bluuebunny/crossref_metadata_2025 huggingface dataset:
Ballooning stability boundaries with plasma compression
Authors: Cooper W. Anthony , Sigmar D. J. , Hirshman S. P.
Published: July 2002

Level curves for the resistive compressible ballooning growth rates and the ideal ballooning stability boundaries with incompressibility and perpendicular compressibility are determined in the parameter space of shear-versus-pressure gradient for realistic tokamak equilibria. The trajectories of these level curves differ from the ideal incompressible stability boundary in this space.
Kinetic theory and simulation of multispecies plasmas in tokamaks excited with electromagnetic waves in the ion-cyclotron range of frequencies
Authors: Kerbel G. D. , McCoy M. G.
Published: July 2002

A description of a bounce-averaged Fokker–Planck quasilinear model for the kinetic description of tokamak plasmas is presented. The nonlinear collision and quasilinear resonant diffusion operators are represented in a form conducive to numerical solution with specific attention to the treatment of the boundary layer separating trapped and passing orbit regions of velocity space. The numerical techniques employed are detailed insofar as they constitute significant departure from those used in the conventional uniform magnetic field case. Examples are given to illustrate the combined effects of collisional and resonant diffusion.
Physical estimates of the number of degrees of freedom in free convection
Authors: Foias C. , Manley O. P. , Temam R.
Published: July 2002

It is shown how simple physical arguments yield estimates of the number of degrees of freedom governing Bénard convection. Those estimates are close to those arrived at by rigorous studies of dynamical systems associated to the partial differential equations of fluid mechanics. It is suggested that, guided by additional physical considerations, the estimate of the sufficient number of degrees of freedom may be reduced further.
Effects of a radial electric field on tokamak edge turbulence
Authors: Chiueh T. , Terry P. W. , Diamond P. H. , Sedlak J. E.
Published: July 2002

Turbulence associated with sheared radial electric fields such as those arising in tokamak edge plasmas is investigated analytically. Two driving mechanisms are considered: in the region of maximum vorticity (maximum electric field shear), the electric field is the dominant driving mechanism. Away from the maximum, turbulence is driven by the density gradient. In the latter case, previous work is extended to include the effects of the electric field on the spatial scales of density correlation in the frequency-Doppler-shifted, density-gradient-driven turbulence. For radial-electric-field-driven turbulence, the effects of magnetic shear on linear instability and on fully developed turbulence are examined. In the case of weak magnetic shear, saturation occurs through an enstrophy cascade process which couples regions of driving and dissipation in wavenumber space. For stronger magnetic shear, such that the width of the dissipation region resulting from parallel resistivity is comparable to the radial electric field scale length, saturation occurs through nonlinear broadening of the mode structure, which pushes enstrophy into the region of dissipation. Estimates of mode widths, fluctuation levels, and scalings are obtained for both mechanisms. Comparison is made with the results of fluctuation measurements in the TEXT tokamak [Phys. Fluids 27, 2956 (1984)].
Contribution to half-harmonic splitting resulting from Raman backscattering in a rippled plasma
Authors: Figueroa H. , Joshi C.
Published: July 2002

The dispersion relation of electron plasma waves in a rippled plasma is analyzed. The ripple couples the plasma wave to its spatial harmonics thereby introducing ‘‘forbidden’’ frequency bands. It is shown that the stimulated Brillouin scattering (SBS) induced ripple can directly influence the frequency matching condition for Raman near the quarter-critical density and thereby lead to an asymmetric splitting of the half-harmonic radiation. The predicted blue-shifted peak is found to be more sensitive to the ripple amplitude than the red-shifted peak. This mechanism may contribute to the half-harmonic splitting that is observed in many experiments.
Magnetohydrodynamic stability of hot-electron stabilized tandem mirrors
Authors: Dobrott D. , Kendrick S. , Freis Robert P. , Cohen Bruce I.
Published: July 2002

The influence of energetic electron rings, or disks, on the magnetohydrodynamic stability of an axisymmetric tandem mirror configuration is studied. Linear stability equations describing interchange and ballooning modes are solved both analytically and numerically using a two-dimensional stability code for axisymmetric mirrors. Finite-ion-Larmor-radius effects and the presence of a lateral wall are included in the calculations. Stable configurations can be obtained with either sufficiently high hot-electron pressure or with sufficiently high center-cell pressure if a lateral conducting wall is close enough to the plasma. In particular, hot electrons might be used to stabilize transiently the plasma during buildup to a high-beta, wall-stabilized configuration.
Injection of an electron beam into a plasma and spacecraft charging
Authors: Okuda H. , Kan J. R.
Published: July 2002

Injection of a nonrelativistic electron beam into a fully ionized plasma from a spacecraft including the effect of charging has been studied using a one-dimensional particle simulation model. It is found that the spacecraft charging remains negligible and the beam can propagate into a plasma, if the beam density is much smaller than the ambient density. When the injection current is increased by increasing the beam density, significant spacecraft charging takes place and the reflection of beam electrons back to the spacecraft reduces the beam current significantly. On the other hand, if the injection current is increased by increasing the beam energy, spacecraft charging remains negligible and a beam current much larger than the thermal return current can be injected. It is shown that the electric field caused by the beam–plasma instability accelerates the ambient electrons toward the spacecraft thereby enhancing the return current.
Solitary waves in the resonant phenomenon between a surface gravity wave packet and an internal gravity wave
Authors: Sepúlveda Nicasio
Published: July 2002

