bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic island control on the Compact Auburn Torsatron*
Authors: Gandy R. F. , Hartwell G. J. , Hanson J. D. , Knowlton S. F. , Lin H.
Published: July 2002

The understanding and control of magnetic islands is an important issue in confinement of toroidal plasmas. A series of recent experiments on the Compact Auburn Torsatron (CAT) [Fusion Technol. 18, 281 (1990)] have investigated the control of magnetic islands. As a by-product, the minimization of the size of magnetic islands has been accomplished through the application of externally applied error fields. Both the phase and amplitude of the applied correction field at a given rational surface are varied in order to control the island chain on a rational surface. The experimental results have been interpreted within the framework of a Hamiltonian description of the magnetic field in toroidal geometry. In particular it has been demonstrated that first order perturbation theory is sufficient to predict the variation of island size for a wide range of externally applied fields. Using this theoretical approach a novel technique of error field determination through the measurement of the local rotational transform has been developed. The physical mechanisms which lead to spatial spreading of the electron beam used in the surface mapping have also been studied.
Hot plasma diagnosis in tokamaks using synchrotron radiation
Authors: Fidone I. , Granata G. , Geniet F.
Published: July 2002

Emission of synchrotron radiation for diagnostic purposes in hot large major radius tokamaks is investigated for arbitrary angles with respect to the magnetic field. It is shown that for given values of the wave frequency, the source of radiation is localized in ordinary and momentum spaces yielding a kind of selective diagnostics of the radiating electrons.
Tearing mode evolution on an invariant manifold
Authors: Wessen Ken
Published: July 2002

The invariant manifold reduction is applied to the problem of nonlinear tearing mode evolution in a plane plasma slab, and the resulting reduced description compared with existing numerical simulation results. The reduced description is found to give a significant degree of insight into the mode competition processes responsible for the major features of the nonlinear evolution, and even provide a degree of quantitative accuracy.
A two-dimensional ion kinetic model of the scrape-off layer of a diverted plasma with a private flux region
Authors: Catto Peter J. , Connor J. W.
Published: July 2002

Earlier two-dimensional (radial and poloidal angle), analytically tractable ion kinetic models of the scrape-off layer (SOL) in which a steady state is achieved by balancing the streaming loss of ions to the divertor target plates with the radial diffusion of ions from the core are unable to distinguish between limited and diverted plasmas. The model presented here removes this limitation while still remaining amenable to a similar Wiener–Hopf solution procedure. To phenomenologically model ion recycling, the boundary conditions employed at the divertor plates allow for partial reflection. The diffusion into the private flux region and the extended divertor channels (all of normalized length d along the magnetic field), as well as the rest of the SOL, is evaluated. The SOL is shown to be asymmetric about the separatrix because ions from the core must stream by the X point be- fore diffusing into the private flux region. The channel or leg SOL width is of order [LD(1+2d)/vi ‖ln γ‖]1/2, where D, L, vi, and γ are the diffusion coefficient, connection length, ion thermal speed, and reflection coefficient, respectively. The SOL in the private flux region is narrower (by [2d/(1+2d)]1/2) with a lower density (by [2d/(1+2d)]3/4) and a stronger poloidal dependence than the region beyond the separatrix. To equalize the heat load between the private flux region and the leg portions of the target, the legs must be longer than the connection length.
Effect of aspect ratio on magnetic field fluctuations in the reversed-field pinch
Authors: Ho Y. L. , Schnack D. D. , Nordlund P. , Mazur S. , Satherblom H.-E. , Scheffel J. , Drake J. R.
Published: July 2002

The effect of aspect ratio on magnetic field fluctuations in reversed-field pinches is investigated using a three-dimensional magnetohydrodynamic code. Configurations with aspect ratios of 1.1, 2.2, and 4.4 are modeled. The results are extrapolated to aspect ratio 8.8 for comparison with the Extrap T1 experiment [Nucl. Fusion 34, 427 (1994)]. It is found that the average modal amplitudes decrease with aspect ratio. However, the spectrum broadens correspondingly, resulting in negligible effect on the magnetic fluctuation level. The computed spectrum dynamics are found to be in good agreement with experimental observations on the T1 experiment. Quantitative evaluations of the field line stochasticity indicate no dependence of the mean magnetic field diffusion rate on aspect ratio.
Effect of E×B drift on divertor plasma flows
Authors: Krasheninnikov S. I. , Sigmar D. J. , Yushmanov P. N.
Published: July 2002

Due to the influence of a sheared E×B drift affecting the inertia term in the plasma momentum equation a strong variation of the plasma pressure along the magnetic field lines can appear similar to experimental observations of the ‘‘detached divertor’’ regimes. The typical radial scale length of plasma parameter variation, such that the E×B drift becomes important, is of the order of the poloidal ion gyroradius.
Response to ‘‘Comment on ‘An analytic treatment of the bounded and free presheaths with arbitrary viscosity in magnetized flowing plasmas’ ’’ [Phys. Plasmas 1, 2864 (1994)]
Authors: Chung Kyu-Sun
Published: July 2002

The comments of Hutchinson (Ref. 1) on the author’s titled paper are discussed. The one-dimensional model of free presheaths does include viscosity and not merely convective momentum transport. Mathematical arguments are given to support this assertion. (AIP)
Mechanism of microwave generation by virtual cathode oscillation
Authors: Jiang W. , Masugata K. , Yatsui K.
Published: July 2002

