bluuebunny/crossref_metadata_2025 huggingface dataset:
Propagation of acoustic waves in disordered flows composed of many vortices. I. General aspects
Authors: Boyer Denis , Baffico Maurizio , Lund Fernando
Published: July 2002

We study the coherent propagation of an acoustic wave through a disordered flow with zero mean velocity. The flow is modeled as an assembly of vortices randomly distributed. The source term of the linear wave equation satisfied by the acoustic pressure must be expanded up to terms of order Mach number of the background flow squared. The complex wave number of the coherent wave is calculated analytically and related to average properties of the flow using multiple scattering theory in a Bourret approximation. The perturbations induced by the fluid motion on the index of refraction and on the attenuation length of the wave are of order Mach number squared. The role of the finite compressibility of the fluid is considered in detail, as well as the need, or lack thereof, to consider widely different time scales for acoustic propagation and flow evolution. For a gas and for long wavelengths, the phase velocity of the coherent wave may become higher than in the fluid at rest.
Scaling laws for layer formation in stably-stratified turbulent flows
Authors: Wunsch Scott
Published: July 2002

Experiments have demonstrated that the stirring of a stably-stratified fluid sometimes generates a series of layers of nearly constant density separated by steep density gradients. In this paper, mixing length arguments based on the Kolmogorov picture of turbulence are used to suggest scalings for the layer sizes, gradients, and buoyancy fluxes in the limit of strong turbulence. Experimentally observed layer sizes are consistent with the proposed scaling law.
Shear-rate dependent transport coefficients in a binary mixture of Maxwell molecules
Authors: Marı́n C. , Garzó V. , López de Haro M.
Published: July 2002

Mass and heat transport in a dilute binary mixture of Maxwell molecules under steady shear flow are studied in the limit of small concentration gradients. The analysis is made from the Gross–Krook kinetic model of the Boltzmann equation. This model is solved by means of a perturbation solution around the steady shear flow solution [Phys. Fluids 8, 2756 (1996)], which applies for arbitrary values of the shear rate. In the first order of the expansion the results show that the mass and heat fluxes are proportional to the concentration gradient but, due to the anisotropy of the problem, mutual diffusion and Dufour tensors can be identified, respectively. Both tensors are explicitly determined in terms of the shear rate and the parameters of the mixture (particle masses, concentrations, and force constants). A comparison with the results derived from the exact Boltzmann equation at the level of the diffusion tensor shows a good agreement for a wide range of values of the shear rate.
Cross-channel advective–diffusive transport by a monochromatic traveling wave
Authors: Sobel Adam H. , Flierl Glenn R.
Published: July 2002

The cross-channel tracer flux due to the combined effects of advection and diffusion is considered for two-dimensional incompressible flow in a channel, where the flow is that due to a monochromatic traveling wave and the boundary conditions at the walls are fixed tracer concentration. The tracer flux is computed numerically over a wide range of the parameters ε=U/c and δ=K/cL, with U the maximum fluid velocity, c the wave phase speed, K the tracer diffusivity, and L the channel width. Prior work has used analytical methods to obtain solutions for δ either infinite (stationary overturning cells) or small. In addition to the full numerical solutions, solutions obtained using mean field theory are presented, as well as a new asymptotic solution for small ε, and one for small δ due to Flierl and Dewar. The various approximations are compared with each other and with the numerical solutions, and the domain of validity of each is shown. Mean field theory is fairly accurate compared to the full numerical solutions for small δ, but tends to underpredict the tracer flux by 30–50% for larger δ. The asymptotic solution derived by Flierl and Dewar for small δ is found to break down when δ∼ε−2 rather than when δ∼1 as suggested by the original derivation, and a scaling argument is presented which explains this.
Interaction between Görtler vortices and two-dimensional Tollmien–Schlichting waves
Authors: Mendonça M. T. , Morris P. J. , Pauley L. L.
Published: July 2002

The nonlinear interaction between Görtler vortices and two-dimensional Tollmien–Schlichting (TS) waves is studied with a spatial, nonparallel model based on the parabolized stability equations. The effect of the TS waves on the development of the vortices is accounted for, showing that TS wave amplitudes of the same order of magnitude as the vortices result in significant nonlinear interaction. The range of governing parameters that has been studied so far is extended and the main effects of Görtler number, spanwise wave number, and initial amplitudes are identified. The study shows that the relative growth rates and initial amplitudes are the two most significant parameters.
Viscosity influence on the stability of a swirling jet with nonrotating core
Authors: Sarasúa L. G. , Schifino A. C. Sicardi
Published: July 2002

The effect of the viscosity on the linear stability of a swirling top hat jet with nonrotating core is studied analytically. Numerical values of the growth rate are obtained. It is shown that the viscosity induces instabilities in the region of wave numbers where no inviscid instability occurs. In the short wave limit, the asymptotic solution reveals that the situation is quite different for the strictly inviscid and the viscous cases. While in the inviscid fluid the growth rate is proportional to the wave number, in the viscous case the growth rate approaches a constant value for large wave numbers.
Formation and ‘self-healing’ of magnetic islands in finite-β Helias equilibria
Authors: Hayashi T. , Sato T. , Merkel P. , Nührenberg J. , Schwenn U.
Published: July 2002

