bluuebunny/crossref_metadata_2025 huggingface dataset:
On interface dynamics
Authors: Arpaci V. S. , Esmaeeli A.
Published: July 2002

An intuitive study is presented for unstable interfacial waves. The maximum wavelength obtained for the most rapid unstable growth is shown to have a universal part which also characterizes the isotropic scales of buoyancy-driven turbulence.
An analytic treatment of the bounded and free presheaths with arbitrary viscosity in magnetized flowing plasmas
Authors: Chung Kyu-Sun
Published: July 2002

Analytic solutions are derived from one-dimensional fluid equations with arbitrary shear viscosity for the free and bounded presheaths in the strongly magnetized flowing plasma. Plasma density and flow velocity are obtained analytically in terms of position along the field line, Mach number (M∞), normalized viscosity (α), and size of bounded presheath. Simple analytic relations between the ratio (R) of sheath current and the flow Mach number [M∞ ≡ Vd/√(ZTe+Ti)/mi] are derived as R=exp(M∞/Mc), where Mc = 1/[1 +√α2/1+α arctan√1+α], with less than 5% accuracy for 0≤M∞≤0.5. By generating two free and two bounded presheaths by two Mach probes within two free presheaths generated by a larger object than Mach probes, one can measure α and M∞ simultaneously. The allowed range of α in terms of M∞ is obtained, and comparisons with previous numerical analyses are also made.
The influence of the spectral properties of electrostatic fluctuations on the ambipolar electric field and plasma rotation
Authors: Voitsekhovitch I. A. , Coronado M. , Martinell J. J.
Published: July 2002

A general system of momentum balance equations that take into account neoclassical effects and the fluctuations of plasma quantities and electromagnetic field is established. This is used to study the influence of the properties of electrostatic fluctuations on the radial anomalous flows and ambipolar electric field. The case when the momentum drag is provided by collisions with neutrals is considered, which is relevant for the edge plasma. It is shown that a change of the spectrum accompanied by a transfer of 40% of the fluctuation energy from the big wavelength region to the small wavelength region of the spectrum can lead to a change of up to 20% in the magnitude of the electric field. As a result the plasma rotation will change. The possibility to induce plasma spin-up through the ambipolar electric field produced by monochromatic waves delivered from an external source is also considered.
Poincaré maps define topography of Vlasov distribution functions consistent with stochastic dynamics
Authors: Bailey A. D. , Bellan P. M. , Stern R. A.
Published: July 2002

In a recent paper [A. D. Bailey et al., Phys. Rev. Lett. 34, 3124 (1993)], the authors presented direct planar laser induced fluorescence measurements of the oscillatory ion fluid velocity field in the presence of a large amplitude drift-Alfven wave. Surprisingly, the measured speeds were an order of magnitude lower than predicted by standard fluid theory, yet the flow pattern was consistent with the fluid theory. A new model, based on the connection between stochasticity and bulk behavior, is presented which gives insights into the cause of this behavior. It is shown that when particle motion is stochastic, invariant sets of a ‘Poincaré map’ define a flat-topped particle distribution function consistent with both the electromagnetic field driving the Vlasov equation and the fine-scale single particle dynamics. The approach is described for the general case and explored for a slab model of the observed drift wave.
Rayleigh–Taylor vortices and their stability in magnetized plasmas
Authors: Chakrabarti Nikhil , Kaw P. K.
Published: July 2002

Recent experiments and computer simulations have shown that the probability distribution function (PDF) of density and potential fluctuations associated with two-dimensional Rayleigh–Taylor turbulence in collisionless plasmas, may be exhibiting significant non-gaussian features. A possible interpretation of these results is that nonlinearly saturated coherent structures are formed which are however transient, because they are themselves unstable to fine scale secondary instabilities. An analytical investigation has been carried out to investigate some of these effects. The two- dimensional nonlinear equations describing the Rayleigh–Taylor turbulence are analytically solved; a dipole vortex solution has been obtained and its stability to secondary perturbations examined. Results show that there is a significant growth of the secondary perturbation due to ellipticity of the vortex.
Application of the invariant manifold reduction to dissipative plasma instabilities
Authors: Wessen Ken
Published: July 2002

The invariant manifold reduction is a technique by which a low-dimensional description of the evolution of a dissipative nonlinear system may be obtained. A simple trapped-ion model is used to demonstrate the technique, since the results obtained are also relevant to many systems that share its O(2) symmetry. A complete description of the nonlinear behavior available to this (reduced) system is constructed, and, in particular, various interesting dynamic, yet stable, states are found.
Negative shear effect on toroidal ion temperature gradient mode
Authors: Kim J. Y. , Wakatani M.
Published: July 2002

A study of the effect of negative shear on the toroidal ion temperature gradient mode is given. It is shown that the negative shear can give a substantial stabilizing effect on the mode when the safety factor q is small.
Kinetic theory of charged particles of variable shape
Authors: Coppa G. G. M. , Riccardo V. , Lapenta G.
Published: July 2002

Recently a new kind of computational technique for kinetic plasma simulation has been developed, making use of particles of variable shape (blobs). The aim of the present work is to derive a kinetic equation governing the evolution of the blobs and solve it in the case of small perturbations. Results are compared with those obtained for the real plasma and for a plasma of particles of fixed shape (the model usually employed in popular particle-in-cell simulations). The comparison is useful to find out whether the additional cost of dealing with more degrees of freedom is balanced by the superior accuracy provided by the new technique in describing the phase-space distribution.
Nonlinear stopping power of ions in plasmas
Authors: Boine-Frankenheim O.
Published: July 2002