A simple model is developed to describe the virtual cathode oscillation. With this model, it is found that the frequency dependence of the microwave power gain is determined by the phase relation between the oscillating electric field and the modulated electron beam current. Analytical expressions are given for the frequency dependence of the gain and for estimation of the maximum efficiency of microwave energy extraction from the electron beam.
Bifurcations from periodic solution in a simplified model of two-dimensional magnetoconvection
Authors: Bekki N. , Karakisawa T.
Published: July 2002

A two-dimensional Boussinesq fluid with nonlinear interaction between Rayleigh–Bénard convection and an external magnetic field is investigated numerically and analytically. A simplified model consisting of a fifth-order system of nonlinear ordinary differential equations with five parameters is introduced and integrated numerically in certain parameter regions. Various types of bifurcations from periodic solutions are found numerically: period-doubling bifurcation, heteroclinic bifurcation, intermittency, and saddle-node bifurcation. A normal form equation is also derived from the fifth-order system, and center manifold theory is applied to it. An expression for the renormalized Holmes–Melnikov boundary for the evaluation of the numerical results is given. It is shown from the normal form equation that each property of the two phase portraits described by the Duffing equation and the van der Pol equation emanates from one common attractor in the five-dimensional space of the fifth-order system.
Nonlinear interactions between high heat flux plasma and electron-emissive hot material surface
Authors: Ye M. Y. , Masuzaki S. , Shiraishi K. , Takamura S. , Ohno N.
Published: July 2002

Nonlinear dynamic behaviors are investigated for a system composed of high heat flux plasma and electron-emissive hot material surface. Phase transitions due to bifurcation have been observed in the experiments and analyzed numerically by using a series of simple equations describing energy balance as well as sheath formation, including a new Child–Langmuir expression for an electron emission from the material surface into the plasma. It is found that the thermal insulation between these two constituent parts depends not only on plasma parameters but also strongly on electron-emission characteristics of the material surface in a nonlinear way. The electron emission leads to a strong reduction of sheath voltage and a large enhancement of heat flux density on the material surface. The plasma is cooled when it is in contact with a strongly electron-emissive material surface.
Magnetic ripple and the modeling of lower-hybrid current drive in tokamaks
Authors: Peysson Yves , Arslanbekov Robert , Basiuk Vincent , Carrasco Joël , Litaudon Xavier , Moreau Didier , Bizarro João P.
Published: July 2002

Using ray tracing, a detailed investigation of the lower-hybrid (LH) wave propagation in presence of toroidal magnetic field ripple is presented. The local ray behavior is first depicted for a cylindrical equilibrium periodically modulated along the axial direction. Variations along ray trajectories in the component of the wave vector parallel to the equilibrium magnetic field are observed, with a maximum relative amplitude that is locally of the order of the ripple level. For the full rippled toroidal equilibrium, a similar local behavior is found when the ray trajectory crosses a high ripple region. Despite the modest amplitude of the local ray perturbation, its global influence on ray trajectories may be strong, as a consequence of the combined effects of toroidal and poloidal inhomogeneities. By coupling ray tracing with a one-dimensional relativistic Fokker–Planck code, simulations of LH experiments have been performed for the TORE SUPRA tokamak [Equipe TORE SUPRA, in Proceedings of the 15th Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville (International Atomic Energy Agency, Vienna, 1995), Vol. 1, p. 105, Paper IAEA-CN-60/A1-5]. It is shown that magnetic ripple may induce significant modifications in the LH power deposition profiles, mainly in the ‘‘few passes’’ regime when the wave makes some, but not many, passes inside the plasma before being absorbed. The effect of magnetic ripple leads then to a broadening of the power deposition profile and a shift towards the center of the plasma, and a better coupling with high energy electrons. This behavior may be explained by an increase in the overall ray stochasticity. Taking into account magnetic ripple in LH simulations, a better agreement is found between numerical predictions and experimental observations.
Current neutralization and focusing of intense ion beams with a plasma-filled solenoidal lens. I
Authors: Oliver B. V. , Sudan R. N.
Published: July 2002

The response of the magnetized plasma in an axisymmetric, plasma-filled, solenoidal magnetic lens, to intense light ion beam injection is studied. The lens plasma fill is modeled as an inertialess, resistive, electron magnetohydrodynamic (EMHD) fluid since characteristic beam times τ satisfy 2π/ωpe,2π/Ωe≪τ≤2π/Ωi (ωpe is the electron plasma frequency and Ωe,i are the electron, ion gyrofrequencies). When the electron collisionality satisfies νe≪Ωe, the linear plasma response is determined by whistler wave dynamics. In this case, current neutralization of the beam is reduced on the time scale for whistler wave transit across the beam. The transit time is inversely proportional to the electron density and proportional to the angle of incidence of the beam with respect to the applied solenoidal field. In the collisional regime (νe>Ωe) the plasma return currents decay on the normal diffusive time scale determined by the conductivity. The analysis is supported by two-and-one-half dimensional hybrid particle-in-cell simulations.
Analysis and simulation of Raman backscatter in underdense plasmas
Authors: Shvets G. , Wurtele J. S. , Shadwick B. A.
Published: July 2002

