bluuebunny/crossref_metadata_2025 huggingface dataset:
Sum rules for zeros of polynomials and generalized Lucas polynomials
Authors: Ricci Paolo Emilio
Published: July 2002

A representation formula in terms of generalized Lucas polynomials of the second kind [see formula (4.3)], for the sum rules Js(i) introduced by Case [J. Math. Phys. 21, 702 (1980)] and studied by Dehesa et al. [J. Math. Phys. 26, 1547 (1985); 26, 2729 (1985)] in order to obtain information about the zeros’ distribution of eigenfunctions of a class of ordinary polynomial differential operators, is derived.
Calogero–Moser hierarchy and KP hierarchy
Authors: Shiota Takahiro
Published: July 2002

Identification of the hierarchy of Calogero–Moser dynamical systems with the dynamical systems of poles of certain solutions to the KP hierarchy is achieved.
Topological entropy for appropriately approximated C*-algebrasa)
Authors: Hudetz Thomas
Published: July 2002

‘‘Classical’’ topological entropy is one of the main numerical invariants in topological dynamics on compact spaces. Here, the author’s recent development of a noncommutative generalization of topological entropy, in the natural setting of general C*-algebras as the noncommutative counterpart of continuous function algebras on compact spaces, is presented in a slightly modified and improved form. This includes both a survey of earlier results with some important corrections, and also new general results in response to (and inspired by) a more recent counterproposal for a noncommutative topological entropy by Thomsen. Finally, some partially new examples for the calculation of the defined topological entropy are shown. The rather self-evident physical interpretation in the framework of (operator-algebraic) quantum statistical mechanics and of ‘‘chaotic’’ quantum dynamical systems is briefly touched upon.
Integrable systems of group SO(1,2) and Green’s functions
Authors: Dane C. , Verdiyev Y. A.
Published: July 2002

Integrable quantum systems related with two-dimensional two- and one-sheeted hyperboloid are considered. The explicit expressions of the Green’s functions of the free particles on the those homogenous spaces are given.
Geometry of spaces with the Jacobi metric
Authors: Szydl/owski Marek , Heller Michael , Sasin Wiesl/aw
Published: July 2002

The generalized Maupertuis principle is formulated for systems with the natural Lagrangian and an indefinite form of the kinetic energy. The generalization is applied to the theory of gravity and cosmology. For such systems, the metric determined by the kinetic energy form has a Lorentz signature. The theorem is proved concerning the behavior of trajectories in a neighborhood of the boundary of the region admissible for motion. This region is not a smooth manifold but turns out to be a differential space of constant differential dimension. This fact allows us to use geometric methods analogous to those elaborated for smooth manifolds. It is shown that singularities of the Jacobi metric are not dangerous for the motion; its trajectories are smooth in the sense of the theory of differential spaces.
Representation of complex probabilities
Authors: Salcedo L. L.
Published: July 2002

Let a “complex probability’’ be a normalizable complex distribution P(x) defined on RD. A real and positive probability distribution p(z), defined on the complex plane CD, is said to be a positive representation of P(x) if 〈Q(x)〉P=〈Q(z)〉p, where Q(x) is any polynomial in RD and Q(z) its analytical extension on CD. In this paper it is shown that every complex probability admits a real representation and a constructive method is given. Among other results, explicit positive representations, in any number of dimensions, are given for any complex distribution of the form Gaussian times polynomial, for any complex distributions with support at one point and for any periodic Gaussian times polynomial.
A canonical realization of the BMS algebra
Authors: Longhi G. , Materassi M.
Published: July 2002

A canonical realization of the BMS (Bondi–Metzner–Sachs) algebra is given on the phase space of the classical real Klein–Gordon field. By assuming the finiteness of the generators of the Poincaré group, it is shown that a countable set of conserved quantities exists (supertranslations); this set transforms under a particular Lorentz representation, which is uniquely determined by the requirement of having an invariant four-dimensional subspace, which corresponds to the Poincaré translations. This Lorentz representation is infinite-dimensional, nonunitary, reducible and indecomposable. Its representation space is studied in some detail. It determines the structure constants of the infinite-dimensional canonical algebra of the Poincaré generators together with the infinite set of the new conserved quantities. It is shown that this algebra is isomorphic with that of the BMS group.
The Lanczos algorithm for extensive many-body systems in the thermodynamic limit
Authors: Witte N. S. , Bessis D.
Published: July 2002

We establish rigorously the scaling properties of the Lanczos process applied to an arbitrary extensive many-body system which is carried to convergence n→∞ and the thermodynamic limit N→∞ taken. In this limit the solution for the limiting Lanczos coefficients are found exactly and generally through two equivalent sets of equations, given initial knowledge of the exact cumulant generating function. The measure and the orthogonal polynomial system associated with the Lanczos process in this regime are also given explicitly. Some important representations of these Lanczos functions are given, including Taylor series expansions, and theorems controling their general properties are proven.
The identities of the algebraic invariants of the four-dimensional Riemann tensor. III
Authors: Sneddon G. E.
Published: July 2002

This paper extends the investigation of the invariants of the Riemann tensor to include the invariants that are of odd degree in the trace-free Ricci tensor. It is shown that these invariants can be expressed in terms of 15 such invariants that are irreducible. As a consequence, it is possible to write down a complete set of invariants of the Riemann tensor. Several syzygies for these invariants have been found and examples of these are given. These syzygies suggest there may be several new syzygies of invariants with even degree in the trace-free Ricci tensor. A large number of these have also been found and are discussed in the paper.
On the contribution of dislocations to the resistivity of niobium
Authors: Sokolenko V. I.
Published: July 2002