The study of the nonlinear stopping power of ions in plasmas is of fundamental importance for various applications. One example is the energy loss of heavy ions passing through a plasma. Due to the high non-equilibrium charge states specific to heavy ions, the plasma regime with coupling parameters 1/ND<1 and Zp/ND≳1 (ND∼ number of electrons in a Debye sphere, Zp charge of the ion) is of interest. In this regime, the Vlasov-Poisson system cannot be linearized, rather a fully nonlinear treatment is required. In the present paper, the Vlasov-Poisson system is solved numerically by using the capability of the new generation of massively parallel supercomputers. The results are compared with the standard dielectric theory and a recent binary collision approach. It is demonstrated that nonlinear effects lead to a strongly reduced Bragg-peak for Zp/ND≳1. In the nonlinear regime, the scaling of the stopping power is close to a Zp3/2 law, which is found to be characteristic for the nonlinear stopping power, if the influence of close collisions on the induced potential is treated properly.
High-frequency fluctuations of a modulated, helical electron beam
Authors: Reynolds M. A. , Morales G. J.
Published: July 2002

The high-frequency electromagnetic field generated by a density-modulated, helical electron beam propagating in a magnetized plasma is calculated. The magnetic fluctuations are found to exhibit spatially localized (evanescent) resonances at harmonics of the electron-cyclotron frequency, whose width is determined by the pitch angle of the beam, and whose existence is a consequence of the helical geometry. In addition, electrostatic modes are radiated near the hybrid frequencies, and electromagnetic modes are radiated above the upper-hybrid frequency. The predicted frequency spectrum and mode structure in configuration space are in good agreement with experimental observations of discrete emission lines at the electron-cyclotron harmonics [Phys. Fluids B 5, 3789 (1993)].
Upper bound for heat transport due to ion temperature gradients
Authors: Kim Chang-Bae , Choi Kang-Oak
Published: July 2002

Turbulent transport due to an ion temperature gradient is studied in the context of a fluid description in slab geometry. An upper bound on the heat transport is obtained through the use of a variational principle. The physical constraint of energy conservation that is included in the principle keeps the bound finite. Additional constraint is needed and employed for the magnetic shear effect to be accounted for. The bounding curve of the heat flux versus the ion temperature gradient, ηi, is presented along with the profiles of the fluctuations. The bound, after an extrapolation, is argued to be in the neighborhood of what numerical simulation predicts.
Second stable regime of nonlocal mixed slab-toroidal anisotropic ion-temperature-gradient-driven mode
Authors: Song H. , Sen A. K. , Rewoldt G.
Published: July 2002

Nonlocal effects on the mixed slab-toroidal ion-temperature-gradient-driven (ITG) instability are investigated with analytical and numerical approaches. Although it is more complex than the local analysis, the nonlocal analysis also leads to the same conclusion that a second stable regime exists at large values of the perpendicular temperature gradient. From a strictly theoretical point of view, this result provides a possible stabilization scheme for the ITG instability.
Electron diamagnetic drift effect on the trapped hot particle-driven resistive interchange mode
Authors: You K.-I. , Choi D.-I. , Kim J.-Y.
Published: July 2002

The linearized flux-surface-averaged resistive magnetohydrodynamic equations, which include the electron diamagnetic drift and hot particle effects, are derived and studied numerically. The effect of electron diamagnetic drift on the trapped hot particle-driven resistive interchange mode is then investigated. It is shown that the effect can have a strong stabilization effect on the mode, similar to the conventional resistive interchange mode case.
Resonantly excited nonlinear ion waves
Authors: Cohen Bruce I. , Lasinski Barbara F. , Langdon A. Bruce , Williams Edward A.
Published: July 2002

One- and two-dimensional simulations and supporting analysis of nonlinear ion acoustic waves as might be associated with the saturation of stimulated Brillouin backscattering (SBBS) are presented. To simulate ion wave phenomena efficiently, while retaining a fully kinetic representation of the ions, a Boltzmann fluid model is used for the electrons, and a particle-in-cell representation is used for the ions. Poisson’s equation is solved in order to retain space-charge effects. We derive a new dispersion relation describing the parametric instability of ion waves, evidence for which is observed in our simulations. One- and two-dimensional simulations of plasma with either initially cold or warm ions (and multi-species ions) exhibit a complex interplay of phenomena that influence the time evolution and relaxation of the amplitude of the excited ion wave: ion trapping, wave steepening, acceleration, heating and tail formation in the ion velocity distribution, parametric decay into longer wavelength ion waves, modulational and filamentation instabilities, and induced scattering by ions. The additional degrees of freedom in two dimensions allow for a more rapid relaxation of the primary ion wave. One-dimensional electrostatic simulations with externally driven ion waves agree qualitatively with electromagnetic simulations in one dimension in which the ponderomotive driving potential is computed self-consistently by solving a Schroedinger-like equation for the electromagnetic waves and calculating the low-frequency ponderomotive force on the electrons.
Propagation properties of a relativistic electron beam with premodulated energy and current
Authors: Uhm Han S.
Published: July 2002