A two-layer inviscid incompressible fluid system of intermediate depth is considered. A multiple-scales perturbation technique is applied to the basic equations and boundary conditions for a two-layer fluid system to derive a system of weakly nonlinear partial integrodifferential equations governing the resonant interaction between a surface gravity wave packet and an internal gravity wave at an intermediate depth, providing a bridge between the existing shallow and deep fluid theories. The convolution integral term in these equations accounts for the dispersion in the lower-layer fluid. An iterative fast Fourier transform scheme is developed to find solitary wave solutions to this system of equations. The overtaking collision of two pairs of solitary waves, simulated using a spectral method, is found to be inelastic. It is found that the amplitude of the solitary waves changes slightly after the collision. The phase shifts these solitary waves undergo was calculated numerically.
The observability of hexagons in modulated Rayleigh–Bénard convection
Authors: Hohenberg P. C. , Swift J. B.
Published: July 2002

The prediction of Roppo, Davis, and Rosenblat [Phys. Fluids 27, 796 (1984)] that hexagons should appear near threshold in a Rayleigh–Bénard cell under external modulation of the temperature is reconsidered with a view to determining the experimental observability of the effect. It is found that in the limit of low-modulation amplitude δ considered by Roppo et al. the effects are qualitatively as given by these authors, but considerably smaller quantitatively. The validity limits of the small-δ expansion are clarified and results of an alternative theory of hexagonal effects, valid at arbitrary amplitude δ and frequency ω, are briefly summarized. Hexagons will only be observable if large amplitudes can be achieved.
Penetrative double-diffusive convection
Authors: Antar Basil N.
Published: July 2002

The onset of double-diffusive convection is studied for an infinite fluid layer possessing a density maximum in its interior. Linear stability analyses are performed on a basic state with constant temperature and concentration gradients in which the density-temperature relationship is quadratic. Regions of stability and instability to both steady and oscillatory modes are delineated in the positive quarterplane of the Rayleigh and solutal Rayleigh numbers plane. It is found that decreasing the position of maximum density in the vertical leads to an increased stability range and also increased regions of oscillatory instability in that quadrant. It is also found that the extent of penetration of the convective motion into the stable region is diminished with increased solute concentration. The effects of both the rigid–rigid and the free–free boundary conditions on the instabilities are investigated. Both conditions lead to dynamically similar motions with significant quantitative differences.
Instability of a deformed liquid drop in an acoustic field
Authors: Shi W. Tao , Apfel Robert E. , Holt R. Glynn
Published: July 2002

The flattening and breakup of an axially symmetric liquid drop in an acoustic standing wave field in air have been studied using a boundary integral method. The interaction between the drop and sound field are crucial to this approach. Our computations are focused on the threshold beyond which the drop loses its static equilibrium, and on the dynamic behavior after it loses its equilibrium up to the point when it breaks up. The numerical results are given in terms of drop size and the strength of sound field and are found to be in good agreement with the measurements of others.
On the alignment of the α-strain and vorticity in turbulent nonpremixed flames
Authors: Boratav O. N. , Elghobashi S. E. , Zhong R.
Published: July 2002

Evolution of the strain and vorticity fields in a turbulent nonpremixed flame is examined using direct numerical simulations. The results show that the flow is strain dominated for both the buoyant and nonbuoyant cases. The evolution equation for the cosine of the angle between the vorticity and the principal strain directions is derived. It is shown that the strain dominance causes vorticity to align with the α-strain, in contrast to the β-strain/vorticity alignment in constant density flows. The divergence term in the evolution equation changes the time-scale of the process. The rotation term (due to the strain coordinates rotation) and the baroclinic term reduce the vorticity/α-strain alignment.
Experimental investigations on the nature of the first wavy instability in liquid-fluidized beds
Authors: Nicolas Maxime , Chomaz Jean-Marc , Vallet Denis , Guazzelli Élisabeth
Published: July 2002

Experiments are described which suggest that the first wavy instability of fluidized beds is convective in nature. In particular, this instability is shown to be sensitive to a harmonic forcing localized at the bottom of the bed.
Experimental and numerical study of Rayleigh-Bénard convection affected by a rotating magnetic field
Authors: Friedrich J. , Lee Y.-S. , Fischer B. , Kupfer C. , Vizman D. , Müller G.
Published: July 2002

In the present paper we experimentally study the effects of a rotating magnetic field (RMF) on the fluid flow in an electrically conducting melt (Gallium), kept in a cylindrical container heated from below (Rayleigh-Bénard configuration). The experimental data are compared to results obtained from three-dimensional, time-dependent numerical calculations. The paper presents the influence of the magnetic induction B, the frequency of the RMF Ω, and the temperature difference ΔT between the hot bottom and cold top of the melt on heat transport and fluid flow, respectively. The results can be summarized in terms of the parameter Nrot, which is defined as the ratio of magnetic Taylor number (∝B2⋅Ω) to Grashof number (∝ΔT). It is shown that for 0.003<Nrot<0.1 large-scale regular thermal waves exist, which travel azimuthally in the same direction as the rotation direction of the RMF. These thermal waves are connected with large-scale temperature fluctuations (amplitude 6%–10% of ΔT). The amplitude decreases with increasing Nrot, whereas the mean frequency increases from 0.001 Hz up to 0.1 Hz for 0.003<Nrot<0.1. For Nrot>0.1 temperature fluctuations with amplitudes smaller than 1%–2% of ΔT and frequencies greater than 0.1 Hz are observed. These oscillations can be attributed to Taylor vortices generated at the vertical cylinder walls. The regions of the different oscillation modes within the parameter space are shown in a stability diagram.
The flow and mixing mechanisms caused by the impingement of an immiscible interface with a vertical jet
Authors: Friedman P. D. , Katz J.
Published: July 2002

