bluuebunny/crossref_metadata_2025 huggingface dataset:
Resistive ballooning modes in helical axis stellarators with longitudinal mass flow
Authors: Cooper W. A. , Depassier M. C.
Published: July 2002

The magnetohydrodynamic equilibrium equation for magnetic confinement systems with helical symmetry and longitudinal plasma flow is derived. The incompressible resistive ballooning mode equation for systems with a coordinate of symmetry and rigid longitudinal flow is also derived. A reduced equation is then obtained by expanding the ballooning mode equation for a system with a large field period length compared with the plasma cross section. An analytic solution is obtained for a model equilibrium with circular flux surfaces. In the limit of small longitudinal velocity, the static mode width and growth rate scaling as the resistivity to the one-third power are recovered. For small growth rates and large longitudinal velocity, these growth rates are driven by the interaction of the magnetic curvature and the centrifugal force with the radial pressure and mass density gradient, respectively, and scale linearly with the resistivity at a reduced level.
Variational quadratic form for low-frequency electromagnetic perturbations. I. Formalism
Authors: Berk H. L. , Lane B. G.
Published: July 2002

A variational formalism is obtained in the limit of large perpendicular wavenumber which simultaneously includes electrostatic and electromagnetic perturbations, finite Larmor radius corrections, equilibrium plasma rotation, and arbitrary particle bounce effects. A tractable final expression is obtained and kinetic integrals are evaluated in special limits. The more accurate noneikonal expression is obtained from the asymptotic matching of the eikonal form to more restrictive noneikonal quadratic forms derived elsewhere.
Effect of induced azimuthal electric fields and radial fueling on neoclassical scaling of tandem transport
Authors: Myra J. R. , Catto Peter J. , Francis G. L.
Published: July 2002

A simple model for tandem mirror transport and ambipolarity is formulated by neglecting temperature gradients. It is shown that the shape of the radial fueling profile in the center cell and the retention of induced azimuthal electric fields are important in determining the self-consistent radial potential drop ΔΦ and the resulting neoclassical confinement time τ⊥. Numerical solution of the transport equations yields the approximate scaling law τ⊥∝(ΔΦ)−2 when eΔΦ>T and T is held fixed.
Ion tail formation by cascade trapping in lower-hybrid heating experiments
Authors: Gell Y. , Nakach R.
Published: July 2002

The possibility of the operation of a multistep trapping process in the interaction of the ions of a plasma with lower–hybrid waves, leading to the formation of a tail in the ion velocity distribution, is investigated. Considering waves propagating perpendicularly to a homogeneous magnetic field, it is found that the spectral nature of the lower-hybrid excitation and the dependency of the perpendicular wave vector on the radial coordinate of the torus are the characteristics of the system which enable this cascading acceleration to take place. The conditions and limitations for the operation of this mechanism are discussed.
Effect of quasiconfined particles and l=2 stellarator fields on the negative mass instability in a modified betatron
Authors: Roberts G. , Rostoker N.
Published: July 2002

A sufficient stability condition for the negative mass instability is derived. This condition is used to show that quasiconfined particles in a modified betatron create fields that can stabilize the negative mass instability. Additionally, it is shown that stellarator fields can inhibit this instability.
Stokes flow in a two-dimensional cavity with moving end walls
Authors: Sturges Leroy D.
Published: July 2002

When fluid in a two-dimensional, rectangular cavity is driven by moving end walls, the resulting flow consists of a pattern of one or more eddies depending on the aspect ratio (ratio of depth to width) of the cavity. For aspect ratios less than 1, the flow pattern consists of a single eddy that fills the entire cavity. As the aspect ratio increases, this eddy pinches off in the center, forming two eddies. Then small eddies form at the center of the side walls, grow toward the center, and finally coalesce into a central, counterrotating eddy.
The influence of plasma-induced energy deposition effects, the equation of state, thermal ionization, pulse shaping, and radiation on ion-beam-driven expansions of plane metal targets
Authors: Long Keith A. , Tahir Naeem A.
Published: July 2002

In a previous paper by Long and Tahir [Phys. Fluids 29, 275 (1986)], the motion of plane targets irradiated by ion beams whose energy deposition was assumed to be independent of the ion energy, and the temperature and density of the plasma, was analyzed. In this paper, the analytic solution is extended in order to include the effects of a temperature-and density-dependent energy deposition as a result of electron excitation, an improved equation of state, thermal ionization, a pulse shape, and radiation losses. The change in the energy deposition with temperature and density leads to range shortening and an increased power deposition in the target. It is shown how the analytic theory can be used to analyze experiments to measure the enhanced energy deposition. In order to further analyze experiments, numerical simulations are presented which include the plasma-induced effects on the energy deposition. It is shown that since the change in the range is due to both decrease in density and the increase in temperature, it is not possible to separate these two effects in present experiments. Therefore, the experiments which measure the time-dependent energy of the ions emerging from the back side of a plane target do not as yet measure the energy loss as a function of the density and temperature of the plasma or of the energy of the ion, but only an averaged loss over certain ranges of these physical quantities.
Convective contributions to local power loss in a bumpy torus
Authors: Hiroe S. , Haste G. R. , Tolliver J. S. , Quon B. H. , Connor K. A. , Goyer J. R. , Solensten L.
Published: July 2002

