bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear spectral dynamics of hypersonic laminar boundary layer flow
Authors: Chokani Ndaona
Published: July 2002

The nonlinear interactions of the instability modes in a hypersonic laminar boundary layer undergoing natural transition are examined using bispectral analysis. The data are from an experiment of a boundary layer flow on a cooled-wall cone in a low-level free-stream disturbance hypersonic wind tunnel, and thus, the bispectral measurements are a good representation of the natural transition processes. The bispectral analysis shows that in the initial stages of transition the dominant nonlinear interaction is forcing by the fundamental to generate a harmonic. Subsequently, mutual forcing of the fundamental and harmonic yield a second harmonic. Difference interactions within the band of unstable disturbances centered on the fundamental and harmonic also generate a low frequency nonlinear interaction. At high amplitudes of the fundamental and harmonic a nonlinear interaction characterized by a low frequency modulation of the fundamental and harmonic then follows. This nonlinear interaction is then the most dominant and precedes the breakdown of the laminar flow.
The coalescence cascade of a drop
Authors: Thoroddsen S. T. , Takehara K.
Published: July 2002

When a drop is deposited gently onto the surface of a layer of the same liquid, it sits momentarily before coalescing into the bottom layer. High-speed video imaging reveals that the coalescence process is not instantaneous, but rather takes place in a cascade where each step generates a smaller drop. This cascade is self-similar and we have observed up to six steps. The time associated with each partial coalescence scales with the surface tension time scale. The cascade will, however, not proceed ad infinitum due to viscous effects, as the Reynolds number of the process is proportional to the square root of the drop diameter. Viscous effects will therefore begin to be important for the very smallest drops. This cascade is very similar to the one observed previously by Charles and Mason [J. Colloid Sci. 15, 105 (1960)] for two immiscible liquids, where one of the liquids replaces the air in our setup.
Spreading and imbibition of viscous liquid on a porous base. II
Authors: Davis S. H. , Hocking L. M.
Published: July 2002

The competition between viscous spreading of a liquid on a substrate and its absorption by a dry porous substrate is studied in two dimensions. The effect of capillary suction into the pores vies with contact-line slip on the substrate to determine the lifetime of the drop.
Weakly nonlinear saturation of short-wave instabilities in a strained Lamb–Oseen vortex
Authors: Sipp Denis
Published: July 2002

A Lamb–Oseen vortex in a planar straining field is known to be subject to 3D (three-dimensional) short-wave instabilities which are due to the resonance of the straining field and two stationary Kelvin waves characterized by the same axial wave number and by azimuthal wave numbers equal to −1 and +1. The linear regime has been described by Moore and Saffman (1975). In this article, we extend this analysis to the weakly nonlinear regime. The emerging eigenmode is characterized by a complex amplitude A=|A|eiφ, whose behavior is governed by an amplitude equation. It is shown that the unstable perturbation corresponds to an oscillation of the vortex in a plane inclined at an angle φ, while the amplitude of these oscillations is proportional to |A|. The vortex centers are defined as the points where the velocity of the vortex is zero, which also corresponds to the points where the pressure is minimum. We show that these instabilities saturate. The saturation amplitudes are evaluated numerically and expressed in terms of oscillation amplitudes of the vortex centers. If a denotes the internal radius of the vortex and if the straining field is due to a counter-rotating vortex of same strength, located at a distance b, then the maximum amplitude Δ of the vortex oscillations is Δ/b=6.1a2/b2. This result is in agreement with those of the experiments of Leweke and Williamson (1998) for which a/b=0.2. It also shows that in aeronautical situations, for which a/b is smaller, i.e., a/b<0.1, the considered short-wave instability will saturate at very low amplitude.
Axisymmetric instabilities of Bödewadt flow
Authors: Fernandez-Feria R.
Published: July 2002

A spatial linear stability analysis of Bödewadt’s self-similar solution for the rotating flow over a flat plate is performed. In particular, considered is the stability of axisymmetric perturbations propagating towards the axis of rotation, which are the most important ones observed experimentally. Viscous and nonparallel effects on the stability of the perturbations are retained up to the order of the inverse of the local Reynolds number R. The resulting parabolic stability equations are solved numerically using a spectral collocation method varying the nondimensional frequency q and R. The instability region on the (q,R)-plane is discussed and compared with existing experimental data and direct numerical simulation results. The circular waves observed experimentally and in numerical simulations are shown to correspond to an inertial instability mode which becomes stabilized as R decreases below a critical value.
Receptivity to harmonic forcing of a finite-width mixing layer just downstream of the trailing edge of a flat plate
Authors: Rabchuk J. A.
Published: July 2002