This paper considers an electron beam whose energy and current are simultaneously premodulated at the injection point of a drift tube. A theoretical model is developed for the subsequent current and energy modulations which propagate downstream. A closed integrodifferential equation for the normalized beam current is obtained in terms of time and propagation distance. To make the nonlinear current modulation analytically tractable, a small signal theory is introduced into the modulation calculation. The current modulation in the linear regime is a linear combination of the forward and backward waves of the initial current and energy modulations. It is shown that the initial energy modulation is a very effective means for downstream current modulation. The downstream energy modulation is also expressed by a linear combination of the forward and backward waves of the initial current and energy modulations. In general, the maximum current modulation occurs at the propagation distance where the energy modulation is minimum. Properties of the current and energy modulations are numerically investigated from the integrodifferential equation for a broad range of system parameters. Numerical data even for a large initial current and energy modulations agree reasonably well with analytical results predicted by the linear theory.
Stationary symmetric magnetohydrodynamic flows
Authors: Goedbloed J. P. , Lifschitz A.
Published: July 2002

The problem of translation symmetric stationary magnetohydrodynamic (MHD) flow is analyzed with respect to the different permitted flow regimes. A minimal set of equilibrium functions is constructed facilitating the complete explicit solution in the case of self-similar flow. It is found that limiting line and Alfvén singularities leads to a division in four distinct velocity regimes which cannot be connected by continuous flows. Instead, the three types of MHD shocks appear, enabling the system to pass from the high to the low speed regimes.
Determination of microturbulence enhanced electron collisionality in magnetized coaxial accelerator channels by direct magnetic field measurement
Authors: Black D. C. , Mayo R. M. , Caress R. W.
Published: July 2002

A miniature magnetic probe array, consisting of 10 spatially separated coils, has been used to obtain profile information on the time varying magnetic field within the 2.54 cm wide flow channel of the coaxial plasma source experiment (CPS-1) [R. M. Mayo et al., Plasma Sources Sci. Technol. 4, 47 (1995)]. The magnetic field data have been used, together with a resistive, Hall magnetohydrodynamic (MHD) model of applied field distortion by the flowing plasma, to obtain estimates of the microturbulent enhancement to electron collisionality within the CPS-1 flow channel. These measurements provide direct experimental evidence of anomalous electron collisionality, a previously predicted effect in these devices. The anomaly parameter, a=νan/νcl, determined both from the distortion of contours of constant magnetic flux, and from local Bθ and Bz measurements scales with the classical electron magnetization parameter (Ωcl=ωce/νecl), indicating that collisionality plays a strong role in determining the level of anomalous transport in the plasma. When this anomaly parameter scaling is cast in terms of the ratio νecl/ωlh, it is found that the resistivity enhancement scales with νecl/ωlh, and becomes significant at νecl/ωlh⩽1, suggesting that a lower hybrid drift instability may be the responsible mechanism for enhanced transport.
Comment on “Onsager symmetry for inhomogeneous magnetized plasmas” [Phys. Plasmas 3, 4325 (1996)]
Authors: Schneider R. S. , Ziebell L. F. , Gaelzer R.
Published: July 2002

An effective dielectric tensor for inhomogeneous magnetized plasmas was previously derived using a modified local mode approximation. We give a brief account of this derivation from a paper by Caldela Filho et al. (1989) to restate their priority. (AIP)
Stringer spin up due to anomalous transport
Authors: Peeters A. G.
Published: July 2002

A simple equation for Stringer spin up due to anomalous transport valid in arbitrary toroidal symmetric geometry is derived. Additional terms are found compared with previous work that can be traced to a different ordering of the parallel velocity. The influence of elongation and Shafranov shift is shown to be small. Neoclassical viscosity is included and the case of both small and large viscosity are discussed.
Excitation of electromagnetic waves in a plasma-filled backward wave oscillator in the Ku region
Authors: Choyal Y. , Maheshwari K. P.
Published: July 2002

The effect of plasma on backward wave oscillator (BWO) emission in a 12–18 GHz frequency range is given. It is shown that the Ku band emission corresponds to the cylindrically symmetric transverse magnetic TM01 and not the higher mode. Due to the slowing down of waves with the increase in plasma density there occurs a BWO mode switching, i.e., BWO switches from a bounded mode to an unbounded one. Here the analysis for the switched BWO in the unbounded mode is given. The expressions for the temporal and spatial growth rates in the presence of plasma are obtained. The efficiency calculations are also given. Comparison of the results with that of a previous experiment [IEEE Trans. Plasma Sci. 18, 497 (1990)] has been carried out.
On spatial structures formation in dusty plasmas
Authors: Mareev E. A. , Sarafanov G. F.
Published: July 2002

A dynamical model is proposed, which treats the formation of spatial structures as the result of a dissipative instability of charged particle distribution in three-component nonideal dusty plasma. Instability arises due to the correlation interaction between dust particles taken into account in the framework of the diffusion approach. The expressions for the characteristic spatial scale and the effective dust diffusion coefficient are found.
Propagation properties of a relativistic prebunched electron beam in a free electron laser
Authors: Ding Wu , Yang Yunyuan
Published: July 2002