Power flow in the ELMO Bumpy Torus [Plasma Physics and Controlled Nuclear Fusion Research, 1974, Tokyo (IAEA, Vienna, 1975), Vol. 2, p. 141; Plasma Phys. 25, 597 (1983)] was investigated by measuring the power received by a limiter. This power was found to be a small fraction of the gyrotron output power (in one case, 13 out of 100 kW). To investigate the reason for the small fraction that appeared on the limiter, power was selectively removed from various cavities, including the cavity containing the limiter. These experiments have demonstrated that the majority of the power is lost locally. Observations of the potential structure demonstrate that asymmetric potential contours are present that can lead to enhanced plasma loss.
Three-halves harmonic measurements on 0.53 μm laser produced plasmas
Authors: Amiranoff F. , Briand F. , Labaune C.
Published: July 2002

Three-halves harmonic spectra measured at three different observation angles from 0.53 μm laser produced plasmas are presented. The results are compared with existing models. On plastic targets a very good fit is obtained with Barr’s model with Te≂1.6 keV and a plasma velocity at nc/4 of v≂8.107 cm/sec. For higher Z material, the electron temperature seems to increase and Landau damping may become significant.
Laser filamentation in a thermally unstable plasma
Authors: Liang Edison P. , Langdon A. Bruce
Published: July 2002

The propagation of laser light in a thermally unstable plasma attributable to potential imbalances in the local heating and cooling rates caused by density or temperature perturbations is considered. The linear growth rates of laser filamentation and thermal instability are governed by a single fifth-order dispersion relation. The behavior of different modes in various limits is discussed in detail and examples of potential applications to laser–target interactions are given.
The hydrodynamics of ponderomotive interactions in a collisionless plasma
Authors: Kono M. , Škorić M. M. , ter Haar D.
Published: July 2002

A hydrodynamic treatment of ponderomotive interactions in a collisionless plasma is presented and it is shown that consistent hydrodynamics leads to the correct expression for the solenoidal ponderomotive electron current density, a result previously thought to be derivable only in the framework of the warm-plasma kinetic theory.
Absorption of waves at the ion-cyclotron frequency range by drifting electrons
Authors: Fruchtman A. , Cummins W. F.
Published: July 2002

Waves in the ion-cyclotron range of frequencies are shown to be Landau damped by drifting electrons in the presence of an equilibrium electric field. Absorption occurs even if both the electric component of the wave parallel to the equilibrium magnetic field and the component of the wave vector perpendicular to the equilibrium magnetic field are zero.
Temperature measurements behind shock waves using a rapid scanning IR-diode laser
Authors: Brandt O. , Roth P.
Published: July 2002

Temperature measurements behind shock waves are possible by applying optical methods. In the case of a thermally relaxing system it is generally difficult to measure the translational and vibrational temperatures separately and in a direct way. In the present case, a rapid scanning infrared-diode laser has been used to study the relaxation behavior of a shock heated CO/Ar mixture. The vibrational temperature can be measured by rapid scanning over two or three adjacent rotational lines from different vibrational transitions of CO. In addition it is possible to determine the translational temperature of the gas mixture from the thermal broadening of a single highly resolved rotational absorption line. The experimentally determined vibrational and translational temperature are in reasonable agreement with the computed values based on shock speed measurements and vibrational relaxation data.
The fine structure of the 2ω time resolved spectrum in laser plasmas
Authors: Min Gu , Weihan Tan , Zunqi Lin , Wuji Bi , Wenyan Yu , Ximing Deng
Published: July 2002

Time resolved spectrum with both high temporal resolution (∼10 psec) and spectral resolution (∼0.6 Å) of the 2ω0 harmonic emission scattered at 90° to the laser axis are presented in this paper. It is shown experimentally that the primary and secondary peaks of the 2ω0 harmonic are actually composed of many fine and bright fringes with intervals of about 1.5∼3 Å for each, the values of which are in close relation to the atomic weight. Finally, analyses and discussions of these are given.
Plasma kinetic theory without the Markovian approximation. II
Authors: Pocobelli G.
Published: July 2002

A generalized, deterministic Fokker–Planck equation with Eulerian coefficients is derived from the Vlasov equation without making use of the Markovian approximation. Such an equation is equivalent to a simpler (no time derivative) propagation equation for the average distribution  f̄ , and identical at order E2 with a previously derived non-Markovian quasilinear equation. By expressing the perturbed distribution in terms of  f̄ , propagation of  f̄  in terms of all of its past history is obtained, with space and time correlations of the orbit perturbations.
Diamagnetic stability limit for a hot particle annulus in a plasma
Authors: Berk H. L. , Zhang Y. Z.
Published: July 2002

The stability of a hot particle annulus in a mirror cell at arbitrary drift reversal is analyzed by using a cylindrical model with embedded currents that simulate the role of field line curvature in actual mirror experiments. New effects are included by taking into account the Landau damping arising from the spread of hot particle drift velocities. It is found that the ‘‘precessional layer’’ mode is destabilized near the pressure gradient value at which drift reversal is achieved. Its growth rate is proportional to N−4, where N−2 is a coupling constant γ2MHD/ ω̃2κ [ω̃κ is the curvature frequency, γMHD is the typical magnetohydrodynamic (MHD) growth rate]. In addition the unstable interaction of the precessional layer mode with surface waves that can propagate below and above the ion-cyclotron frequency is described. Below the ion-cyclotron frequency the surface wave resembles the Alfvén wave, while above the ion-cyclotron frequency the surface wave resembles a whistler wave.
Poloidally asymmetric electrostatic potentials in closed line bumpy toroids
Authors: Hedrick C. L. , Owen L. W.
Published: July 2002