A new formalism to describe the spatiotemporal evolution of relativistic Raman backscatter (RBS) of ultrashort laser pulses in underdense plasma has been developed. This theory is based on an eikonal representation for the RBS field and averaging over the oscillation frequency. Equations are derived for the evolution of the RBS radiation field amplitude and phase and for particle motion in the ponderomotive potential of the pump and RBS fields. A technique similar to that used in Raman free electron lasers is used to include the plasma density oscillation and its electric field in the particle equations. The equations have been solved numerically in a code that provides an accurate description of the nonlinear electron motion on a short spatial scale of half the radiation wavelength, while at the same time evolving the laser field on a much coarser scale, characteristic of the growth rate of the instability. Two-dimensional nonlinear effects, such as the return current, are analytically estimated and phenomenologically incorporated in the numerical code. The results of recent (RBS) experiments are compared with the analytical and numerical predictions of the theory. A statistical theory of the linear RBS instability growing from plasma noise is developed. It is found that in the nonlinear regime backscattered radiation, seeded by random density fluctuations in the plasma, breaks up into spikes, which exhibit superradiant behavior. These effects might explain the spikiness of the RBS spectra in the nonlinear regime.
Ion densities in a high-intensity, low flow nitrogen–argon plasma
Authors: Brussaard G. J. H. , van de Sanden M. C. M. , Schram D. C.
Published: July 2002

The plasma density in an expanding thermal plasma was determined using planar Langmuir probe measurements. The arc plasma was operated at low flow (500 standard cm3 per minute). It is shown that the decrease of density with increasing distance from the nozzle of the arc in an argon plasma can be explained by diffusion away from the expansion axis. The determined decay length is 10 cm. In the case that nitrogen is injected in the arc, the plasma density is lowered considerably due to charge exchange and dissociative recombination in the expansion. Because of the low electron density (1017 m−3) at a partial nitrogen flow larger than 10%, the dissociative recombination becomes slow. The main loss process of N2+ ions in this case is diffusion away from the plasma axis. The effective decay length found in the nitrogen plasma is 9 cm.
Response to “Comment on ‘Onsager symmetry for inhomogeneous magnetized plasmas”’ [Phys. Plasmas 3, 4325 (1996)]
Authors: Nambu M.
Published: July 2002

The modified local mode approximation was used to obtain a dielectric tensor for inhomogeneous magnetized plasmas which satisfies both the Hermitian and Onsager symmetry properties. (AIP)
Fundamental heating with stellarator wave modes
Authors: Svidzinski V. A. , Swanson D. G.
Published: July 2002

A perturbation method is developed to find the structure of Alfvén wave modes in a cylindrical waveguide filled with a cold, collisional, uniform plasma with a vacuum layer between the plasma and a conducting wall when the magnetic field is a superposition of a uniform and an inhomogeneous l=2 field created by helical windings. The influence of the helical field on the wave mode structure is treated as a perturbation. It is shown that the m=−1 azimuthal component of a modified m=+1 fast Alfvén wave is left-hand polarized in the central part of the plasma. This implies a coupling between the m=+1 fast (right-hand polarized) wave and m=−1 slow (left-hand polarized) waves due to the inhomogeneity of the l=2 fields. The coupling efficiency is examined for different plasma parameters. Results demonstrate that efficient coupling between the modes occurs for appropriate plasma parameters in this model, indicating that efficient plasma heating at the fundamental ion cyclotron frequency is possible in stellarators.
The derivation of equations for fluctuations and transport in flux-tube geometries
Authors: Martinell J. J. , Guzdar P. N. , Hassam A. B.
Published: July 2002

A self-consistent set of equations for the fast space–time evolution of fluctuations and the slow space–time evolution of density and flows in a toroidal plasma, relevant for simulations using field-aligned coordinates in thin flux tubes, has been derived. The methodology for the derivation of these equations is outlined for a model set of equations for the plasma edge, specific to resistive ballooning modes but readily adaptable to other instabilities. The derivation proceeds by first writing the axisymmetric and fluctuating equations in the usual toroidal coordinate system. These are then transformed to the twisted coordinate flux-tube system. Most simulations which use twisted flux-tube computational grids transform to the field-aligned coordinate system first and then take averages to obtain the slow evolution. They however miss some terms since the two operations, namely, multiscale separation and coordinate transformation, do not necessarily commute, because of subsidiary assumptions on the box size. In the present formulation, all the relevant neoclassical effects such as the Pfirsch–Schlüter current and the Stringer spin-up as well as the toroidal Reynolds stress are properly included. This set of multiscale equations is appropriate for the study of the formation and evolution of transport barriers.
Modulational instability of incoherent photons in high-temperature electron-positron plasmas
Authors: Shukla P. K. , Stenflo L.
Published: July 2002

The nonlinear interaction between random phase photons and sound waves in ultra-relativistically hot electron-positron plasmas is considered. The modulational instability of a broad photon spectrum is investigated. Explicit expressions for the growth rates are obtained in limiting cases.
Internal disruptions in Heliotron E*
Authors: Carreras B. A. , Lynch V. E. , Zushi H. , Ichiguchi K. , Wakatani M.
Published: July 2002