The propagation properties of a prebunched electron beam in a free electron laser is investigated in this paper. Results show the following: As the beam propagates downstream, the amplitude of the fundamental modulation current is enhanced and its phase remains unchanged, while both the amplitude and phase of the high harmonic modulation current remain unchanged.
Dynamic orientation phenomena for direct 1s→2p±1 electron–ion excitations in a generalized Lorentzian (kappa) distribution plasma
Authors: Yoon Jung-Sik , Jung Young-Dae
Published: July 2002

Orientation phenomena for 1s→2p±1 transitions in electron–ion collisional excitations in a generalized Lorentzian (kappa) distribution plasma are investigated using the dynamic interaction potential. Scaled transition probabilities for 1s→2p±1 excitations are obtained using the plasma dielectric function. The semiclassical straight-line trajectory approximation is applied to the motion of the projectile electron in order to visualize the variation of the dynamic orientation parameter as a function of impact parameter, collision energy, and spectral index κ. When the electron thermal velocity is greater than the projectile velocity, the propensity of the m=−1 transition in the Lorentzian plasma is found to be smaller than that in the static model plasma. However, when the thermal velocity is smaller than the projectile velocity, the orientation parameters for various κ values are almost identical and the propensity of the m=−1 transition in Lorentzian plasma is stronger than that in the static model plasma. The propensity of the m=−1 transition decreases with increasing the projectile energy. For a large velocity ratio of the electron thermal velocity to the projectile velocity, the propensity of the m=−1 transition decreases with decreasing the spectral index κ.
Analytic expressions of neoclassical transport coefficients for fusion-born alpha particles in a tokamak reactor
Authors: Jhang Hogun
Published: July 2002

Analytic approximate expressions of neoclassical transport coefficients for energetic alpha particles are presented. The energy scattering effect, as well as the usual slowing-down and pitch angle scattering effects, is retained. The formulas are valid for arbitrary aspect ratios and agree well with the previous results, which numerically evaluated the coefficients using an eigenfunction expansion technique. Application of the newly developed formulas to the International Thermonuclear Experimental Reactor Design Activity (ITER EDA) [J. C. Wesley and ITER Joint Central Team, Phys. Plasmas 4, 2642 (1997)] plasma condition is also given.
What is the “beta-induced Alfvén eigenmode?”
Authors: Heidbrink W. W. , Ruskov E. , Carolipio E. M. , Fang J. , van Zeeland M. A. , James R. A.
Published: July 2002

An instability with a lower frequency than the toroidicity-induced Alfvén eigenmode was initially identified as a beta-induced Alfvén eigenmode (BAE). Instabilities with the characteristic spectral features of this “BAE” are observed in a wide variety of tokamak plasmas, including plasmas with negative magnetic shear. These modes are destabilized by circulating beam ions and they transport circulating beam ions from the plasma core. The frequency scalings of these “BAEs” are compared to theoretical predictions for Alfvén modes, kinetic ballooning modes, ion thermal velocity modes, and energetic particle modes. None of these simple theories match the data.
Effect of toroidal rotation on the localized modes in low beta circular tokamaks
Authors: Zheng L.-J. , Chu M. S. , Chen Liu
Published: July 2002

Effects of finite toroidal rotation on the stability of radially localized ideal magnetohydrodynamic (MHD) modes are investigated analytically for low-beta, large aspect-ratio circular tokamaks. Specifically, it is found that, for the Mercier interchange mode, a finite rotation shear is generally stabilizing. For the double internal kink mode, however, finite toroidal rotation shear further enhances the ballooning destabilizing effect.
Fast particle finite orbit width and Larmor radius effects on low-n toroidicity induced Alfvén eigenmode excitation
Authors: Gorelenkov N. N , Cheng C. Z. , Fu G. Y.
Published: July 2002

The effects of finite drift orbit width (FOW) and Larmor radius (FLR) of fast particles on the stability of low-n toroidicity-induced Alfvén eigenmodes (TAE) are studied. The formulation is based on the solution of the low frequency gyrokinetic equation (ω≪ωc, where ωc is particle cyclotron frequency). A quadratic form has been derived in terms of invariant variables; energy E, magnetic moment μ, and toroidal angular momentum Pφ. The growth rate of the TAE is computed perturbatively using numerical averaging over the fast particle drift orbit. This new computational capability improves the NOVA-K code [G. Y. Fu, C. Z. Cheng, and K. L. Wong, Phys. Fluids B 5, 4040 (1994)] which included FOW effects in the growth rate calculation based on small radial orbit width approximation. The new NOVA-K version has been benchmarked for different regimes of particle TAE excitation. It is shown that both FOW and FLR effects are typically stabilizing; the TAE growth rate can be reduced by as much as a factor of 2 for tokamak fusion test reactor supershots [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)]. However, FOW may be destabilizing for the global modes, which are localized at the plasma edge.
Spontaneous hole–clump pair creation
Authors: Berk H. L. , Breizman B. N. , Candy J. , Pekker M. , Petviashvili N. V.
Published: July 2002

Numerical simulations and quantitative theoretical explanations are presented for the spontaneous formation of a hole–clump pair in phase space. The equilibrium is close to the linear threshold for instability and the destabilizing resonant kinetic drive is nearly balanced by either extrinsic dissipation or a second stabilizing resonant kinetic component. The hole and clump, each support a nonlinear wave where the trapping frequency of the particles is comparable to the kinetic linear growth rate from the destabilizing species alone. The power dissipated is balanced by energy extracted by trapped particles locked to the changing wave-phase velocities. With extrinsic dissipation, phase space structures always form just above the linear instability threshold. With a stabilizing kinetic component, an electrostatic interaction is considered with varying mass ratios of the stabilizing and destabilizing species together with collisional effects. With these input parameters, various nonlinear responses arise, only some of which sweep in frequency.
Plasma–wall interaction in an oblique magnetic field: Model of the space-charge sheath for large potentials and small Debye lengths
Authors: Ahedo E.
Published: July 2002