Analytical solutions for the receptivity of a piecewise linear mixing layer to harmonic disturbances at the trailing edge of a flat plate are presented. The solution is for an ideal fluid, with viscous effects at the trailing edge being accounted for by imposing an unsteady Kutta condition. The wake from the flat plate is included in the model. Initial mixing layer response for the discrete and continuous modes of instability is determined as a function of forcing strength, Strouhal number, and cross stream location. Forcing frequencies throughout the unstable regime are considered. The various components of the perturbation field and their contributions to the perturbation velocity and vorticity are investigated in the near field of the trailing edge. In the limit that the forcing frequency approaches zero, the amplification of the streamwise velocity and vorticity associated with the growing instability mode is shown to be inversely proportional to the forcing frequency, in agreement with previous results obtained using the vortex sheet approximation. In contrast, the initial cross stream velocity of the disturbance is quite small for low frequencies, and reaches a peak at the most amplified frequency of the mixing layer profile. The analysis also shows that at higher frequencies the continuous modes of instability become increasingly important channels whereby vorticity is carried away from the trailing edge. The differences in wave number between the various competing components at these higher frequencies leads to the appearance of nodes in the time-averaged perturbation quantities downstream. The entire perturbation is examined at the most amplified frequency.
A thickened flame model for large eddy simulations of turbulent premixed combustion
Authors: Colin O. , Ducros F. , Veynante D. , Poinsot T.
Published: July 2002

A subgrid scale model for large eddy simulations of turbulent premixed combustion is developed and validated. The approach is based on the concept of artificially thickened flames, keeping constant the laminar flame speed sl0. This thickening is simply achieved by decreasing the pre-exponential factor of the chemical Arrhenius law whereas the molecular diffusion is enhanced. When the flame is thickened, the combustion–turbulence interaction is affected and must be modeled. This point is investigated here using direct numerical simulations of flame–vortex interactions and an efficiency function E is introduced to incorporate thickening effects in the subgrid scale model. The input parameters in E are related to the subgrid scale turbulence (velocity and length scales). An efficient approach, based on similarity assumptions, is developed to extract these quantities from the resolved velocity field. A specific operator is developed to exclude the dilatational part of the velocity field from the estimation of turbulent fluctuations. The combustion model is then implemented in a compressible parallel finite volume–element solver able to handle hybrid grids to simulate a lateral injections combustor (LIC). Results are in agreement with the available experimental data.
On the Grad–Shafranov equation as an eigenvalue problem, with implications for q solvers
Authors: LoDestro L. L. , Pearlstein L. D.
Published: July 2002

It is shown that the Grad–Shafranov equation for toroidally symmetric ideal-magnetohydrodynamic (MHD) equilibria is a conventional albeit nonlinear eigenvalue problem. That this has been generally overlooked with limited consequences has been made possible by the existence of a scale-invariant transformation of the equation. If the safety factor q is chosen in place of the toroidal field as one of the free flux functions specifying the source (numerical Grad–Shafranov solvers with this capability are called ‘‘q solvers’’), the eigenvalue is 1 and the scale-transformation factor drops out of the problem. It is shown how this is responsible for the numerical problems that have plagued a class of q solvers, and a simple remedy is suggested. This has been implemented in Livermore’s toroidal equilibrium code (TEQ), and as an example, a quasistatically evolved vertical event is presented.
On the role of neutral particles on edge turbulence and electric fields in the Advanced Toroidal Facility
Authors: Hidalgo C. , Brañas B. , Uckan T. , Harris J. H. , Isler R. , Ritz Ch. P. , Wootton A.
Published: July 2002

Measurements of edge plasma turbulence properties have been carried out in the Advanced Toroidal Facility (ATF) [Fusion Technol. 10, 179 (1986)] during experiments in which hydrogen and neon gas puffing are used to vary the edge temperature in the range 5<Te<40 eV in a controlled way. In the temperature range where the rate coefficients for ionization processes are strongly temperature dependent (5<Te<15 eV) the electron temperature plays an important role in determining the level of edge turbulence and the value of the self-generated radial electric fields. These results provide evidence of edge turbulence and flows modified by the presence of neutral particles.
Stability of force-free plasma–vacuum equilibria
Authors: Lortz D. , Spies G. O.
Published: July 2002

The magnetohydrodynamic stability of force-free plasma–vacuum systems (curl B=μB in the plasma, with constant μ) is studied in circular cylinders with identified ends (topological torus). A necessary stability criterion is derived by considering large poloidal mode numbers. This takes a simple form (the magnetic rotation numbers at the axis and plasma–vacuum interface must have opposite signs) if the magnetic field lines in the interface are not closed. If they are closed, then violation of this simple condition does not imply instability unless the aspect ratio exceeds some value which depends on both the numerator and denominator of the rational magnetic rotation number at the interface. For aspect ratios greater than unity, combination with the criterion for stability to internal kinks implies that the inhomogeneity parameter ‖μ‖ must be above the threshold for reversal of the toroidal current density, but below that for reversal of the poloidal one. This condition is independent of the wall radius, in contrast to the well-known necessary and sufficient stability criterion in the limit of infinite aspect ratio, which remains sufficient for arbitrary aspect ratios, and which requires that ‖μ‖ be in a smaller interval that does depend on the wall radius.
New configuration of a virtual cathode oscillator for microwave generation
Authors: Jiang W. , Masugata K. , Yatsui K.
Published: July 2002

