bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental determination of the conservation of magnetic helicity from the balance between source and spheromak
Authors: Barnes Cris W. , Fernández J. C. , Henins I. , Hoida H. W. , Jarboe T. R. , Knox S. O. , Marklin G. J. , McKenna K. F.
Published: July 2002

The conjecture that magnetic helicity (linked flux) is conserved in magnetized plasmas for time scales that are short compared to the resistive diffusion time is experimentally tested in the CTX spheromak [Phys. Rev. Lett. 45, 1264 (1980); 51, 39 (1983); Nucl. Fusion 24, 267 (1984)]. Helicity is created electrostatically by current drawn from electrodes. The magnetized plasma then flows into a conducting flux conserver where the energy per helicity of the plasma is minimized and a spheromak is formed on a relaxation time scale of many Alfvén times. The magnetic field strength of the equilibrium is subsequently increased and sustained. The amount of helicity created by the magnetized coaxial plasma source, the helicity content of the spheromak equilibrium, and the resistive loss of the helicity are measured to determine the balance of helicity between source and spheromak with a ±16% uncertainty. In CTX the amount of energy that must be rapidly dissipated within the conducting boundary while conserving helicity in the process of sustaining the spheromak is experimentally controllable, and has varied from 1.8 times the spheromak magnetic energy to greater than 10 times. The relaxation, or minimization of the energy-to-helicity ratio, determines the gross structure (the normalized spatial profile) of the spheromak, while the conservation of helicity itself determines the magnitude and time dependence of the magnetic fields of the spheromak equilibrium. Helicity balance tests are done by individually varying the sign and magnitude of the source voltage and flux, and by observing sustainment of spheromaks with fields opposing those of the source. A threshold for helicity injection from the source is measured and related to the source and entrance region size. During times short compared to resistive

diffusion time scales the helicity is shown to be conserved with a ±12% uncertainty using no free parameters. For longer times the resistive dissipation Its value is independently measured and appears to be related to the expected classical resistive decay. Absolutely calibrated bolometer measurements are consistent with excess source energy heating the spheromak plasma during the sustainment by electrostatic helicity injection.
Steady flow past sudden expansions at large Reynolds number. II. Navier–Stokes solutions for the cascade expansion
Authors: Milos Frank S. , Acrivos Andreas , Kim John
Published: July 2002

In the steady laminar flow past a sudden expansion at large Reynolds number R, the equations of motion reduce to the boundary-layer equations as R→∞ if the longitudinal length scale of the separated eddy increases linearly and indefinitely with R. In part I of this series [Phys. Fluids 29, 1353 (1986)], several sudden expansion geometries were considered, and in each case, when the inflow was uniform, steady solutions to the boundary-layer equations were found to exist provided that the expansion ratio remained above a critical value where the pressure gradient became singular near the reattachment point of the eddy. These results suggested that for uniform inflows and smaller values of the expansion ratio, the eddy length could not continue to increase linearly with R if the latter were sufficiently large. In the present work a global Newton method was employed to obtain finite-difference solutions to the steady Navier–Stokes equations up to R=1000 for a uniform inflow past a cascade of sudden expansions. The calculations show that for large values of the expansion ratio, the eddy length increases linearly with R, and that the main features of the flow approach those predicted by the boundary-layer solutions, including the existence of large pressure gradients near the reattachment point, as the expansion ratio is reduced toward the critical value. However, for smaller values of the expansion ratio where solutions to the boundary-layer equations could not be found, the steady solutions to the Navier–Stokes equations approach, with increasing R, the limit of an inviscid eddy O(1) in length, with the main features of the flow conforming to the theoretical model of Batchelor [J. Fluid Mech. 1, 388 (1956)] for an inviscid separated eddy behind a bluff body.
Lower bounds on permeability
Authors: Rubinstein Jacob , Keller Joseph B.
Published: July 2002

A method is presented for obtaining lower bounds on the permeability of a porous medium. It is applied to media composed of periodic and random configurations of spheres and cylinders.
Particle simulations of plasma and dielectric Čerenkov masers
Authors: Pointon T. D. , De Groot J. S.
Published: July 2002

A new particle simulation model has been developed to investigate Čerenkov masers. The novel aspects of the code are briefly described, and results of simulations of two types of Čerenkov masers are presented. The first device uses a conventional dielectric lining as its slow-wave structure. The second is a new type of Čerenkov maser in which a circular waveguide is partially filled with a dense annular plasma instead of a dielectric layer. Both simulations agree well with experimental results and linear theory calculations. Saturation of the instability is shown to be due to trapping of the beam electrons. The relative merits of each system are discussed.
Direct numerical simulations of the turbulent mixing of a passive scalar
Authors: Eswaran V. , Pope S. B.
Published: July 2002

The evolution of scalar fields, of different initial integral length scales, in statistically stationary, homogeneous, isotropic turbulence is studied. The initial scalar fields conform, approximately, to ‘‘double-delta function’’ probability density functions (pdf ’s). The initial scalar-to-velocity integral length-scale ratio is found to influence the rate of the subsequent evolution of the scalar fields, in accord with experimental observations of Warhaft and Lumley [J. Fluid Mech. 88, 659 (1978)]. On the other hand, the pdf of the scalar is found to evolve in a similar fashion for all the scalar fields studied; and, as expected, it tends to a Gaussian. The pdf of the logarithm of the scalar-dissipation rate reaches an approximately Gaussian self-similar state. The scalar-dissipation spectrum function also becomes self-similar. The evolution of the conditional scalar-dissipation rate is also studied. The consequences of these results for closure models for the scalar pdf equation are discussed.
Finite-aspect-ratio effects on the bootstrap current in tokamaks
Authors: Hirshman S. P.
Published: July 2002

