bluuebunny/crossref_metadata_2025 huggingface dataset:
Ampère and Hall nonlinearities in the magnetic dynamo in the Arnol’d–Beltrami–Childress (ABC) flow
Authors: Galanti B. , Kleeorin N. , Rogachevskii I.
Published: July 2002

The effect of different nonlinearities (Ampère force and Hall effect) on the saturation of a magnetic field generated by flows of conducting fluid is studied by means of numerical simulations. A three-fluid (i.e., ions, electrons, and neutral particles) model is considered. The velocity field of the neutral particles is a prescribed, deterministic, incompressible three-dimensional field in the form of the Arnol’d–Beltrami–Childress (ABC) flow. The dynamics of the charged components of fluid is determined by two-fluid magnetohydrodynamics when ion–neutral particle collisions are taken into account. Four typical regimes of the nonlinear evolution of the magnetic field, corresponding to different types of nonlinearities (Ampère force or Hall effect) and different types of collisions (ion–ion collisions or ion–neutral particle collisions) are found. The transitions between these regimes, the structure of the saturated magnetic field, and the evolution of the magnetic field in these regimes are studied. Scaling estimates of the level of the saturated magnetic field and conditions obtained for the different regimes of the magnetic field evolution are in agreement with the results of the numerical simulations.
Nonlinear evolution of the transverse electron velocity in a helical wiggler with a positive axial guide magnetic field at magnetoresonance
Authors: Yu Junsheng
Published: July 2002

Coupled nonlinear evolution equations are derived for the transverse velocity in a helical wiggler with positive axial guide magnetic field at magnetoresonance. The nonlinear oscillator representation is adopted, without having to impose the usual restriction of the slow variation on the amplitude and the phase. Numerical calculations show that the evolution equations are quite appropriate to describe the envelope of the transverse velocity amplitude. The maximum transverse velocity and the maximum axial velocity spread can be estimated straightforwardly from the evolution equations.
Measurement of the parallel velocity distribution function of the electron beam in a quasi-optical gyrotron by Electron Cyclotron Emission
Authors: Soumagne G. , Alberti S. , Hogge J. P. , Pedrozzi M. , Siegrist M. R. , Tran M. Q. , Tran T. M.
Published: July 2002

The parallel velocity distribution function of the weakly relativistic electron beam of a quasi-optical gyrotron has been determined by measuring the Doppler-shifted Electron Cyclotron Emission (ECE) at an angle θ=15° with respect to the external magnetic dc-field. Due to the Doppler shift, the frequency of the spontaneous cyclotron emission at the fundamental (ν0=100 GHz) is upshifted to 140 GHz. A broadening of the spectrum up to 10 GHz [full width at half maximium (FWHM)] was measured. The measured mean frequency agrees well with the theoretical predictions, but the observed line-width, and hence the parallel velocity distribution function, is 2–3 times larger than expected. Considerations on ECE-measurements of the electron beam energy spread, performed at larger angles θ, are also discussed.
A possible hybrid dissipative trapped electron ion temperature gradient mode
Authors: Bai L. , Qiu X. M. , Huang L. , Song X. M.
Published: July 2002

The effects of dissipative trapped electrons on ion temperature gradient-driven instability (ηi mode) in tokamak plasmas are considered. A sheared slab geometry is adopted and a linearized fluid model of ion temperature gradient (ITG) mode including dissipative trapped electrons, which are described by means of the well-known formula of the nonadiabatic electron response [P. L. Similon and P. H. Diamond, Phys. Fluids 27, 916 (1984)], is presented. Results show that in tokamak plasmas not only is there a modification of the dissipative trapped electrons on the ηi mode, but also there may exist an intrinsic oscillation mode, namely, a hybrid dissipative trapped electron ion temperature gradient mode. The higher the dissipative trapped electron fraction, the more it drives the ηi mode, that is, when the trapped electron fraction is sufficient high and the trapped electrons are dissipated strongly, the mode is dominated by the trapped electron dynamics and propagates in the electron diamagnetic direction. These analytical results can be reduced to the usual predictions of the ion temperature gradient-driven instability in the absence of the dissipative trapped electron. Numerical results further show that (a) there may be a hybrid dissipative trapped electron ITG mode and the dissipative trapped electron effect is a destabilizing effect on this mode; and (b) when the trapped electron fraction is sufficiently high and the trapped electrons are dissipated strongly, the mode is determined by the trapped electron dynamics. These conclusions are in agreement with the experimental observations in the latest simulated tokamak experiment on the Columbia Linear Machine [J. Chen and A. K. Sen, Phys. Rev. Lett. 72, 3997 (1994)].
Evidence of low-frequency oscillations in heavy ion plasmas heated by electron cyclotron resonance
Authors: Lamoureux M. , Girard A. , Pras R. , Charles P. , Khodja H. , Bourg F. , Briand J. P. , Melin G.
Published: July 2002

Time-resolved experiments (bremsstrahlung, diamagnetism, electron, and ion end-loss currents) have been carried out at the Electron Cyclotron Resonance Ion Source Quadrumafios [A. Girard, et al., Rev. Sci. Instrum. 65, 1714 (1994)] continuously operated with heavy neutral gases. All the diagnostics reveal oscillations (on the time scale of about 1 s) which could be characterized by variations of the hot electron perpendicular temperature. The onset of this low-frequency periodic regime with increasing heating powers and/or decreasing gas injection pressures is shown to limit the highly charged ion currents available. The addition of a lighter gas, e.g., He into Kr, delays the appearance of the periodic regime and enables one to extract higher currents.
Filamentation instability of upper hybrid wave in magnetoplasmas
Authors: Kuo S. P. , Huang Joe
Published: July 2002