Here an analytic expression is developed for the poloidal asymmetry in the electrostatic potential observed experimentally in the T mode of operation in the Elmo Bumpy Torus (EBT) [Phys. Fluids 28, 2848 (1985)]. A multiple fluid treatment for the ‘‘cool,’’ ‘‘warm,’’ and ‘‘hot’’ electrons is used. The central idea is that the ‘‘warm’’ electrons dominate both the radial and poloidal structure of the electrostatic potential in the T mode of operation. The expression for the poloidal asymmetry in the electrostatic potential, which agrees reasonably well with experiment, is also applied to a modification of the EBT magnetic configuration. It is found that a substantial reduction in the asymmetry of the electrostatic potential below that of the Elmo Bumpy Torus is possible in the modification of the EBT magnetic configuration.
Observations of the dependence of unstable drift cyclotron loss cone mode characteristics on plasma density
Authors: McCarrick M. J. , Booske J. H. , Ellis R. F.
Published: July 2002

Results of precise measurements on the variation of drift cyclotron loss cone frequency and perpendicular wavenumber as a function of ion density are reported. Careful quantitative comparisons to theory are made. In particular, it was found that a local linear slab model accurately predicted the unstable mode frequencies, even under conditions where nonlocal effects should be significant—i.e., when characteristic gradient scale lengths are on the order of an unstable mode’s wavelength. Inaccurate predictions of unstable perpendicular wavelength, however, may be a consequence of nonlocal effects.
Refueling pellets as a source of charge exchange neutrals for alpha particle measurements
Authors: Gerdin G.
Published: July 2002

Alpha interactions with neutrals in the ablation cloud of a refueling pellet are considered as a fast alpha diagnostic through charge-capture radiation (ACCR) by the alphas or their complete neutralization and escape. Calculations of pellet penetration into a reactor plasma indicate that regions of r/a>0.61 could be reached by refueling pellets with initial radii of 4 mm, hence this would be an outer core diagnostic. A spherically symmetric neutral-gas shielding model (SSMNGSM) in which ionization is treated self-consistently and governed by local thermodynamic equilibrium is used in these calculations. Collisional processes are found to govern the alpha–cloud interactions and local equilibrium calculations were performed in which the charge exchange and ionization cross sections are used to obtain the state fractions of the alphas in the cloud. At 1640 Å the ratio of ACCR to cloud bremsstrahlung is found to be 10−5 whereas at 304 Å this ratio ranges from 3×10−3 to 0.5 for the cases considered; no neutralized alphas escape the cloud. Magnetic field effects are considered and they appear to reduce the bremsstrahlung from the cloud by a factor of 300 at 304 Å in regions perpendicular to the magnetic field from the pellet’s surface; escape of neutralized alphas may be possible in these directions. Alpha refueling pellet interactions are potentially useful as outer core fast alpha diagnostics and this could be tested in near term experiments.
Generation of magnetic vortices by electromagnetic waves in an inhomogeneous plasma
Authors: Yu M. Y. , Shukla P. K. , Stenflo L.
Published: July 2002

It is shown that a high power laser can parametrically generate magnetic electron drift motion in a high density, nonuniform collisional plasma. The nonlinear dispersion relation as well as the growth rates of the decay and modulational instabilities are obtained. Possible application of these results in laser–plasma interaction is discussed.
Influence of velocity-magnetic field correlations on decaying magnetohydrodynamic turbulence with neutral X points
Authors: Pouquet A. , Sulem P. L. , Meneguzzi M.
Published: July 2002

Direct numerical simulations at resolutions up to (512)2 of developed turbulence in nondriven two-dimensional incompressible magnetohydrodynamic (MHD) flows are presented. In all cases, there is no background magnetic field. Nevertheless, periodic exchanges of kinetic and magnetic energies are observed, an effect that is reminiscent of interference of linear Alfvén waves; three-dimensional runs also display such oscillations. This effect is inhibited by the velocity-magnetic field correlations. It is also found that the correlations do not sizably affect the characteristic transfer time of energy to the small scales, but significantly reduce the maximum amplitude of velocity and magnetic field gradients. It is shown that velocity-magnetic field correlation also affects inertial range spectral indices, a result that validates predictions based on two-point closures [Astron. Astrophys. 126, 151 (1983)].
Observation of radio-frequency field induced plasma loss in a simple mirror
Authors: Kuthi A. , Olson L. , Lam K. L. , Zwi H. , Wong A. Y.
Published: July 2002

High-frequency parallel electric fields applied to plasmas confined in a simple magnetic mirror cause enhanced plasma loss. The plasma potential rises because of a population of electrons in the magnetic loss cone that are accelerated to parallel energies significantly higher than the bulk electron temperature. The radio-frequency induced plasma loss has a nonambipolar component, with ions lost predominantly in the radial direction.
Second harmonic emission from a picosecond laser-produced plasma
Authors: Takada Youichi , Nakano Noboru , Kuroda Hiroto
Published: July 2002