The behavior of finite-pressure-induced magnetic islands is numerically analyzed for three-dimensional magnetohydrostatic equilibria of the Helias configuration by using a three- dimensional equilibrium code. It is found that an island chain is generated on the 5/6 rational surface, when such a surface appears in the plasma region of the finite-β equilibrium. The island chain, however, is not so dangerous as to destroy the plasma confinement even if it appears in a vanishingly small shear region. Thus, a high β equilibrium with clear magnetic surfaces can be realized. Moreover, it is definitely confirmed that the finite pressure effect sometimes exhibits an unexpectedly good aspect, namely, that the vacuum islands are removed as β increases, which can be called ‘self-healing’ of islands. This property can be explained by the numerically discovered fact that the phases of islands induced by the finite-pressure effect are always locked in the same phase regardless of β.
Both temperature and relativistic effects on the Langmuir–Child equation
Authors: Qian Bao-Liang , Liu Yong-Gui , Li Chuan-Lu
Published: July 2002

A modified Langmuir–Child equation, including both temperature and relativistic effects, is derived and studied using kinetic theory. The current densities of charged particles are analyzed numerically, and the results clearly show that the temperature and relativistic effects can be easily determined through the parameters δ=T/2mc2 and α=q(Vp−V)/2mc2.
Some properties of rotating coherent structures in a non-neutral plasma column
Authors: Aly J. J.
Published: July 2002

Coherent structures rotating at the angular velocity ω in a two-dimensional pure electron plasma confined inside a conducting grounded cylinder are considered. These structures are described by a streamfunction ψ, obeying a nonlinear boundary value problem. It is shown, in particular, that (i) this problem has a minimal cylindrically symmetric solution if it has a solution at all. (ii) For a nonsymmetric solution, 2ω is necessarily comprised between the minimum and maximum values taken by the local diocotron frequency in the cylinder. (iii) Bifurcation of a symmetric radially decreasing solution to a nonsymmetric one can occur only when 2ω is equal to the mean diocotron frequency.
Improved solutions to the nonlinear one-dimensional Debye screening in plasmas with two temperatures
Authors: Pérez Ricardo J. , Martín Pablo
Published: July 2002

Three new approximants to the solution of the one-dimensional nonlinear Debye screening potential have been obtained. The ions and electrons are in steady-state equilibrium situations with different temperatures. Therefore, the potential satisfies the nonlinear Poisson equation with a Boltzmann factor for both the electronic and ion densities. To obtain the approximants, the original equation has been written in a suitable compact way. The variables have been transformed in such a way that the two-point quasifractional approximation technique can be applied. The simplest approximant is almost as simple as the usual linear one, but it has an additional coefficient and it is more accurate. The other two approximants, more accurate than the simplest one, are exact for the limit of equal temperatures. All the approximants obtained here give better accuracy than the third-order approximation obtained previously by Clemente and Martín [J. Phys. Soc. Jpn. 61, 34 (1992)].
Electrostatic turbulence in the Z pinch corona
Authors: Terry R. E.
Published: July 2002

The microstability of electrostatic waves in the periphery of a Z pinch is formulated and resolved for a new class of Vlasov meta-equilibria admitting self-similar solutions in the electron gyrokinetic limit. These equilibria are subject to strong radial motions, and a mild deviation from charge neutrality arises to maintain ion acceleration close to that of the magnetized electrons. A unique class of profiles in density, axial current, temperature, and drift speed defines these equilibria. They are characterized by (i) the interior pinch current, (ii) the interior number density, (iii) the parallel and perpendicular temperatures, (iv) the exterior axial electric field value at the initial time, and (v) the radial ion acceleration relative to that of the electrons. Unstable ion sound waves arise in this medium by coupling radial and axial free energy to azimuthal longitudinal oscillations. The waves grow only for a limited range of radial or axial E×b̂/B drift speeds and electron temperatures. The growth rate, which can be as large as 0.115ωpi, is found to scale proportional to plasma frequency over the density range from 1012 to 1018 ions cm−3.
Quasilinear stochastic electron energy diffusion driven by an intense cyclotron wave in oblique propagation
Authors: Farina Daniela , Pozzoli Roberto , Romé Massimiliano
Published: July 2002

The dynamics of electrons in a magnetically confined plasma under the action of an intense monochromatic wave is analyzed by means of a relativistic Hamiltonian treatment. Oblique wave propagation with respect to the magnetic field is considered. It is found that in the case of global stochasticity the particle acceleration has a diffusive nature. It is shown that in a proper space the energy diffusion equation is one dimensional. The relevant diffusion coefficient is given by a well defined local quasilinear expression. The theoretical model is validated up to high values of the perturbation parameter by comparison with long time numerical simulations of the electron motion.
Effects of nonlinear electron dynamics in a fluid model of collisionless trapped-electron mode turbulence
Authors: Craddock G. G. , Koniges A. E. , Crotinger J. A. , Diamond P. H. , Newman D. E. , Terry P. W.
Published: July 2002