The change of the resistivity increment Δρ/Nd normalized to the average density of dislocations in the core of niobium single crystals is studied in different structural states (after rolling deformation at 20 K and consecutive removal of surface layers). For a homogeneous structure with a high density of uniformly distributed dislocations (Nd=13×1010 cm−2), the value of resistivity produced by a single dislocation rd≃(9±3)⋅10−19 Ω⋅cm3 is determined, taking into account the contribution from vacancies to Δρ. A quantitative analysis of the value of rd for niobium is carried out on the basis of models of resonant electron scattering by dislocations.
Variational methods for studying tokamak stability in the presence of a thin resistive wall
Authors: Haney S. W. , Freidberg J. P.
Published: July 2002

External magnetohydrodynamic modes stabilized by the presence of a close-fitting perfectly conducting wall become destabilized when the wall is assumed to possess finite resistivity. A simple variational principle giving an estimate for the resulting growth rate and the threshold for stability is derived in terms of quantities relating to the ideal system with and without a perfectly conducting wall. This variational principle is valid for an arbitrary three-dimensional external mode in an arbitrarily shaped plasma possessing an arbitrarily shaped, but thin, resistive wall. As an example of the utility of the method, the variational principle is used to investigate the axisymmetric (n=0) stability of straight, zero pressure elliptical tokamaks with arbitrary current density profiles in the presence of a resistive wall.
Formation of streamers in plasma with an ion temperature gradient
Authors: Dimits A. M. , Drake J. F. , Hassam A. B. , Meerson B.
Published: July 2002

The nonlinear growth and saturation of the negative compressibility ηi instability in an inhomogeneous plasma confined by a straight magnetic field is investigated. A set of nonlinear equations that describe the two-dimensional fluid motion along and across (in the direction of ∇⊥Ti ) the local magnetic field is derived. These equations are basically the same as those describing a one-dimensional nonlinear sound wave, but the compressibility is negative for ηi=̇d log(Ti)/log(ni) exceeding a threshold. The nonlinearity in the equations arises from the parallel convective motion of the fluid. The solution of these equations evolves to a final state in which plasma flows are driven into a low temperature region along the local magnetic field and then jet across the magnetic field along ∇⊥Ti in narrow streams. The structure of these streams is consistent with previous two-dimensional simulations. Results for the scaling of the transport due to these streams are presented.
Ablation scaling in laser-produced plasmas with laser intensity, laser wavelength, and target atomic number
Authors: Dahmani Faiz
Published: July 2002

Layered target experiments at 1.06 μm have been performed in order to measure the mass-ablation rate ṁ and the ablation pressure Pa as a function of absorbed laser intensity Ia, laser wavelength λL, and target atomic number Z in steady-state ablation dominated by nonlocalized inverse bremsstrahlung absorption. The results can be expressed as ṁexp(kg/ sec cm2)≂65 [Ia(W/cm2)/1013]5/9 λ−4/9L(μm)Z1/4 and Pa(Mbar)≂2.5[Ia(W/ cm 2)/ 1013]7/9 λ−2/9L(μm)Z1/8. The measured results show good agreement with theory for nonlocalized absorption and with data of others.
Collisional equipartition rate for a magnetized pure electron plasma
Authors: Glinsky Michael E. , O’Neil Thomas M. , Rosenbluth Marshall N. , Tsuruta Kenji , Ichimaru Setsuo
Published: July 2002

The collisional equipartition rate between the parallel and perpendicular velocity components is calculated for a weakly correlated electron plasma that is immersed in a uniform magnetic field. Here, parallel and perpendicular refer to the direction of the magnetic field. The rate depends on the parameter κ̄=(b̄/rc)/, where rc=(T/m)1/2/Ωc is the cyclotron radius and b̄=2e2/T is twice the distance of closest approach. For a strongly magnetized plasma (i.e., κ̄≫1), the equipartition rate is exponentially small (ν∼exp[−5(3πκ̄)2/5/6]). For a weakly magnetized plasma (i.e., κ̄≪1), the rate is the same as for an unmagnetized plasma except that rc/b̄ replaces λD/b̄ in the Coulomb logarithm. (It is assumed here that rc<λD; for rc>λD, the plasma is effectively unmagnetized.) This paper contains a numerical treatment that spans the intermediate regime κ̄∼1, and connects onto asymptotic results in the two limits κ̄≪1 and κ̄≫1. Also, an improved asymptotic expression for the rate in the high-field limit is derived. The present theoretical results are in good agreement with recent measurements of the equipartition rate over eight decades in κ̄ and four decades in the scaled rate ν/nv̄b̄2, where n is the electron density and v̄=(2T/m)1/2.
Decay instabilities for inhomogeneous plasmas: WKB analysis and absolute instability
Authors: Picard G. , Johnston T. W.
Published: July 2002

A general approximate criterion is developed (via WKB analysis) for the absolute instability of two parametrically pumped decay waves with one-dimensional inhomogeneity. The absolute instability threshold pump power proves to be higher than that required for amplitude amplification R by a factor πk′l2/27R, when k′ is the phase mismatch dΣki/dx at the location of maximum instability and l−2 is the effective curvature of the necessarily second-order expansion about that point. Previous results are included as special cases.
Asymptotic analysis of radio frequency heated collisional plasma
Authors: Fisch Nathaniel J. , Karney Charles F. F.
Published: July 2002