Several kinds of experiments have been done relating to the second harmonic emission from a plasma produced by a 1 μm laser pulse of 30 psec. Complicated spectra of the second harmonic have been observed above ∼1013  W/cm2. It has been concluded that their production mechanisms are dependent on the self-focusing of the laser beam. In the absence of the self-focusing of the laser beam, it was determined that the complicated spectrum was caused by the parametric decay instability of an electron plasma wave excited by the resonance absorption. In the presence of the self-focusing of the laser beam, on the other hand, other mechanisms have been found to be dominant. It was tentatively concluded that they are the unstable motion of the critical surface of the plasma or the self-phase modulation of the laser pulse.
Efficient acceleration of planar foils driven by ion beams
Authors: Piriz A. R. , Ng A.
Published: July 2002

A previous model [Phys. Fluids 31, 658 (1988)] has been generalized and modified to describe ion beam interaction with planar foils in which the energy absorption region is composed of two materials. The model provides a unified description of foils with thicknesses larger and smaller than the ion range (super-range and subrange foils). The hydrodynamic efficiency of the unheated part of the foil, the payload, is studied for different foil structures and a set of parameters that determine the dynamics of the interaction is found.
The first Landau constant in a viscous free shear layer
Authors: Fujimura Kaoru
Published: July 2002

The first Landau constant of a free shear layer with parallel velocity profile U=tanh y is calculated numerically according to the amplitude expansion formulation proposed by Herbert [J. Fluid Mech. 126, 167 (1983)]. The numerical results strongly support Gotoh’s asymptotic expression [J. Phys. Soc. Jpn. 24, 1137 (1968)] of the first Landau constant λ1 for large Reynolds number R when the amplification factor ci is smaller than R−1/3: λ1=−8.177R1/3(1+0.25Rci). It is also concluded that the third-order nonlinear term in the Landau equation acts as the stabilizing effect throughout the parameter range where the numerical calculation was performed.
Effects of excitation level on the stability of an axisymmetric mixing layer
Authors: Samet M. M. , Petersen R. A.
Published: July 2002

The effect of various levels of excitation on the stability and development of an axisymmetric mixing layer was studied experimentally. The flow was excited axisymmetrically by a single speaker placed at the base of the plenum chamber. Measurements of mean and phase-averaged velocity profiles were made using an array of hot-wire probes. The measured profiles were compared to eigenfunctions calculated from linear, viscous stability theory. It is shown that the theoretical predictions, based on measured profiles of mean velocity, compare very well with the phase-averaged measurements, even when the local disturbance reaches levels as high as 24% of the jet speed. The cumulative effect of excitation on the mean flow is examined as a function of local Strouhal number as well as excitation level.
The renormalization group method in statistical hydrodynamics
Authors: Eyink Gregory L.
Published: July 2002

This paper gives a first principles formulation of a renormalization group (RG) method appropriate to study of turbulence in incompressible fluids governed by Navier–Stokes equations. The present method is a momentum-shell RG of Kadanoff–Wilson type based upon the Martin–Siggia–Rose (MSR) field-theory formulation of stochastic dynamics. A simple set of diagrammatic rules are developed which are exact within perturbation theory (unlike the well-known Ma–Mazenko prescriptions). It is also shown that the claim of Yakhot and Orszag (1986) is false that higher-order terms are irrelevant in the ε expansion RG for randomly forced Navier–Stokes (RFNS) with power-law force spectrum F̂(k)=D0k−d+(4−ε). In fact, as a consequence of Galilei covariance, there are an infinite number of higher-order nonlinear terms marginal by power counting in the RG analysis of the power-law RFNS, even when ε≪4. The difficulty does not occur in the Forster–Nelson–Stephen (FNS) RG analysis of thermal fluctuations in an equilibrium NS fluid, which justifies a linear regression law for d≳2. On the other hand, the problem occurs also at the nontrivial fixed point in the FNS Model A, or its Burgers analog, when d<2. The marginal terms can still be present at the strong-coupling fixed point in true NS turbulence. If so, infinitely many fixed points may exist in turbulence and be associated to a somewhat surprising phenomenon: nonuniversality of the inertial-range scaling laws depending upon the dissipation-range dynamics.
Spontaneous rupture of thin liquid films due to thermocapillarity: A full-scale direct numerical simulation
Authors: Krishnamoorthy S. , Ramaswamy B. , Joo S. W.
Published: July 2002

Rupture dynamics of a thin liquid film on a heated substrate has been studied by integrating the fully-coupled nonlinear system of continuity, momentum, and energy equations. The system is solved using finite element method in an Arbitrary-Lagrangian-Eulerian (ALE) frame of reference to update precisely the motion of the moving free-surface. The results of these simulations are compared with those from the long-wave theory. Fully nonlinear process of spontaneous rupture due to thermocapillarity is shown for the first time.
Nonlinear evolution of waves on falling films at high Reynolds numbers
Authors: Yu L.-Q. , Wasden F. K. , Dukler A. E. , Balakotaiah V.
Published: July 2002