An analytical model of the space-charge sheath around a planar wall is derived for the case of (i) uniform magnetic field B incident at any angle into the wall, (ii) large wall potentials (relative to the plasma temperature): φW≫1, and (iii) Debye length, λd, much smaller than thermal Larmor radius of the attracted species, λm. It is found that, irrespective of the angle of incidence: (i) The potential threshold for a magnetized sheath is φW=O(λd4/λm4); (ii) the characteristic magnetic length in the sheath is λm3/λd2, much larger than λm and proportional to |B|−3 and the plasma flow into the sheath; (iii) the electric field E increases towards the wall and produces a noncycloidal plasma drift that breaks down magnetic insulation. Plasma dynamics in a magnetized sheath consists of an E-aligned region, a drift region, and a B-aligned region. The drift region is relevant only for angles far from normal incidence and the density profile presents spatial oscillations there; for grazing incidence, the B-aligned region is not found. Different scaling laws of the sheath thickness versus wall potential and incidence angle are obtained; in particular, the thickness of a magnetized sheath at parallel incidence is the local Larmor radius at the wall. The applicability of the model to experiments in the ionosphere is commented.
Ultra-high-beta spherical tokamak formation by use of an oblate field-reversed configuration
Authors: Ono Y. , Inomoto M.
Published: July 2002

A new slow formation of oblate field-reversed configuration (FRC) has been developed in the Tokyo University Spherical Torus No. 3 (TS-3) merging experiment using two merging spheromaks with opposing toroidal field. This unique technique was extended to a novel formation of ultra-high-beta (50%–70%) spherical tokamak (ST) by applying an external toroidal field Bt,ext to the FRC so produced. The high-beta ST was found to have a diamagnetic toroidal field in sharp contrast with low-beta STs with strong paramagnetic toroidal fields. High-beta properties of FRCs including their hollow current profile were maintained during the equilibrium transition, suggesting a close relationship between FRCs and high-beta STs in the second stability regime.
Radial displacement of pellet ablation material in tokamaks due to the grad-B effect
Authors: Parks P. B. , Sessions W. D. , Baylor L. R.
Published: July 2002

During pellet injection in tokamaks, a rapid movement of pellet ablation substance towards the low-field or outward major radius R direction is observed, favoring pellet injection from the high-field side in order to promote deeper fuel penetration. The motion has been attributed to a vertical curvature and ∇B drift current induced inside the ionized ablated material by the 1/R toroidal field variation. The uncompensated vertical drift current inside the weakly diamagnetic (β<0.1) ablation cloud will cause charge separation at the boundary. The resulting electrostatic field induces the E×B drift to the large-R side of the torus. The calculated fuel penetration depth is consistent with inside launched pellet experiments on the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. The dependence of the penetration depth with plasma parameters suggests that low velocity inside launched pellets may provide a unique solution to the refueling problem in larger and hotter machines of the future.
Sheath superheat transmission due to redeposition of thermally emitted material
Authors: Brooks J. N. , Naujoks D.
Published: July 2002

A self-consistent three-dimensional numerical analysis using the newly developed BPHI-3D code was performed for the Debye/magnetic tokamak-type oblique incidence magnetic field sheath, with near-surface ionization and transport of thermally emitted surface material. The analysis uses Monte Carlo, kinetic treatment for deuterium–tritium and impurity ions/neutrals, Boltzmann/guiding-center electrons, and particle-in-cell time-independent Poisson solver. For typical predicted fusion edge plasma conditions for a liquid lithium divertor most evaporated lithium atoms—from a ∼1 cm2 overheated spot–are ionized in the ∼1 mm-wide magnetic sheath. These ions are strongly redeposited due to the sheath electric field. While this redeposition minimizes core plasma contamination, it increases the peak heat flux to the surface. A runaway situation is then possible due to superheating/evaporation positive feedback. Carbon may behave likewise as seen in code results obtained for a TORE SUPRA [Aymar et al., Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1989)] carbon limiter. A semianalytic formula for sheath parameters as a function of emitted surface material flux is developed and verified with the code.
Electron current to a probe in a magnetized, collisional plasma
Authors: Charro M. , Sanmartı́n J. R.
Published: July 2002

A collisional analysis of electron collection by a probe in a strongly magnetized, fully ionized plasma is carried out. A solution to the complete set of macroscopic equations with classical transport coefficients that is wholly consistent in the domain 1≪R2/le∞2≪(mi/me)3/2 is determined; R and le∞ are probe radius and electron gyroradius, respectively. If R2/le∞2 is large compared with mi/3me (probe large compared with ion gyroradius), ion–electron energy exchange—rather than electron heat diffusion—keeps electrons isothermal. For smaller probes at negative bias, however, electron cooling occurs in the plasma beyond the sheath, with a potential overshoot lying well away from it. The probe characteristic in the electron-retarding range may then mimic the characteristic for a two electron-temperature plasma and lead to an overestimate of electron temperature; the validity of these results for other transport models is discussed.
Fourier analysis for the isothermal capacitance transient spectroscopy signal
Authors: Ishikawa Toru , Kwon Young Kyu , Kuwano Hiroshi
Published: July 2002

