through the use of an initial condition: the amplitude of a random density perturbation imposed on the initial plasma shell. The perturbations seed the development of magnetically driven Rayleigh–Taylor instabilities which greatly affect the dynamics of the implosion and the resulting production of radiation. Analysis of such simulations allows insights into the physical processes by which these calculations reproduce the experimental results. As examples, the insights gained from the simulations of Sandia “Z” accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] experiments have allowed for the investigation of possible physical processes which produce high powers in “nested array” implosions and high temperatures within “dynamic hohlraum” loads. Building on these insights, the 2D code has been used in designing new experiments to optimize the desired physical conditions and in interpreting the experimental results obtained. These examples and others will be discussed as well as examples of simulation results where improvement is needed and what steps are being taken to make that improvement.
Electron–anode interactions in particle-in-cell simulations of applied-B ion diodes
Authors: Vesey R. A. , Pointon T. D. , Cuneo M. E. , Mehlhorn T. A. , Bailey J. E. , Johnson D. J. , Stygar W. A.
Published: July 2002

Particle-in-cell simulations of applied-B ion diodes using the QUICKSILVER code [D. B. Seidel et al., in Proceedings of the Europhysics Conference on Computational Physics, Amsterdam, 1990, edited by A. Tenner (World Scientific, Singapore, 1991), p. 475] have been augmented with Monte Carlo calculations of electron–anode interactions (reflection and energy deposition). Extraction diode simulations demonstrate a link between the instability evolution and increased electron loss and anode heating. Simulations of radial and extraction ion diodes show spatial nonuniformity in the predicted electron loss profile leading to hot spots on the anode that rapidly exceed the 350 °C–450 °C range, known to be sufficient for plasma formation on electron-bombarded surfaces. Thermal desorption calculations indicate complete desorption of contaminants with 15–20 kcal/mole binding energies in high-dose regions of the anode during the power pulse. Comparisons of parasitic ion emission simulations and experiment show agreement in some aspects, but also highlight the need for better ion source, plasma, and neutral gas models.
Stability of Bernstein–Greene–Kruskal modes
Authors: Manfredi Giovanni , Bertrand Pierre
Published: July 2002

The stability of Bernstein–Greene–Kruskal (BGK) modes is investigated in the limit of small electric potential (weak inhomogeneity). It is proven that one-hole BGK modes can be unstable, contrarily to what was observed in previous numerical simulations. A simple stability criterion is derived. In particular, it is proven that the velocity distribution must have at least three maxima for instability to occur. Numerical simulations confirm the analytical results, and extend them to the nonlinear and strongly inhomogeneous regimes. In particular, it is shown that a strong inhomogeneity has a stabilizing effect.
Doping level selective photochemical dry etching of GaAs
Authors: Ashby C. I. H.
Published: July 2002

The first observation of a highly selective photochemical dry etching process which discriminates between semiconductor materials differing in dopant concentrations by less than a factor of 5 is reported here. A photochemical reaction of GaAs with gas phase reactive Cl species occurs when the surface is irradiated with low-intensity light of band gap or greater quantum energies. Application of an appropriate negative bias to a GaAs sample can almost totally suppress the photochemical reaction of heavily doped n-GaAs, while less heavily doped n-GaAs or p-GaAs continue to etch at undiminished rates under the same conditions. This is the first reported etching process to produce greater than 20:1 selectivity for doping levels differing by less than a factor of 5. A mechanism which may explain the origin of the photochemical reaction and the voltage-controlled doping level selectivity which it displays is presented here. The potential significance of these observations for other semiconductor materials, such as Si, is discussed.
Efficient second harmonic generation of 10-μm radiation in AgGaSe2
Authors: Eckardt R. C. , Fan Y. X. , Byer R. L. , Route R. K. , Feigelson R. S. , van der Laan Jan
Published: July 2002

AgGaSe2 crystals for nonlinear infrared applications are being grown reproducibly. Using high-quality, 2-cm-long crystals, 14% energy and 60% peak intensity conversion efficiency have been demonstrated for second harmonic generation of the pulsed output of a CO2 miniature transversely excited atmospheric laser.
Characterization of reactively sputtered ruthenium dioxide for very large scale integrated metallization
Authors: Krusin-Elbaum L. , Wittmer M. , Yee D. S.
Published: July 2002

We have investigated reactively sputtered films of RuO2 for possible application in very large scale integrated circuits. Sputtering yields stoichiometric ruthenium dioxide in a large window of oxygen pressures and the films are reasonably low stressed in the 10−9 dyn cm−2 range. The resistivity of as deposited films is 40 μΩ cm. The films are excellent barriers against interdiffusion of Si and Al.
Short pulse amplification in the presence of absorption
Authors: Tilleman M. M. , Jacob J. H.
Published: July 2002