It is shown that a distribution of electrons in resonance with traveling waves, but colliding with background distributions of electrons and ions, evolves to a steady state. Previously, the existence of such solutions had been assumed, but not proved, in numerical and other calculations. Details of the steady state are given analytically in the asymptotic limit of high electron energy and are compared with numerical solutions. The asymptotic analytic solution may be useful for quickly relating emission data to likely excitations and is more reliable than conventional numerical solutions at high energy. A method of improving numerics at high energy is suggested.
Generalized nonlinear harmonic gyrotron theory
Authors: Danly B. G. , Temkin R. J.
Published: July 2002

The nonlinear efficiency for a gyrotron oscillator operating at harmonics of the cyclotron frequency has been calculated and is presented as a function of generalized parameters for the second through fifth harmonics. The numerical results are valid for a wide range of operating conditions, including voltage, current, beam radius, cavity dimensions, and operating mode. Relatively high efficiencies are found even at high harmonics; the maximum transverse efficiencies for harmonics 2, 3, 4, and 5 are 0.72, 0.57, 0.45, and 0.36, respectively. The calculation of the efficiency in terms of generalized parameters allows the straightforward design and optimization of harmonic gyrotrons. The influence of the axial profile of the rf field in the gyrotron cavity on the efficiency is also investigated. Improved efficiency can be achieved with asymmetric field profiles. The implications of these results for the generation of millimeter and submillimeter wave radiation by harmonic emission are discussed.
Effects of small, intense laser spots on thin films
Authors: Estabrook Kent
Published: July 2002

Two-dimensional hydrodynamic simulations show that small diameter laser spots burn through flat foils considerably faster than larger spots. The physical mechanisms are (1) more nearly spherical divergence from the smaller spots which allows the debris to rapidly convect radially away from the small volume of laser heated plasma and (2) a variation on thermal self-focusing in which light is refracted into this volume, increasing the local heating rate.
The potential form of the fourth-order evolution equation for deep-water gravity–capillary waves
Authors: Hogan S. J.
Published: July 2002

The fourth-order evolution equation for the wave envelope of a train of deep-water gravity–capillary waves is derived in terms of the complex velocity potential Ψ(x,t). Despite a difference between Ψ(x,t) and the complex amplitude a(x,t) to this order, the evolution equations are curiously almost identical.
The expansion of the cathode spot plasma in vacuum arc discharges
Authors: Wieckert Christian
Published: July 2002

A stationary one-dimensional two-fluid model for the expansion and corresponding acceleration of the dense cathode spot plasma in vacuum arc discharges is evaluated. The equations exhibit a saddle point at the ion sound velocity. For an acceleration to supersonic velocities, the effect of Joule heating due to electron–ion collisions has to compensate for the effect of geometrical dilution at the sonic point. As a consequence, a transsonic flow is possible for a specific ion starting velocity only. This model shows that large final ion velocities observed in experiments are mainly caused by the directed friction force due to electron–ion collisions. A small potential hump has only a minor effect for the momentum gain of the ions.
Determination of the macroscopic (partial) slip boundary condition for a viscous flow over a randomly rough surface with a perfect slip microscopic boundary condition
Authors: Jansons Kalvis M.
Published: July 2002

Consider a viscous fluid, at zero Reynolds number, moving over a solid surface flat except for a random array of microscopic defects having a small area fraction c. Assuming a microscopic boundary condition of perfect slip, the macroscopic boundary condition is determined from first principles. The asymptotic structure of the solution for a random surface with finite slope is quite different from those of earlier studies in the limit of an ‘‘almost flat’’ surface. The results of this study show that very small amounts of roughness can well approximate a no-slip boundary condition macroscopically, for example, one defect of the order of 10−9 m per (10−7 m)2 gives a slip length of only 10−5 m.
Electron cyclotron heating correlation with tokamak toroidal plasma rotation
Authors: Lieber A. J. , Snider R. T. , Lee Ping , Wojtowicz S. S.
Published: July 2002

Toroidal rotation of the central plasma core has been observed prior to and during electron cyclotron heating (ECH) in the Doublet III tokamak [in Plasma Physics and Controlled Nuclear Fusion Research 1984, Proceedings of the Tenth International Conference, London (IAEA, Vienna, 1985), Vol. 1, p. 131]. Measurements were made using a curved-crystal Bragg x-ray spectrometer with spectral resolution of λ/Δλ of greater than 19 000 and temporal resolution of 20 msec. Ion rotation velocity was determined from the Doppler shift of x rays emitted from the plasma impurity Ti XXI resonance line. Ion temperatures were determined from the Doppler broadening of this line. A study of purely Ohmic discharges suggests that nonrotating discharges exhibit magnetohydrodynamic activity other than sawtooth oscillations, while those that rotate exhibit sawteeth. Discharges with plasma rotation prior to the appliction of ECH appear to heat more effectively than those that do not. Velocity and temperature histories show that there are temporal characteristics corresponding to whether the plasma was or was not effectively heated.
Experimental examination of Eulerian frequency spectra in zero-mean-shear turbulence
Authors: Kit E. , Fernando H. J. S. , Brown J. A.
Published: July 2002