The bootstrap current in tokamaks is determined in the low collision frequency regime for arbitrary values of the aspect ratio and effect charge. The resulting expression should be useful for a quantitative analysis of currents in tokamaks.
Comparison of spectral method and lattice Boltzmann simulations of two-dimensional hydrodynamics
Authors: Martínez D. O. , Matthaeus W. H. , Chen S. , Montgomery D. C.
Published: July 2002

Numerical solutions of the two-dimensional Navier–Stokes equations are presented by two methods; spectral and the novel lattice Boltzmann equation (LBE) scheme. Very good agreement is found for global quantities as well as energy spectra. The LBE scheme is, indeed, providing reasonably accurate solutions of the Navier–Stokes equations with an isothermal equation of state, in the nearly incompressible limit. Relaxation to a previously reported ‘‘sinh-Poisson’’ state is also observed for both runs.
Film entrained by a fiber quickly drawn out of a liquid bath
Authors: Koulago Alexander , Shkadov Victor , Quéré David , de Ryck Alain
Published: July 2002

The thickness of the film entrained by a fiber quickly withdrawn out of a bath of wetting liquid is of interest. For velocities larger than a threshold (usually of the order of 1 m/s), inertia can no longer be neglected and must be incorporated in a generalized form of the classical Landau–Levich–Deryaguin model. It is shown here that the effect of inertia is to make the film thicker, which agrees with recent observations.
Self-induced vortex ring dynamics in subsonic rectangular jets
Authors: Grinstein Fernando F.
Published: July 2002

The development in space and time of vortex rings in low aspect-ratio (AR) rectangular jets is investigated. By design, the present studies isolate the self-induced ring dynamics from effects of unsteady events otherwise present upstream and downstream of the rings in developed jets. The simulations show that the vortex rings undergo quite regular self-induced nonplanar deformations, approximately recovering their shape and flatness with axis rotated with respect to their initial configuration. The axis-rotation periods are in good agreement with previously reported data for pseudoelliptic rings, and exhibit nearly linear growth rate as a function of AR. For the larger aspect-ratio case studied (AR=4), bifurcation of the ring due to vortex reconnection into roughly round rings is observed, followed by collision of the split rings and a new reconnection process, suggesting pathways for transition to turbulence based on self-induced vortex deformations and reconnections.
Vortex sinks with axial flow: Solution and applications
Authors: Shtern Vladimir , Borissov Anatoly , Hussain Fazle
Published: July 2002

In this paper we develop a new class of analytical solutions of the Navier–Stokes equations and suggest ways to predict and control complex swirling flows. We consider vortex sinks on curved axisymmetric surfaces with an axial flow and obtain a five-parameter solution family that describes a large variety of flow patterns and models fluid motion in a cylindrical can, whirlpools, tornadoes, and cosmic swirling jets. The singularity of these solutions on the flow axis is removed by matching them with swirling jets. The resulting composite solutions describe flows, consisting of up to seven separation regions (recirculatory “bubbles” and vortex rings), and model flows in the Ranque–Hilsch tube, in the meniscus of electrosprays, in vortex breakdown, and in an industrial vortex burner. The analytical solutions allow a clear understanding of how different control parameters affect the flow and guide selection of optimal parameter values for desired flow features. The approach permits extension to swirling flows with heat transfer and chemical reaction, and have the potential of being significantly useful for further detailed investigation by direct or large-eddy numerical simulations as well as laboratory experimentation.
Shear instability of two-fluid parallel flow in a Hele–Shaw cell
Authors: Gondret P. , Rabaud M.
Published: July 2002

We study experimentally the parallel flow in a Hele–Shaw cell of two immiscible fluids, a gas and a viscous liquid, driven by a given pressure gradient. We observe that the interface is destabilized above a critical value of the gas flow and that waves grow and propagate along the cell. The experimental threshold corresponds to a velocity difference of the two fluids in good agreement with the inviscid Kelvin–Helmholtz instability, while the wave velocity corresponds to a pure viscous theory deriving from Darcy’s law. We report our experimental results and analyze this instability by the study of a new equation where the viscous effects are added to the Euler equation through a unique drag term. The predictions made from the linear stability analysis of this equation agree with the experimental measurements.
On the breakdown of vortex rings from inclined nozzles
Authors: Lim T. T.
Published: July 2002