In an isothermal capacitance transient spectroscopy (ICTS) measurement, analysis of the results is based on the assumption of an exponential capacitance transient. We propose the use of Fourier analysis for the ICTS signal which arises from nonexponential transient capacitance. The method allows us to determine deep levels which have adjacent emission time constants.
Configurationally multistable defect in silicon
Authors: Chantre A. , Kimerling L. C.
Published: July 2002

We report the isolation of a new defect in n-type silicon following room-temperature electron irradiation. Using deep level transient spectroscopy (DLTS) combined with minority-carrier injection at 250 K, we show that the DLTS peak usually ascribed to the phosphorus-vacancy pair hides a signal arising from this defect in its stable configuration. We find that the defect can exist in three other configurations which can be individually studied by DLTS. It is proposed that this defect involves a lattice vacancy and a phosphorus atom, plus a third unidentified defect or impurity.
Imaging in real time with the tunneling microscope
Authors: Bryant A. , Smith D. P. E. , Quate C. F.
Published: July 2002

We report the achievement of real-time video rate images of the atomic surface of graphite using the scanning tunneling microscope (STM). In this high-speed mode of operation the topographic information is extracted directly from variations in the tunneling current. We find that this high-speed technique offers the following advantages: the microscope becomes insensitive to most low frequency vibrations; piezoelectric scanner hysteresis appears to be reduced; and the effect of 1/f noise, which is conjectured to arise from diffusion of molecular species, is reduced by shifting the data to higher frequencies. The real-time technique offers the opportunity to observe dynamic atomic processes which occur on time scales previously incaccessible to the STM.
Plastic optical fibers for near-infrared transmission
Authors: Kaino Toshikuni
Published: July 2002

A new plastic optical fiber using pentafluoro trideutero styrene as a core has been prepared which enables stable transmission of near-infrared (near-IR) optical signals. Replacement of hydrogen in the core polymer with a combination of fluorine and deuterium effectively reduces molecular vibrational absorption from the near-IR to the visible region. A 174 dB/km attenuation loss was obtained in the 850 nm region. Fluorination of aromatic hydrogen of styrene suppresses the water vapor absorption very effectively, and stable optical attenuation under high humidity is achieved.
Rapid thermal annealing of Mg++As+ dual implants in GaAs
Authors: Patel Kalpesh K. , Sealy Brian J.
Published: July 2002

Mg+ and As+ ions were implanted into GaAs to study the effects of dual implantation on the electrical properties. Increased electrical activity and significantly less redistribution of the magnesium were observed for the dual implants compared with the single implants. A maximum activity of 40% with a sheet resistivity as low as 185 Ω/⧠ was obtained for the dual implant compared to an activity of 27% with a resistivity of 250 Ω/⧠ for the single implant at a dose of 1015 cm−2. A peak hole concentration approaching 4×1019 cm−3 was recorded for the dual implant annealed at 900 °C for 30 s.
Amorphization of transition metal Zr alloys by mechanical alloying
Authors: Hellstern E. , Schultz L.
Published: July 2002

Amorphous alloy powders of the types NiZr, CoZr, FeZr, and CuZr are produced by mechanical alloying from crystalline elemental powders. The alloying and amorphization process is monitored by microstructural investigations, x-ray diffraction, and, where applicable, by magnetization measurements. The crystallization temperatures, determined by differential scanning calorimetry, are comparable to those measured for rapidly quenched and solid state reacted amorphous metals of the same compositions.
Negative absolute mobility of holes in n-doped GaAs quantum wells
Authors: Höpfel R. A. , Shah J. , Wolff P. A. , Gossard A. C.
Published: July 2002

We observe the effect of negative absolute mobility for minority holes in n-modulation-doped GaAs quantum wells at low temperatures. Photoinjected minority holes drift towards the positive electrode, which is shown experimentally using a photoluminescence imaging technique. The effect is attributed to ‘‘carrier drag’’ by the high-mobility electron plasma via electron-hole scattering. From a quantitative analysis we are able to determine electron-hole momentum relaxation times.
Electrochromic hydrated nickel oxide coatings for energy efficient windows: Optical properties and coloration mechanism
Authors: Svensson J. S. E. M. , Granqvist C. G.
Published: July 2002

Durable electrochromic coatings of hydrated nickel oxide were produced by reactive rf magnetron sputtering of Ni followed by treatment in KOH. Spectrophotometry was used to assess the achievable modulation of luminous and solar transmittance and to verify that the studied material is interesting for ‘‘smart window’’ applications. 15N nuclear reaction analysis suggested that coloration occurred upon hydrogen extraction.
Novel electrostatic mechanism in the thermal instability of z-cut LiNbO3 interferometers
Authors: Skeath Perry , Bulmer C. H. , Hiser S. C. , Burns W. K.
Published: July 2002