This paper summarizes a series of experiments to determine the effects of impinging a water and diesel fuel interface with a water jet. Four distinct flow regimes and associated mixing mechanisms were identified as primarily a function of an interface Richardson number, Rii and to a lesser extent the Reynolds number, Re. In flow regime 1, which occurred at Rii>15, the jet impinged the interface and formed a slight deformation. Flow behaved similarly to a jet impinging a solid boundary and no mixing was observed. Regime 2 occurred at 15>Rii>1.1. With decreasing Rii, the interfacial deformation became taller and steeper and a flow separation developed at its edge. Mixing resulted from the formation of a fuel lip at the flow separation point. In regime 3, which occurred at Rii<1.1, the water jet penetrated deeply into the fuel layer causing an unstable deformation that successively collapsed and reformed. Mixing resulted from the impact of the collapsing deformation. Finally, in regime 4, the jet’s momentum was sufficient to impact the solid boundary at the top of the fuel layer, creating a cloud of water droplets.
Calculations of longitudinal and transverse velocity structure functions using a vortex model of isotropic turbulence
Authors: He Guowei , Doolen Gary D. , Chen Shiyi
Published: July 2002

The longitudinal structure function (LSF) and the transverse structure function (TSF) in isotropic turbulence are calculated using a vortex model. The vortex model is composed of the Rankine and Burgers vortices which have the exponential distributions in the vortex Reynolds number and vortex radii. This model exhibits a power law in the inertial range and satisfies the minimal condition of isotropy that the second-order exponent of the LSF in the inertial range is equal to that of the TSF. Also observed are differences between longitudinal and transverse structure functions caused by intermittency. These differences are related to their scaling differences which have been previously observed in experiments and numerical simulations.
Applications of intense pulsed ion beam to materials science*
Authors: Yatsui K. , Kang X. D. , Sonegawa T. , Matsuoka T. , Masugata K. , Shimotori Y. , Satoh T. , Furuuchi S. , Ohuchi Y. , Takeshita T. , Yamamoto H.
Published: July 2002

In addition to being initially developed as an energy driver for an inertial confinement fusion, an intense, pulsed, light-ion beam (LIB) has been found to be applied to materials science. If a LIB is used to irradiate targets, a high-density ‘‘ablation’’ plasma is produced near the surface since the range of the LIB in materials is very short. Since the first demonstration of quick preparation of thin films of ZnS by an intense, pulsed, ion-beam evaporation (IBE) using the LIB-produced ablation plasma, various thin films have been successfully prepared, such as of ZnS:Mn, YBaCuO, BaTiO3, cubic BN, SiC, ZrO2, ITO, B, C, and apatite. Some of these data will be presented in this paper, with its analytic solution derived from a one-dimensional, hydrodynamic, adiabatic expansion model for the IBE. The temperature will be deduced using ion-flux signals measured by a biased ion collector. Reasonable agreement is obtained between the experiment and the simulation. High-energy LIB implantation to make chemical compounds and the associated surface modification are also discussed.
Velocity-shear-induced ion-cyclotron turbulence: Laboratory identification and space applications
Authors: Koepke M. E. , Amatucci W. E. , Carroll J. J. , Gavrishchaka V. , Ganguli G.
Published: July 2002

Laboratory measurements are reported that identify a new class of plasma oscillation driven by the inhomogeneity in wave energy density caused by transverse-velocity shear [Ganguli et al., Phys. Fluids 31, 823 (1988)]. The experiments concentrate on a branch of oscillation in the ion-cyclotron range of frequencies that results from the coupling of the magnetic-field-aligned current and the inhomogeneous dc electric field localized in a layer thicker than the ion gyroradius. The observed transition between the well-known current-driven electrostatic ion-cyclotron mode and this inhomogeneous energy–density-driven mode is related to the ratio of the azimuthal and axial Doppler shifts. The mode characteristics associated with the two instabilities have significantly different properties. For conditions of large transverse-velocity shear, turbulence is generated with a broadband, spiky spectrum around the ion-cyclotron frequency at small values of the magnetic-field-aligned current. The experimental identification is reinforced with numerical results from a nonlocal eigenvalue condition.
Transverse dynamics of a short, relativistic electron bunch in a plasma lens
Authors: Hairapetian G. , Davis P. , Clayton C. E. , Joshi C. , Pellegrini C. , Katsouleas T.
Published: July 2002

Dynamic focusing of a 3.8 MeV, 25 ps long, full width at half-maximum (FWHM), electron bunch by an overdense (np≫nb, where np and nb are the plasma and bunch densities, respectively) plasma lens has been studied experimentally. The plasma focused the bunch from an initial transverse size of approximately 2.4 mm (FWHM) to about 0.5 mm, 21 cm downstream of the plasma. The sharp rise time (7 ps 10%–90%) of the electron bunch, excites a large-amplitude (<1 MeV/m) plasma wave (plasma wake field). The peak focusing force of the lens is partly (60%) due to the beam-generated, azimuthal magnetic field and partly (40%) due to the radial component of the electrostatic wake field.
Fast flow phenomena in a toroidal plasma
Authors: Den Hartog D. J. , Almagri A. F. , Chapman J. T. , Ji H. , Prager S. C. , Sarff J. S. , Fonck R. J. , Hegna C. C.
Published: July 2002