A four-wave parametric coupling leading to the filamentation of a large amplitude upper hybrid wave and the generation of magnetic field-aligned density striation in collisional magnetoplasmas is studied. The dispersion relation is derived including the nonlinear effects of ponderomotive force, thermal pressure force, and beating currents. It is analyzed to obtain the threshold field and the growth rate which are applied to the high latitude ionospheric modifications by powerful high frequency waves.
Symmetric simple map for a single-null divertor tokamak
Authors: Punjabi Alkesh , Ali Halima , Boozer Allen
Published: July 2002

A new map called the symmetric simple map is introduced to represent the chaotic trajectories of magnetic field lines in the scrape-off layer of a single-null divertor tokamak. Good surfaces of this map are very nearly axisymmetric. Therefore it gives a far better representation of the magnetic topology of a single-null divertor tokamak. The map is investigated in detail and used to analyze the generic features of the field line trajectories and their footprint on the divertor plate. The map is employed to calculate the variations in the fraction of magnetic flux from the stochastic layer diverted onto plate, in the footprint and in related parameters as the map parameter is varied. The Lyapunov exponents and the field diffusion coefficients are calculated. The low mode number map and the dipole map are introduced to include the effects of low and high mode number perturbations in the new map.
Finite Larmor radius stabilization of the Rayleigh–Taylor turbulent mixing width
Authors: Mikaelian Karnig O.
Published: July 2002

A diffusion model for turbulent mix [C. Cherfils and K. O. Mikaelian, Phys. Fluids 8, 522 (1996)] is compared with recent two-dimensional magnetohydrodynamic simulations by Huba [Phys. Plasmas 3, 2523 (1996)]. The model accounts for density gradient stabilization and for finite Larmor radius stabilization, thus suppressing the Rayleigh–Taylor mixing width to below its classical value. The model, which has no free parameters, appears to be in good agreement with Huba’s numerical simulations.
Response to “Comment on ‘Magnetohydrodynamic simulations of direct current helicity injection for current drive in tokamaks’ ’’ [Phys. Plasmas 3, 1038 (1996)]
Authors: Sovinec C. R. , Prager S. C.
Published: July 2002

We agree that additional effects (toroidicity, Hall and rotamak effects, and Lundquist number scaling) will alter the physics and magnetohydrodynamic simulation of HIT experiments. Future research will clarify the importance of these effects. (AIP)
Energy principles for linear dissipative systems with application to resistive MHD stability
Authors: Pletzer A.
Published: July 2002

A formalism for the construction of energy principles for dissipative systems is presented. It is shown that dissipative systems satisfy a conservation law for the bilinear Hamiltonian provided the Lagrangian is time invariant. The energy on the other hand, differs from the Hamiltonian by being quadratic and by having a negative definite time derivative (positive power dissipation). The energy is a Lyapunov functional whose definiteness yields necessary and sufficient stability criteria. The stability problem of resistive magnetohydrodynamic (MHD) is addressed: the energy principle for ideal MHD is generalized and the stability criterion by Tasso [Phys. Lett. 147, 28 (1990)] is shown to be necessary in addition to sufficient for real growth rates. An energy principle is found for the inner layer equations that yields the resistive stability criterion DR<0 in the incompressible limit, whereas the tearing mode criterion Δ′<0 is shown to result from the conservation law of the bilinear concomitant in the resistive layer.
Asynchronous cycling as a convergence acceleration method in particle simulation of direct current glow discharges
Authors: Meyer Peter , Wunner Günter
Published: July 2002

The problem of multiple time scales in modeling plasmas by dynamical particle simulation methods, such as the particle-in-cell (PIC) method, is well known. One important cause is the large ratio of the ion and electron mass. Ways to overcome this problem are the implicit PIC, or simply, the use of a reduced ion–electron mass ratio. However, these methods are not acceptable in modeling dc glow discharges by the PIC-Monte Carlo hybrid simulation technique. Therefore, a new method called asynchronous cycling was developed, which manipulates the synchronization of the electron and ion simulation cycles. It allows a 50 times faster convergence due to direct reduction of the different time scales for situations in which the changes of macroscopic quantities are slower than the ion movement. This is demonstrated by the modeling of a complete one-dimensional dc glow discharge including cathode fall, plasma bulk, and anode fall.
Plasma biasing by radial electric fields induced in front of lower hybrid grills via poloidal ponderomotive forces
Authors: Petržı́lka V. , Tataronis J. A. , Söldner F. X. , Krlı́n L.
Published: July 2002

This article treats the stationary radial electric field that the lower hybrid wave induces in front of the grill of the exciting antenna structure. The source of the induced electric field is the poloidal ponderomotive force of lower hybrid waves. It is shown that electric fields of the order of tens of volts per cm are possible in tokamak discharges. The required poloidal ponderomotive force would be possible if the mutual phasing of the horizontal waveguide rows of the grill were of a suitable value. It has been found that the induced radial electric field and the associated poloidal and toroidal plasma rotation will undergo sign changes if the poloidal phasing of the grill is altered appropriately.
Transport processes close to magnetic axis in quasihelically symmetric stellarators
Authors: Shaing K. C.
Published: July 2002