Eulerian frequency spectra were measured in zero-mean-shear (oscillating-grid induced) turbulent flows and were compared with the spectral form proposed by Tennekes [J. Fluid Mech. 67, 561 (1975)]. A satisfactory agreement between the theoretical prediction and experimental results were obtained in a limited frequency range. The empirical constants pertinent to the spectral law, obtained experimentally, were in good agreement with the numerical simulation results.
Conditional sampling of dissipation in moderate Reynolds number grid turbulence
Authors: Thoroddsen S. T.
Published: July 2002

Multi-point hot-wire measurements in grid turbulence have been used to conditionally average the dissipation over inertial-scale features of the turbulent field. Velocities were measured at 5 separate spatial points obtaining six simultaneous velocity components, at turbulent Rey- nolds numbers Reλ of 160 and 230. The dissipation was approximated with pseudo- or surro- gate dissipation εr′ based on one or two velocity gradients. The conditioned dissipation εr′ inside the most intense large-scale rotational regions of the flow is more than twice as large as the expected value of εr′ over the whole domain. Conditional averaging was also carried out over strong large-scale fronts showing smaller excess in dissipation.
System identification and control of a turbulent boundary layer
Authors: Rathnasingham Ruben , Breuer Kenneth S.
Published: July 2002

An experimental investigation is made into the active control of the near-wall region of a turbulent boundary layer using a linear feed-forward control algorithm. A wall-based detection scheme is described which effectively detects coherent structures and predicts downstream flow behavior. A simple demonstration, using three wall-based sensors and a single actuator, achieves a maximum of 31% reduction in urms and 17% reduction in prms.
Motion of spheroidal particles in vertical shear flows
Authors: Broday David , Fichman Mati , Shapiro Michael , Gutfinger Chaim
Published: July 2002

The motion of non-neutrally buoyant prolate spheroidal particles in vertical shear flows is investigated. Using the generalized Faxen law, we calculate the hydrodynamic forces and moments acting on such inertial and inertialess particles, and their trajectories. The calculations are done for (i) freely rotating particles, and (ii) particles with orientations fixed by means of an external torque exerted by a strong orienting field. Inertial particles are found to migrate across the streamlines, and their trajectories differ considerably from those calculated for inertialess particles. Neutrally buoyant spheroids, inertial or not, which either freely rotate or have fixed orientations in shear flows, translate along the streamlines. Non-neutrally buoyant inertialess spheroids freely moving in simple shear flow translate along periodic trajectories with no net lateral drift. In contrast, inertial particles under similar flow conditions drift laterally toward locations characterized by higher local velocities in a direction opposing gravity. The motion of non-neutrally buoyant inertial particles with fixed orientations may be unstable with the drift velocity growing exponentially with time. Conditions for the occurrence of this unstable motion are formulated analytically in terms of particle and flow parameters. In general, the rate of drift depends on particle shape, via its aspect ratio, and its inertia.
Intermittency and scaling laws for wall bounded turbulence
Authors: Benzi R. , Amati G. , Casciola C. M. , Toschi F. , Piva R.
Published: July 2002

Well defined scaling laws clearly appear in wall bounded turbulence, very close to the wall, where a distinct violation of the refined Kolmogorov similarity hypothesis (RKSH) occurs together with the simultaneous persistence of scaling laws. A new form of RKSH for the wall region is here proposed in terms of the structure functions of order two which, in physical terms, confirms the prevailing role of the momentum transfer towards the wall in the near wall dynamics.
Multistage gas–surface interaction model for the direct simulation Monte Carlo method
Authors: Yamanishi Nobuhiro , Matsumoto Yoichiro , Shobatake Kosuke
Published: July 2002

A multistage (MS) gas–surface interaction model for a monatomic/diatomic gas molecule interacting with a solid surface is presented, based on the analysis of molecular dynamics (MD) simulations and a model equation derived from the classical theory of an ellipsoid hitting a hard cube. This model is developed for use with the direct simulation Monte Carlo (DSMC) method and belongs to the thermal scattering regime. The molecular dynamics method is used for the molecular-level understanding of the scattering of O2, N2, and Ar from a graphite surface. The basic idea of the model is to separate the collision into three stages. At stages 1 and 2, the energy and scattering direction are determined by the model equation. At stage 3, according to the translational energy, the molecule is determined to scatter, re-enter or be trapped by the surface. Re-entering molecules return to stage 1. The model parameters are determined from our MD database. Experiments are also performed by scattering a supersonic O2 molecular beam from a clean graphite surface in an ultrahigh vacuum chamber. The in-plane scattering distribution, out-of-plane scattering distribution, and in-plane velocity distribution of the model show good agreement with those of molecular beam experiments. A model equation was included in the MS model to maintain thermal equilibrium between a gas and a surface at the same temperature when applied to DSMC simulations and the results are also shown. The high accuracy of the model clearly shows that such multiple-scale analysis can lead to the development of realistic models of the gas–surface interaction.
Electron-cyclotron absorption and emission: ‘‘Vexatae quaestiones’’
Authors: Bornatici M. , Engelmann F.
Published: July 2002