In Heliotron E [K. Uo, Nucl. Fusion 25, 1243 (1985)] shifted-in vacuum magnetic field configuration, the q profile varies from just above 2 at the magnetic axis to 0.4 at the plasma edge. For low-β plasmas, resistive interchange modes are the dominant low-n instabilities at the plasma core. They saturate at low fluctuation levels. Above a threshold value, the ideal m/n=2/1 modes become unstable. They can be resonant or nonresonant modes depending on the value of q(0). In either case, their nonlinear evolution leads to a sudden crash of the pressure within the r/a=0.3 radius (sawtooth oscillation). When beta is increased further, the q=2 surface moves out of the plasma, and the ideal m/n=2/1 modes are effectively stabilized when q(0)<1.85. As a result, the sawtooth oscillations are suppressed.
Fast heat pulse propagation in hot plasmas
Authors: Lazzaro Enzo , Wilhelmsson Hans
Published: July 2002

It is found that traveling wave solutions of the diffusion equation can have propagation times, related to the scale length of the perturbation, which can be considerably less than the characteristic diffusion times. This provides a possible model for the interpretation of recent experiments of fast “hot” and “cold” pulses in magnetically confined plasmas.
A unified self-consistent model for calculating ion stopping power in ICF plasma
Authors: Wang P. , Mehlhorn T. M. , MacFarlane J. J.
Published: July 2002

A new unified self-consistent ion stopping power model for use in ion-driven inertial confinement fusion (ICF) target design has been developed. This model includes sophisticated treatments for electron density distribution of an atom in plasmas and a full random phase approximation stopping function that extrapolates the zero temperature Lindhard stopping function to arbitrary temperatures. It is shown that this model provides accurate ion stopping power for cold materials, including both low-Z and high-Z elements. For finite temperature plasmas, the model accounts for the stopping effects due to electrons in ground states, excited states, resonance states, and continuum states in a self-consistent manner. There is no separation treatment for “bound” and “free” electrons. Hence, the model enables the calculations of ion stopping power to be made within a single model for a wide range of beam and target conditions relevant to ICF studies.
Ion pressure induced bending of the virtual-cathode in multistage ion diodes
Authors: Slutz Stephen A.
Published: July 2002

The electric force that accelerates ions in a magnetically insulated diode is transferred to the electrodes by the magnetic field. This force, which appears as an ion pressure term in the pressure balance equation, causes a bending of the virtual-cathode and consequently a lensing effect. Analytic calculations of the shape of the virtual-cathode resulting from this ion pressure are presented and a simple formula for the effective focal length resulting from the bending of the virtual-cathode is given. The focussing is a function of the accelerating voltage and the injected beam kinetic energy and current density. Thus a relationship between these parameters must be maintained to keep the beam on target.
Characterization of energy flow and instability development in two-dimensional simulations of hollow z pinches
Authors: Peterson D. L. , Bowers R. L. , McLenithan K. D. , Deeney C. , Chandler G. A. , Spielman R. B. , Matzen M. K. , Roderick N. F.
Published: July 2002

A two-dimensional (2-D) Eulerian Radiation-Magnetohydrodynamic (RMHD) code has been used to simulate imploding z pinches for three experiments fielded on the Los Alamos Pegasus II capacitor bank [J. C. Cochrane et al., Dense Z-Pinches, Third International Conference, London, United Kingdom 1993 (American Institute of Physics, New York, 1994), p. 381] and the Sandia Saturn accelerator [R. B. Spielman et al., Dense Z-Pinches, Second International Conference, Laguna Beach, 1989 (American Institute of Physics, New York, 1989), p. 3] and Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)]. These simulations match the experimental results closely and illustrate how the code results may be used to track the flow of energy in the simulation and account for the amount of total radiated energy. The differences between the calculated radiated energy and power in 2-D simulations and those from zero-dimensional (0-D) and one-dimensional (1-D) Lagrangian simulations (which typically underpredict the total radiated energy and overpredict power) are due to the radially extended nature of the plasma shell, an effect which arises from the presence of magnetically driven Rayleigh–Taylor instabilities. The magnetic Rayleigh–Taylor instabilities differ substantially from hydrodynamically driven instabilities and typical measures of instability development such as e-folding times and mixing layer thickness are inapplicable or of limited value. A new measure of global instability development is introduced, tied to the imploding plasma mass, termed “fractional involved mass.” Examples of this quantity are shown for the three experiments along with a discussion of the applicability of this measure.
Solitary kinetic Alfvén waves with finite ion gyroradius
Authors: Wang Xu-yu , Wang Xue-yi , Liu Zhen-xing , Li Zhong-yuan
Published: July 2002

A solitary kinetic Alfvén wave (SKAW) with a finite ion gyroradius is obtained by using kinetic theory. It is found that the existence of SKAW depends not only on the parallel wave number but also on the perpendicular wave number. In contrast to the results with cold ions, both the super-Alfvénic and the sub-Alfvénic solitons exist in the density compressive region. The amplitude and the parallel propagation velocity of the solitons are found to be enhanced, while the width is narrowed due to the finite ion gyrodius effect.
Parametric instability on an annular beam induced by a periodic beam-channel boundary
Authors: Lazaros A. , Vomvoridis J. L.
Published: July 2002

In the framework of the general Nishikawa formalism for parametric instabilities the coupling is examined of the plasma oscillations to the density modulations of an annular electron beam, modulated by an axially periodic beam-channel boundary commonly used in microwave sources. It is found that the instability is present when the distance between the beam and the channel boundary is larger than the periodic length of the boundary. It is also found that the growth rate of the instability is independent of the beam current and depends only on the beam velocity and the periodic length of the beam-channel boundary.
Intermittent particle transport in two-dimensional edge turbulence
Authors: Sarazin Y. , Ghendrih Ph.
Published: July 2002