The role of the electron nonlinearity on saturation levels and particle transport in collisionless trapped electron mode turbulence is examined numerically using a two-dimensional fluid model. It is found that the removal of the electron nonlinearity results in an order of magnitude increase in fluctuation and particle transport levels. In addition, the qualitative behavior of the saturated state changes distinctly, including a decrease in RMS wave number in both cross-field directions, deviations from isotropy, changes in transport scalings, and changes in the spectral flow of energy. These results indicate that the electron nonlinearity is responsible for an efficient transfer of internal energy to small dissipation scales, thus resulting in lower fluctuation levels than predicted by ion mode coupling alone. The electron nonlinearity also decreases the phase angle between the density and potential leading to a decrease in the driving source. These results underscore the importance of the cross-correlation on saturation and demonstrate that numerical or theoretical use of ‘‘iδ’’ models to describe weakly collisional or collisionless trapped-electron mode turbulence is not appropriate.
Imaging of laser-irradiated targets at a wavelength of 33.8 Å using a normal-incidence multilayer mirror
Authors: Seely J. F. , Brown C. M. , Holland G. E. , Lee R. W. , Moreno J. C. , MacGowan B. J. , Back C. A. , Da Silva L. B. , Wan A. S.
Published: July 2002

The morphology of the 33.8 Å emission from laser-irradiated targets was studied using a concave mirror with a W/B4C multilayer coating. The mirror had peak normal-incidence reflectance of 1.8% at a wavelength of 33.8 Å. The emissions from a variety of targets were imaged on film with a spatial resolution of 30 μm in the target plane. Radiatively heated, low-density plastic and silica foams, x-ray laser targets, and a gas-filled enclosure were imaged. Several targets were simultaneously imaged at wavelengths of 33.8 and 130 Å using two normal-incidence microscopes.
Lorentz ratio of quantum plasmas
Authors: Khalfaoui Abdelhamid , Bennaceur Djamila
Published: July 2002

A quantum collective approach is developed to investigate linear transport properties of a system of highly degenerate weakly coupled electrons and strongly coupled semiclassical ions. The basic formalism rests upon suitable extention of the Boltzmann–Bloch quantum transport equation. The model considers electron–ion (e–i) and electron–electron (e–e) collisions in a unified scheme of both long- and short-range Coulomb interactions. The e–e collisions contribute to the thermal conductivity calculation in the low coupling regime. Even though they can be insignificant for strongly coupled systems, the extensively used Lorentz gas approximation cannot be justified for plasmas of astrophysical interests. It is shown that the Lorentz ratio of high-density plasma may exhibit substantial negative deviation from the ideal Sommerfeld value, due to some nonidealities, such as e–e interaction and quantum effects. Results are presented under analytical and compact forms allowing numerical applications, as well as comparisons with existing theories.
On the launching loss and the Weibel instability of the gyrotron traveling wave tube
Authors: Kou C. S.
Published: July 2002

A linear theory and a self-consistent theory are used to study the launching loss and the Weibel instability of the gyrotron traveling wave tube (gyro-TWT). Numerical results and a simplified analytical model both show that the launching loss is inversely proportional to the growth rate of the wave, and proportional to the difference between the propagation constant of the forward constant amplitude wave and that of the forward growing wave. In addition, the prediction of the Weibel instability in a fast wave interaction structure by the dispersion relation is verified to be valid even though the assumption of close resonance condition of the linear theory breaks down in this region. Nevertheless, this instability is overwhelmed by the launching loss.
The High-Yield Lithium-Injection Fusion-Energy (HYLIFE)-II inertial fusion energy (IFE) power plant concept and implications for IFE
Authors: Moir Ralph W.
Published: July 2002

In the High-Yield Lithium-Injection Fusion-Energy (HYLIFE) power plant design, lithium is replaced by molten salt. HYLIFE-II [Fusion Technol. 25, 5 (1994)] is based on nonflammable, renewable-liquid-wall fusion target chambers formed with Li2BeF4 molten-salt jets, a heavy-ion driver, and single-sided illumination of indirect-drive targets. Building fusion chambers from existing materials with life-of-plant structural walls behind the liquid walls, while still meeting non-nuclear grade construction and low-level waste requirements, has profound implications for inertial fusion energy (IFE) development. Fluid-flow work and computational fluid dynamics predict chamber clearing adequate for 6 Hz pulse rates. Predicted electricity cost is reduced about 30% to 4.4¢/kWh at 1 GWe and 3.2¢/kWh at 2 GWe. Development can be foreshortened and cost reduced by obviating expensive neutron sources to develop first-wall materials. The driver and chamber can be upgraded in stages, avoiding separate and sequential facilities. Important features of a practical IFE power plant are ignition and sufficient gain in targets; low-cost, efficient, rep-ratable driver; and low-cost targets.
Hot-ion Bernstein wave with finite k∥
Authors: Ignat D. W. , Ono M.
Published: July 2002

The complex roots of the hot plasma dispersion relation in the ion cyclotron range of frequencies have been surveyed. Consistent with current understanding, the cold plasma fast wave appears at low values of perpendicular wave number k⊥, followed by the well-known Bernstein wave at higher k⊥. At still higher k⊥ there can be two previously unappreciated hot plasma waves with relatively little sensitivity to frequency, in contrast to the Bernstein wave which is characterized by large changes in k⊥ for small changes in frequency or magnetic field. The latter waves exist only for relatively large k∥, the wave number parallel to the magnetic field. Both waves are strongly absorbed if the electron temperature is near the ion temperature, but not in a hot-ion, cold-electron, plasma.
Intermittency in tokamak edge turbulence: An artifact of kurtosis analysis
Authors: Sethia Gautam C. , Chenna Reddy D.
Published: July 2002