In a recent experimental investigation, Webster and Longmire [Phys. Fluids 9, 655 (1997)] reported that the large-scale jet structures from inclined nozzles, which consisted of continuous inclined vortex rings, would undergo breakdown if the inclined angle of the nozzle was sufficiently large. They attributed the breakdown to the presence of longitudinal vorticity, but did not elaborate on the mechanism involved. In this paper, we examined the above issue by focusing primarily on the large-scale structures of the inclined jet (i.e., the inclined vortex rings). To the author’s knowledge, this area of research remains relatively unexplored. A study of it would certainly help to shed light on the mechanism involved in the breakdown of the inclined jets. Here we investigated the effects of the Reynolds number, the nozzle’s inclined angle, and L/D (see below) on the evolution of inclined vortex rings. Nozzles with the inclined angle of 5°, 10°, 20°, and 45° were considered, and the Reynolds number of the flow ranged from 1447 to 4824. The L/D was varied from 0.77 to 1.92, where L is the length of the slug of fluid ejected through the nozzle of diameter D. The results showed that when inclined vortex rings were formed, they were subjected to a differential rate of vortex stretching, due in part to a nonaxisymmetric vortex roll-up. As a result, circumferential flow was produced in the vortex core which increased with the nozzle angle and the Reynolds number. If sufficiently large, the circumferential flow was found to lead to core breakdown by initiating a wavy instability in the vortex filament which subsequently developed into a “bubble-type” breakdown and then a “double-helix”-type breakdown before the ring disintegrated into a chaotic motion. A simple physical model describing this transition process is proposed. However, if the circumferential flow was low, core bulging (or core swelling) might occur instead. A breakdown chart plotted using the Reynolds number and L/D for different nozzle angles is presented. The chart enables one to determine the flow conditions under which the core breakdown would occur.
Trajectory and entrainment of a round jet in crossflow
Authors: Yuan Lester L. , Street Robert L.
Published: July 2002

This paper examines the trajectory and entrainment characteristics of a round jet in crossflow. A series of large eddy simulations was performed at Reynolds numbers of 1050 and 2100 and at jet to crossflow velocity ratios of 2.0 and 3.3. Trajectories, which are defined based on the mean streamlines on the centerplane, all collapse to a single curve far from the jet exit, and this curve can be represented with a power law fit. Within this power law region, entrainment of crossflow fluid is shown to be the primary mechanism by which the jet trajectory is determined. Upstream of the power law region, near the jet exit, jet trajectory varies from changes in pressure drag and from differences in the turbulence intensities in the incoming pipe flow.
Mechanism of elastic instability in Couette flow of polymer solutions: Experiment
Authors: Groisman Alexander , Steinberg Victor
Published: July 2002

Experiments on flow stability and pattern formation in Couette flow between two cylinders with highly elastic polymer solutions are reported. It is found that the flow instabilities are determined by the elastic Deborah number, De, and the polymer concentration only, while the Reynolds number becomes completely irrelevant. A mechanism of such “purely elastic” instability was suggested a few years ago by Larson, Shaqfeh, and Muller [J. Fluid Mech. 218, 573 (1990)], referred to as LMS. It is based on the Oldroyd-B rheological model and implies a certain functional relation between De at the instability threshold and the polymer contribution to the solution viscosity, ηp/η, that depends on the polymer concentration. The elastic force driving the instability arises when perturbative elongational flow in radial direction is coupled to the strong primary azimuthal shear. This force is provided by the “hoop stress” that develops due to stretching of the polymer molecules along the curved streamlines. It is found experimentally that the elastic instability leads to a strongly nonlinear flow transition. Therefore, the linear consideration by LMS is expanded to include finite amplitude velocity perturbations. It is shown that the nature of the elastic force implies major asymmetry between inflow and outflow in finite amplitude secondary flows. This special feature is indeed exhibited by the experimentally observed flow patterns. For one of the flow patterns it is also shown that the suggested elastic force should be quite efficient in driving it, which is important evidence for the validity of the mechanism proposed by LMS. Further, the predicted relation between De and ηp/η is tested. At fixed ηp/η the elastic instability is found to occur at constant Deborah number in a broad range of the solution relaxation times in full agreement with the theoretical prediction. The experimentally found dependence of the Deborah number on ηp/η also agrees with the theoretical prediction rather well if a proper correction for the shear thinning is made. This provides further support to the proposed instability mechanism.
Velocity fluctuations in a turbulent soap film: The third moment in two dimensions
Authors: Belmonte A. , Goldburg W. I. , Kellay H. , Rutgers M. A. , Martin B. , Wu X. L.
Published: July 2002

Quasi-two-dimensional decaying turbulence is studied in a flowing soap film by measuring the moments of the probability density function P(δv(r)) for the longitudinal velocity differences δv(r) on a scale r. As in three-dimensional (3-D) turbulence, P becomes non-Gaussian with decreasing r. The third moment S3(r)≡〈(δv(r))3〉 is small and negative at small scales, but becomes positive at larger scales. The exact calculation of S3(r) for 2-D homogeneous isotropic turbulence relates this change in sign to the development of the velocity correlation function as the turbulence decays.
Turbulent boundary layer control utilizing the Lorentz force
Authors: Berger Timothy W. , Kim John , Lee Changhoon , Lim Junwoo
Published: July 2002

Direct numerical simulations (DNS) of a turbulent channel flow at low Reynolds number (Reτ=100,200,400, where Reτ is the Reynolds number based on the wall-shear velocity and channel half-width) are carried out to examine the effectiveness of using the Lorentz force to reduce skin friction. The Lorentz force is created by embedding electrodes and permanent magnets in the flat surface over which the flow passes. Both open-loop and closed-loop control schemes are examined. For open-loop control, both temporally and spatially oscillating Lorentz forces in the near-wall region are tested. It is found that skin-friction drag can be reduced by approximately 40% if a temporally oscillating spanwise Lorentz force is applied to a Reτ=100 channel flow. However, the power to generate the required Lorentz force is an order of magnitude larger than the power saved due to the reduced drag. Simulations were carried out at higher Reynolds numbers (Reτ=200,400) to determine whether efficiency, defined as the ratio of the power saved to the power used, improves with increasing Reynolds number. We found that the efficiency decreases with increasing Reynolds number. An idealized wall–normal Lorentz force is effected by detecting the near-wall turbulent events responsible for high-skin friction. It is found that the drag can be significantly reduced with a greater efficiency than that produced by the spanwise open-loop control approach. This result suggests that, when employed with a closed-loop control scheme, the Lorentz force might result in a net decrease of power required to propel objects through viscous conducting fluids.
Density ratio dependence of Rayleigh–Taylor mixing for sustained and impulsive acceleration histories
Authors: Dimonte Guy , Schneider Marilyn
Published: July 2002