Experimental data from z-cut LiNbO3 Mach–Zehnder interferometers show that electrical contact to the device electrodes leads to the large thermal instability of the intrinsic phase bias. Withdrawing contact from the electrodes causes the thermal instability to virtually disappear. A model is proposed in which contact to the electrodes creates strong gradients in the local surface potential near the electrodes when unscreened pyroelectrically induced surface charges are present, independent of whether the probes are floating or grounded. The resulting differential electric field strength in the interferometer branches gives rise to the observed thermal instability, which is five orders of magnitude larger than the calculated instability due to the pyroelectric effect alone. The model of the instability mechanism is used to suggest methods of reducing the instability.
Extension of bearing endurance life by ion implantation
Authors: Hirano Motohisa , Miyake Shojiro
Published: July 2002

The oscillatory characteristics of ion-implanted ball bearings are described. N-implanted bearings are found to have lower starting torque and longer endurance life both in a vacuum as well as in air than unimplanted bearings. Typical starting torque in air can be reduced by 0.8 thus extending bearing life from 104 to 106 cycles. The effect of ion implantation is discussed with respect to the mechanism of fretting wear in ball bearings subjected to relatively small-angle oscillatory motion.
Survey of defect-mediated recombination lifetimes in GaAs epilayers grown by different methods
Authors: Yablonovitch E. , Bhat R. , Harbison J. P. , Logan R. A.
Published: July 2002

In GaAs double-heterostructure potential wells at moderate injection levels, surface, radiative, and Auger recombination can all be suppressed. This leaves only Shockley–Read–Hall recombination which is mediated by defects in the crystallographic structure. In a survey of organometallic chemical vapor deposition (OMCVD), liquid phase epitaxy, and molecular beam epitaxy, we find that all the growth methods produce better than expected material quality with recombination lifetimes at room temperature as high as ∼2 μs observed in the best OMCVD material. If this lifetime were to be maintained during device processing, then material quality would not be the limiting factor in the implementation of certain novel device concepts.
Low-frequency dielectric properties of Co-Al2O3 composite films
Authors: Niklasson G. A. , Brantervik K.
Published: July 2002

We have studied the dielectric permittivity in the frequency range 10−4–102 Hz for Co-Al2O3 cermets with cobalt volume fractions below 0.15. The films were produced by simultaneous electron beam evaporation from two sources. The ac results show an anomalous low-frequency dispersion. We interpret the data in the framework of a random walk on a percolation cluster with traps. A trap distribution parameter, as derived from the ac data, is in qualitative agreement with that derived from current-voltage curves.
Reproducible technique for fabrication of thin films of high transition temperature superconductors
Authors: Mankiewich P. M. , Scofield J. H. , Skocpol W. J. , Howard R. E. , Dayem A. H. , Good E.
Published: July 2002

We report on a new process to make films of Y1Ba2Cu3O7 using coevaporation of Y, Cu, and BaF2 on SrTiO3 substrates. The films have high transition temperatures (up to 91 K for a full resistive transition), high critical current densities (106 A/cm2 at 81 K), and a reduced sensitivity to fabrication and environmental conditions. Because of the lower reactivity of the films, we have been able to pattern them in both the pre-annealed and post-annealed states using conventional positive photoresist technology.
Picosecond pulse generation (<1.8 ps) in a quantum well laser by a gain switching method
Authors: Arakawa Y. , Sogawa T. , Nishioka M. , Tanaka M. , Sakaki H.
Published: July 2002

A picosecond pulse (<1.8 ps) at 8570 Å is successfully generated by a gain switching method in an optically pumped GaAs/AlGaAs multiquantum well laser with a cavity length of 155 μm. This is the narrowest pulse width so far achieved in semiconductor lasers without the external cavity. We believe that this short pulse generation results from the enhanced differential gain due to the two-dimensional properties of the carriers in the quantum wells.
Generation of blue cw coherent radiation by sum frequency mixing in KTiOPO4
Authors: Baumert J.-C. , Schellenberg F. M. , Lenth W. , Risk W. P. , Bjorklund G. C.
Published: July 2002

The wavelength, angle, and temperature tolerances of a new room-temperature noncritically phase-matched frequency mixing process involving type II frequency mixing of 1064 and 809 nm radiation in potassium titanyl phosphate (KTiOPO4) are measured. The angular and temperature bandwidths are found to be unusually wide. Device applications for upconversion of diode pumped neodymium:yttrium aluminum garnet laser sources to the blue spectral region are explored.
Gallium arsenide photoluminescence under picosecond-laser-driven shock compression
Authors: Lu X. Z. , Garuthara R. , Lee S. , Alfano R. R.
Published: July 2002

A pump-and-probe technique was used to investigate shock effects on the photoluminescence spectra (∼833 nm) at T=80 K due to the direct transition E0 from the Γ6 conduction band to the Γ8 fourfold degenerate top valence band in GaAs. Under the shock loading condition, the photoluminescence peak was observed to blue shift and split into two components, corresponding to the transitions from the Γ6 conduction band to the valence heavy- and light-hole subbands, because of symmetry breaking by the uniaxial shock compression along the [001] direction. From the blue shift of the photoluminescence peaks, we deduced our picosecond-laser-driven shock pressure of ∼10 kbar.
Zinc-enhanced beryllium redistribution in GaAs/GaAlAs grown by molecular beam epitaxy
Authors: Houston P. A. , Shepherd F. R. , SpringThorpe A. J. , Mandeville P. , Margittai A.
Published: July 2002