Kurtosis or flatness factor of an experimental time series is considered to be one of the important computed parameters in turbulence studies. The analysis itself is of a simple nature but like any other statistical analysis it demands extreme caution in the absence of which one is prone to draw false conclusions. An earlier kurtosis based statistical analysis [Phys. Rev. Lett. 69, 1375 (1992)] on the reported first measurement of intermittency in tokamak edge turbulence using density and potential fluctuation data has been found to be questionable on a number of counts (‘‘Comment’’ submitted to Phys. Rev. Lett.). This has necessitated a detailed reanalysis of the same data which has now been carried out and the results are presented in this paper. It is found that most of the density datasets and all the potential datasets exhibit a significant peak in their autospectra at about 4–6 kHz and all other datasets with significant non-Gaussianity are found to be either having plasma current associated high amplitude transients or are found to be nonstationary due to a trend in the variance. The datasets are close to Gaussian with kurtosis values approaching zero if these factors are properly taken care of.
Analytical and numerical studies of ion mobility near the tokamak plasma edge
Authors: Xiao H. , Hazeltine R. D. , Valanju P. M.
Published: July 2002

The effects of radial electric field on charged particle motion and transport in the toroidal magnetic system have been studied both analytically and numerically. The effects of radial electric field on particle orbits are examined, allowing for the relatively large and strongly sheared field observed in some experiments. It is found that ion radial mobility due to the combined effects of radial electric field and charge exchange collisions can dramatically affect the ion transport and orbit loss near the tokamak edge. These properties may help understand the formation of transport barrier near the tokamak plasma edge during high confinement mode (H-mode) discharge and explain the asymmetry between bias voltage and confinement in biased-electrode-induced H-mode.
Shear Alfvén waves with Landau and collisional effects
Authors: Hedrick C. L. , Leboeuf J.-N. , Spong D. A.
Published: July 2002

Shear Alfvén waves can be driven unstable by hot particles such as alpha particles in an ignited fusion device or hot ions in existing devices. Motivated by rather collisional Wendelstein 7 Advanced Stellarator (W7-AS) [Phys. Rev. Lett. 72, 1220 (1994)] beam-driven global Alfvén instability experiments, the effect of electron and ion collisions on these modes has been examined. Collisions broaden and suppress the peak associated with Landau effects. This broadening makes ion damping more important, while the electron damping is suppressed. Additional resistive effects provide increased damping for the main part of the spectrum, which can have a rather high phase velocity. Of more general interest is the fact that collisional and collisionless resistivity has a numerically stabilizing effect that is known to be important for nonlinear resistive magnetohydrodynamics (MHD). This can preclude the need for introducing and testing the sensitivity to similar ad hoc effects. Numerical and analytic results for both a particle-conserving Krook collision operator and a Lorentz (pitch angle) collision operator are compared and contrasted.
Study of efficiency enhancement through magnetic field profiling in cyclotron autoresonance maser
Authors: Chen S. H. , Dawn T. Y.
Published: July 2002

Phase control and the increase of the available free energy are two basic mechanisms that enhance the efficiency of a cyclotron autoresonance maser (CARM) device through linearly tapering the externally applied magnetic field. Based on these two mechanisms, a nonlinearly profiled magnetic field, as formed by adding a positively three-quarter sine profile onto a uniform magnetic field, applied on CARM is proposed in this study. Numerical results show that applications of the nonlinearly profiled magnetic field can raise the efficiency of CARM to around 50% over a wide range of frequency detunings. The reduction of efficiency sensitivity to the beam velocity spread can also be achieved for the increase of the available free energy with the use of the nonlinearly profiled magnetic field. Furthermore, the nonlinearly profiled magnetic field is also far more effective and practical than the linearly tapered magnetic field in efficiency enhancement.
Coordinate system for use around an X point
Authors: Mattor Nathan
Published: July 2002

Standard flux coordinate systems are singular around an X point, where the poloidal flux function has a saddle. In this paper a coordinate system that is well behaved in this region is presented. The system resembles Cartesian coordinates, with one axis along each separatrix branch, and is nearly orthogonal in a neighborhood around the separatrix.
Thermonuclear microexplosion ignition by imploding a disk of relativistic electrons
Authors: Winterberg F.
Published: July 2002

A new ignition concept for thermonuclear reactions is described, in which an electron cloud produced by inductive charge injection and reaching the Brillouin limit, is magnetically compressed inside a long cylindrical solenoid. For sufficiently fast compression, the front of the cloud becomes a relativistically contracted annular disk of high-energy density, which upon impact on a grounded target leads to a cylindrical implosion easily exceeding the power fluxes required for the ignition of thermonuclear microexplosions. Unlike concepts proposed in the past to ignite thermonuclear microexplosions by relativistic electron beams, the energy delivered to the target is not in the form of kinetic particle energy but in the form of an intense electromagnetic pulse.
Modification of emission from the X-mode at the third electron cyclotron harmonic from mode conversion and reflection
Authors: Swanson D. G. , Ng C. S.
Published: July 2002