Interchange turbulence in two dimensions is investigated in the scrape-off layer (SOL) of fusion devices, when driven by a constant core particle influx. Contrary to the standard gradient-driven approach, density is allowed to fluctuate around its average profile. Transverse transport exhibits some of the features of self-organized critical systems, namely inward and outward avalanches, together with a frequency spectrum decrease in 1/f and f−2 at intermediate and high frequencies, respectively. An avalanche occurs when the local radial density gradient exceeds the critical one. A self-sustained particle flux then follows the large radial structures of the electric potential. As observed experimentally, the radial profile of density relative fluctuations decreases from the wall into the core plasma, while that of electric potential relative fluctuations peaks inside the SOL. Equilibrium density exhibits the experimental exponential decrease. An analytical expression of the SOL width ΔSOL is obtained, which maximizes the linear growth rate, when the poloidal modulation of electric potential equilibrium is taken into account. The parametric dependencies of ΔSOL are compared to experimental data.
Edge resonant fluctuations and particle transport in a reversed-field pinch
Authors: Möller A.
Published: July 2002

Electrostatic fluctuations are measured in the Extrap T2 reversed-field pinch [J. R. Drake et al., in Plasma Physics and Controlled Nuclear Fusion Research 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 2, pp. 193–199] using a Langmuir probe array. The electrostatic fluctuation, driven particle transport ΓnΦ is derived and found to constitute a large fraction of the total particle transport. The spectral density of all measured quantities exhibits a peak in the frequency range 100–250 kHz, which originates from fluctuations that are resonant close to the edge [n=−(40–80)]. This peak contains only about 10–20% of the total fluctuation power, but is shown to dominate ΓnΦ. The main reason for this is the high toroidal mode number as compared with internally resonant magnetohydrodynamic fluctuations. The edge resonant fluctuations also features a higher coherence (γ=0.5) and close to 90° phase shift between density and potential fluctuations.
Nonlinear electronic waves in warm plasmas
Authors: Ciurea-Borcia Rodica , Ignat Margareta
Published: July 2002

The properties of nonlinear longitudinal electronic waves propagating through warm plasma are examined. Thus, using the kinetic theory and the reductive perturbation method in the third order of approximation, it is shown that the nonlinear wave–wave interactions lead to phase velocity shifts. When the thermal velocity, νT, is much smaller than the phase velocity, νf (νT≪νf), an energy transfer appears from the wave to the plasma electrons, together with a decrease in the phase velocity. However, when electrons with high velocities Vz≫νf are injected in plasma, they determine an energy transfer from the particles to the wave and an increase in the phase velocity, too.
Simulation of photon acceleration in a plasma wake
Authors: Solodov A. A. , Mora P. , Chessa P.
Published: July 2002

The frequency shifting of an ultrashort (femtosecond) low intensity laser pulse in the presence of a plasma wave is investigated using particle simulations. One-dimensional simulations confirm the existence of photon trajectories similar to the trajectories of trapped or untrapped charged particles in a plasma wave. In the case when a plasma wake is produced by a relativistically intense laser pulse with a duration of the order of one plasma period, some full oscillations of the photon frequency take place only for the untrapped photons moving backward in the plasma wave frame before the intense laser pulse depletion due to the energy transfer to the plasma wake occurs. An analytical estimate of the probe pulse phase and frequency shift in two-dimensional (2-D) axially symmetrical plasma wake is performed. Axially symmetrical particle simulations in 2-D, with experimentally attainable parameters, display a frequency shift of the probe pulse of a few percent. In these analytical estimates and 2-D simulations special attention has been given to the modeling of the probe pulse optical collecting line that is typically used in plasma diagnostics.
First adiabatic invariant of a charged particle modified in a time-dependent magnetic field
Authors: Olszewski S. , Roliński T.
Published: July 2002

The mechanics of the change of the first adiabatic invariant of a nonrelativistic charged particle due to the linear change with time of the spatially-uniform magnetic induction is studied with the aid of an approximate analytic formalism. The same formalism enables one also to undertake an analytic study of the motion of the guiding center of the gyrating particle. The accuracy of the formalism is checked by comparing it with numerical calculations. The results obtained within the formalism are thoroughly compared with a former study of the mechanics of the change of the first adiabatic invariant done by Borovsky and Hansen [Phys. Rev. A 43, 560 (1991)] mainly on the basis of computer simulations. The examined changes of the magnetic field are small compared to the original field and occupy a time period much longer than one gyroperiod.
A test-bed for Langmuir wave turbulence modeling of stimulated Raman backscatter
Authors: Rose Harvey A.
Published: July 2002