The bulk fluid velocity is measured spectroscopically with 10 μs time resolution in the Madison Symmetric Torus (MST) reversed-field pinch (RFP) [Fusion Technol. 19, 131 (1991)], a diagnostic capability used to study the fast flow dynamics associated with locked modes and the RFP dynamo. The phase velocity of the tearing modes and the fluid velocity accelerate between sawtooth events, reaching a maximum speed of about 20 km/s in a few ms. Both slow down at the sawtooth crash in ≊100 μs. This deceleration time scale is as calculated for the tearing modes from the action of electromagnetic torque on the magnetic islands, but is much faster than expected from the viscous torque on the bulk fluid. In the RFP, correlated fluctuations in the tearing modes and fluid velocity probably also generate current via the ‘‘RFP dynamo,’’ 〈ũ×B̃〉, where u is the bulk fluid velocity. Initial data indicate a possible increase in 〈ũφB̃r〉 during sawtooth events, coincident with toroidal flux generation.
Local ideal magnetohydrodynamic stability of a quasihelically symmetric stellarator
Authors: Talmadge J. N. , Cooper W. A.
Published: July 2002

The ideal magnetohydrodynamic stability with respect to localized ballooning modes and the Mercier criterion of a quasihelically symmetric stellarator configuration is investigated. Configurations with an enhanced magnetic mirror and a deeper magnetic well that can be achieved by energizing a set of auxiliary coils have also been analyzed. In all cases, the Mercier criterion imposes a slightly stricter β limit than the ballooning modes. Using relatively broad pressure profiles, the Mercier criterion yields β=0.4% for the standard configuration, β=0.3% for the high mirror configuration and β=1.3% for the deep well configuration. The comparable ballooning limits are β=0.7%, β=0.6%, and β=1.66% for the standard, high mirror, and deep well configurations, respectively. With a more peaked pressure profile, ballooning instabilities near the center of the plasma and Mercier modes near the edge of the plasma limit the performance of the standard configuration at β≂0.8%.
Physics of the steady-state advanced tokamak
Authors: Goldston R. J.
Published: July 2002

The tokamak reactor becomes a more attractive fusion power source if it can operate in steady state, and at high fusion power density, with low recirculating power. This implies that a ‘‘steady-state advanced tokamak’’ must achieve both high beta and high confinement, consistent with a high fraction of the total plasma current being carried by the bootstrap effect. The most attractive mode of operation to fulfill these requirements involves a reversal of the global magnetic shear, dq/dr, in the plasma core. This allows self-consistency between the radial profile of the bootstrap current and that of the total current, while simultaneously reducing turbulent transport in the plasma core and increasing magnetohydrodynamic (MHD) stability. In this paper both theoretical and experimental work on the steady-state advanced tokamak are reviewed, and we point to new research areas that need to be pursued to make this concept a reality. Presently operating devices can make strong contributions in this research area, and future devices should be designed with the capability to access, investigate, and exploit this operating mode.
Experimental investigation on tilt stabilizing effect of external toroidal field in low aspect ratio tokamak
Authors: Morita A. , Ono Y. , Katsurai M. , Yamada M. , Yoshikawa S.
Published: July 2002

This paper describes experimental investigations on the equilibrium and global stability of low aspect ratio tokamaks with different aspect ratios ranging from 1.1 to 1.9. The Z-θ pinch spheromak formation technique is used to produce low aspect ratio tokamaks in an external toroidal field generated by the center conductor. Using this operation, the plasma stability has been investigated in the transition regime from tokamaks to spheromaks. It has been found that there exists a lower critical value of the center conductor current to surpress the global plasma instability of the n=1 tilt and/or shift modes. The ratio of this critical current to plasma current is experimentally measured for the first time as a function of the aspect ratio. Glass-tube cylindrical limiters with different radii are installed along the symmetric center axis of the Spherical Torus-3 device [Y. Ono et al., Phys. Fluids B 5, 3691 (1993)]. As the aspect ratio is decreased from 1.9 to 1.1, it is observed that the critical ratio of the center conductor current to plasma current decreases from 1.2 to 0.2. The safety factor q at the plasma edge corresponding to this critical current is roughly 1.5 to 3.0. Similar experiments are also carried out with a thin metal cover surrounding the surface of the glass tube limiter. The thin metal cover permits the decrease in the critical current and the corresponding edge q value of q∼1. These experimental results of the critical current ratio are found to be comparable to that predicted from theoretical models where the restoring force against the tilt motion is considered to be generated by the interaction of the external toroidal field with the n=1 induced surface currents in the tilt motion.
Origin of diffusion in Hamiltonian dynamics
Authors: Bénisti D. , Escande D. F.
Published: July 2002