Energy amplification of short pulses in the presence of distributed absorption is analyzed and discussed. A short pulse is defined as a pulse length that is much shorter than the lasing state lifetime. It is shown that the energy density of a short pulse cannot be amplified indefinitely in the presence of nonsaturable absorption. In an active medium having a small signal gain g0 and a nonsaturable absorption α, the maximum attainable energy density is (Esat g0/α), where Esat is the medium saturation energy. High-energy densities are obtained at the expense of the extraction efficiency. An amplifier design can be optimized by choosing the appropriate parameters such as small signal gain and active medium gain length.
Stable photoinduced paramagnetic defects in hydrogenated amorphous silicon nitride
Authors: Krick David T. , Lenahan P. M. , Kanicki J.
Published: July 2002

Room-temperature stable paramagnetic defects have been induced in hydrogenated amorphous silicon nitride films by exposure to sub-band-gap ultraviolet light. These defects are almost certainly trivalent silicon centers which can be completely annealed at temperatures higher than 250 °C. These defects seem similar in chemical origin to light-induced centers in hydrogenated amorphous silicon and defects in irradiated metal-oxide-semiconductor field-effect transistors.
Bias dependence of instability mechanisms in amorphous silicon thin-film transistors
Authors: Powell M. J. , van Berkel C. , French I. D. , Nicholls D. H.
Published: July 2002

We have measured the bias dependence of the threshold voltage shift in a series of amorphous silicon-silicon nitride thin-film transistors, where the composition of the nitride is varied. There are two distinct instability mechanisms: a slow increase in the density of metastable fast states and charge trapping in slow states. State creation dominates at low fields and charge trapping dominates at higher fields. The state creation is found to be independent of the nitride composition, whereas the charge trapping depends strongly on the nitride composition. This is taken as good evidence that state creation takes place in the hydrogenated amorphous silicon (a-Si:H) layer, whereas the charge trapping takes place in the a-SiN:H. The metastable states are suggested to be Si dangling bonds in the a-Si:H, and the state creation process similar to the Staebler–Wronski effect. The confirmation of state creation in a thin-film transistor means that states can be created simply by populating conduction-band states in the undoped material. The slow states are also thought to be Si dangling bonds, but located in the silicon nitride matrix.
GexSi1−x strained-layer heterostructure bipolar transistors
Authors: Temkin H. , Bean J. C. , Antreasyan A. , Leibenguth R.
Published: July 2002

Double heterostructure bipolar transistors with the base region consisting of a p-Ge0.5Si0.5 strained-layer superlattice have been grown by molecular beam epitaxy. At a wavelength of 1.3 μm, optical gain as high as 52 has been achieved in two-terminal phototransistors. The large photocurrent is inferred to be a product of the transistor gain, on the order of 20, and avalanche multiplication. A differential current gain of 10 has been obtained in the three-terminal bipolar transistors. The incorporation of a narrow band-gap GexSi1−x superlattice base is expected to result in higher emitter injection efficiency as compared to Si bipolar transistors.
Measurement of ultrafast hot-carrier relaxation in silicon by thin-film-enhanced, time-resolved reflectivity
Authors: Doany F. E. , Grischkowsky D.
Published: July 2002

Time-resolved reflectivity measurements with ∼100 fs resolution have revealed an initial 350 fs relaxation time in silicon, believed to be the time it takes hot, photoinjected carriers to relax to the band edge. The measurements were made at low carrier densities (∼1017 cm−3) for which carrier-carrier processes are negligible, and were facilitated by the greater than order of magnitude enhancement of the change in reflectivity signals that can be produced by the use of thin films.
Optical generation and detection of surface acoustic waves on a sphere
Authors: Royer D. , Dieulesaint E. , Jia X. , Shui Y.
Published: July 2002

Surface acoustic waves (SAW’s) have been optically generated and detected on metallic spheres. The source was a focused yttrium aluminum garnet (YAG) laser operated in the Q-switched mode and the probe was a sensitive optical heterodyne interferometer. The curvature of the surface gives rise to dispersive effects. The variation of the SAW group velocity is deduced from the evolving waveforms of pulses detected after propagating a few turns around the sphere. The observation of SAW pulse waveforms at positions away from the pole (a point diametrically opposite to the source) reveals that the spectral components undergo a π-phase shift when passing through the pole.
A short segment within the cytoplasmic domain of the neural cell adhesion molecule (N-CAM) is essential for N-CAM-induced NF-κB activity in astrocytes
Authors: Little Edward B. , Crossin Kathryn L. , Krushel Leslie A. , Edelman Gerald M. , Cunningham Bruce A.
Published: July 2002