Transport processes in the region close to the magnetic axis in quasihelically symmetric stellarators are investigated by solving fluid and kinetic equations. It is found that the isomorphic transformation between transport fluxes in quasihelically symmetric stellarators and those in tokamaks is still valid if the dependence of the Fourier amplitude of the magnetic field spectrum on the flux function does not transform accordingly, and if the absolute values are employed in the transformations of orbit size and fraction of trapped particles. It is also found that orbit squeezing effects are less effective in quasihelically symmetric stellarators because the size of the orbit is smaller than that in tokamaks. The bootstrap current density does not vanish on the magnetic axis if dp/dψ remains finite. Here, ψ is the toroidal flux function and p is the plasma pressure. The direction of the bootstrap current density is also opposite to that in tokamaks similar to that in the core region.
Alfvén wave forces, affecting the tokamak edge plasma in the presence of impurities or dust
Authors: Tsypin V. S. , Vladimirov S. V. , Elfimov A. G. , Tendler M. , de Assis A. S. , de Azevedo C. A.
Published: July 2002

Dust particles appear on the edge of tokamak plasmas as a result of disruptions and plasma interactions with divertor plates, limiters, or blankets, as well as of pellet injections. It is shown that the presence of the dust impurities can lead to an effective decrease of the slow Alfvén wave absorption in the region. These waves are used for heating, current drive, and control of the low-to-high (L–H) confinement transition. Radio-frequency forces can also be strongly changed in the presence of dust particles.
On the stiffness of the sand pile profile
Authors: Carreras B. A. , Lynch V. E. , Diamond P. H. , Medvedev M.
Published: July 2002

The existence of a critical gradient is an important feature of many transport models. [M. Kotschenreuther et al., Phys. Plasmas 2, 2381 (1995)]. However, fundamental differences exist in the dynamics near marginal stability, depending on whether the transport phenomena are controlled by strict (linear) marginal stability or by a self-organized criticality. One of the most striking differences is in the stiffness of the profiles. In this paper, a sand pile model is used to gain some basic understanding of the stiffness of the profile under different dynamics.
Role of E×B flow shear on confinement enhancement in DIII-D high internal inductance discharges with high-confinement edge
Authors: Lao L. L. , La Haye R. J. , Burrell K. H. , Chan V. S. , Ferron J. R. , Rettig C. L. , Rewoldt G. , Scoville J. T. , Staebler G. M. , Strait E. J. , Tang W. M. , Taylor T. S. , Wròblewski D.
Published: July 2002

The role of E×B flow shear on confinement enhancement in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] high internal inductance discharges with high-confinement edge is investigated experimentally using a nonaxisymmetric poloidal magnetic-field perturbation from an external coil to drag down the plasma toroidal rotation. At similar values of internal inductance, discharges which rotate faster and have a stronger E×B flow shear have better confinement. These results indicate that E×B flow shear likely plays an important role in the confinement enhancement of these discharges.
Transport of recycled deuterium to the plasma core in the Tokamak Fusion Test Reactor
Authors: Skinner C. H. , Bell M. G. , Budny R. V. , Jassby D. L. , Park H. , Ramsey A. T. , Stotler D. P. , Strachan J. D.
Published: July 2002

Fueling of the plasma core by recycling in the Tokamak Fusion Test Reactor [Phys. Plasmas 2, 2176 (1995)] has been studied. In plasmas fueled by deuterium recycled from the limiter and tritium-only neutral beam injection, the DT neutron rate provides a measure of the deuterium influx into the core plasma. A reduced influx with plasmas using lithium pellet conditioning and with plasmas of reduced major (and minor) radius is found. Modeling with the DEGAS [D. P. Stotler et al., Phys. Plasmas 3, 4084 (1996)] neutrals code shows that the dependence on radius can be related to the penetration of neutrals through the scrape-off layer.
Feasibility experiments for electron ripple injection on current drive experiment-upgrade
Authors: Choe W. , Ono M. , Hwang Y. S. , Chang C. S.
Published: July 2002

In search of a method to generate a radial electric field in tokamak plasmas, an experimental study has been performed to investigate the possibility of inducing radial electrical current. An external coil array has been used to create a local magnetic ripple well and the electron cyclotron resonance heating (ECH) has been used to trap some electrons that will then be subject to rapid vertical drifts into the plasma. Using a simplified experimental arrangement with only a toroidal magnetic field, an ECH-driven radial electrical current has been observed. The ECH-driven elecron temperature anisotropy, which is necessary for ripple trapping and electron drifts, has been determined by several different methods. The perpendicular temperature can be shown to be as large as 11 times the parallel temperature, which should yield a significant amount of ripple trapping and radial current.
A spectral method for magnetohydrodynamic stability
Authors: Morse Edward C.
Published: July 2002