Both absorption and emission of the X-mode in the vicinity of the third electron cyclotron harmonic (ω∼3ωce) in a weakly inhomogeneous magnetic field are analyzed. The mode conversion problem is solved both without absorption and with synchrotron absorption and emission. The transmission is shown to be independent of absorption, and the emission is related to absorption exclusively through the reflection coefficient, which is calculated. The numerical values for the reflection coefficient are obtained from a sum that is exact, and quickly gives high accuracy values within its radius of convergence. The effects of reflection are shown to be generally neglectible for most laboratory plasmas, but potentially important in some space plasmas.
Quasilinear analysis of absorption of ion Bernstein waves by electrons
Authors: Cardinali A. , Paoletti F. , Bernabei S. , Ono M.
Published: July 2002

The effects induced on plasma electrons by an externally launched ion Bernstein wave (IBW), in the presence of a lower hybrid wave (LHW) in the current drive regime, are studied by analytical integration of the IBW ray-tracing equations, along with the amplitude transport equation (Poynting theorem). The electric field amplitude parallel and perpendicular to the external magnetic field, the quasilinear diffusion coefficient, and the modified electron distribution function are analytically calculated in the case of IBW. The analytical calculation is compared to the numerical solution obtained by using a two-dimensional (2-D) Fokker–Planck code for the distribution function, without any approximation for the collision operator. The synergy between the IBW and LHW can be accounted for, and the absorption of the IBW power when the electron distribution function presents a tail generated by the LHW in the current drive regime can be calculated.
Smoothing of nonuniformity in a planar ablative flow
Authors: Murakami M.
Published: July 2002

Smoothing of perturbations in a steady-state ablative flow is studied with an analytical model, in which obviously invalid boundary conditions commonly used in previous analytical models are corrected. It turns out that, through the ablation layer, the momentum flux (hence pressure) perturbation smoothes down even more appreciably than ones in the previous models. On the contrary, the mass flux perturbation decays much less effectively than the momentum flux perturbation, and its dependency on the transverse wave number, k, can be roughly reproduced with the simple cloudy day model ∝exp(−kD) for kD≤3 (D is the ablation layer thickness). It is shown that the smoothing effect is due to the interplay between the lateral fluid flow and heat conduction.
Plasma maser with cyclotron resonance
Authors: Nambu M. , Sakai J. I.
Published: July 2002

The plasma-maser process, including whistler mode turbulence and a test Langmuir wave, is considered for weakly magnetized plasma ωpe>Ωe, where ωpe and Ωe are the electron plasma and cyclotron frequency, respectively. The cyclotron resonant interaction between electrons and whistler mode is effective for the growth of the Langmuir wave through the plasma-maser process. The unstable test Langmuir wave and the whistler mode propagate in opposite directions. The direct coupling contribution exactly cancels out with the reverse absorpting effect due to the quasi-linear effect. The polarization coupling contribution gives the plasma-maser effect. The theory can explain a recent numerical simulation.
Experiments on ion wake fields
Authors: Kim Hyun-Soo , Yi Seungjun , Amin Ali , Carpenter Raymon T. , Lonngren Karl E.
Published: July 2002

Ion wake fields are excited and investigated in quasineutral two component plasmas in a standard double plasma machine. The period of the oscillating wake field decreases as the plasma density increases. The wake field amplitude depended upon the resonance between the perturbation duration and the wake field period rather than the perturbation amplitude. The experimental results are in agreement with the theoretical model that we have recently proposed.
Investigation of the first wall reflectivity and related synchrotron radiation loss and current drive in tokamaks
Authors: Fidone I. , Granata G.
Published: July 2002

The first wall reflectivity and the correlated synchrotron radiation loss and current drive in high central electron temperature tokamak plasmas are investigated. It is shown that the reflection coefficient can be determined from the frequency of maximum emission. In particular, the differential reflectivity with respect to the magnetic field correlated with the toroidal current generated by a fish-scale structure is proportional to the relative shift of frequencies. It is pointed out that experimental investigation of the first wall reflectivity suited for synchrotron radiation current drive can be performed in existing devices at moderate electron temperatures.
Kinetic toroidal Alfvén eigenmodes in finite-β tokamak plasmas
Authors: Zheng L. -J. , Chen Liu
Published: July 2002

Kinetic toroidal Alfvén eigenmodes (KTAEs) in finite-β circular tokamak plasmas are investigated. Here, β is the ratio between plasma and magnetic pressures, and, formally, β∼O(r/Rq2), with q being the safety factor, r and R denoting, respectively, minor and major radii. A new effect associated with finite parallel electric field effect due to the ion magnetic drift (IMD) is discovered, which is of the same order as that due to the well-known effect associated with the finite ion Larmor radii. The IMD-induced parallel electric field effect is shown to contribute to potential wells for the eigenmodes. Therefore, the IMD-induced parallel electric field effect can discretize the lower Alfvén continuum along the real eigenfrequency. Subsequently, this new branch of KTAEs could be readily destabilized by the energetic ions.
Formation of core transport barriers by parallel flow
Authors: Sen S.
Published: July 2002