Without invoking any kind of “loss of memory” hypothesis, a diffusion equation is derived for the Hamiltonian dynamics defined by H=p2/2+(K/4π2)∑m=−MM cos(q−mt+φm), where the φms are fixed random phases. The key point of the derivation is a property of locality for the waves inducing transport. Using perturbation theory, it is shown that only waves whose phase velocities satisfy |m−p(t)|⩽α(K/4π2)2/3, where α is a constant, approximately 5, play a relevant role for the statistical properties of the dynamics. This implies scaling properties for the dynamics, and leads to the understanding of the origin of force decorrelation and of diffusion, and to the prediction of their occurrence in time. Moreover, the convergence of the diffusion coefficient to its quasilinear value when K→∞ is shown, and is interpreted as the consequence of the crossover between two regimes of decorrelation that are of different natures.
Structure of kinetic Alfvén waves with small transverse scale length
Authors: Morales G. J. , Maggs J. E.
Published: July 2002

This analytical study illustrates the spatial pattern of kinetic Alfvén waves excited by a current-modulating disk whose dimension a, transverse to the confining magnetic field, is comparable to the ion sound gyroradius cs/Ωi, where cs is the sound speed and Ωi the ion cyclotron frequency. The radial structure of the wave azimuthal magnetic field is found to consist of four regions: a Bessel function behavior for r<a, a near null at r≅a, and a driven Airy pattern for r≫a which merges onto the 1/r asymptotic region. The pattern spreads at an angle given by tan θ=(ω/Ωi)(cs/vA)/2.6, where ω is the modulation frequency and vA is the Alfvén speed. This behavior arises because there is a maximum value at finite k⊥ for the ratio of the perpendicular to parallel group velocity, which differs from the cone spreading [G. J. Morales et al., Phys. Plasmas 1, 3765 (1994)] associated with inertial Alfvén waves.
Chaotic flows and kinematic magnetic dynamos: A tutorial review
Authors: Ott Edward
Published: July 2002

This paper reviews results on fast kinematic dynamo theory, emphasizing the recent realization that Lagrangian chaos of the underlying flow is the key element for understanding of the problem. Simple models for singular behavior of the magnetic field in the large magnetic Reynolds number limit are described and used to illustrate the tendency for fractal magnetic field distributions with extreme cancellation properties. The relation of ergodic properties of the chaotic flow to properties of the dynamo (e.g., growth rate, fractal dimension) are also discussed.
A full wave theory of high-harmonic fast wave absorption in high-beta plasmas
Authors: Lashmore-Davies C. N. , Fuchs V. , Cairns R. A.
Published: July 2002

A theory of fast wave absorption in a high-beta plasma is given. A reduced, second-order, ordinary differential equation has been used which includes all collisionless electron dissipation mechanisms and ion cyclotron damping over many harmonics. This is relevant to the high-harmonic fast wave heating scheme proposed by Ono [Phys. Plasmas 2, 4075 (1995)]. The parameters appropriate to the National Spherical Tokamak Experiment NSTX [J. Spitzer et al., Fusion Technol. 30, 1337 (1996)] have been used to investigate the absorption characteristics of electrons and ions under high-beta, high-harmonic conditions.
High-harmonic fast magnetosonic wave coupling, propagation, and heating in a spherical torus plasma
Authors: Menard J. , Majeski R. , Kaita R. , Ono M. , Munsat T. , Stutman D. , Finkenthal M.
Published: July 2002

A novel rotatable two-strap antenna has been installed in the current drive experiment upgrade (CDX-U) [T. Jones, Ph.D. thesis, Princeton University (1995)] in order to investigate high-harmonic fast wave coupling, propagation, and electron heating as a function of strap angle and strap phasing in a spherical torus plasma. Radio-frequency-driven sheath effects are found to fit antenna loading trends at very low power and become negligible above a few kilowatts. At sufficiently high power, the measured coupling efficiency as a function of strap angle is found to agree favorably with cold plasma wave theory. Far-forward microwave scattering from wave-induced density fluctuations in the plasma core tracks the predicted fast wave loading as the antenna is rotated. Signs of electron heating during rf power injection have been observed in CDX-U with central Thomson scattering, impurity ion spectroscopy, and Langmuir probes. While these initial results appear promising, damping of the fast wave on thermal ions at high ion-cyclotron-harmonic number may compete with electron damping at sufficiently high ion β—possibly resulting in a significantly reduced current drive efficiency and production of a fast ion population. Preliminary results from ray-tracing calculations which include these ion damping effects are presented.
Nonlinear dynamics of pressure driven magnetic islands in low aspect ratio tokamaks
Authors: Hegna C. C.
Published: July 2002

An analytic theory of nonlinear pressure driven magnetic island evolution is presented which includes both neoclassical and curvature effects. This work differs from prior analytic treatments where large aspect ratio, small beta expansions were employed that are not appropriate for tight aspect ratio applications. In this work, this calculation is revisited by deriving and analyzing an island Grad–Shafarnov equation using an asymptotic expansion based solely on a small island width expansion. In conventional high-temperature tokamaks, the stabilizing curvature effects are weaker than the destabilizing neoclassical tearing mode instability drive in regions of monotonically increasing q-profile. At low aspect ratio, these two effects become comparable which suggests that tight aspect ratio configurations are less susceptible to the neoclassical tearing mode than are conventional tokamaks.
Nonlinear hybrid Boltzmann–particle-in-cell acceleration algorithm
Authors: Cartwright K. L. , Verboncoeur J. P. , Birdsall C. K.
Published: July 2002