The eigenfunctions of the equation ∇×bn=λnbn are the solutions of a Sturm-Liouville operator and form an orthonormal basis for the expansion of magnetic fields on any closed domain. Equilibria can be formed by summing individual eigenfunctions bn. The so-called Taylor state is a member of the general class of equilibria consisting entirely of a single mode with the lowest eigenvalue. It is interesting to note that the more general configurations formed by sums of these eigenfunctions are not necessarily zero pressure, nor are they necessarily stable. Physical insight can be gained from decomposition in this way. Stability calculations are greatly simplified in this choice of basis functions, especially for incompressible modes with ∇⋅ξ=0. The magnetohydrodynamic (MHD) δW equation can be written as a bilinear form δW=an*Wnmam, where the Wnm matrix can be composed from the integrated scalar triple products of the m=0 equilibrium basis curl-eigenfunction states and the forward and adjoint curl-eigenfunction magnetic perturbations. The problem of ideal MHD stability in simple geometries (but general equilibria) becomes tractable for PC solutions. The general procedure for stability analysis in arbitrary geometries is given, and detailed calculations for spherical equilibria are given.
Diagnostics of plasma emission spectra during electron assisted chemical vapor deposition of diamond films
Authors: Zhu Xiao-Dong , Hu Min , Zhan Ru-Juan , Wen Xiao-Hui , Zhou Hai-Yang
Published: July 2002

Diamond films were deposited by hot cathode direct current discharge plasma chemical vapor deposition from a CH4–H2 gas mixture. Optical emission spectroscopy was employed to investigate in situ the plasma emission characterization during diamond synthesis. The dependence of plasma emission spectra on the input CH4/H2 ratio and the substrate temperature was investigated. A significant variation in the emission intensity of the CH radical was measured with a change in the CH4 concentration. C2 was detected only at high CH4 concentration. In addition, the relative emission intensity of the C2 species was sensitive to a high substrate temperature. The correlation between the spectra of some species and the quality of diamond films was studied. These results suggest that CH and CH+ are all important precursor species in the diamond deposition reaction, while C2 is associated with the presence of a nondiamond phase in the films.
Response to “Comment on ‘A Lyapunov functional for stability of ideal magnetohydrodynamic systems in arbitrary motion’ ” [Phys. Plasmas 5, 1566 (1998)]
Authors: Berge Gerhard
Published: July 2002

In this response I will try to clarify the issues brought forward by this comment (hereafter referred to as H&I).
Linear and nonlinear dispersive Alfvén waves in two-ion plasmas
Authors: Faria R. T. , Mirza Arshad M. , Shukla P. K. , Pokhotelov O. A.
Published: July 2002

A set of coupled nonlinear equations for dispersive Alfvén waves (DAWs) in nonuniform magnetoplasmas with two-ion species is derived by employing a multifluid model. The DAW frequency is assumed to lie between the gyrofrequencies of the light and heavy ion impurities. In the linear limit, a local dispersion relation (LDR) is derived and analyzed. The LDR admits a new type of DAW in two-ion plasmas. Furthermore, it is found that stationary solutions of the nonlinear mode coupling equations in two-ion plasmas can be represented in the form of different types of coherent vortex structures. The relevance of our investigation to space and laboratory plasmas is pointed out.
Suppression of stimulated Brillouin scattering by seeded ion wave mode coupling
Authors: Cohen B. I. , Lasinski B. F. , Langdon A. B. , Williams E. A. , Baldis H. A. , Labaune C.
Published: July 2002

Particle simulations and the analytical solution of coupled mode equations are used to demonstrate that the mode coupling of a primary ion acoustic wave produced by stimulated Brillouin backscatter with a secondary finite-amplitude ion wave propagating at a finite relative angle can reduce the amplitude of the primary ion wave and the backscatter. The simulations and analysis give results that are in qualitative agreement with experimental observations [Baldis et al., Phys. Rev. Lett. 77, 2957 (1996)] of the partial suppression of stimulated Brillouin backscattering (SBBS) when there are simultaneous, overlapping SBBS processes at a finite relative angle, which may influence SBBS reflectivities in other current and future laser-fusion experiments with multiple overlapping beams.
Effects of wiggler and axial guide fields on wave propagation in a free-electron laser
Authors: Mehdian Hassan , Willett Joseph E. , Aktas Yildirim
Published: July 2002

An analysis of the propagation of space-charge and electromagnetic waves through combined helical wiggler and axial guide magnetic fields is presented. A convenient method using complex basis vectors is used to derive a dispersion relation for coupled electrostatic and electromagnetic waves. Numerical results for space-charge waves differ markedly from the conventional approximation. For example, the effective electron-density factor Φ is double valued, becomes complex in some parameter regimes and large in others, and does not become singular near the stability limit for group I orbits. A numerical study of right circularly polarized electromagnetic waves also shows interesting effects of the combined wiggler and guide fields on the dispersion.
Resistive wall mode in rotating plasma with multiple walls
Authors: Lee D.-Y.
Published: July 2002

Resistive wall kink mode is studied in cylindrical plasma that is surrounded by two resistive walls. The outer wall is regarded as active coils for feedback stabilization, which can be made to fake-rotate. The impact of plasma rotation on such a scheme is investigated. It is found that plasma rotation at some sufficient rate can destabilize the resistive wall mode, which would otherwise remain stable by the fake-rotating coil feedback scheme.
Radial electric field and ion confinement in a toroidal heliac
Authors: Lewandowski J. L. V.
Published: July 2002