Stimulated Raman backscatter (SRS) may incorporate several, qualitatively different regimes of Langmuir wave dynamics, as it grows convectively in space. These typically include a strictly linear regime at the far end of the plasma from the laser, where SRS comes up from thermal Langmuir wave fluctuations; which may progress to a regime where the primary SRS daughter Langmuir wave is unstable to the Langmuir wave decay instability (LDI); and perhaps to a regime of strong Langmuir wave turbulence (SLT). The accurate description of the spatial transition between these regimes, which may involve large Langmuir wave correlation lengths, is a great challenge for turbulence modeling. In this paper a highly idealized model of SRS in periodic geometry is introduced which allows for the presence of a unique Langmuir wave regime for a given set of physical parameters, and therefore presents the minimal challenge for a turbulence model. One- and two-dimensional simulations of this SRS model, which allows for LDI and SLT as described by Zakharov’s model of nonlinear Langmuir wave dynamics, are compared with the predictions of a recently introduced turbulence model, and quantitative agreement is obtained, without the use of any ad hoc parameters, for the SRS reflectivity and correlation length, and Langmuir and acoustic wave energy densities, over an order of magnitude variation of SRS growth rate and ion acoustic damping rate.
Comment on “Parametric decays of a circularly polarized electromagnetic wave in an electron–positron plasma” [Phys. Plasmas 5, 3171 (1998)]
Authors: Shukla P. K. , Stenflo L.
Published: July 2002

It is shown that the nonlinear dispersion equation of Muñoz and Gomberoff [Phys. Plasmas 5, 3171 (1998)] has to be reinterpreted. The physics of the parametric instabilities of circularly polarized electromagnetic waves in an unmagnetized electron–positron plasma, as well as a new nonlinear dispersion relation for modulational interactions of the waves, are presented.
Destabilization of diocotron modes inside structured anode cavities
Authors: Riyopoulos Spilios
Published: July 2002

Previous stability analysis of low space charge ωe2/Ω2≪1, E×B drifting flows inside smooth anode cavities has shown that they are practically stable against slow wave ω/ck≪1 diocotron instabilities when the flow touches the cathode (cathode layer). It is shown here that the presence of periodically structured anode destabilizes cathode layer modes when their phase velocity matches that of an empty cavity eigenmode, as structured cavities support slow waves in vacuum. An analytic dispersion relation that self-consistently includes the structured anode effects is obtained in the guiding center approximation. The growth rate, related to the onset of magnetron oscillations, scales as γ/ω∼ωD/ω≃1/kd; it is independent of, and remains finite as, ωe2/Ω2→0. The most unstable frequency corresponds to a resonant layer below the flow surface (slightly below the Buneman–Hartree frequency) and the growth rate is found symmetric relative to the detuning from resonance.
Ultra-powerful compact amplifiers for short laser pulses
Authors: Malkin V. M. , Shvets G. , Fisch N. J.
Published: July 2002

Laser energies and powers, significantly much higher than available now through the most advanced chirped pulse amplifiers, might be achieved in much smaller devices. The working medium in such devices is plasma, capable of tolerating ultrahigh laser intensities within times shorter than it takes for filamentation instabilities to develop. The ultrafast amplification mechanism that outruns filamentation instabilities is the transient Raman backscattering of a laser pump in plasma. In principle, this mechanism is fast enough to reach nearly relativistic pumped pulse intensities, like 1017 W/cm2 for λ=1 μm wavelength radiation. Such a nonfocused intensity would be 105 times higher than currently available. This mechanism also produces complete pump depletion. Many amplifiers with expensive and fragile meter-size gratings might then be replaced by a single amplifier comprised of a 1 cm size plasma layer. Raman instabilities of the pump to noise, as the pump traverses plasma layer towards the seed pulse, can be suppressed by detuning the resonance appropriately, even as the desired amplification process persists with high efficiency due to nonlinear resonance broadening. Moreover, since the peak intensity scales like 1/λ2, even much higher laser intensities might become feasible when appropriate x-ray pump lasers are developed.
Interaction of plasmas with intense lasers
Authors: Kruer William L.
Published: July 2002

The interaction of plasmas with intense lasers is an excellent example of how different fields of physics are interconnected. Invention of the laser and its ongoing development has allowed the creation and study of high temperature, dense matter in the laboratory. The results both advance the underlying plasma science and are relevant to many fields ranging from astrophysics to fusion and nonlinear physics. A brief overview of the interaction physics is given. Selected topics are discussed to illustrate the exciting progress in experimental, theoretical, and computational investigations with focused laser intensities up to 1021 W/cm2.
Plasma screening effects on resonant Compton scattering of photons by hydrogenic ions in strongly coupled plasmas
Authors: Jung Young-Dae , Song Mi-Young
Published: July 2002

Plasma screening effects on resonant Compton scattering of photons by bound atomic electrons from the ground state of hydrogenic ions in uniformly distributed strongly coupled plasmas are investigated. The interaction energy in strongly coupled plasmas is given by the ion-sphere model potential. The screened radial atomic wave functions and energy eigenvalues for the 1s and 2p states of the target hydrogenic ion in strongly coupled plasmas are obtained by the variation and perturbation methods. The transition matrix element near resonance is obtained by the lowest-order photon-perturbation Hamiltonian in the electronic dipole representation. The resonant Compton scattering cross section including the plasma screening effect is found to be smaller than that neglecting the plasma screening effect since the resonance frequency is increased due to the plasma screening effect.
Langmuir wave turbulence transition in a model of stimulated Raman scatter
Authors: Rose Harvey A.
Published: July 2002