The salient features of the theory of electron-cyclotron absorption and emission for the parameter range for which the radiation spectrum has a line structure are reviewed and contrasted with erroneous results of a recent paper by Arunasalam [Phys. Fluids B 4, 1643 (1992)]. In particular, (i) the relativistic and nonrelativistic forms of the dielectric tensor relevant to describing the electron-cyclotron absorption are considered, their range of validity is displayed, and on this basis an incorrect suggestion in the above paper on how to obtain the relativistic dielectric tensor from the nonrelativistic one is refuted; (ii) the crucial role of the thermal effects on the mode polarization, which in turn affects the absorption profile around the fundamental frequency, is emphasized, showing that using a ‘‘Trubnikov O-mode factor’’ as advocated in the above paper is not correct; (iii) the profile of the absorption around the fundamental frequency results to be single humped in all cases, in contrast to the assertion in the above paper that the absorption profile of the first harmonic O mode is double humped; (iv) the radiation from a nonuniformly magnetized plasma is evaluated for a bi-Maxwellian electron distribution showing, in particular, that the nonrelativistic treatment used in the above paper is incorrect for perpendicular propagation; (v) an expression for the local radiation temperature obtained in the above paper and upon which the analysis of the emission spectra of supershot plasmas in the Tokamak Fusion Test Reactor (TFTR) has been based by Taylor et al. [Proceedings of the Eighth Workshop on Electron Cyclotron Emission and Heating, Gut Ising, Germany, 1992, (Max-Planck Institut für Plasmaphysik, Garching, 1993), IPP III/186, Vol. 1, p. 277] is shown not to be valid.
Alfvén ‘resonance’ reconsidered: Exact equations for wave propagation across a cold inhomogeneous plasma
Authors: Bellan P. M.
Published: July 2002

Previous discussions of Alfvén wave propagation across an inhomogeneous plasma predicted that shear Alfvén waves become singular (resonant) at the ω=kzvA layer and that there is a strong wave absorption at this layer giving localized ion heating. In this paper the three standard derivations of the Alfvén ‘resonance’ (incompressible magnetohydrodynamics, compressible magnetohydrodynamics, and two-fluid) are re-examined and shown to have errors and be mutually inconsistent. Exact two-fluid differential equations for waves propagating across a cold inhomogeneous plasma are derived; these show that waves in an ideal cold plasma do not become ‘resonant’ at the Alfvén layer so that there is no wave absorption or localized heating. These equations also show that the real ‘shear’ Alfvén wave differs in substance from both the ideal MHD and earlier two-fluid predictions and, in the low density, high field region away from the ω=kzvA layer, is actually a quasielectrostatic resonance cone mode. For ω≪ωci and ky=0, the ω=kzvA layer turns out to be a cutoff (reflecting) layer for both the ‘shear’ and compressional modes (and not a resonance layer). For finite ω/ωci and ky=0 this layer becomes a region of wave inaccessibility. For ω≪ωci and finite ky there is strong coupling between shear and compressional modes, but still no resonance.
Enhanced performance of deuterium–tritium-fueled supershots using extensive lithium conditioning in the Tokamak Fusion Test Reactor
Authors: Mansfield D. K. , Strachan J. D. , Bell M. G. , Scott S. D. , Budny R. , Marmar E. S. , Snipes J. A. , Terry J. L. , Batha S. , Bell R. E. , Bitter M. , Bush C. E. , Chang Z. , Darrow D. S. , Ernst D. , Fredrickson E. , Grek B. , Herrmann H. W. , Hill K. W. , Janos A. , Jassby D. L. , Jobes F. C. , Johnson D. W. , Johnson L. C. , Levinton F. W. , Mikkelsen D. R. , Mueller D. , Owens D. K. , Park H. , Ramsey A. T. , Roquemore A. L. , Skinner C. H. , Stevenson T. , Stratton B. C. , Synakowski E. , Taylor G. , von Halle A. , von Goeler S. , Wong K. L. , Zweben S. J.
Published: July 2002

In the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] a substantial improvement in fusion performance has been realized by combining the enhanced confinement due to tritium fueling with the enhanced confinement due to extensive conditioning of the limiter with lithium. This combination has resulted in not only significantly higher global energy confinement times than have previously been obtained in high current supershots, but also in the highest central ratio of thermonuclear fusion output power to input power observed to date.
The ion velocity (Bohm–Chodura) boundary condition at the entrance to the magnetic presheath in the presence of diamagnetic and E×B drifts in the scrape-off layer
Authors: Stangeby P. C. , Chankin A. V.
Published: July 2002

In the absence of drifts, the Bohm–Chodura criterion gives that the fluid velocity parallel to the magnetic field reaches the ion acoustic speed at the entrance to the magnetic presheath. The changes to this criterion are derived for the situation where poloidal E×B drifts are present. (It was shown earlier that diamagnetic drifts do not influence this criterion.) Here E×B drifts can cause the parallel fluid velocity at the magnetic presheath entrance to become supersonic, subsonic, or reversed (negative).
The new phase of the Joint European Torus and prospects for future operation
Authors: Tubbing B. J. D. ,   the JET Team
Published: July 2002

The Joint European Torus (JET) [Fusion Technol. 11, 1 (1987)] restarted operations in its new pumped divertor configuration. The main elements of the new configuration are four divertor coils, an inertially cooled divertor structure, and an internal cryogenic vacuum pump. Two major differences were observed with respect to the ‘‘old’’ JET. First, the heat load capability of the divertor is dramatically improved. Second, in the high confinement mode (H mode) the occurrence of edge localized modes (ELMs), is more prevalent. Quasi-steady-state ELMy H modes are now obtained in a wide variety of conditions. In the 1994 experimental campaign maximum plasma currents of 4 MA have been obtained. Steady-state H-mode operation has been obtained for a duration of 20 s. Detached divertor operation was established in Ohmic and L-mode plasmas, but not yet in H mode. In low current discharges a poloidal beta βP=2.6 has been obtained in transient. The high βP regime has been extended to long pulse ELMy H mode. Preliminary experiments have not shown an effect of magnitude shear reversal on the confinement. Using the new internal saddle coils, powered by high-frequency amplifiers, toroidal Alfvén eigenmodes (TAE) were excited. The first direct measurements of the damping of TAE modes were carried out.
Simulation of experimentally achieved DIII-D detached plasmas using the U
E
D
G
E code
Authors: Porter Gary D. , Allen S. L. , Brown M. , Fenstermacher M. E. , Hill D. N. , Jong R. A. , Leonard A. W. , Nilson D. , Rensink M. E. , Rognlien T. D. , Smith G. R. ,   the DIII-D Team
Published: July 2002