A model to describe an unmagnetized (ρi/a∼1; ρi is the thermal ion gyro-radius and a is the plasma minor radius) plasma in a three-dimensional, inhomogeneous magnetic topology is presented. Ion trajectories are integrated numerically and all finite-Larmor radius effects are retained exactly. Numerical simulations relevant to the low and high confinement modes of the toroidal heliac H1-National Facility [Hamberger et al., Fusion Technol. 17, 123 (1990)] are presented and discussed.
Numerical study of the nonlinear evolution of toroidicity-induced Alfvén eigenmodes
Authors: Chen Yang , White Roscoe B. , Fu Guo-yong , Nazikian Raffi
Published: July 2002

A simulation code for the nonlinear evolution of the toroidicity-induced Alfvén eigenmodes (TAE) is implemented based on a collisional δf simulation method [Phys. Plasmas 4, 3591 (1997)] and a Hamiltonian formulation of the guiding-center motion [Phys. Fluids 27, 2455 (1984)]. The nonlinear behavior of TAE in the regime of weak damping is studied numerically and compared with the predictions of theoretical analysis. The theoretical prediction for the scaling of the saturation amplitude with the linear growth rate and the collision rate is well confirmed. The enhancement of the saturation level due to resonance overlapping when there are multiple modes is observed and analyzed. The simulation code is applied to the alpha-driven TAE’s in the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 26, 11 (1984)]. An estimate of the background damping rate is obtained from the simulation result.
Discrete charging model and parametric excitation of the so-called acoustic mode in dusty plasmas
Authors: Dwivedi C. B.
Published: July 2002

A simple harmonic oscillator model has been considered for the statistical charging process of the dust grainlike impurity ions. Treating it as a driver eigenmode and the plasma eigenmodes under consideration (the acoustic modes and plasma oscillations) as the idler modes, a simple process of parametric excitation of the idler modes has been discussed. It is suggested that the proposed model could provide a seeding mechanism for resonant excitation of the electrostatic plasma modes.
Nonperiodicity in space of the magnetic moment series
Authors: Weitzner Harold , Pfirsch Dieter
Published: July 2002

The first term in the magnetic moment adiabatic invariant series which is explicitly not periodic in space is calculated. The implications of the existence of such a term are discussed.
Enhancement of the space-charge limited current of relativistic electron beams
Authors: Uhm Han S.
Published: July 2002

The space-charge limited current is caused by the electrostatic potential originated from the beam’s self-space charge field. A grounded conductor near the beam neutralizes the self-potential, enhancing the limiting current. The presence of a positively charged conductor near the beam increases the axial energy of beam electrons, enhancing the space-charge limited current. It is also shown that increase of the space-charge limited current for an electron beam propagating through an ion channel is due to the electrostatic potential caused by ions.
Comment on “Calculation of electron transport in Ar–N2 and He–Kr gas mixtures—implications for validity of Blanc’s law method” [Phys. Plasmas 4, 551 (1997)]
Authors: Chiflikian R. V.
Published: July 2002

It is shown that the claim of Y. Wang and R. J. Van Brunt in Phys. Plasmas 4, 551 (1997) about >40% accuracy of the analog of Blanc’s law for electrons (ABLE) developed by R. V. Chiflikian in Phys. Plasmas 2, 3902 (1995) is totally unjustified. The erroneous inferences drawn by the authors of the former paper are based both on the logically incorrect procedure of comparison and the fatal fallacy of the computational method itself (more than 50% error as compared with well-established Boltzmann calculation) used by them for testing of the ABLE. It is shown that the ABLE has great potential in predicting electron transport in gas mixtures.
Sheared-flow modes in toroidal geometry
Authors: Lewandowski J. L. V. , Lin Z. , Lee W. W. , Hahm T. S.
Published: July 2002

Using a Fourier–Bessel representation for the fluctuating (turbulent) electrostatic potential, an equation governing the sheared-flow modes in toroidal geometry is derived from the gyrokinetic Poisson equation, where both the adiabatic and nonadiabatic responses of the electrons are taken into account. It is shown that the principal geometrical effect on sheared-flow modes of the electrostatic potential is due to the flux-surface average of 1/B, where B is the magnetic field strength.
Poloidal field effects on fundamental minority ion cyclotron resonance heating in a tokamak plasma
Authors: Jun S. C. , Imre Kaya , Stevens D. C. , Weitzner Harold , Chang C. S.
Published: July 2002

Minority ion fundamental cyclotron resonance is studied in a large tokamak in which the geometrical optics approximation applies off resonance and the minority average speed is less than the wave phase speeds. Poloidal equilibrium magnetic field effects are included, which lead to nontrivially nonlocal integrodifferential equations for the wave fields. Exact reciprocity relation is given as well as explicit analytic solutions for the transmission coefficients for both the high and low field side incidences. Numerical solutions are needed only for the high field side incident reflection coefficient. Numerical schemes are described and numerical results are presented together with a reliable error bound. Typically, energy absorption increases with poloidal field. The energy absorption increases with minority density at low values of minority density. However, it decreases at high minority density. Poloidal field effects weaken the dependence of energy absorption on the toroidal wave number.
Space-time evolution of beam-plasma instabilities
Authors: Rostomian E. V.
Published: July 2002