In a one-dimensional stationary slab model, it is found that once the stimulated Raman scatter (SRS) homogeneous growth rate, γ0, exceeds a threshold value, γT, there exists a local, finite amplitude instability, which leads to Langmuir wave turbulence (LWT). Given energetic enough initial conditions, this allows forward SRS, a linearly convective instability, to be nonlinearly self-sustaining for γ0>γT. Levels of forward scatter, much larger than predicted by the linear amplification of thermal fluctuations, are then accessible. The Stochastic quasilinear Markovian (SQM) model of SRS interacting with LWT predicts a jump in the value of 〈ε〉, the mean energy injection rate from the laser to the plasma, across this threshold, while one-dimensional plasma slab simulations reveal large fluctuations in ε, and a smooth variation of 〈ε〉 with γ0. Away from γT, 〈ε〉 is well predicted by the SQM. If a background density ramp is imposed, LWT may lead to loss of SRS gradient stabilization for γ0≪γT.
Room-temperature cw operation of InGaPAs/GaAlAs visible light double heterojunction lasers
Authors: Kaneiwa Shinji , Takiguchi Haruhisa , Hayakawa Toshiro , Yamamoto Saburo , Hayashi Hiroshi , Yano Seiki , Hijikata Toshiki
Published: July 2002

InGaPAs/GaAlAs double heterojunction (DH) lasers operated continuously at room temperature in the spectral range of 720–730 nm. The DH structure was grown on a GaAs substrate by liquid phase epitaxy, and the laser diode fabrication was the same as V-channeled substrate inner stripe lasers. The lowest values of a threshold current (Ith) were 145 and 186 mA under pulsed and cw operation, respectively. The characteristic temperature T0 of the Ith was typically 120 K. It was confirmed that the InGaPAs/GaAlAs system has sufficient band-gap energy difference between the active layer and the cladding layer. Multiple longitudinal and fundamental transverse modes were obtained.
Localized epitaxial growth of hexagonal and tetragonal MoSi2 on (111) Si
Authors: Lin W. T. , Chen L. J.
Published: July 2002

Both hexagonal and tetragonal forms of MoSi2 (h-MoSi2 and t-MoSi2) were grown epitaxially on (111)Si for the first time. The best epitaxy was obtained in samples annealed at 1050 °C for 1 h. The orientation relationships between epitaxial h-MoSi2 and Si were determined to be [0001]MoSi2//[111]Si and (202̄0)MoSi2//(202̄)Si whereas those between epitaxial t-MoSi2 and Si were analyzed to be [110]MoSi2//[111]Si and (004)MoSi2//(2̄02)Si. Regular interfacial dislocations, 100 Å in spacing, were identified to be of edge type with (1)/(6) 〈112〉 Burgers vectors. Preliminary investigations indicated that a number of other technologically important refractory silicides can all be grown epitaxially on silicon. The results present an exciting prospect to fabricate novel classes of devices with desirable characteristics.
Stripe-geometry AlxGa1−xAs-GaAs quantum well heterostructure lasers defined by Si diffusion and disordering
Authors: Meehan K. , Gavrilović P. , Holonyak N. , Burnham R. D. , Thornton R. L.
Published: July 2002

The use of Si diffusion and impurity-induced layer disordering, via a Si3N4 mask pattern, to construct stripe-geometry AlxGa1−xAs-GaAs quantum well heterostructure lasers on n-type substrates is described. This leads to a convenient form of index-guided buried-heterostructure laser that is easily constructed and replicated (in various geometries) on commonly available n-type GaAs substrate.
Selective area growth of GaAs and In0.53Ga0.47As epilayer structures by chemical beam epitaxy using silicon shadow masks: A demonstration of the beam nature
Authors: Tsang W. T.
Published: July 2002

Selective area growth of GaAs and In0.53Ga0.47As epilayer structure with well-defined smooth edges has been achieved with a Si mask shadowing technique for chemical beam epitaxy (CBE). Epilayer stripes with widths as narrow as 2–5 μm have been replicated. An experiment was also conducted to unequivocally establish the beam nature of CBE and the absence of a stagnant gas boundary layer above the substrate surface during growth which is present in both atmospheric and low-pressure metalorganic chemical vapor deposition.
Laser-initiated deposition reactions: Microchemistry in organogold polymer films
Authors: Gross M. E. , Fisanick G. J. , Gallagher P. K. , Schnoes K. J. , Fennell M. D.
Published: July 2002

Micron-sized gold features have been produced by the cw Ar+ laser-initiated decomposition of thin organogold polymer films. These features may exhibit pronounced periodic structure under a wide variety of experimental conditions, resulting from propagation of the reaction front ahead of the laser by the heat of reaction associated with the chemical transformation. Gold features formed by irradiation of gold chloride-doped polymer films further demonstrate these thermal effects. Thermoanalytical studies provide pertinent data for selection of suitable precursor materials.
Generation of intense tunable picosecond pulses in the far-infrared
Authors: Berg M. , Harris C. B. , Kenny T. W. , Richards P. L.
Published: July 2002

Continuously tunable far-infrared (FIR) picosecond pulses have been generated from 20 to 200 cm−1 through the parametric interaction of two visible pulses. Pulse energies were generated up to 3 nJ, which are sufficient for time-resolved spectroscopy. The quantum efficiency of 0.1–0.3% was surprisingly constant over this frequency range considering the large variations in FIR absorption, diffraction, and phonon resonance enhancement. The depletion of the visible pulse and the dependence of the FIR energy on pump power were typical of a saturated process. However, the FIR energy showed unexpectedly large pulse-to-pulse fluctuations. Understanding the origin of these fluctuations may lead to substantial improvements in the average efficiency.
Rapid thermal nitridation of thin thermal silicon dioxide films
Authors: Nulman J. , Krusius J. P.
Published: July 2002