The well-known instability associated with the drift-ballooning modes is shown to be stabilized in the presence of a radially varying parallel flow profile. This is contrary to the usual belief that the parallel flow shear is destabilizing for the drift-like microinstabilities. The scale length of the flow profile required for this stabilization is rather modest and is usually observed in the toroidal flow profile measured during various improvement modes in the tokamak core. This shows that purely parallel flow can be used to create transport barriers to reduce the loss of particle and energy from the plasma. The improved mode formed by the parallel flow will, unlike the reverse shear mode, be nontransient in nature.
Sudden transition to chaos in plasma wave interactions
Authors: López-Rebollal O. , Sanmartı́n J. R. , del Rı́o E.
Published: July 2002

The coherent three-wave interaction, with linear growth in the higher frequency wave and damping in the two other waves, is reconsidered; for equal dampings, the resulting three-dimensional (3-D) flow of a relative phase and just two amplitudes behaved chaotically, no matter how small the growth of the unstable wave. The general case of different dampings is studied here to test whether, and how, that hard scenario for chaos is preserved in passing from 3-D to four-dimensional flows. It is found that the wave with higher damping is partially slaved to the other damped wave; this retains a feature of the original problem (an invariant surface that meets an unstable fixed point, at zero growth rate) that gave rise to the chaotic attractor and determined its structure, and suggests that the sudden transition to chaos should appear in more complex wave interactions.
Hot plasma effects on the polarization of electron cyclotron waves
Authors: Dumont R. , Giruzzi G.
Published: July 2002

The evolution of the wave polarization during the propagation of electron cyclotron waves in hot plasmas is investigated. The coupled mode equations are solved in slab geometry, including the relativistic corrections to the dielectric tensor. This formalism is applied to purely ordinary or extraordinary modes, propagating obliquely with respect to the magnetic field in tokamak plasmas, in order to determine the rate of change of the mode purity due to finite temperature effects. It is found that such an alteration of the mode purity can be one to three orders of magnitude larger than in a cold plasma. However, the overall effect remains tolerable for most applications and typical parameters of magnetically confined plasmas.
Ballooning modes on open magnetic field lines
Authors: Hameiri Eliezer
Published: July 2002

The ballooning instability on open magnetic field lines is given a thorough mathematical analysis. It is shown that resistive bounding ends (endplates) induce the same stability properties as insulating ends. When unstable, the maximal growth rate increases monotonically with boundary resistivity. An interchange instability may be present, and one necessary condition for its stability is that ∫dl/B be constant on pressure surfaces. (This is an equilibrium existence condition for systems with closed magnetic field lines.) Another necessary condition for interchange stability has the same form as in the closed line case. Precise necessary and sufficient stability criteria are given for various types of bounding ends, including insulating, resistive, and perfectly conducting.
On the realization of the current-driven dust ion-acoustic instability
Authors: Ostrikov K. N. , Yu M. Y. , Vladimirov S. V. , Ishihara O.
Published: July 2002

The current-driven dust ion-acoustic instability in a collisional dusty plasma is studied. The effects of dust-charge variation, electron and ion capture by the dust grains, as well as various dissipative mechanisms leading to the changes of the particles momenta, are taken into account. It is shown that the threshold for the excitation of the dust ion-acoustic waves can be high because of the large dissipation rate induced by the dusts.
Influence of particle trapping on the penetration of high-frequency electromagnetic field into a semibounded plasma
Authors: Rouhani M. R. , Tsintsadze N. L. , Tskhakaya D. D.
Published: July 2002

Penetration of a high-frequency (hf) electromagnetic field into a semibounded plasma in the presence of trapped particles is considered. It is shown that the trapping of particles by the hf field considerably changes the condition for its propagation. In the stationary case, with increasing the hf field amplitude at the boundary, the frequency above which the plasma is transparent continually decreases. But the trapping of particles was found to give a lower limit for this critical frequency, i.e., this lower limit goes to zero when there is no trapping continually as the incident field amplitude increases, meanwhile the length of penetration increases and approaches a finite limit, in contrast to the untrapped case, where it increases monotonically. Moreover, when the hf field amplitude at the boundary exceeds some critical value, the plasma will become transparent and the localized structure of the hf field will change to an oscillatory structure.
Modeling of nonlocal slow-electron kinetics in a low-pressure negative-glow plasma
Authors: Arslanbekov Robert R. , Kudryavtsev Anatoly A.
Published: July 2002