The introduction of a divertor Thomson scattering system in DIII-D [J. Luxon et al., International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1986), p. 159] has enabled accurate determination of the plasma properties in the divertor region. Two plasma regimes are identified: detached and attached. The electron temperature in the detached regime is about 2 eV, much lower than 5–10 eV determined earlier. Fluid models of the DIII-D scrape-off layer plasma successfully reproduce many of the features of these two regimes, including the boundaries for transition between them. Detailed comparison between the results obtained from the fluid models and experiment suggest the models underestimate the spatial extent of the low-temperature region associated with the detached plasma mode. Low-temperature atomic physics processes that are not included in the present models may account for this discrepancy.
Recent progress of implosion experiments with uniformity-improved GEKKO XII laser facility at the Institute of Laser Engineering, Osaka University
Authors: Mima K. , Kato Y. , Azechi H. , Shigemori K. , Takabe H. , Miyanaga N. , Kanabe T. , Norimatsu T. , Nishimura H. , Shiraga H. , Nakai M. , Kodama R. , Tanaka K. A. , Takagi M. , Natatsuka M. , Nishihara K. , Yamanaka T. , Nakai S.
Published: July 2002

The irradiation uniformity of the GEKKO XII laser [C. Yamanaka et al., IEEE J. Quantum Electron. QE-17 1639 (1981)] has been improved by using partially coherent light with angular spectral dispersion and by improving power balance among the beams. Implosion experiments with the uniformity-improved GEKKO XII have been carried out in order to demonstrate stable formation of the hot spark. The isentrope of the compressed pellet shell is controlled by adding a prepulse to enhance the ablative stabilization of the Rayleigh–Taylor instability. The implosion dynamics has been investigated by x-ray imaging and neutron diagnostics. The experimental results are compared with the mix model prediction based on the one-dimensional hydrodynamic simulation. The degradation of neutron yield is attributed to the low-mode asymmetry. Finally, recent results of planer target experiments on the ablative stabilization of the Rayleigh–Taylor instability are also presented. The observed linear growth rate was about 50% of the classical growth rate and was lower than that of the theoretical prediction, based on the Takabe formula combined with the one-dimensional simulation.
The Joint European Torus (JET) pumped divertor results and their significance for the International Thermonuclear Experimental Reactor (ITER)
Authors: Watkins M. L. ,   the JET Team
Published: July 2002

The effectiveness of the pumped divertor during the 1994/95 experimental campaign of the Joint European Torus (JET) [P.-H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 (1985)] has allowed the pursuit of a broad-based research program that is highly relevant to the International Thermonuclear Experimental Reactor (ITER) [K. Tomabechi and the ITER Team, Nucl. Fusion 31, 1135 (1991)]. High-performance hot-ion discharges with high confinement (H-modes) free of edge localized modes (ELMs) have set a JET record neutron rate in deuterium, but are limited by various magnetohydrodynamic (MHD) phenomena to βN<1.8, where βN=β/(I/aB), β is the ratio of the plasma pressure to the toroidal field pressure, I is the plasma current, B is the toroidal field, and a is the horizontal minor radius of the plasma. Quasi-steady-state ELMy H-modes have also been studied at high power, high current, and high β. The underlying energy transport exhibits a gyro-Bohm dependence that is lost close to the H-mode threshold and at high β. ELMy H-modes with detached divertor plasmas and radiative power exhaust (the operating regime foreseen for ITER) reduce the power loading to the targets, but at the expense of main plasma confinement and purity. Beryllium has been compared with carbon fiber composite as a divertor target material and melting has been induced at ITER reference off-normal heat loads, but only a moderate degree of self-protection of the beryllium target was found.
Progress in confinement and stability with plasma shape and profile control for steady-state operation in the Japan Atomic Energy Research Institute Tokamak-60 Upgrade
Authors: Koide Yoshihiko ,   JT-60 Team
Published: July 2002