The electrical characteristics of thin nitrided thermal silicon dioxide films prepared by a new rapid thermal process are described. This process is performed in a controlled gas ambient under an incoherent photon flux with a duration typically less than 60 s. It allows the incorporation of a controlled nitride layer in the bulk (oxinitride), on the surface, or at the silicon interface of the oxide. Gate dielectrics with superior electrical and chemical properties have been obtained. Applications in metal-oxide-semiconductor integrated circuit technologies are suggested.
Manufacturing process for interconnected submodules of hydrogenated amorphous silicon photovoltaic panels
Authors: Gau S. C. , Volltrauer H. , Faras F. , Delahoy A. E. , Eser E. , Kiss Z.
Published: July 2002

Large scale deposition of hydrogenated amorphous silicon (a-Si) for photovoltaic panels is discussed. The ‘‘box carrier’’ concept is introduced for hydrogenated a-Si deposition. The spectral response studies prove that cross contamination is not present. Using this new concept, the average of 72 panels in one run is 5.6%. An I-V curve for a square foot panel with an efficiency of 7.2% is shown. The utilization efficiency of silane is much higher (61%) than what has previously been reported (8%).
Acceleration sensitivities of surface acoustic waves propagating on a cantilever quartz beam
Authors: Hauden D. , Loewenguth B.
Published: July 2002

High accuracy quartz accelerometers are built on the principle of an electronic servoloop circuit. We study the performance of open loop quartz accelerometers which have surface acoustic wave (SAW) delay lines (or resonator) as sensitive component mounted in an oscillator. Sensitivities versus bending forces and compression force are calculated as a function of quartz crystal anisotropy. Comparison with experimental values obtained with SAW oscillator shows that transverse acceleration has a very low sensitivity for single rotated cut (Y±θ).
Self-defocusing and self-phase modulation in InSb measured with picosecond infrared pulses
Authors: Elsaesser T. , Lobentanzer H. , Kaiser W.
Published: July 2002

The nonlinear distortion of the beam profile by self-defocusing is studied at the absorption edge of InSb with tunable picosecond pulses. In addition, the rapid change of the refractive index Δn is monitored by self-phase modulation of the infrared pulses. A quantitative analysis of the results gives a Δn≂4×10−2 at a pulse energy of 10−6 J. The optical nonlinearity occurs on a time scale shorter than 4 ps.
Diffusion of zinc into Ga1−xAlxAs
Authors: Ageno S. K. , Roedel R. J. , Mellen N. , Escher J. S.
Published: July 2002

A study of the diffusion of zinc into Ga1−xAlxAs with a ‘‘leaky tube’’ diffusion furnace has been carried out. The Ga1−xAlxAs epitaxial layers were grown with liquid phase epitaxy, and the aluminum concentration was varied from 0 to 44% in 4% increments; diffusions were performed in the temperature range 550–650 °C. It was found that the junction depth versus aluminum concentration was not a simple, monotonically increasing curve; rather, dramatic decreases in the apparent diffusion coefficient at xAl≊0.05 and xAl≊0.20 suggest that point defects, possibly including the DX center, are playing a significant role.
Comparison of electronic effects and stress effects in enhancing regrowth rate of ion-implanted amorphous Si
Authors: Pai C. S. , Lau S. S. , Suni I. , Csepregi L.
Published: July 2002

The electronic effects and stress effects for epitaxial regrowth rate enhancement in ion-implanted Si were investigated. By co-doping B and Ge into Si layers at the same time, the strain field introduced by these impurities in Si can be compensated. Significantly enhanced regrowth rates were obtained in B-Ge co-doped and stress-free Si samples. In contrast, only a slight increase in regrowth rate was observed in the strained Si samples doped only with Ge. These results lead us to conclude that the regrowth rate enhancement of ion-implanted Si is due mainly to the electronic effects.
Ghost proximity correction in an electron image projector
Authors: Nicholas K. H. , Ford R. A.
Published: July 2002

The electron image projector is capable of defining submicron integrated circuit patterns but proximity effects are observed with very fine geometries. The possibility of using ‘‘Ghost’’ proximity corrections has been investigated. It has been shown that features down to 0.3 μm can be reproduced without measurable proximity effects even with fully isolated features.
Flux annihilation in superconducting tubes
Authors: Clem John R. , Fang M. M. , Miller S. L. , Ostenson J. E. , Zhao Z. X. , Finnemore D. K.
Published: July 2002

A study of the motion of quantized vortices in cylindrical superconducting thin films was undertaken to determine the feasibility of constructing a magnetic shield in which the trapped transverse magnetic flux could be systematically swept out. The idea is to induce vortex-antivortex annihilation along one side of the cylinder under the influence of the Lorentz force using a transport current parallel to the axis of the cylinder. It is found that for a granular aluminum film approximately 1500 Å thick, there is a narrow temperature window near the critical temperature in which the vortices and antivortices can indeed be swept to one side and the internal field significantly reduced.
Amplified spontaneous emission of F+3 color centers in a LiF crystal
Authors: Zheng Lixing , Guo Shaozhang , Wan Liangfeng
Published: July 2002

This letter reports the observation of one- and two-dimensional amplified spontaneous emission of F+3 color centers in a LiF crystal and a measurement of its optical gain coefficient.