The kinetics of slow electrons is studied in a low-pressure negative-glow plasma (NGP). A method based on the nonlocal approach is proposed, which allows the nonlocal (nonequilibrium) nature of slow electrons to be accounted for in a physically transparent and numerically efficient manner. The slow electrons are divided into trapped (cold, Maxwellian) and free (superthermal, non-Maxwellian). It is shown that the superthermal (free) electrons are particularly important because they carry current and supply energy to the system of cold (trapped) electrons. A nonlocal energy-balance equation for the trapped electrons is derived, in which heating by superthermal electrons and diffusion cooling are found to be among the most important mechanisms. Simple expressions for the diffusion-cooling rate and the wall potential are determined. The proposed method is validated by the numerical solution of the full kinetic equation for a NGP in Ar. The energy and space distributions of electron fluxes are analyzed, and flux reversal (in energy space) is observed and explained. A comparison to experiment is carried out and close agreement is obtained. The proposed method can be useful in building fully kinetic, self-consistent models of various NGP-based discharge devices.
Tokamak plasmas with dominant electron cyclotron heating; evidence for electron thermal transport barriers
Authors: de Baar M. R. , Beurskens M. N. A. , Hogeweij G. M. D. , Lopes Cardozo N. J.
Published: July 2002

In the Rijnhuizen Tokamak Project [RTP, A. J. H. Donné, the RTP Team, Plasma Phys. Rep. 20, 192 (1994)], experiments have been carried out with Electron Cyclotron Heating (ECH) at four to seven times the Ohmic power, with a deposition width <10% of the minor radius. Central deposition leads to sharp electron temperature (Te) gradients near the q=1 surface. Off-axis deposition results in steady-state hollow Te profiles. A radial scan of the ECH deposition reveals a discrete, stepwise response of Te profile. The transitions occur for a displacement of the deposition by less than 1 mm. Each transition is due to the loss of a transport barrier. The transitions occur when the minimum value of q passes through the values 1, 4/3, 5/2, 2, 7/2 and 3. A numerical model featuring transport barriers near those q values gives good reproduction of the experimental data. The possible role of the topological properties of the magnetic field is discussed.
Spectroscopic measurements of target preheating on OMEGA
Authors: Elton R. C. , Griem H. R. , Iglesias E. J.
Published: July 2002

The preheating of laser-heated microballoon targets has been measured by time-resolved x-ray and extreme ultraviolet (euv) spectroscopy on the 30 kJ, 60-beam (maxima), 351 nm, laser-fusion system at the University of Rochester Laboratory for Laser Energetics. Thin coatings of aluminum overcoated with magnesium served as indicators. Both the sequence of the x-ray line emission and the intensity of euv radiation were used to determine a preheating peaking at ∼10 nsec prior to onset of the main laser pulse, with a power density ≲1% of the main pulse. The measurements are supported by numerical modeling. Further information is provided by absorption spectra from the aluminum coating, backlighted by continuum from the heated surface. The exact source of the preheating energy remains unknown at present, but most likely arrives from early laser leakage through the system. The present target diagnostic is particularly useful when all beams cannot be monitored directly at all laser wavelengths.
Liquid metal flow encasing a magnetic cavity
Authors: Hassam A. B. , Drake J. F. , Goel Deepak , Lathrop D. P.
Published: July 2002

A stationary equilibrium of a liquid metal flowing past a cylindrical magnetic cavity is presented. The cavity has an azimuthal magnetic field and can also have an axial field. The liquid metal flow can be maintained by a sufficiently high pressure head. The scheme could be used to support a flowing liquid wall for systems producing high heat fluxes.
Observations on intensity oscillations in reflection high-energy electron diffraction during chemical beam epitaxy
Authors: Tsang W. T. , Chiu T. H. , Cunningham J. E. , Robertson A.
Published: July 2002

We report the observation of reflection high-energy diffraction (RHEED) intensity oscillations during the growth of GaAs using triethylgallium in chemical beam epitaxy (CBE). The oscillation period corresponds exactly to the time required for the growth of one monolayer. RHEED oscillation studies also suggest the absence of flux transients due to switching of gas flows, abrupt and complete initiation and termination of growth with submonolayer resolution, and that CBE is capable of thickness control with submonolayer precision when coupled with the use of in situ RHEED intensity monitoring technique. The temperature and flux dependence of growth rates are also studied using RHEED oscillations. Results indicate that CBE growth is predominantly via a two-dimensional layer-by-layer mechanism.
New metastable defects in GaAs
Authors: Buchwald W. R. , Johnson N. M. , Trombetta L. P.
Published: July 2002

Two previously unreported metastable defects in n-type GaAs epitaxial layers grown by metalorganic chemical vapor deposition have been identified by deep level transient spectroscopy. These levels, here labeled M3 and M4, have activation energies for thermal emission of electrons of 0.31 and 0.61 eV, respectively. Formation of the M4 center and annealing of the M3 center are observed to take place when a reverse bias is applied to Schottky diodes at elevated temperatures. The reverse transition is observed under zero bias at elevated temperatures (e.g., 400 K). An additional metastable defect, labeled M2, with transformation properties similar to those of the M4 center and an emission activation energy of 0.64 eV, was observed in samples which received a rapid thermal anneal at temperatures of 900 °C and above.
Characterization of the pulsed laser evaporation process: Selenium thin-film formation
Authors: Hansen S. G. , Robitaille T. E.
Published: July 2002

Selenium (Se) films were prepared by pulsed laser evaporation of the elemental powder in vacuum. Film growth rates as a function of average laser power (0.1–5 W/cm2) and wavelength (266–1064 nm) were measured. The angular distribution of evaporated material was found to be unusually narrow for wavelengths shorter than the Se absorption edge (∼600 nm). The photon efficiency for evaporation ranged from 0.4 to 0.1 atoms/photon for 266 to 1064 nm light, respectively.
Laser induced fluorescence detection of CF and CF2 radicals in a CF4/O2 plasma
Authors: Booth J. P. , Hancock G. , Perry N. D.
Published: July 2002