A study of the effect of zinc diffusion into beryllium- and silicon-doped GaAs/Ga0.7Al0.3As structures has been carried out. The beryllium dopant was found to diffuse very rapidly as a result of the presence of diffusing zinc, while the silicon remained unaffected. A mechanism is proposed whereby competition for the gallium sites causes the beryllium to move interstitially wherever zinc is present.
Preparation of superconducting Y-Ba-Cu-O films by a reactive plasma evaporation method
Authors: Terashima Kazuo , Eguchi Keisuke , Yoshida Toyonobu , Akashi Kazuo
Published: July 2002

Y-Ba-Cu-O superconducting films were prepared by a reactive plasma evaporation method, in which mixed powders were coevaporated in a thermal radio-frequency Ar+O2 plasma and the ternary composition controlled high-temperature metallic vapors were codeposited onto a substrate. The deposition rate was much more than 10 μm/min, which is several orders of magnitude higher than those reported by the other methods previously. The structure of the prepared films was identified as an orthorhombic oxygen-deficient perovskite phase and some films showed the preferred orientation of (001). The as-deposited film without post-annealing showed a superconducting transition temperature Tcm (midpoint) of 94 K.
γ-Glutamyl carboxylation: An extracellular posttranslational modification that antedates the divergence of molluscs, arthropods, and chordates
Authors: Bandyopadhyay Pradip K. , Garrett James E. , Shetty Reshma P. , Keate Tyler , Walker Craig S. , Olivera Baldomero M.
Published: July 2002

The posttranslational γ-carboxylation of glutamate residues in secreted proteins to γ-carboxyglutamate is carried out by the vitamin K-dependent enzyme γ-glutamyl carboxylase. γ-Carboxylation has long been thought to be a biochemical specialization of vertebrates, essential for blood clotting. Recently, a γ-carboxylase was shown to be expressed in
Drosophila
, although its function remains undefined in this organism. We have characterized both cDNA and genomic clones for the γ-glutamyl carboxylase from the marine mollusc,
Conus
, the only nonvertebrate organism for which γ-carboxyglutamate-containing proteins have been biochemically and physiologically characterized. The predicted amino acid sequence has a high degree of sequence similarity to the
Drosophila
and vertebrate enzymes. Although γ-carboxylases are highly conserved, the
Conus
and mammalian enzymes have divergent substrate specificity. There are striking parallels in the gene organization of
Conus
and human γ-carboxylases. Of the 10
Conus
introns identified, 8 are in precisely the same position as the corresponding introns in the human enzyme. This remarkable conservation of intron/exon boundaries reveals that an intron-rich γ-carboxylase was present early in the evolution of the animal phyla; although specialized adaptations in mammals and molluscs that require this extracellular modification have been identified, the ancestral function(s) and wider biological roles of γ-carboxylation still need to be defined. The data raise the possibility that most introns in the genes of both mammals and molluscs antedate the divergence of these phyla.
Critical contribution of liver natural killer T cells to a murine model of hepatitis
Authors: Takeda Kazuyoshi , Hayakawa Yoshihiro , Van Kaer Luc , Matsuda Hironori , Yagita Hideo , Okumura Ko
Published: July 2002

Natural killer T (NKT) cells constitute a distinct subpopulation of T cells with a unique antigen specificity, prompt effector functions, and an unusual tissue distribution. NKT cells are especially abundant in the liver, but their physiological function in this organ remains unclear. In the present study, we examined the possible contribution of NKT cells to a murine model of hepatitis induced by i.v. injection of Con A. CD1-deficient mice lacking NKT cells were highly resistant to Con A-induced hepatitis. Adoptive transfer of hepatic NKT cells isolated from wild-type mice, but not from FasL-deficientgldmice, sensitized CD1-deficient mice to Con A-induced hepatitis. Furthermore, adoptive transfer of hepatic mononuclear cells from wild-type mice, but not from CD1-deficient mice, sensitizedgldmice to Con A-induced hepatitis. Upon Con A administration, hepatic NKT cells rapidly up-regulated cell surface FasL expression and FasL-mediated cytotoxicity. At the same time, NKT cells underwent apoptosis leading to their rapid disappearance in the liver. These results implicated FasL expression on liver NKT cells in the pathogenesis of Con A-induced hepatitis, suggesting a similar pathogenic role in human liver diseases such as autoimmune hepatitis.
A B-cell receptor-specific selection step governs immature to mature B cell differentiation
Authors: Levine Matthew H. , Haberman Ann M. , Sant'Angelo Derek B. , Hannum Lynn G. , Cancro Michael P. , Janeway Charles A. , Shlomchik Mark J.
Published: July 2002

Seventy percent of peripheral immature conventional (B2) B cells fail to develop into mature B cells. The nature of this cell loss has not been characterized; the process that governs which immature B cells develop into long-lived peripheral B cells could be either stochastic or selective. Here, we demonstrate that this step is in fact selective, in that the fate of an immature B cell is highly dependent on its Ig receptor specificity. A significant skewing of the B cell receptor repertoire occurs by the time cells enter the mature B cell fraction, which indicates that there is selection of only a minority of immature B cells to become mature B cells. Because only a few heavy-light chain pairs are enhanced of the diverse available repertoire, we favor the idea that selection is positive for these few heavy-light chain pairs rather than negative against nearly all others. Because most immature B cells are lost at this transition, this putative positive selection event is likely to be a major force shaping the mature B cell receptor repertoire available for adaptive immune responses.