The evolution of initial perturbation in the case of electron beam instability development in various systems is investigated generally. The approach is based on the equation for slowly varying amplitude of an induced wave packet. It is shown that the space-time dynamics of fields in the case of beam instability development is described by a partial differential equation of third order independently on system geometry, presence of external fields, specific parameters, etc. This equation is solved and the analytical expression for the fields’ space-time structure is obtained and analyzed. The shape of the induced wave packet is presented. The velocities of unstable perturbations vary between the group velocity of the resonant wave (without beam) and the beam velocity. The dependence of the growth rates of the perturbations from their velocities is obtained. The well-known maximal growth rate of resonant beam instability is nothing else than the growth rate in the peak of the wave packet. Previous investigations on beam instability dynamics actually are particular cases of the present result. The dynamics of beam instability in other cases also may be obtained from this one by specifying the system parameters. For a given system, only two parameters must be specified (in a system with dissipation—three). Some often encountered examples are presented: boundless beam-plasma system, magnetized beam-plasma waveguide, dissipative beam instability, rippled-wall waveguide. Obtained results are valid both for plasma-filled and vacuum systems for a large class of monoenergetic electron beam instabilities: Cherenkov-type, cyclotron-type, beam instability in periodical structures, etc.
Subharmonic autoresonance of the diocotron mode
Authors: Friedland L. , Fajans J. , Gilson E.
Published: July 2002

This paper investigates the excitation and control of the driven l=1 diocotron mode in a pure electron plasma by adiabatic passage through higher order resonances. The excitation takes place when the driving frequency ωd is swept such that its nth harmonic passes through the linear diocotron mode frequency ω0; ωd≈ω0/n, n=2,3,…. Once past the resonant region, the mode enters the autoresonant regime characterized by persisting phase locking and strong nonlinearities. The transition to autoresonance occurs provided that the driving amplitude exceeds a sharp threshold, which scales as A3/(4n), where A/2π is the driving frequency chirp rate. The theory of these thresholds for n=2 and n=3 is developed and experimental results for n=1,2,3,4 and 5 are presented.
Dynamic model of a plasma structure with an intermediate double-layer, formed outside an anodic plasma contactor
Authors: Lapuerta V. , Ahedo E.
Published: July 2002

The plasma structure around an electron-collecting, plasma contactor consists of two quasineutral regions separated by an intermediate, free double layer(DL). A dynamic model of this three-region structure is here established, based in a linear perturbation of the complete steady solution. Time-dependent conditions at the mobile DL are obtained; these consist of Bohm and Langmuir laws, jump conditions on the accelerated species and barrier conditions on the confined ones. Boundary conditions at the contactor surface and at the undisturbed, ambient plasma are analyzed in detail. The dynamic response to perturbations in plasma emission is the combination of a quasisteady electron mode and several ion–electron modes, with different characteristics in the core and presheath. Mode interaction takes place through the DL and the electric potential. At large frequencies, the DL displacement is restricted by ambient ions, which tend to be rigid. The stability of the steady solution is studied through a nonlocal dispersion relation. It is found that this plasma structure does not develop the radial, ion–electron (macro)instability, and, in particular, the Buneman instability.
Generation of auroral kilometric radiation by electron horseshoe distributions
Authors: Bingham R. , Cairns R. A.
Published: July 2002

High time resolution of rocket and satellite electron distribution functions within the source region of auroral kilometric radiation display a characteristic crescent shaped or horseshoe distribution. Such distribution functions are created by a field aligned electron beam moving into an increasing magnetic field, conservation of the first adiabatic invariant causes an increase of their pitch angle. This produces a broad region on the distribution function where ∂fe/∂v⊥>0, and is a possible source of free energy leading to radio wave emission by the cyclotron maser instability, which is more efficient than the conventional loss-cone maser instability. The stability of these electron horseshoe distribution functions is examined for right-hand extraordinary mode (R–X mode) radiation close to the electron cyclotron frequency propagating perpendicular to the magnetic field.
Plasma screening effects on electron–ion Coulomb bremsstrahlung in uniformly magnetized plasmas
Authors: Jung Young-Dae
Published: July 2002

Plasma screening effects are investigated on classical bremsstrahlung in electron–ion Coulomb scattering in uniformly magnetized plasmas. The calculation is for the limited case of projectile motion along the magnetic field. The electron–ion interaction potential in uniformly magnetized plasmas has been obtained by the introduction of the longitudinal plasma dielectric function. The classical straight-line trajectory method is applied to the path of the projectile electron in order to investigate the variation of the bremsstrahlung cross section as a function of the impact parameter and plasma parameters. The results shows that the bremsstrahlung cross section is increased with increasing the strength of the magnetic field. The bremsstrahlung cross section for the soft photon radiation is found to be greater than that for the hard photon radiation.
Instability of shear waves in an inhomogeneous strongly coupled dusty plasma
Authors: Mishra Amruta , Kaw P. K. , Sen A.
Published: July 2002

It is demonstrated that low frequency shear modes in a strongly coupled, inhomogeneous, dusty plasma can grow on account of an instability involving the dynamical charge fluctuations of the dust grains. The instability is driven by the gradient of the equilibrium dust charge density and is associated with the finite charging time of the dust grains. The present calculations, carried out in the generalized hydrodynamic viscoelastic formalism, also bring out important modifications in the threshold and growth rate of the instability due to collective effects associated with coupling to the compressional mode.
A three-fluid system of equations describing dusty magnetoplasmas with dynamically important dust and ion components
Authors: Schröer A. , Kopp A.
Published: July 2002