The turbulent Rayleigh–Taylor instability is investigated over a comprehensive range of fluid density ratio (R)1.3⩽R⩽50 [0.15⩽A=(R−1)/(R+1)⩽0.96] and different acceleration histories g(t) using the Linear Electric Motor. The mixing layer is diagnosed with backlit photography and laser-induced fluorescence. For a constant acceleration, the bubble (2) and spike (1) amplitudes are found to increase as hi=αiAgt2 with α2∼0.05±0.005 and α1∼α2RDα with Dα∼0.33±0.05. For temporally varying accelerations Ag(t)>0, this can be generalized to hi=2αiAS using S=[∫gdt]2/2 rather than the displacement Z=∫∫gdt′ dt. For impulsive accelerations, S remains constant during the coast phase and the amplitudes obey a power law hi∼tθi with θ2∼0.25±0.05 and θ1∼θ2RDθ with Dθ∼0.21±0.05. These values of Dα and Dθ compare favorably with numerical simulations and mix models. The average diameter at the mixing front for bubbles is found to increase as d2∼h2(1+A)/4 in qualitative agreement with “merger” models, but the associated dhi/dt is two times larger than the terminal velocity of an isolated bubble. The spikes become relatively narrow at large R, yet they still grow as gt2.
The interaction of vorticity and rate-of-strain in homogeneous sheared turbulence
Authors: Nomura K. K. , Diamessis P. J.
Published: July 2002

The coupled interaction of vorticity ω and rate-of-strain S in homogeneous sheared turbulence is investigated using direct numerical simulation. Conditional sampling and comparison with linear simulations reveal various aspects of the structure and dynamics. Due to the influence of the imposed ω and S, distinct directional features develop. Initial stretching of fluctuating ω by mean extensional strain and the presence of mean vorticity establish a predominant misalignment of ω with respect to the principal axes of S. The associated locally induced rotation of the S axes results in preferred orientations in ω and S. In high amplitude rotation-dominated regions of the flow, distinct characteristics are exhibited by the pressure Hessian Π due to the presence of small-scale spatial structure. Nonlocally induced S axes rotation through Π tends to counteract locally induced rotation in these regions. These features are absent in the linear flow which suggests a lack of spatial coherence in the corresponding intense ω2 regions. High amplitude strain-dominated and comparable rotation-strain regions are also considered. In general, the high amplitude conditional samples capture the main features of the flow. The underlying behavior of ω and S is essentially the same as in isotropic turbulence; the directional preferences observed in shear flow demonstrate the physical implications of the associated mechanisms. Although there is greater directional variation in flows with high Reλ/Sh, results indicate the significance of the persistence of mean shear.
The biorthogonal decomposition as a tool for investigating fluctuations in plasmas
Authors: Dudok de Wit T. , Pecquet A.-L. , Vallet J.-C. , Lima R.
Published: July 2002

The investigation of fluctuation phenomena in plasmas often necessitates the analysis of spatiotemporal signals. It is shown how such signals can be analyzed using the biorthogonal decomposition, which splits them into orthogonal spatial and temporal modes. The method, also referred to as the singular value decomposition, allows complex spatiotemporal patterns to be decomposed into a few coherent modes that are often easier to interpret. This is illustrated with two applications to fluctuating soft x-ray and magnetic signals, as measured in a tokamak. Emphasis is given to the physical interpretation of the biorthogonal components and their link with known physical models is discussed. It is shown how new insight can be gained in the interpretation of spatiotemporal plasma dynamics.
Stimulated Raman scattering in a plasma channel
Authors: Liu C. S. , Tripathi V. K.
Published: July 2002

An intense laser beam propagating in a low-density plasma channel undergoes stimulated Raman forward scattering, producing a plasma wave and two electromagnetic sideband waves. The electromagnetic waves are localized within a width ∼(ca/ωp0)1/2, where a is the radius of the channel, ωp0 is the plasma frequency on the axis of the channel, and c is the velocity of light, whereas the localization of the plasma wave is dependent on the growth rate of the Raman process. The localization effects reduce the region of interaction and the growth rate. At moderate intensities the growth rate scales as 2/3 power of intensity, however, at higher intensities it varies less rapidly.
Response to ‘‘Comment on ‘On the Alfvén resonance and its existence’ ’’ [Phys. Plasmas 2, 340 (1995)]
Authors: Rauf S. , Tataronis J. A.
Published: July 2002

The spatial singularity of the shear Alfvén wave in MHD plasma is a delta function. We show that this delta function solution makes physical sense and explore its characteristics. (AIP)
Two-dimensional modeling of magnetically driven Rayleigh–Taylor instabilities in cylindrical Z pinches
Authors: Peterson D. L. , Bowers R. L. , Brownell J. H. , Greene A. E. , McLenithan K. D. , Oliphant T. A. , Roderick N. F. , Scannapieco A. J.
Published: July 2002