This paper describes the latest achievements of the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U) [H. Kimura and the JT-60 Team, Phys. Plasmas 3, 1943 (1996)] for the establishment of the physics and technology basis for steady-state, fully noninductive current-drive plasmas with high fusion performance. Recent results are highlighted by (1) high performance reversed-magnetic-shear discharges [an idealized equivalent QDT of 1.05, a confinement-enhancement factor (H factor) of 3.23, and a normalized beta value (βN) of 1.88 at Ip=2.8 MA/Pabs=17 MW]; the first observation of thermal transport barriers both for electrons and ions, (2) the first injection of negative-ion-based neutral deuterium beams in the world for studies of current drive, heating, and high-energy particle behavior (achieved so far: 2.5 MW/400 keV, design values: 10 MW/500 keV); high neutralization efficiency of 60% at 370 keV; high current drive efficiency of ηCD=8×1018 m−2 A/W, (3) improved giant edge-localized-mode (ELM) limit and normalized beta limit by high triangularity (δ) shaping; the quasi-steady-state ELMy plasmas with integrated high performance were sustained for 8 times the energy confinement time (δ=0.35, H=2.5, βN=2.5 at Ip=1.5 MA and Bt=3.6 T), and (4) enhanced divertor radiation and detached divertor in reversed-shear discharges with neon puffing, while the internal transport barrier was sustained.
Two-dimensional mapping of the plasma density in the upper atmosphere with computerized ionospheric tomography (CIT)
Authors: Bernhardt Paul A. , McCoy R. P. , Dymond K. F. , Picone J. M. , Meier R. R. , Kamalabadi F. , Cotton D. M. , Charkrabarti S. , Cook T. A. , Vickers J. S. , Stephan A. W. , Kersley L. , Pryse S. E. , Walker I. K. , Mitchell C. N. , Straus P. R. , Na Helen , Biswas C. , Bust G. S. , Kronschnabl G. R. , Raymund T. D.
Published: July 2002

Tomographic imaging of the ionosphere is a recently developed technique that uses integrated measurements and computer reconstructions to determine electron densities. The integral of electron density along vertical or oblique paths is obtained with radio transmissions from low-earth-orbiting (LEO) satellite transmitters to a chain of receivers on the earth’s surface. Similar measurements along horizontal paths can be made using transmissions from Global Position System (GPS) navigation satellites to GPS receivers on LEO spacecraft. Also, the intensities of extreme ultraviolet (EUV) emissions can be measured with orbiting spectrometers. These intensities are directly related to the integral of the oxygen ion and electron densities along the instrument line of sight. Two-dimensional maps of the ionospheric plasma are produced by analyzing the combined radio and EUV data using computerized ionospheric tomography (CIT). Difficulties associated with CIT arise from the nonuniqueness of the reconstructions, owing to limited angle measurements or nonoptimal receiver location. Improvements in both reconstruction algorithms and CIT measurement systems are being implemented to overcome these difficulties. New imaging systems being developed employ CIT for large area mapping of the plasma densities in the ionosphere.
Thermal equilibria and thermodynamics of trapped plasmas with a single sign of charge
Authors: O’Neil T. M. , Dubin Daniel H. E.
Published: July 2002

Plasmas consisting exclusively of particles with a single sign of charge (e.g., pure electron plasmas and pure ion plasmas) can be confined by static electric and magnetic fields (e.g., in a Penning trap) and also be in a state of global thermal equilibrium. This important property distinguishes these totally un-neutralized plasmas from neutral and quasineutral plasmas. This paper reviews the conditions for and structure of the thermal equilibrium states and then develops a thermodynamic theory of the trapped plasmas. Thermodynamics provides hundreds of general relations (Maxwell relations) between partial derivatives of thermodynamic variables with respect to one another. Thermodynamic inequalities place general and useful bounds on various quantities. General and relatively simple expressions are provided for fluctuations of the thermodynamic variables. In practice, trapped plasmas are often made to evolve through a sequence of thermal equilibrium states through the slow addition (or subtraction) of energy and angular momentum (say, by laser cooling and torque beams). A thermodynamic approach to this late time transport describes the evolution through coupled ordinary differential equations for the thermodynamic variables, which is a huge reduction in complexity compared to the partial differential equations typically required to describe plasma transport. These evolution equations provide a theoretical basis for the dynamical control of the plasmas.
Toroidal visco-resistive magnetohydrodynamic steady states contain vortices
Authors: Bates Jason W. , Montgomery David C.
Published: July 2002

Poloidal velocity fields seem to be a fundamental feature of resistive toroidal magnetohydrodynamic (MHD) steady states. They are a consequence of force balance in toroidal geometry, do not require any kind of instability, and disappear in the “straight cylinder” (infinite aspect ratio) limit. If a current density j results from an axisymmetric toroidal electric field that is irrotational inside a torus, it leads to a magnetic field B such that ∇×(j×B) is nonvanishing, so that the Lorentz force cannot be balanced by the gradient of any scalar pressure in the equation of motion. In a steady state, finite poloidal velocity fields and toroidal vorticity must exist. Their calculation is difficult, but explicit solutions can be found in the limit of low Reynolds number. Here, existing calculations are generalized to the more realistic case of no-slip boundary conditions on the velocity field and a circular toroidal cross section. The results of this paper strongly suggest that discussions of confined steady states in toroidal MHD must include flows from the outset.
Anomalous resistivity and heating in current-driven plasma thrusters
Authors: Choueiri E. Y.
Published: July 2002

A theory is presented of anomalous resistivity and particle heating in current-driven plasma accelerators such as the magnetoplasmadynamic thruster (MPDT). An electromagnetic dielectric tensor is used for a current-carrying, collisional and finite-beta plasma and it is found that an instability akin to the generalized lower hybrid drift instability (GLHDI) exists for electromagnetic modes (i.e., with finite polarization). Weak turbulence theory is then used to develop a second-order description of the heating rates of particles by the waves and the electron-wave momentum exchange rate that controls the anomalous resistivity effect. It is found that the electron Hall parameter strongly scales the level of anomalous dissipation for the case of the MPDT plasma. This scaling has recently been confirmed experimentally [Phys. Plasmas 5, 3581 (1997)]. Polynomial expressions of the relevant transport coefficients cast solely in terms of macroscopic parameters are also obtained for including microturbulence effects in numerical plasma fluid models used for thruster flow simulation.
Shock structuring due to fabrication joints in targets
Authors: Goldman S. R. , Caldwell S. E. , Wilke M. D. , Wilson D. C. , Barnes Cris W. , Hsing W. W. , Delamater N. D. , Schappert G. T. , Grove J. W. , Lindman E. L. , Wallace J. M. , Weaver R. P. , Dunne A. M. , Edwards M. J. , Graham P. , Thomas B. R.
Published: July 2002