In this work a closed set of differential equations is derived which allows for the description of multifluid plasmas consisting of electrons, ions, and electrically charged dust particles. An additional neutral gas component has been omitted here for clarity. The main feature of this set of equations is the consideration of the dynamics of an inert ion component as an improvement of recently published models, where only the dynamics of the heavy dust particles have been included. At the same time the assumption of quasineutrality becomes expendable and may be replaced by the full set of continuity equations. This new set of equations considerably expands the range of possible applications, especially in the field of astrophysical dusty plasmas. Moreover, the mass of the dust particles is no longer required to be large compared to the ion mass, thus also allowing for the modeling of plasmas consisting of two ion species. Following previously published methods, the set of equations may be solved numerically and can easily be extended to include more complicated features like nonlinear collision terms or the additional neutral gas component of these models.
Muonium as a probe for defects in electron irradiated silicon
Authors: Albert E. , Barth S. , Möslang A. , Recknagel E. , Weidinger A. , Moser P.
Published: July 2002

Electron irradiated silicon was investigated by the muon spin rotation (μSR) technique. We found a strong correlation between the muon spin relaxation rate and the annealing behavior of irradiation induced defects (E and A centers).
Si/SiO2 interface roughness: Structural observations and electrical consequences
Authors: Carim A. H. , Bhattacharyya Anjan
Published: July 2002

Thin oxides (on the order of 80–90 Å) grown at 900 °C in dry O2 diluted with Ar were examined in cross section by high-resolution transmission electron microscopy. Varying degrees of roughness at the substrate/oxide interface were observed, depending on the processing sequence employed. Nonuniformity of the interface was found to correlate with undesirable electrical characteristics. By adding an intermediate annealing step to the oxidation process, the quality of the film was greatly improved. A qualitative model for the observed behavior based on stress effects and viscous flow considerations is presented.
Crystallographic slip in GaAs wafers annealed using incoherent radiation
Authors: Blunt R. T. , Lamb M. S. M. , Szweda R.
Published: July 2002

Reflection x-ray topography has been used to show that incoherent radiation annealing of complete 2-in.-diam GaAs wafers results in extensive crystallographic slip networks. These slip lines are believed to be due to small temperature nonuniformities induced during the anneal by both edge radiation and thermal mass effects. The use of a series of annular guard rings around the wafer during the anneal is shown to reduce both the severity and the number of slip lines.
Room-temperature operation of optically pumped InGaAsSb/AlGaAsSb double-heterostructure laser at ≂2 μm
Authors: Dutt B. V. , Temkin H. , Kolb E. D. , Sunder W. A.
Published: July 2002

Optically induced room-temperature lasing at ≂2 μm of the InGaAsSb/AlGaAsSb double heterostructures is reported. The growth of the quaternary antimonide structures lattice matched to GaSb substrates by liquid phase epitaxy is described. The optical thresholds and their temperature dependence are comparable to the InGaAsP devices operating at 1.55-μm wavelength regions.
Hydrogen injection and neutralization of boron acceptors in silicon boiled in water
Authors: Tavendale A. J. , Williams A. A. , Pearton S. J.
Published: July 2002

Hydrogen injection and neutralization of boron acceptors are observed in p-type crystalline Si boiled in water. Electrical neutralization of acceptors by hydrogen was measured by capacitance-voltage profiling of Al-contact Schottky diodes from material boiled in light or heavy water, and confirmed by secondary ion mass spectrometry depth profiling of deuterium. Boiling 1 Ω cm p-Si in water for 6 h (dark conditions) shows significant neutralization to ∼1 μm depth; a drastic reduction in neutralization occurs under strong white light illumination. Hydrogen injection efficiency reduces with increasing resistivity and boiling time, the latter effect suggesting that a surface limiting reaction (for example oxidation) may be involved in the generation of injection sites. Field drift of the donorlike, hydrogen-related neutralizing defect was detected in reverse-biased diodes.
Polarity determination of CdTe(111) orientation grown on GaAs(100) by molecular beam epitaxy
Authors: Hsu C. , Sivananthan S. , Chu X. , Faurie J. P.
Published: July 2002

Since the crystallographic polarity of the CdTe(111) buffer layer can change the growth conditions of HgCdTe, HgMnTe, and HgZnTe, it is important to know the polarity of the terminating of the CdTe(111)∥GaAs(100) substrates. It is especially important since this composite substrate appears as the candidate for replacement of CdTe for epitaxial growth of Hg-based II-VI compounds. The identification of the orientation is reported here for the first time by x-ray photoelectron spectroscopy, chemical etching, and in situ electron diffraction. From all three techniques, the results show unambiguously that the CdTe(1̄1̄1̄)B face (Te face) is the terminating surface for CdTe(111) growing on GaAs(100) substrates.
Direct observation of resolidification from the surface upon pulsed-laser melting of amorphous silicon
Authors: Bruines J. J. P. , van Hal R. P. M. , Boots H. M. J. , Sinke W. , Saris F. W.
Published: July 2002

Amorphized Si has been irradiated using a 7.5-ns frequency-doubled neodymium:yttrium aluminum garnet laser. For low energy density pulses, time-resolved reflectivity measurements and Rutherford backscattering spectrometry of Cu implantation profiles show that the melted layer solidifies from the surface as well as from the liquid-solid interface. From interferences in the reflectivity, growth from the surface is found to occur at a velocity of 1.5 m/s. At higher energy densities sufficient to obtain epitaxial regrowth of the amorphous layer, solidification from the surface does not occur.
Anomalous temperature dependence of threshold for thin quantum well AlGaAs diode lasers
Authors: Zory P. S. , Reisinger A. R. , Waters R. G. , Mawst L. J. , Zmudzinski C. A. , Emanuel M. A. , Givens M. E. , Coleman J. J.
Published: July 2002