A two-dimensional computational methodology has been developed that uses a phenomenological representation of initial perturbations to model the evolution of magnetically driven Rayleigh–Taylor instabilities in a hollow Z pinch. The perturbed drive current waveform and x-ray output obtained from the two-dimensional models differ qualitatively from the results of unperturbed (one-dimensional) models. Furthermore, the perturbed results reproduce the principle features measured in a series of capacitor bank-driven pulsed power experiments. In this paper we discuss the computational approach and the computational sensitivity to initial conditions (including the initial perturbations). Representative examples are also presented of instability evolution during implosions, and the results are compared with experimentally measured current waveforms and visible framing camera images of perturbed implosions. Standard magnetohydrodynamic modeling, which includes instability growth in two dimensions, is found to reproduce the features seen in experiments.
Saturation of stimulated Brillouin scatter by self-consistent flow profile modification in laser hot spots
Authors: Rose Harvey A.
Published: July 2002

Scattering instabilities, such as stimulated Brillouin scatter (SBS) and stimulated Raman scatter (SRS), transfer momentum to the plasma which leads to flow inhomogeneities in laser hot spots that may significantly reduce the level of SBS. Simple estimates and simulations shown that the magnitude of flow fluctuations can reach Mach numbers of order unity in a time scale of hundreds of picoseconds.
Two-dimensional plasma flow past a laser beam
Authors: Ghosal Sandip , Rose Harvey A.
Published: July 2002

Analytical results are presented for laser beam deflection rate due to plasma flow when the ponderomotive force (PMF) is static and given. Explicit expressions are obtained in various parameter regimes including that of weak PMF for the case of a coherent (diffraction limited) beam and a beam whose fluctuations are spatially homogeneous, as in the case of a model random phase plate beam. When the Landau damping coefficient, γ0, is negligible and the beam is either coherent and cylindrically symmetric, or random with isotropic fluctuations, the deflection rate is obtained as a closed form function of plasma flow Mach number, M. For finite damping, results are expressed in terms of a universal, one dimensional integral parameterized by M and γ0. For arbitrary PMF and M small, the problem is identified with one in the theory of random dielectric media.
Nonlinear tearing modes in the presence of resistive wall and rotation
Authors: Finn John M. , Sovinec Carl R.
Published: July 2002

Nonlinear simulations of tearing modes with a resistive wall and plasma rotation in a two-dimensional (2-D) incompressible slab are presented. The regimes of interest include cases (1) in which a tearing mode is unstable even for a perfectly conducting wall; (2) in which a resistive wall tearing mode is unstable but may be stabilized by rotation. In (1), the mode can lead to a wall locking process, in which both the flow and the mode’s phase velocity are slowed, allowing flux diffusion through the wall and a wider island. There is a bifurcation with hysteresis between locked and unlocked states. If the decay of total momentum is large, the range with multiple states is smaller. In (2) resistive wall modes grow and lock for all parameters with linear instability. And even if rotation gives linear stability, there is nonlinear instability. The bifurcation diagram is similar to that for case (1), but in which the zero island width state takes the place of the unlocked state. Results are shown for case (2) giving the critical field error to drive nonlinear instability and begin locking.
Observations of two-dimensional magnetic field evolution in a plasma opening switch
Authors: Shpitalnik R. , Weingarten A. , Gomberoff K. , Krasik Ya. , Maron Y.
Published: July 2002

The time dependent magnetic field distribution was studied in a coaxial 100-ns positive-polarity Plasma Opening Switch (POS) by observing the Zeeman effect in ionic line emission. Measurements local in three dimensions are obtained by doping the plasma using laser evaporation techniques. Fast magnetic field penetration with a relatively sharp magnetic field front (⩽1 cm) is observed at the early stages of the pulse (t≲25). Later in the pulse, the magnetic field is observed at the load-side edge of the plasma, leaving “islands” of low magnetic field at the plasma center that last for about 10 ns. The two-dimensional (2-D) structure of the magnetic field in the r,z plane is compared to the results of an analytical model based on electron-magneto-hydrodynamics, that utilizes the measured 2-D plasma density distribution and assumes fast magnetic field penetration along both POS electrodes. The model results provide quantitative explanation for the magnetic field evolution observed.
Optical probing of fiber z-pinch plasmas
Authors: Tatarakis M. , Aliaga-Rossel R. , Dangor A. E. , Haines M. G.
Published: July 2002

An experimental study of optical probing of a dense z-pinch plasma using the MAGPIE (mega-ampere generator for plasma implosion experiments) generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)] is reported. The generator was operated with a peak current of 1.1 MA rising in 150 ns (10%–90%). The loads were 33 μm diam carbon fibers. Faraday rotation was used to investigate the distribution of the current flowing in the plasma. A measurable Faraday rotation angle was observed only in a time window from 50 to 60 ns after the current start, due to the fact that this effect depends on a combination of the magnetic-field strength and electron number density. A new type of self-referencing cyclic radial shear interferometer was used to evaluate the plasma density profiles which are necessary for the reconstruction of the current distribution. It was calculated that ∼110 kA was flowing in the plasma at 52 ns after the current start. Shadowgraphy was used to study the dynamics of the plasma and to investigate the formation of instabilities. Plasma instabilities were observed at very early times (∼5 ns). These instabilities appeared to be not entirely axi-symmetric implying the existence of m=1 and maybe higher modes as well as m=0. The perturbations increased with time and evolved into density islands (isolated plasma fragments) distributed along the axis at late times (∼70 ns).
Nonlinear electrodynamics of the interaction of ultra-intense laser pulses with a thin foil
Authors: Vshivkov V. A. , Naumova N. M. , Pegoraro F. , Bulanov S. V.
Published: July 2002