Kinetic simulation of plasmas in which equilibrium occurs over ion time scales poses a computational challenge due to disparity with electron time scales. Hybrid electrostatic particle-in-cell (PIC) algorithms are presented in which most of the electrons reach thermodynamic equilibrium [Maxwell–Boltzmann (MB) distribution function] each time step. Conservation of charge enables convergence of the nonlinear Poisson equation. Energy conservation is used to determine the temperature of the Boltzmann species. This article first develops an algorithm where all the electrons have a MB energy distribution, either with a full MB distribution or with a truncation of the high energy tail. Second, high energy PIC electrons are added to the truncated distribution so that high energy electrons are modeled kinetically by PIC and low energy electrons (the majority) are modeled by the MB distribution. Collisions for PIC electrons are included via a Monte Carlo model, while for the MB electrons, the distributions are integrated with energy dependent cross sections. The MB model is not constrained by the electron time scale which decreases the required computer time by about the square root of the mass ratio of ion to electron. However, the hybrid boundary conditions are more complex and the simulation is not quite self-consistent. Comparison between full PIC and the PIC–MB hybrid is made for simulations of photo-ionized sustained discharges and current-driven dc discharges.
Device quality growth and characterization of (110) GaAs grown by molecular beam epitaxy
Authors: Allen L. T. P. , Weber E. R. , Washburn J. , Pao Y. C.
Published: July 2002

Device quality (110)GaAs has been reproducibly grown by molecular beam epitaxy (MBE) for the first time. Angling of the substrate to expose stable, Ga-rich ledges on the (110) surface has been shown to be the necessary condition for two-dimensional growth. The layers exhibit a room-temperature electron mobility of ∼5700 cm2/V s for NSi∼4×1015 and a strong exciton photoluminescence emission at 4 K. This breakthrough in MBE growth of III-V compounds allows for fabrication of (110) GaAs devices which will take advantage of the unique properties of this orientation.
Cooling of hot carriers in Ga0.47In0.53As
Authors: Lobentanzer H. , Polland H.-J. , Rühle W. W. , Stolz W. , Ploog K.
Published: July 2002

Carrier cooling is measured in an Al0.48In0.52As/Ga0.47In0.53As/InP heterostructure by time-resolved photoluminescence experiments for excitation densities between 5×1016 and 2.2×1018 cm−3. For low excitation density, the energy-loss rate by emission of optical phonons is close to theoretical bulk value, whereas a reduction of the energy-loss rate by a factor of 100 is found for the highest excitation density.
Secretory responses of intact glomus cells in thin slices of rat carotid body to hypoxia and tetraethylammonium
Authors: Pardal Ricardo , Ludewig Uwe , García-Hirschfeld Julia , López-Barneo José
Published: July 2002

We have developed a thin-slice preparation of whole rat carotid body that allows us to perform patch-clamp recording of membrane ionic currents and to monitor catecholamine secretion by amperometry in single glomus cells under direct visual control. In normoxic conditions (
P

O
2

≈ 140 mmHg; 1 mmHg = 133 Pa), most glomus cells did not have measurable secretory activity, but exposure to hypoxia (
P

O
2

≈ 20 mmHg) elicited the appearance of a large number of spike-like exocytotic events. This neurosecretory response to hypoxia was fully reversible and required extracellular Ca
2+
influx. The average charge of single quantal events was 46 ± 25 fC (
n
= 218), which yields an estimate of ≈140,000 catecholamine molecules per vesicle. Addition of tetraethylammonium (TEA; 2–5 mM) to the extracellular solution induced in most (>95%) cells tested (
n
= 32) a secretory response similar to that elicited by low
P

O
2

. Cells nonresponsive to hypoxia but activated by exposure to high external K
+
were also stimulated by TEA. A secretory response similar to the responses to hypoxia and TEA was also observed after treatment of the cells with iberiotoxin to block selectively Ca
2+
- and voltage-activated maxi-K
+
channels. Our data further show that membrane ion channels are critically involved in sensory transduction in the carotid body. We also show that in intact glomus cells inhibition of voltage-dependent K
+
channels can contribute to initiation of the secretory response to low
P

O
2

.
Testosterone prevents the heat shock-induced overactivation of glycogen synthase kinase-3β but not of cyclin-dependent kinase 5 and c-Jun NH
2
-terminal kinase and concomitantly abolishes hyperphosphorylation of τ: Implications for Alzheimer's disease
Authors: Papasozomenos Sozos Ch. , Shanavas Alikunju
Published: July 2002

We have shown previously that glycogen synthase kinase-3β (GSK-3β), cyclin-dependent kinase 5, and c-Jun NH
2
-terminal kinase become overactivated and hyperphosphorylate τ in heat-shocked female rats. This hyperphosphorylation of τ is estrogen-independent, prevented by androgens, and similar to Alzheimer's disease. In this study, ovariectomized (OVX) Sprague-Dawley rats (
n
= 75) received daily injections of 10 μg of 17β-estradiol benzoate (EB), or 250 μg of testosterone propionate (TP), or both EB and TP, or sesame oil (SO) vehicle for 4–6 weeks. In kinase assays of forebrain homogenates, overactivation of GSK-3β at 0–6 h after heat shock toward human recombinant τ, bovine τ, and phosphoglycogen synthase peptide 2 was prevented in OVX + TP and OVX + (EB + TP) but not in sham-OVX + SO, OVX + SO, and OVX + EB. Abs against inactive (pSer
9
) and activity-enhanced (pTyr
216
) GSK-3β showed marked increase of pSer
9
- and decrease of pTyr
216
-GSK-3β in both OVX + TP and OVX + (EB + TP) but not in sham-OVX + SO, OVX + SO, and OVX + EB. EB enhanced the overactivation of cyclin-dependent kinase 5. The activity of c-Jun NH
2
-terminal kinase was gonadal hormone-independent. The serum concentrations of testosterone and 17β-estradiol were 2.53 ng/ml and 201 pg/ml in OVX + TP and OVX + EB, respectively. These findings demonstrate that testosterone prevents the hyperphosphorylation of τ by inhibiting the heat shock-induced overactivation of GSK-3β and suggest that androgens given to aging men or, in combination with estrogens, to postmenopausal women could prevent or delay Alzheimer's disease.
The gene coding for the Hrp pilus structural protein is required for type III secretion of Hrp and Avr proteins in
Pseudomonas syringae
pv.
tomato
Authors: Wei Wensheng , Plovanich-Jones Anne , Deng Wen-Ling , Jin Qiao-Ling , Collmer Alan , Huang Hsiou-Chen , He Sheng Yang
Published: July 2002