The neural cell adhesion molecule (N-CAM) is expressed on the
surface of astrocytes, where its homophilic binding leads to the
activation of the transcription factor NF-κB. Transfection of
astrocytes with a construct encompassing the transmembrane region and
the cytoplasmic domain of N-CAM (designated Tm-Cyto, amino acids
685–839 in the full-length molecule) inhibited this activation up to
40%, and inhibited N-CAM-induced translocation of NF-κB to the
nucleus. N-CAM also activated NF-κB in astrocytes from N-CAM knockout
mice, presumably through binding to a heterophile. This activation,
however, was not blocked by Tm-Cyto expression, indicating that the
inhibitory effect of the Tm-Cyto construct is specific for cell surface
N-CAM. Deletions and point mutations of the cytoplasmic portion of the
Tm-Cyto construct indicated that the region between amino acids 780 and
800 were essential for inhibitory activity. This region contains four
threonines (788, 793, 794, and 797). Mutation to alanine of T788, T794,
or T797, but not T793, abolished inhibitory activity, as did mutation
of T788 or T797 to aspartic acid. A Tm-Cyto construct with T794 mutated
to aspartic acid retained inhibitory activity but did not itself induce
a constitutive NF-κB response. This result suggests that
phosphorylation of T794 may be necessary but is not the triggering
event. Overall, these findings define a short segment of the N-CAM
cytoplasmic domain that is critical for N-CAM-induced activation of
NF-κB and may be important in other N-CAM-mediated signaling.
The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: Structure, quantum-theoretical analysis, and simultaneity principles
Authors: Catalán Javier , Pérez Pilar , del Valle Juan Carlos , de Paz J. L. G. , Kasha Michael
Published: July 2002

Six stable dimer models for 7-azaindole (including the classic
C
2h
doubly hydrogen-bonded, coplanar, centrosymmetric dimer) are considered to be observable in adiabatic nozzle jet molecular beams. They are analyzed by hybrid density functional theory (DFT), the MP2
ab initio
method for the ground electronic state, and the single-excitation configuration interaction (CIS) (over frozen ground state optimized geometries obtained from DFT) excited state calculations, for global potential minima and proton-transfer potential energy curves. Three simultaneity principles are stated: (
i
) intermolecular coherent excitation molecular exciton simultaneity, (
ii
) intramolecular acid–base change simultaneity at the pyrrolo-N-H and aza-N proton-donor, proton-acceptor sites, and (
iii
) intermolecular simultaneity of catalytic proton-donor, proton-acceptor action. It is suggested that the formation of the classic
C
2h
dimer of 7-azaindole, which is considered exclusively by previous researchers, can be formed from at least one of the several card-pack hydrogen-bonded dimers in a secondary slower step approaching a microsecond scale, instead of the picosecond events at the supersonic nozzle. It is proposed that the complexity of dimerization modes is the basis of the postexcitation, postionization diverse kinetic isotope results.
The ricinosomes of senescing plant tissue bud from the endoplasmic reticulum
Authors: Schmid Markus , Simpson David J. , Sarioglu Hakan , Lottspeich Friedrich , Gietl Christine
Published: July 2002

The ricinosome (synonym, precursor protease vesicle) is a novel
organelle, found so far exclusively in plant cells. Electron
microscopic studies suggest that it buds off from the endoplasmic
reticulum in senescing tissues. Biochemical support for this unusual
origin now comes from the composition of the purified organelle, which
contains large amounts of a 45-kDa cysteine endoprotease precursor with
a C-terminal KDEL motif and the endoplasmic reticulum lumen residents
BiP (binding protein) and protein disulfide isomerase. Western blot
analysis, peptide sequencing, and mass spectrometry demonstrate
retention of KDEL in the protease proform. Acidification of isolated
ricinosomes causes castor bean cysteine endopeptidase activation, with
cleavage of the N-terminal propeptide and the C-terminal KDEL motif. We
propose that ricinosomes accumulate during senescence by programmed
cell death and are activated by release of protons from acidic
vacuoles.
MEKK1 is essential for cardiac hypertrophy and dysfunction induced by Gq
Authors: Minamino Tetsuo , Yujiri Toshiaki , Terada Naohiro , Taffet George E. , Michael Lloyd H. , Johnson Gary L. , Schneider Michael D.
Published: July 2002

Signaling via mitogen-activated protein kinases is implicated in heart failure induced by agonists for G protein-coupled receptors that act via the G protein Gαq. However, this assertion relies heavily on pharmacological inhibitors and dominant-interfering proteins and not on gene deletion. Here, we show that endogenous cardiac MAPK/ERK kinase kinase-1 (MEKK1)/(MAP3K1), a mitogen-activated protein kinase kinase kinase, is activated by heart-restricted overexpression of Gαq in mice. In cardiac myocytes derived from embryonic stem cells in culture, homozygous disruption of MEKK1 selectively impaired c-Jun N-terminal kinase activity in the absence or presence of phenlyephrine, a Gαq-dependent agonist. Other terminal mitogen-activated protein kinases were unaffected. In mice, the absence of MEKK1 abolished the increase in cardiac mass, myocyte size, hypertrophy-associated atrial natriuretic factor induction, and c-Jun N-terminal kinase activation by Gαq, and improved ventricular mechanical function. Thus, MEKK1 mediates cardiac hypertrophy induced by Gαqin vivoand is a logical target for drug development in heart disease involving this pathway.