Thermally induced threshold wavelength shifts of 50 nm have been observed in short cavity length diode lasers fabricated from thin quantum well AlGaAs. Analysis suggests that the high-energy radiation is generated by transitions between the n=2 level in the conduction band and the n=2 heavy hole level in the valence band. The threshold characteristic temperature (T0) of the laser material is found to be a strong function of cavity length.
New silicon-on-insulator technology using a two-step oxidation technique
Authors: Lin T. L. , Wang K. L.
Published: July 2002

Large silicon-on-insulator (SOI) structures have been obtained using a new two-step oxidation technique on a silicon molecular epitaxial film grown onto a porous silicon sample. The first low-temperature oxidation step oxidizes the large-pore silicon underlayer to avoid the collapse of the porous structure and thus secure the lateral oxygen supply channels in the porous oxide. In the second step, a higher temperature is used to oxidize the remaining porous silicon and the bottom part of the Si molecular beam epitaxy overlayer with the oxygen supplied through the oxidized porous silicon underlayer. A new porous silicon structure used is composed of a top porous silicon layer which has a smaller porosity for the ease of subsequent silicon epitaxial growth, and a second layer with a larger porosity which not only gives oxygen supply channels during the SOI oxidation but also allows the volume expansion of the porous silicon oxide to be accommodated. SOI sizes of 325 μm×2 mm have been successfully fabricated. The breakdown voltage of the SOI structures is about 400 V and the leakage current densities between the SOI layer and the substrate are below 3 nA cm−2 with a 10-V bias. n-channel metal-oxide-silicon field-effect transistors have been fabricated on the SOI structures with good characteristics, indicating that this technology may be suitable for very large scale integrated circuits applications.
Direct observation of effective mass filtering in InGaAs/InP superlattices
Authors: Lang D. V. , Sergent A. M. , Panish M. B. , Temkin H.
Published: July 2002

By making transport measurements on InGaAs/InP superlattices we have been able to demonstrate a regime of rapid electron tunneling perpendicular to the layers in the presence of quantum hole localization, i.e., effective mass filtering. The experiments were conducted on multiple quantum well and single quantum well samples in the form of p+-n junctions and involved low-temperature photoinduced capacitance and ac conductivity measurements, which easily resolved the 205 Å superlattice period, and deep level transient spectroscopy observations of hole trapping in 60–80 Å quantum wells.
Thermal resistance of ridge-waveguide lasers mounted upside down
Authors: Amann Markus-Christian
Published: July 2002

The heat dissipation in upside down mounted ridge-waveguide lasers equipped with a double-channel structure is analyzed by a simplified device model. Assuming an isothermal active region, the thermal resistance is obtained by means of conformal mapping. A comparison to published experimental results shows good agreement.
Hydrogenation of GaAs on Si: Effects on diode reverse leakage current
Authors: Pearton S. J. , Wu C. S. , Stavola Michael , Ren F. , Lopata J. , Dautremont-Smith W. C. , Vernon S. M. , Haven V. E.
Published: July 2002

Plasma hydrogenation for 3 h at 250 °C of GaAs layers grown directly on Si substrates by metalorganic chemical vapor deposition, followed by a 5-min, 400 °C anneal to restore the passivated shallow donor electrical activity, increases the reverse breakdown voltage of Schottky diode structures from 2.5 to 6.5 V. This improvement appears to be a result of the passivation by atomic hydrogen of defects such as threading dislocations caused by the large (4%) lattice mismatch between GaAs and Si. A reduced Schottky barrier height is exhibited by hydrogenated samples, consistent with As depletion of the surface occurring during the long duration plasma processing.
Rapid thermal anneal induced effects in polycrystalline silicon gate structures
Authors: Kamgar Avid , Hillenius S. J.
Published: July 2002

Effects of dynamic temperature nonuniformities during rapid thermal anneal (RTA) cycles of layered structures have been investigated. Silicon gate capacitors (4000 Å, As doped, polycrystalline Si on 175 Å thermal oxide) were subjected to temperatures 450–1200 °C using incoherent radiation from W lamps. Capacitance-voltage measurements before and after RTA showed that due to RTA, interface traps were generated at temperatures as low as 600 °C, but that they were passivated by a standard post-metallization anneal. High-temperature RTA induced, in addition, a residual flatband voltage shift which was not removed by hydrogen anneal. The residual shift can be due to either a change in oxide fixed charges or changes in the characteristics of the polycrystalline Si gate. Faster temperature ramp rates increased the generation of interface traps confirming the dynamic nature of stress, but did not affect the residual shift.
Hot-electron flow in an inhomogeneous field
Authors: Artaki Michael
Published: July 2002

The drift velocity for hot electrons is expected to depend on the gradient of an inhomogeneous field, dF/dx, through a term proportional to a phenomenological length constant. Estimates suggest that the field gradient effect can be significant in short channel devices. Monte Carlo calculations of this length coefficient have been made, and numerical results are presented for electrons in silicon.