Bacterial surface appendages called pili often are associated with DNA and/or protein transfer between cells. The exact function of pili in the transfer process is not understood and is a matter of considerable debate. The Hrp pilus is assembled by the Hrp type III protein secretion system of
Pseudomonas syringae
pv.
tomato
(
Pst
) strain DC3000. In this study, we show that the
hrpA
gene, which encodes the major subunit of the Hrp pilus, is required for secretion of putative virulence proteins, such as HrpW and AvrPto. In addition, the
hrpA
gene is required for full expression of genes that encode regulatory, secretion, and effector proteins of the type III secretion system.
hrpA
-mediated gene regulation apparently is through effect on the mRNA level of two previously characterized regulatory genes,
hrpR
and
hrpS
. Ectopic expression of the
hrpRS
gene operon restored gene expression, but not protein secretion, in the
hrpA
mutant. Three single amino acid mutations at the HrpA carboxyl terminus were identified that affect the secretion or regulatory function of the HrpA protein. These results define an essential role of the Hrp pilus structural gene in protein secretion and coordinate regulation of the type III secretion system in
Pst
DC3000.
Whole-body and intravital optical imaging of angiogenesis in orthotopically implanted tumors
Authors: Yang Meng , Baranov Eugene , Li Xiao-Ming , Wang Jin Wei , Jiang Ping , Li Lingna , Moossa A. R. , Penman Sheldon , Hoffman Robert M.
Published: July 2002

The development of drugs for the control of tumor angiogenesis
requires a simple, accurate, and economical assay for tumor-induced
vascularization. We have adapted the orthotopic implantation model to
angiogenesis measurement by using human tumors labeled with
Aequorea victoria
green fluorescent protein for grafting
into nude mice. The nonluminous induced capillaries are clearly visible
against the very bright tumor fluorescence examined either intravitally
or by whole-body luminance in real time. The orthotopic implantation
model of human cancer has been well characterized, and fluorescence
shadowing replaces the laborious histological techniques for
determining blood vessel density. Intravital images of orthotopically
implanted human pancreatic tumors clearly show angiogenic capillaries
at both primary and metastatic sites. A quantitative time course of
angiogenesis was determined for an orthotopically growing human
prostate tumor periodically imaged intravitally in a single nude mouse
over a 19-day period. Whole-body optical imaging of tumor angiogenesis
was demonstrated by injecting fluorescent Lewis lung carcinoma cells
into the s.c. site of the footpad of nude mice. The footpad is
relatively transparent, with comparatively few resident blood vessels,
allowing quantitative imaging of tumor angiogenesis in the intact
animal. Capillary density increased linearly over a 10-day period as
determined by whole-body imaging. Similarly, the green fluorescent
protein-expressing human breast tumor MDA-MB-435 was orthotopically
transplanted to the mouse fat pad, where whole-body optical imaging
showed that blood vessel density increased linearly over a 20-week
period. These powerful and clinically relevant angiogenesis mouse
models can be used for real-time
in vivo
evaluation of
agents inhibiting or promoting tumor angiogenesis in physiological
microenvironments.
Reciprocal electromechanical properties of rat prestin: The motor molecule from rat outer hair cells
Authors: Ludwig Jost , Oliver Dominik , Frank Gerhard , Klöcker Nikolaj , Gummer Anthony W. , Fakler Bernd
Published: July 2002

Cochlear outer hair cells (OHCs) are responsible for the exquisite
sensitivity, dynamic range, and frequency-resolving capacity of the
mammalian hearing organ. These unique cells respond to an electrical
stimulus with a cycle-by-cycle change in cell length that is mediated
by molecular motors in the cells' basolateral membrane. Recent work
identified prestin, a protein with similarity to pendrin-related anion
transporters, as the OHC motor molecule. Here we show that
heterologously expressed prestin from rat OHCs (rprestin) exhibits
reciprocal electromechanical properties as known for the OHC motor
protein. Upon electrical stimulation in the microchamber configuration,
rprestin generates mechanical force with constant amplitude and phase
up to a stimulus frequency of at least 20 kHz. Mechanical stimulation
of rprestin in excised outside-out patches shifts the voltage
dependence of the nonlinear capacitance characterizing the electrical
properties of the molecule. The results indicate that rprestin is a
molecular motor that displays reciprocal electromechanical properties
over the entire frequency range relevant for mammalian hearing.
Unspecific hydrophobic stabilization of folding transition states
Authors: Viguera Ana Rosa , Vega Cristina , Serrano Luis
Published: July 2002