An analytical description of the interaction of laser light with a foil, described as a thin slab of overdense plasma, is presented together with the results of multidimensional particle in cell simulations. The matching conditions at the foil result in nonlinear boundary conditions for the wave equation. The conditions for relativistic transparency are given. The interaction with the foil leads to shaping of the laser pulse. In the case of oblique incidence of a relativistically intense pulse, nonlinear coupling modifies the pulse polarization and causes emission of high harmonics and generation of an electric current. Strong focalization of the reflected pulse, in particular in three-dimensional simulations, is observed for normal and oblique incidence due to the induced distortion of the foil surface.
A review of rotating magnetic field current drive and the operation of the rotamak as a field-reversed configuration (Rotamak-FRC) and a spherical tokamak (Rotamak-ST)
Authors: Jones Ieuan R.
Published: July 2002

The physics underlying the rotating magnetic field current drive technique is presented. The rotamak is a compact torus configuration having the unique and distinctive feature that the toroidal plasma current is driven in a steady-state, noninductive fashion by means of the application of a rotating magnetic field. In its basic form, the rotamak is operated as a field-reversed configuration (Rotamak-FRC). However, by means of a simple modification, a steady toroidal magnetic field can be added to the basic rotamak apparatus and the configuration then becomes that of a spherical tokamak (Rotamak-ST). The performance of a 50-liter rotamak device, both as an FRC and as an ST, is described. Toroidal currents of over 10.5 kA have been achieved with input powers of 300 kW (at 0.5 MHz). Hydrogen plasmas with ne≈7×1018 m−3 and Te≈35 eV have been obtained. The noteworthy reproducibility of the rotamak discharge has enabled the magnetic field lines of an ST to be directly reconstructed from experimental data for the first time. Attention is drawn to the fact that a fair evaluation of the rotamak concept requires experimentation at higher radio-frequency power levels than are presently available.
Advances in plasma-filled microwave sources
Authors: Goebel Dan M. , Carmel Yuval , Nusinovich Gregory S.
Published: July 2002

Significant improvements in the performance of microwave sources have been achieved in recent years by the introduction of the appropriate amount of plasma into tubes designed to accommodate plasma. Plasma filling has been credited with increasing the electron beam current, bandwidth, efficiency and reducing or eliminating the need for guiding magnetic fields in microwave sources. Neutralization of the e-beam space charge by a plasma enhances the current capability and beam propagation, and the generation of hybrid waves in plasma-filled sources increases the electric field on axis and improves the coupling and efficiency. Control of the plasma density in these microwave sources is often required to avoid instabilities and variations in the output power level and pulse length. Recent experimental and theoretical advances in this field, and the benefits and limitations of plasma filling of several different types of microwave sources, will be discussed.
The physics of spherical torus plasmas
Authors: Peng Y.-K. M.
Published: July 2002

Broad and important progress in plasma tests, theory, new experiments, and future visions of the spherical torus (ST, or very low aspect ratio tokamaks) have recently emerged. These have substantially improved our understanding of the potential properties of the ST plasmas, since the preliminary calculation of the ST magnetohydrodynamic equilibria more than a decade ago. Exciting data have been obtained from concept exploration level ST experiments of modest capabilities (with major radii up to 35 cm), making important scientific contributions to toroidal confinement in general. The results have helped approval and construction of new and/or more powerful ST experiments, and stimulated an increasing number of theoretical calculations of interest to magnetic fusion energy. Utilizing the broad knowledge base from the successful tokamak and advanced tokamak research, a wide range of new ST physics features has been suggested. These properties of the ST plasma will be tested at the 1 MA level with major radius up to ∼80 cm in the new proof of principle devices National Spherical Torus Experiment (NSTX, U.S.) [M. Peng et al., European Conf. Abst. 22C, 451 (1998); S. M. Kaye et al., Fusion Technol. 36, 16 (1999); M. Ono et al., “Exploration of Spherical Torus Physics in the NSTX Device,” 17th IAEA Fusion Energy Conf., paper IAEA-CN-69/ICP/01 (R), Yokohama, Japan (1998)], Mega Ampere Spherical Tokamak (MAST, U.K.) [A. C. Darke et al., Fusion Technol. 1, 799 (1995); Q. W. Morris et al., Proc. Int. Workshop on ST (Ioffe Inst., St. Petersburg, 1997), Vol. 1, p. 290], and Globus-M (R.F.) [V. K. Gusev et al., European Conf. Abst. 22C, 576 (1998)], which have just started full experimental operation. New concept exploration experiments, such as Pegasus (University of Wisconsin) [R. Fonck and the PEGASUS Team, Bull. Am. Phys. Soc. 44, 267 (1999)], Helicity Injected Tokamak-II (HIT-II, University of Washington) [T. R. Jarboe et al., Phys. Plasmas 5, 1807 (1998)], and Current Drive Experiment-Upgrade (CDX-U, Princeton Plasma Physics Laboratory) [M. Ono et al., Proceedings of the 16th International Conference on Fusion Energy (IAEA, Vienna, 1997), Vol. 2, p. 71] and other experiments in Japan and Brazil, etc., present additional opportunities for important progress. This tutorial paper summarizes our understanding and projections of the physics of the ST plasmas, the investigation of which will hopefully bring new enthusiasm and advancements for fusion energy science research in the U.S. and the world.
Detection of thermal waves through optical reflectance
Authors: Rosencwaig Allan , Opsal Jon , Smith W. L. , Willenborg D. L.
Published: July 2002