A new model describing the dynamics of large-amplitude waves on laminar falling wavy films at high Reynolds numbers (Re≳300) is presented. The model is based on second-order boundary layer theory and includes the pressure variation across the film as well as higher-order viscous terms. The consistency and accuracy of the model is verified by comparing the linear stability results with Kapitza’s classical boundary layer model and Orr–Sommerfeld studies of the two-dimensional Navier–Stokes equations. Numerical integration of a traveling wave simplification of the model predicts the existence of chaotic large-amplitude, nonperiodic waves, as observed in the experiments. The computed wave statistics such as wave celerities, root-mean-square (RMS) values of film thickness, probability density function (PDF), and film thickness power spectrum using the present model are in reasonable agreement with those measured on naturally excited fully developed flows at Re≳300. The present model also overcomes the main deficiency of the classical boundary layer model (namely, negative wall shear stress) predicts large-amplitude waves (with peak to substrate ratios of 3 to 4) and gives better agreement with data.
Nonlocal nature of vortex stretching in an inviscid fluid
Authors: Ohkitani Koji , Kishiba Seigo
Published: July 2002

Three-dimensional Euler equations are studied numerically and analytically to characterize intense vortex stretching in an inviscid fluid. Emphasis is put on the nonlocal effects stemming from the pressure term. The purpose of this paper is twofold. One is to give numerically a detailed characterization of vortex structures on the basis of previously proposed two eigenvalue problems associated with vorticity. The other is to give some mathematical analyses which highlight the role of the pressure Hessian in vortex dynamics, especially in connection with a possible singularity. Also discussed are the differences in local and global (possible) blowups. The blowup problem is not directly discussed by the present numerics at moderate resolution.
One-way flow of a rarefied gas induced in a channel with a periodic temperature distribution
Authors: Sone Yoshio , Waniguchi Yorifumi , Aoki Kazuo
Published: July 2002

A rarefied gas in a channel of two parallel plane walls with a spatially periodic steady temperature distribution, on which a series of ditches is dug periodically, is considered, and the behavior of a flow induced by the temperature variation on the walls is investigated numerically for various Knudsen numbers and ditch sizes by the standard direct simulation Monte Carlo method (Bird’s method). It is found that a steady one-way flow is induced through the channel without any pressure gradient being applied externally. The pumping effect owing to the one-way flow is discussed.
Investigation of the stochastic collisions of drops produced by Rayleigh breakup of two laminar liquid jets
Authors: Brenn G. , Kalenderski St. , Ivanov I.
Published: July 2002

The stochastic collisions of drops of two intersecting streams were investigated experimentally. The drop streams were produced by Rayleigh breakup of two laminar jets of propanol-2 and were arranged spatially so that they lie in one plane and intersect at an angle which was varied in the experiments. The collisional interactions of the drops were visualized using video equipment. In the zone between the drop streams downstream of the intersection point new drops occur which are formed by the collisions. The visualization showed that these new drops may be produced either by the merging of two colliding drops or by the breakup of liquid bridges formed between drops after off-center collisions. Measurements of velocity and size of the drops in the flow field were carried out using a phase-Doppler anemometer (PDA). These data and the frequency of drop arrival in the measurement control volume of the PDA give insight into the drop formation processes caused by the collisions and enable the computation of the collision frequency.
Dispersion in two-dimensional periodic porous media. Part II. Dispersion tensor
Authors: Amaral Souto Hélio P. , Moyne Christian
Published: July 2002

The dispersion tensor of two-dimensional periodic porous media is investigated numerically. The theory is first briefly reviewed using the volume averaging method. Then, with the help of the hydrodynamics determined in Part I of this study, the dispersion tensor is calculated both for ordered (in-line or staggered square cylinders) and disordered (randomly distributed square cylinders) varying both the particle Péclet number (0<Pep<104), the particle Reynolds number, from the Stokes flow to the laminar–inertial regime (0<Rep<100), and the direction of the average flow with the axes of the unit cell. The influence of order and spatial periodicity is discussed. Lastly the results are compared with those for “real” porous media.
On the pinch-off of a pendant drop of viscous fluid
Authors: Henderson Diane M. , Pritchard William G. , Smolka Linda B.
Published: July 2002

The pinch-off of a drop of viscous fluid is observed using high-speed digital imaging. The behavior seen by previous authors is observed here; namely, the filament that attaches the drop to the orifice evolves into a primary thread attached to a much thinner, secondary thread by a slight bulge. Here, we observe that the lengths of the primary and secondary threads are reproducible among experiments to within 3% and 10%. The secondary thread becomes unstable as evidenced by wave-like disturbances. The actual pinch-off does not occur at the point of attachment between the secondary thread and the drop. Instead, it occurs between the disturbances on the secondary thread. After the initial pinch-off, additional breaks occur between the disturbances, resulting in several secondary satellite drops with a broad distribution of sizes. The pinch-off of the thread at the orifice is similar to that at the drop with one main difference: there is no distinct secondary thread. Instead, the primary thread necks down monotonically until wave-like disturbances form, resulting in pinch-off at multiple sites in between. The speed of the tips of the retreating, secondary threads after pinch-off are reported and discussed in the context of various scaling laws.
Nonaxisymmetric flow in a finite-length cylinder with a rotating magnetic field
Authors: Martin Witkowski L. , Walker J. S. , Marty P.
Published: July 2002