The use of copper-doped beryllium ablators on National Ignition Facility [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] targets, in place of plastic, can require the bonding together of hemispheres with a joint of differing composition. Indirect drive experiments have been conducted on the Nova laser [J. L. Emmet, W. F. Krupke, and J. B. Trenholme, Sov. J. Quantum Electron. 13, 1 (1983)], and the resulting shock structuring compared with code simulations. It is concluded that one of the available codes, the RAGE code [R. M. Baltrusaitis et al., Phys. Fluids 8, 2471 (1996)] provides useful insight into the effect of joints. This code is then employed to obtain a physical picture of the shock front nonuniformity in terms of a secondary rarefaction and an oblique shock interacting with the main shock that propagates in the absence of the joint. A simple analysis reinforces this picture.
Beta-dependent upper bound on ion temperature anisotropy in a laboratory plasma
Authors: Keiter Paul A. , Scime Earl E. , Balkey Matthew M. , Boivin Robert , Kline John L. , Gary S. Peter
Published: July 2002

Laser induced fluorescence measurements of ion temperatures, parallel and perpendicular to the local magnetic field, in the Large Experiment on Instabilities and Anisotropies space simulation chamber (a steady-state, high beta, argon plasma) display an inverse correlation between the upper bound on the ion temperature anisotropy and the parallel ion beta (β=8πnkT/B2). These observations are consistent with in situ spacecraft measurements in the Earth’s magnetosheath and with a theoretical/computational model that predicts that such an upper bound is imposed by scattering from enhanced fluctuations due to growth of the ion cyclotron anisotropy instability (the Alfvén ion cyclotron instability).
Nonlinear excitonic absorption in (Zn,Mn)Se superlattices and ZnSe films
Authors: Andersen D. R. , Kolodziejski L. A. , Gunshor R. L. , Datta S. , Kaplan A. E. , Nurmikko A. V.
Published: July 2002

Optical transmission studies have been performed on ZnSe thin films and two (Zn,Mn)Se superlattices (Eg≂2.8 eV) of varying barrier compositions and well widths grown by molecular beam epitaxy. The excitons in these samples ranged in character from three dimensional to quasi two dimensional. The spectra, obtained at 77 K as a function of incident light intensity, clearly show the saturation of the excitonic resonance. This is the first demonstration of nonlinear excitonic absorption in the (Zn,Mn)Se system. Mechanisms for the saturation are discussed with estimates for the relative contributions from Coulombic screening and phase space filling.
New efficient phosphor material ZnS:Sm,P for red electroluminescent devices
Authors: Tohda T. , Fujita Y. , Matsuoka T. , Abe A.
Published: July 2002

Thin-film electroluminescent devices employing a new phosphor material ZnS:Sm,P have been found to exhibit bright red emission. Luminous efficiency of ZnS:Sm,P phosphor films is higher than that of ZnS:Sm phosphor films in the range of annealing temperature above 500 °C. A brightness of 1000 cd/m2 and an efficiency of 8×10−2 lm/W have been obtained in the devices with ZnS:Sm(1 at. %),P(0.5 at. %) phosphor films annealed at 600 °C. These results indicate that P is an efficient co-activator for Sm in ZnS.
Performance of quarter-micron GaAs metal-semiconductor field-effect transistors on Si substrates
Authors: Aksun M. I. , Morkoç H. , Lester L. F. , Duh K. H. G. , Smith P. M. , Chao P. C. , Longerbone M. , Erickson L. P.
Published: July 2002

Metal-semiconductor field-effect transistors (MESFET’s) having quarter-micron gate lengths were fabricated in GaAs films grown directly on 〈100〉 silicon tilted towards 〈110〉 by 4°. Following the growth of 2 μm undoped GaAs buffer layer a 3 μm GaAs doped with Si to a level of 3×1017 cm−3 was grown all by molecular beam epitaxy. Devices showed good dc characteristics with extrinsic transconductances as high as 360 mS/mm. Extrapolation of the short-circuit current gain calculated from scattering parameters measured up to 20 GHz indicated a current gain cut-off frequency of 55 GHz, which is comparable to the best MESFET on GaAs substrates. The noise figure measured at 18 GHz was 2.8 dB, which is about 1.4 dB higher than that on GaAs substrates.
Hot-electron capture to D
X centers in AlxGa1−xAs at low Al mole fractions (x<0.2)
Authors: Theis T. N. , Parker B. D. , Solomon P. M. , Wright S. L.
Published: July 2002

We report quantitative studies of hot-electron trapping by DX centers in short channel GaAs/n-AlxGa1−x As field-effect transistors. A remarkable result is that persistent hot-electron capture occurs even at very low values of the Al mole fraction, x≲0.2, where thermal capture is not observed. Thermal emission studies confirm that the trap state is associated with the DX center. Thus, for x≲0.2 the trap state is metastable, but can be persistently populated by heating the free electrons with an electric field. This and other features of the capture process support a large lattice relaxation model of the center.