We show that thermal wave detection and analysis can be performed, in a noncontact and highly sensitive manner, through the dependence of sample optical reflectance on temperature. Applications to the study of microelectronic materials are illustrated by an example of measuring the thickness of thin metal films.
Reactivity of Dy2O3 and Tb4O7 with Nd15Fe77B8 powder and the coercivity of the sintered magnets
Authors: Ghandehari M. H.
Published: July 2002

The reaction of Dy2O3 or Tb4O7 with Nd15Fe77B8 powder during the sintering increases the coercivity of the sintered magnets. These oxides are reduced to metal during the sintering process, resulting in magnets with properties similar to those prepared from the Nd13.5Dy1.5Fe77B8 alloy.
Multiple quantum well 10 μm GaAs/AlxGa1−xAs infrared detector with improved responsivity
Authors: Choi K.-K. , Levine B. F. , Bethea C. G. , Walker J. , Malik R. J.
Published: July 2002

We have achieved a high responsivity, R=1.9 A/W, 10 μm infrared detector using intersubband absorption in GaAs/AlxGa1−xAs quantum well superlattices. The photocurrent is produced by intersubband absorption followed by efficient photoexcited tunneling. This responsivity is nearly four times higher than our previous results and has been obtained by using thicker and higher AlxGa1−xAs superlattice barriers thereby reducing the dark current and allowing the detector to be operated at higher biases.
Acoustic wave viscosity sensor
Authors: Ricco A. J. , Martin S. J.
Published: July 2002

An acoustic wave device utilizing plate modes having components of displacement parallel to a crystal surface has been demonstrated to be an effective sensor of liquid shear viscosity (η) over a wide viscosity range. When a liquid is present on the sensor surface, the propagation loss of the acoustic wave depends upon η in a calculable fashion. Because the device functions at 159 MHz, liquid relaxation effects occur with high viscosity liquids, causing the propagation loss to saturate. Viscosity is sampled in the 50-nm-thick liquid layer which couples to the acoustic wave, so that only a single drop of liquid is required for measurement.
New technique of photodisplacement imaging using one laser for both excitation and detection
Authors: Chen L. , Yang K. H. , Zhang S. Y.
Published: July 2002

A new interferometry technique has been developed that uses only a single laser for both exciting and detecting photothermal displacement, which greatly simplifies the system and the optical alignment. The minimum detectable displacement 10−2 Å/(Hz)1/2 is obtained. To examine this setup, we have performed a simulated experiment. And also, we have succeeded in imaging the subsurface flaw in a copper block. The preliminary results demonstrate that this new method is very promising for nondestructive evaluation and other applications.
Stability of superconducting YBa2Cu3O7 in the presence of water
Authors: Barns R. L. , Laudise R. A.
Published: July 2002

The new high transition temperature superconductors such as YBa2Cu3O7 in powdered form have been observed to react vigorously at room temperature with water producing a nonsuperconducting cuprate with copper in oxidation state +2, O2, CuO, and Ba(OH)2. Similar reactions will probably occur with nearly equal rapidity in all cuprate superconductors. We have used room-temperature and liquid N2 resistance measurements to study the stability of smaller surface area specimens (thin sheets). We have found that such specimens react slowly with humid air at room temperature and rapidly in air at 85 °C and 85% rel. humidity, the standard conditions used for accelerated humidity testing. The resultant end products are nonsuperconducting phases formed in from hours to days, depending on conditions. Furthermore, we have found that, while a plastic encapsulant considerably slowed such reactions, it did not eliminate them. The implications for both scientific experiments to characterize high transition temperature superconductors and for applications as well as possible remedies are discussed.
Atomic oxygen and the thermal oxidation of silicon
Authors: Hoff Andrew M. , Ruzyllo Jerzy
Published: July 2002

Both molecular and atomic oxygen have been postulated to react with silicon in the thermal oxidation process. At temperatures below 700 °C very little film growth is observed in molecular oxygen. When silicon is allowed to react thermally with a flowing afterglow containing atomic oxygen an enhancement in film growth is observed. Film growth in this mode exhibits little temperature activation and is speculated to be due to a constant concentration of atomic oxygen.
The structural mechanism of GTP stabilized oligomerization and catalytic activation of the
Toxoplasma gondii
uracil phosphoribosyltransferase
Authors: Schumacher Maria A. , Bashor Caleb J. , Song Minsun Hong , Otsu Kanao , Zhu Shuren , Parry Ronald J. , Ullman Buddy , Brennan Richard G.
Published: July 2002