This paper treats the flow of an electrically conducting liquid in an insulating cylinder with a spatially uniform, transverse, rotating magnetic field. The frequency of the externally applied magnetic field is sufficiently low that this field penetrates throughout the liquid. Previous researchers have treated the steady, axisymmetric flow driven by the azimuthal average of the electromagnetic body force. This paper presents an analytical solution for the nonaxisymmetric flow perturbation driven by the deviation of the body force from its azimuthal average. For the magnetic field strengths and frequencies currently used in crystal-growth processes, the values of the interaction parameter N and the Reynolds number Re, both based on the frequency of the magnetic field, are small and large, respectively. The results of the appropriate asymptotic analysis show that the ratio of the nonaxisymmetric flow to the axisymmetric one is extremely small, thus validating the common neglect of the nonaxisymmetric flow. Therefore a rotating magnetic field may provide beneficial stirring without disturbing the desirable axisymmetric distribution of additives in the liquid.
Investigation of magnetic reconnection during a sawtooth crash in a high-temperature tokamak plasma
Authors: Yamada M. , Levinton F. M. , Pomphrey N. , Budny R. , Manickam J. , Nagayama Y.
Published: July 2002

In this paper a laboratory investigation is made on magnetic reconnection in high-temperature Tokamak Fusion Test Reactor (TFTR) plasmas [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 51]. The motional Stark effect (MSE) diagnostic is employed to measure the pitch angle profile of magnetic field lines, and hence the q profile. An analytical expression that relates pitch angle to q profile is presented for a toroidal plasma with circular cross section. During the crash phase of sawtooth oscillations in plasma discharges, the ECE (electron cyclotron emission) diagnostic measures a fast flattening of the two-dimensional (2-D) electron temperature profile in a poloidal plane, an observation consistent with the Kadomtsev reconnection theory. On the other hand, the MSE measurements indicate that central q values do not relax to unity after the crash, but increase only by 5%–15%, typically from 0.7 to 0.8. The latter result is in contradiction with the 2-D models of Kadomtsev and/or Wesson. In the present study this puzzle is addressed by a simultaneous analysis of electron temperature and q profile evolutions. Based on a heuristic model for magnetic reconnection during the sawtooth crash, the small change of q, i.e., partial reconnection, is attributed to the precipitous drop of pressure gradients that drive the instability and the reconnection process, as well as flux conserving plasma dynamics.
Trapping and acceleration in nonlinear plasma waves
Authors: Esarey Eric , Pilloff Mark
Published: July 2002

The trapping and acceleration of a test electron in a nonlinear plasma wave is analyzed in one dimension using Hamiltonian dynamics. The plasma wave is described by a nonlinear, cold fluid model. The maximum energy gain and the minimum energy required for trapping of the test electron are determined. The separatrix is plotted for several values of plasma wave amplitude. In the large wave amplitude limit, the maximum energy of a trapped electron scales as 2γ2pE2z, where γp is the relativistic factor associated with plasma wave phase velocity and Ez is the electric field amplitude of the nonlinear plasma wave. This is in contrast to the well-known results for a sinusoidal wave, in which the maximum energy scales as 4γ2pEz. As the nonlinear plasma wave approaches wavebreaking, the maximum energy is given by γmax→4γ3p−3γp, where γmax is the relativistic factor of the trapped electron.
Physics of helicon discharges
Authors: Chen Francis F.
Published: July 2002

Because of their high density, low-pressure discharges ionized by helicon waves are being studied for their possible use in cluster tools for the fabrication of next-generation computer chips. How helicon waves are related to whistler waves and waves in a plasma-filled waveguide is explained, and the mystery of the high ionization efficiency is outlined. Experimental data on the waves and the equilibrium properties of the discharge are shown, and the status of our current understanding of the physical processes therein is summarized. The importance of kinetic effects and of a short-wavelength mode arising at low magnetic fields is evaluated. Applications of helicon discharges to such diverse fields as plasma accelerators, microwave generators, and tokamak physics are illustrated. Low-temperature plasma physics is often considered a discipline so different from high-temperature plasma physics that there is little overlap, but these studies show that the techniques developed in fusion and space plasma physics can be applied to technological plasmas as well.
Characteristics of sheath instability in a double plasma device
Authors: Sarma A. , Bailung H. , Chutia Joyanti
Published: July 2002

Observations are carried out in a double-plasma device where the instabilities are excited around the negatively biased grid when there is sufficient plasma density difference between the two chambers and an ion beam is injected from the source to the target section. If the density difference between the two chambers is slowly decreased, the instabilities exhibit oppositely different characteristics after a critical value. Below the critical value of the density ratio, the energy resonance of the background ions and the beam ions is a typical condition for the excitation of such instabilities. Chaotic phenomena occur when there is a hump in the plasma potential profile near the sheath edge. Above the critical value of the density ratio, the instability occurs due to the interaction of the three beams that arise due to asymmetry of the sheath potential.
Tearing mode analysis in tokamaks, revisited
Authors: Nishimura Y. , Callen J. D. , Hegna C. C.
Published: July 2002