Here we present a method for determining the inference of non-native conformations in the folding of a small domain, α-spectrin Src homology 3 domain. This method relies on the preservation of all native interactions after Tyr/Phe exchanges in solvent-exposed, contact-free positions. Minor changes in solvent exposure and free energy of the denatured ensemble are in agreement with the reverse hydrophobic effect, as the Tyr/Phe mutations slightly change the polypeptide hydrophilic/hydrophobic balance. Interestingly, more important Gibbs energy variations are observed in the transition state ensemble (TSE). Considering the small changes induced by the H/OH replacements, the observed energy variations in the TSE are rather notable, but of a magnitude that would remain undetected under regular mutations that alter the folded structure free energy. Hydrophobic residues outside of the folding nucleus contribute to the stability of the TSE in an unspecific nonlinear manner, producing a significant acceleration of both unfolding and refolding rates, with little effect on stability. These results suggest that sectors of the protein transiently reside in non-native areas of the landscape during folding, with implications in the reading of φ values from protein engineering experiments. Contrary to previous proposals, the principle that emerges is that non-native contacts, or conformations, could be beneficial in evolution and design of some fast folding proteins.
A wholly empirical explanation of perceived motion
Authors: Yang Zhiyong , Shimpi Amita , Purves Dale
Published: July 2002

Because the retinal activity generated by a moving object cannot
specify which of an infinite number of possible physical displacements
underlies the stimulus, its real-world cause is necessarily uncertain.
How, then, do observers respond successfully to sequences of images
whose provenance is ambiguous? Here we explore the hypothesis that the
visual system solves this problem by a probabilistic strategy in which
perceived motion is generated entirely according to the relative
frequency of occurrence of the physical sources of the stimulus. The
merits of this concept were tested by comparing the directions and
speeds of moving lines reported by subjects to the values determined by
the probability distribution of all the possible physical displacements
underlying the stimulus. The velocities reported by observers in a
variety of stimulus contexts can be accounted for in this way.
Distinct gene-specific mechanisms of arrhythmia revealed by cardiac gene transfer of two long QT disease genes,
HERG
and
KCNE1
Authors: Hoppe Uta C. , Marbán Eduardo , Johns David C.
Published: July 2002

The long QT syndrome (LQTS) is a heritable disorder that
predisposes to sudden cardiac death. LQTS is caused by mutations in ion
channel genes including HERG and KCNE1, but the precise mechanisms
remain unclear. To clarify this situation we injected adenoviral
vectors expressing wild-type or LQT mutants of HERG and KCNE1 into
guinea pig myocardium. End points at 48–72 h included
electrophysiology in isolated myocytes and electrocardiography
in vivo
. HERG increased the rapid component,
I
Kr
, of the delayed rectifier current,
thereby accelerating repolarization, increasing refractoriness, and
diminishing beat-to-beat action potential variability. Conversely,
HERG-G628S suppressed
I
Kr
without
significantly delaying repolarization. Nevertheless, HERG-G628S
abbreviated refractoriness and increased beat-to-beat variability,
leading to early afterdepolarizations (EADs). KCNE1 increased the slow
component of the delayed rectifier,
I
Ks
,
without clear phenotypic sequelae. In contrast, KCNE1-D76N suppressed
I
Ks
and markedly slowed repolarization,
leading to frequent EADs and electrocardiographic QT prolongation.
Thus, the two genes predispose to sudden death by distinct mechanisms:
the KCNE1 mutant flagrantly undermines cardiac repolarization, and
HERG-G628S subtly facilitates the genesis and propagation of premature
beats. Our ability to produce electrocardiographic long QT
in
vivo
with a clinical KCNE1 mutation demonstrates the utility of
somatic gene transfer in creating genotype-specific disease models.
Visualization of Ca2+entry through single stretch-activated cation channels
Authors: Zou Hui , Lifshitz Lawrence M. , Tuft Richard A. , Fogarty Kevin E. , Singer Joshua J.
Published: July 2002

Stretch-activated channels (SACs) have been found in smooth muscle and are thought to be involved in myogenic responses. Although SACs have been shown to be Ca2+permeable when Ca2+is the only charge carrier, it has not been clearly demonstrated that significant Ca2+passes through SACs in physiological solutions. By imaging at high temporal and spatial resolution the single-channel Ca2+fluorescence transient (SCCaFT) arising from Ca2+entry through a single SAC opening, we provide direct evidence that significant Ca2+can indeed pass through SACs and increase the local [Ca2+]. Results were obtained under conditions where the only source of Ca2+was the physiological salt solution in the patch pipette containing 2 mM Ca2+. Single smooth muscle cells were loaded with fluo-3 acetoxymethyl ester, and the fluorescence was recorded by using a wide-field digital imaging microscope while SAC currents were simultaneously recorded from cell-attached patches. Fluorescence increases at the cell-attached patch were clearly visualized before the simultaneous global Ca2+increase that occurred because of Ca2+influx through voltage-gated Ca2+channels when the membrane was depolarized by inward SAC current. From measurements of total fluorescence (“signal mass”) we determined that about 18% of the SAC current is carried by Ca2+at membrane potentials more negative than the resting level. This would translate into at least a 0.35-pA unitary Ca2+current at the resting potential. Such Ca2+currents passing through SACs are sufficient to activate large-conductance Ca2+-activated K+channels and, as shown previously, to trigger Ca2+release from intracellular stores.