Laser induced fluorescence has been used to detect ground-state CF and CF2 radicals in a CF4/O2 plasma etching reactor. Measurements are reported of the spatial variation of CF2 concentrations, of the CF radical rotational state distributions, and of the variation of both species with O2 content of the plasma.
Electrolyte electroreflectance of HgCdTe at low temperatures
Authors: Berlouis L. E. A. , Peter L. M. , Astles M. G. , Humphreys R. G.
Published: July 2002

Electrolyte electroreflectance measurements in ethanol/lithium perchlorate solutions over the temperature range 300–156 K have been used to derive the temperature dependence of the E1 transition of Hg1−xCdxTe (x=0.23–0.36). The temperature coefficient dE1/dT derived by third derivative line-shape fitting (6.6±0.5×10−4 eV K−1) is considerably higher than values reported previously in the literature.
Defect structures at the GaAs/Si interface after annealing
Authors: Tsai H. L. , Lee J. W.
Published: July 2002

High-resolution electron microscopy was applied to investigate the effect of post-annealing on the defect structure at the GaAs/Si interface. This study indicates that annealing results in dislocation rearrangement at the interface to form the majority of Lomer’s dislocations with their Burgers vectors parallel to the interface. Dislocations with inclined Burgers vectors (type 2) at the interface after annealing are often observed at steps introduced by the substrate surface roughness. This observation is discussed in terms of the shrinking of stacking faults and microtwins and the preferential nucleation of both stacking faults (or microtwins) and type 2 dislocations at surface steps.
Picosecond relaxation of hot-carrier distributions in GaAs/GaAsP strained-layer superlattices
Authors: Edelstein D. C. , Tang C. L. , Nozik A. J.
Published: July 2002

We have studied the energy relaxation of hot carriers in GaAs/GaAs0.5P0.5 strained-layer superlattices following picosecond photoexcitation. A significant enhancement of highly excited carrier distribution lifetimes over those of bulk GaAs was observed at cryogenic as well as room temperatures. These results support the explanation of photocurrent results reported for solar photoelectrodes made with these structures. Comparison results were also obtained for a GaAs/AlGaAs multiple quantum well structure which showed even longer hot-carrier lifetimes.
Novel method to determine capture cross-section activation energies by deep-level transient spectroscopy techniques
Authors: Criado J. , Gomez A. , Calleja E. , Muñoz E.
Published: July 2002

Deep-center characterization by deep-level transient spectroscopy (DLTS) allows a direct determination of the trap thermal emission activation energy. However, capture barrier energy measurements, based on trap partial filling by pulses of increasing width, require a quite different experimental processing and pose some hardware difficulties. In this letter we present a new method to determine the trap capture barrier energy, one that requires constant-width filling pulses and obtains capture information from standard DLTS data. This technique has been applied to Te-, Sn-, and Si-related DX centers in AlGaAs alloys.
Laser-induced degradation of GaAs photoluminescence
Authors: Raja M. Y. A. , Brueck S. R. J. , Osinski M. , McInerney J.
Published: July 2002

We report experimental studies of laser-induced degradation of surface photoluminescence efficiency in n-GaAs, p-GaAs, semi-insulating Cr-doped, and undoped liquid-encapsulated Czochralski-grown GaAs for excitation intensities ranging from ∼0.1 to ∼20 kW/cm2. The data suggest a contribution from optically induced defects in the bulk material. The time dependence of photoluminescence efficiency was fitted to a simple power-law expression. A previously unreported fast decay of photoluminescence, occurring in 1–2 s immediately following the cleaving of a fresh surface, was observed in p- and n-type samples.
Conformational landscape of cytochrome
c
folding studied by microsecond-resolved small-angle x-ray scattering
Authors: Akiyama Shuji , Takahashi Satoshi , Kimura Tetsunari , Ishimori Koichiro , Morishima Isao , Nishikawa Yukihiro , Fujisawa Tetsuro
Published: July 2002

To investigate protein folding dynamics in terms of compactness, we developed a continuous-flow mixing device to make small-angle x-ray scattering measurements with the time resolution of 160 μs and characterized the radius of gyration (
R
g
) of two folding intermediates of cytochrome
c
(cyt
c
). The early intermediate possesses ≈20 Å of
R
g
, which is smaller by ≈4 Å than that of the acid-unfolded state. The
R
g
of the later intermediate is ≈18 Å, which is close to that of the molten globule state. Considering the α-helix content (
f
H
) of the intermediates, we clarified the folding pathway of cyt
c
on the conformational landscape defined by
R
g
and
f
H
. Cyt
c
folding proceeds with a collapse around a specific region of the protein followed by a cooperative acquisition of secondary structures and compactness.
Density per particle as a descriptor of Coulombic systems
Authors: Ayers Paul W.
Published: July 2002

It is shown that for finite Coulombic systems the density per particle, σ ≡ ρ/N, determines the value of any observable quantity. The associated variational principle is derived.