A new Δ′ shooting code has been developed to investigate tokamak plasma tearing mode stability in a cylinder and large aspect ratio (ε⩽0.25) toroidal geometries, neglecting toroidal mode coupling. A different computational algorithm is used (shooting out from the singular surface instead of into it) to resolve the strong singularities at the mode rational surface, particularly in the presence of the finite pressure term. Numerical results compare favorably with Furth et al. [H. P. Furth et al., Phys. Fluids 16, 1054 (1973)] results. The effects of finite pressure, which are shown to decrease Δ′, are discussed. It is shown that the distortion of the flux surfaces by the Shafranov shift, which modifies the geometry metric elements, stabilizes the tearing mode significantly, even in a low-β regime before the toroidal magnetic curvature effects come into play.
Intermittency of plasma edge fluctuation data: Multifractal analysis
Authors: Carreras B. A. , Lynch V. E. , Newman D. E. , Balbı́n R. , Bleuel J. , Pedrosa M. A. , Endler M. , van Milligen B. , Sánchez E. , Hidalgo C.
Published: July 2002

Plasma edge fluctuations show a degree of intermittency similar to fluid turbulence. Using fluctuation measurements obtained with Langmuir probe data from two confinement devices, it is shown that plasma fluctuations have a multifractal character over the fluctuation range of scales with intermittency levels comparable to the levels measured in neutral fluid turbulence. In the mesoscale range, that is, for time scales between 10 times the turbulence decorrelation time and plasma confinement time, plasma fluctuations have a structure closer to a monofractal with very low intermittency.
Light emission from zero-dimensional excitons—Photoluminescence from quantum wells in strong magnetic fields
Authors: Sakaki H. , Arakawa Y. , Nishioka M. , Yoshino J. , Okamoto H. , Miura N.
Published: July 2002

We report, for the first time, the spontaneous light emission from ‘‘zero-dimensional exciton states’’ in GaAs/GaAlAs quantum wells under a high magnetic field at 80 K. The formation of such fully quantized states is evidenced by observing the wavelength shift Δ λ of the spontaneous emission peak as well as the narrowing of the spectrum width as the magnetic field is raised up to 15 T. The observed shift Δλ is shown to be well explained by the theory in which the high magnetic field effect on two-dimensional hydrogenic exciton is taken into account. The formation of such a novel state is further evidenced by the strong anisotropy of the photoluminescence spectrum, which depends on the direction of the magnetic field.
Photoluminescence in CdTe grown on GaAs by metalorganic chemical vapor deposition
Authors: Wang C. H. , Cheng K. Y. , Yang S. J.
Published: July 2002

Epitaxial layers of CdTe grown on (100) GaAs substrates by metalorganic chemical vapor deposition at temperatures between 320 and 410 °C were investigated by photoluminescent measurements at 14 K. The sharp bound exciton-related peak at 1.594 eV, the band-edge emission located near 1.557 eV, and the weak defect-related extrinsic band at 1.476 eV, with their well-resolved longitudinal optical phonon replicas were the observed major photoluminescence peaks. The variations of these peak intensities are a strong function of the epilayer growth temperatures. The intensity ratio of the defect-related emission peak to the bound exciton peak decreases to less than 1/10 as the growth temperature is adjusted within 360 and 390 °C. For the CdTe epitaxial layer grown at 375 °C, the peak related to defects is completely eliminated.
Energy band discontinuities in heterojunctions measured by internal photoemission
Authors: Heiblum M. , Nathan M. I. , Eizenberg M.
Published: July 2002

A novel method involving internal photoemission has been developed to determine the conduction band discontinuity Δ Ec of heterojunctions. The method is straightforward, accurate, and assumes minimum unknowns; and has been applied to GaAs/AlxGa1−xAs heterojunctions. We have found for x<0.4 that Δ Ec≂0.62 Δ Eg, where Δ Eg is the band-gap difference. For x>0.4, the apparent Δ Ec is considerably smaller.
Role of mechanical stress in the light-induced degradation of hydrogenated amorphous silicon
Authors: Stutzmann Martin
Published: July 2002

The correlation between the high intrinsic mechanical stresses in plasma-deposited hydrogenated amorphous silicon and the magnitude of the light-induced degradation in this material (Staebler–Wronski effect) has been studied for films with different thicknesses and for various substrate materials. The experimental results suggest that the creation of metastable defects occurs mainly in the strained region of the films near the substrate, and that the number of these states is roughly proportional to the total stress in a given sample.
Generation of subpicosecond electrical pulses on coplanar transmission lines
Authors: Ketchen M. B. , Grischkowsky D. , Chen T. C. , Chi C-C. , Duling I. N. , Halas N. J. , Halbout J-M. , Kash J. A. , Li G. P.
Published: July 2002

Electrical pulses shorter than 0.6 ps were generated by photoconductively shorting a charged coplanar transmission line with 80 fs laser pulses. After propagating 8 mm on the line the electrical pulses broadened to only 2.6 ps.
A 1.55-μm semiconductor-optical fiber ring laser
Authors: Jopson R. M. , Eisenstein G. , Whalen M. S. , Hall K. L. , Koren U. , Simpson J. R.
Published: July 2002

We demonstrate the first semiconductor-optical fiber ring laser. The laser uses an InGaAsP traveling wave optical amplifier as the gain medium and a 1-m length of highly birefringent fiber as the feedback loop. Output coupling is provided by a fiber directional coupler. The highly birefringent fiber is used as an intracavity wavelength filter, enabling single-frequency oscillation with a narrow linewidth at a wavelength near 1.55 μm.
Decomposition of palladium acetate films with a microfocused ion beam
Authors: Harriott L. R. , Cummings K. D. , Gross M. E. , Brown W. L.
Published: July 2002