Uracil phosphoribosyltransferase (UPRT) is a member of
a large family of salvage and biosynthetic enzymes, the
phosphoribosyltransferases, and catalyzes the transfer of ribose
5-phosphate from
α-
d
-5-phosphoribosyl-1-pyrophosphate (PRPP) to the N1
nitrogen of uracil. The UPRT from the opportunistic pathogen
Toxoplasma gondii
represents a promising target for
rational drug design, because it can create intracellular, lethal
nucleotides from subversive substrates. However, the development of
such compounds requires a detailed understanding of the catalytic
mechanism. Toward this end we determined the crystal structure of the
T. gondii
UPRT bound to uracil and cPRPP, a
nonhydrolyzable PRPP analogue, to 2.5-Å resolution. The structure
suggests that the catalytic mechanism is substrate-assisted, and a
tetramer would be the more active oligomeric form of the enzyme.
Subsequent biochemical studies revealed that GTP binding, which has
been suggested to play a role in catalysis by other UPRTs, causes a
6-fold activation of the
T. gondii
enzyme and strikingly
stabilizes the tetramer form. The basis for stabilization was revealed
in the 2.45-Å resolution structure of the UPRT–GTP complex, whereby
residues from three subunits contributed to GTP binding. Thus, our
studies reveal an allosteric mechanism involving nucleotide
stabilization of a more active, higher order oligomer. Such regulation
of UPRT could play a role in the balance of purine and pyrimidine
nucleotide pools in the cell.
Functional self-association of von Willebrand factor during platelet adhesion under flow
Authors: Savage Brian , Sixma Jan J. , Ruggeri Zaverio M.
Published: July 2002

We have used recombinant wild-type human von Willebrand factor
(VWF) and deletion mutants lacking the A1 and A3 domains, as well as
specific function-blocking monoclonal antibodies, to demonstrate a
functionally relevant self-association at the interface of soluble and
surface-bound VWF. Platelets perfused at the wall shear rate of
1,500 s
−1
over immobilized VWF lacking A1 domain function
failed to become tethered to the surface when they were in a
plasma-free suspension with erythrocytes, but adhered promptly if
soluble VWF with functional A1 domain was added to the cells. The same
results were observed when VWF was immobilized onto collagen through
its A3 domain and soluble VWF with deleted A3 domain was added to the
cells. Thus, VWF bound to glass or collagen sustains a process of
homotypic self-association with soluble VWF multimers that, as a
result, can mediate platelet adhesion. The latter finding demonstrates
that direct immobilization on a substrate is not a strict requirement
for VWF binding to platelet glycoprotein Ibα. The dynamic and
reversible interaction of surface-bound and soluble VWF appears to be
specifically homotypic, because immobilized BSA, human fibrinogen, and
fibronectin cannot substitute for VWF in the process. Our findings
highlight a newly recognized role of circulating VWF in the initiation
of platelet adhesion. The self-assembly of VWF multimers on an injured
vascular surface may provide a relevant contribution to the arrest of
flowing platelets opposing hemodynamic forces, thus facilitating
subsequent thrombus growth.
β3: An additional auxiliary subunit of the voltage-sensitive sodium channel that modulates channel gating with distinct kinetics
Authors: Morgan Kevin , Stevens Edward B. , Shah Bhaval , Cox Peter J. , Dixon Alistair K. , Lee Kevin , Pinnock Robert D. , Hughes John , Richardson Peter J. , Mizuguchi Kenji , Jackson Antony P.
Published: July 2002

The voltage-sensitive sodium channel confers electrical
excitability on neurons, a fundamental property required for higher
processes including cognition. The ion-conducting α-subunit of the
channel is regulated by two known auxiliary subunits, β1 and β2. We
have identified rat and human forms of an additional subunit, β3. It
is most closely related to β1 and is the product of a separate gene
localized to human chromosome 11q23.3. When expressed in
Xenopus
oocytes, β3 inactivates sodium channel opening
more slowly than β1 does. Structural modeling has identified an amino
acid residue in the putative α-subunit binding site of β3 that may
play a role in this difference. The expression of β3 within the
central nervous system differs significantly from β1. Our results
strongly suggest that β3 performs a distinct neurophysiological
function.
Human
DINB1
-encoded DNA polymerase κ is a promiscuous extender of mispaired primer termini
Authors: Washington M. Todd , Johnson Robert E. , Prakash Louise , Prakash Satya
Published: July 2002

Both in yeast and humans, DNA polymerase (Pol) η functions in the error-free replication of UV-damaged DNA, and Polη has the unique ability to efficiently replicate through a cis-syn thymine–thymine (T–T) dimer by inserting two As opposite the two Ts of the dimer. Although human
DINB1
-encoded Polκ belongs to the same protein family as Polη, Polκ shows no ability to bypass this DNA lesion and its biological function has remained unclear. Here, we examine Polκ for its ability to extend from primer-terminal mispairs opposite nondamaged and damaged DNA templates. We find that Polκ is a promiscuous extender of primer-terminal mispairs opposite nondamaged DNA templates, and interestingly, it is also very efficient at extending from a G opposite the 3′T of a T–T dimer. These observations provide biochemical evidence for a role of Polκ in the extension of mismatched base pairs during normal DNA replication, and in addition, they implicate Polκ in the mutagenic bypass of T–T dimers. In its proficient mismatch extension ability, Polκ is more similar to the unrelated DNA polymerase ζ than it is to the phylogenetically related Polη or Polι. Thus, in humans, Polκ would compete with Polζ for the extension of mismatched base pairs on damaged and undamaged DNAs.