Submicron Pd features have been fabricated on Si and SiO2 substrates by microfocused Ga+ ion beam exposure of spin-on palladium acetate, [Pd(O2CCH3)2]3, films. Electrical conductivity measurements were made on the exposed features as a function of ion dose for nominal linewidths of 1 and 10 μm. The sheet conductivity in the two cases is comparable and increases dramatically in the exposure dose range between 2×1014 and 5×1014 ions/cm2. The conductivity of the exposed lines is further increased after heating in a hydrogen furnace. Potential applications of this process include mask repair and integrated circuit modification.
Passivation of Si donors and D
X centers in AlGaAs by hydrogen plasma exposure
Authors: Nabity J. C. , Stavola Michael , Lopata J. , Dautremont-Smith W. C. , Tu C. W. , Pearton S. J.
Published: July 2002

The effect of hydrogen plasma exposure upon shallow donors and DX centers in silicon-doped AlGaAs has been investigated by deep level transient spectroscopy and capacitance versus voltage measurements. Following exposure to a hydrogen plasma for 30 min at 250 °C, the shallow level and DX center activity are reduced by an order of magnitude throughout a 1.6-μm-thick layer of molecular beam epitaxially grown AlGaAs. Isochronal annealing studies showed that both the shallow donor and DX center electrical activity recover together at about 400 °C. The shallow donor recovery mimics the behavior of donors in GaAs and has an activation energy of 2.0 eV. The DX center recovery shows a distribution of activation energies centered at 2.1 eV with a full width at half-maximum of 0.25 eV. The hydrogen passivation chemistry of DX centers and shallow donors support models in which isolated Si impurities give rise to both DX behavior and shallow levels.
Explosive crystallization in amorphous Si initiated by long pulse width laser irradiation
Authors: Peercy P. S. , Tsao J. Y. , Stiffler S. R. , Thompson Michael O.
Published: July 2002

Explosive crystallization of amorphous Si is studied in a new heat-flow regime using long pulse (45 ns full width at half-maximum) ruby laser irradiation. In this regime, previously unobserved phenomena are found. Nucleation of crystalline Si is observed while the amorphous phase is melting at high (m/s) velocity and epitaxy from the underlying crystalline substrate is observed even under conditions for which the amorphous Si is never at any instant fully melted. These measurements, combined with previous measurements using short pulse irradiation, lead to development of a more complete model of explosive crystallization.
Observation of a halide (F/Cl) stabilized, new perovskite phase in superconducting Y2Ba5Cu7Ox films
Authors: Kwo J. , Hong M. , Fleming R. M. , Hebard A. F. , Mandich M. L. , DeSantolo A. M. , Davidson B. A. , Marsh P. , Hobbins N. D.
Published: July 2002

A new, ordered perovskite structure was stabilized as a majority phase in superconducting Y2Ba5Cu7O20−δ′(F/Cl)δ″ films epitaxially grown on SrTiO3(100). The new phase was formed via fluorination or chlorination introduced during the post-deposition O2 furnace anneal for recrystallization. The structure is determined by x-ray diffraction to be an orthorhombic unit cell [a∼b=3.86(1) Å, c=27.24(6) Å] with reflection conditions consistent with a fully twinned A-centered cell. Films containing primarily the new phase show a lower resistivity ρ(300 K)∼177 μΩ cm, a comparable ratio of ρ(300 K)/ρ(100 K)∼3.0, and a lower superconducting temperature of 78–74 K when compared to Y1Ba2Cu3O7.
Casein kinase I phosphorylates and destabilizes the β-catenin degradation complex
Authors: Gao Zhong-Hua , Seeling Joni M. , Hill Virginia , Yochum April , Virshup David M.
Published: July 2002

Wnt signaling plays a key role in cell proliferation and development. Recently, casein kinase I (CKI) and protein phosphatase 2A (PP2A) have emerged as positive and negative regulators of the Wnt pathway, respectively. However, it is not clear how these two enzymes with opposing functions regulate Wnt signaling. Here we show that both CKIδ and CKIɛ interacted directly with Dvl-1, and that CKI phosphorylated multiple components of the Wnt-regulated β-catenin degradation complex
in vitro
, including Dvl-1, adenomatous polyposis coli (APC), axin, and β-catenin. Comparison of peptide maps from
in vivo
and
in vitro
phosphorylated β-catenin and axin suggests that CKI phosphorylates these proteins
in vivo
as well. CKI abrogated β-catenin degradation in
Xenopus
egg extracts. Notably, CKI decreased, whereas inhibition of CKI increased, the association of PP2A with the β-catenin degradation complex
in vitro
. Additionally, inhibition of CKI
in vivo
stabilized the β-catenin degradation complex, suggesting that CKI actively destabilizes the complex
in vivo
. The ability of CKI to induce secondary body axes in
Xenopus
embryos was reduced by the B56 regulatory subunit of PP2A, and kinase-dead CKIɛ acted synergistically with B56 in inhibiting Wnt signaling. The data suggest that CKI phosphorylates and destabilizes the β-catenin degradation complex, likely through the dissociation of PP2A, providing a mechanism by which CKI stabilizes β-catenin and propagates the Wnt signal.