A new configuration of a virtual cathode oscillator is proposed for high-power microwave generation. Analysis and simulation are carried out to investigate the characteristics of this configuration. In the numerical simulation, the microwave output power of ∼564 MW is obtained with an electron beam of 300 keV in electron energy and 15 kA in beam current, giving a beam-to-microwave efficiency of ∼12.5%. The significant features of the new configuration are (1) the microwave output power is not affected by an electron energy spread up to 6%; and (2) no significant change of the oscillation frequency is observed when the beam current changes from 10 to 20 kA.
Quasi-equilibria: A special class of time-dependent solutions of the two-dimensional magnetohydrodynamic equations
Authors: Neukirch T.
Published: July 2002

A special method is presented to construct exact time-dependent solutions of the two-dimensional ideal magnetohydrodynamic (MHD) equations for which plasma elements experience no acceleration. The momentum equation then contains the time merely parametrically and assumes the structure of an equilibrium equation. For a special form of the pressure profile p(A), for which the corresponding quasi-equilibrium equation is a completely integrable non-linear elliptic equation that is invariant under conformal transformations, these invariance properties are then used to determine the possible time-dependences of the solutions. Contrary to the common use of the term quasi-equilibrium arbitrarily large plasma velocities are allowed in the present treatment. In polar coordinates, the time evolution turns out to be self-similar in the radial coordinate, but it is in general not self-similar in the azimuthal coordinate. The adiabatic exponent of the plasma is found to be equal to one, which means that the plasma is isothermal. Explicit examples of solutions are discussed.
Formation and evolution of envelope shocks of ion plasma waves
Authors: Honzawa Tadao , Hoshina Takao , Saitou Yoshifumi
Published: July 2002

It is experimentally shown that envelope shocks of low frequency ion plasma waves are formed in a plasma. If the wave amplitude is large enough, a broad packet form of externally excited ion plasma wave first steepens at its leading edge. Thereafter, the wave envelope near the edge evolves into a series of wave packets, forming an envelope shock. A precursor due to reflected ions is also observed ahead of the shock.
The perpendicular electron energy flux driven by magnetic fluctuations in the edge of the Texas Experimental Tokamak
Authors: Fiksel G. , Bengtson Roger D. , Prager S. C. , Wootton A. J.
Published: July 2002

A fast bolometer was used for direct measurements of parallel electron energy flux in the edge of the Texas Experimental Tokamak (TEXT-U) [K. W. Gentle, Nucl. Technol. Fusion 1, 479 (1981)]. The fluctuating component of the parallel electron energy flux, combined with a measurement of magnetic fluctuations, provides an upper limit to the perpendicular electron flux. This magnetically driven energy flux cannot account for the observed energy flux.
On spurious modes for finite Larmor radius models in the ion cyclotron range of frequencies
Authors: Wang C. Y. , Batchelor D. B. , Jaeger E. F.
Published: July 2002

Spurious resonances exist in the dispersion relations for finite Larmor radius models in plasma heating in the ion cyclotron range of frequencies. The spurious resonances occur when the highest-order coefficient in a fourth-order equation vanishes. It is shown that the spurious resonances cannot be removed by using a sixth-order equation because the highest-order coefficient in the sixth-order equation also vanishes at the same location as the highest-order coefficient in the fourth-order equation. However, the spurious modes are often not excited when solving the corresponding ordinary differential equations (ODEs) for an incident fast wave so that the solutions from the ODEs are often accurate.
Collisionless ballooning modes in a toroidal two-ion-component plasma
Authors: Jarmén A. , Malinov P.
Published: July 2002

A toroidal two-fluid model allowing for arbitrary magnetic field curvature is used to describe a contaminated plasma consisting of two ion species. A higher-order ballooning mode is identified in the high- and low-α regimes, and coexists with the ordinary ballooning mode in the medium-α regime (α=ballooning parameter). In the low-α regime the temperature-gradient-driven ηi mode is present. The impurities are, in general, found to be stabilizing—lowering the growth rate and narrowing the instability regime. Comparisons are made with ideal magnetohydrodynamics (MHD) and with a semilocal approximation of the eigenfunction, which are found to give a higher growth rate.
Experimental study of a mixed slab and toroidal ion-temperature-gradient mode
Authors: Chen J. , Sen A. K.
Published: July 2002

The first study of the transition of the slab toward the toroidal branch of the ion-temperature-gradient (ITG) driven mode in the Columbia Linear Machine (CLM) [Plasma Phys. 24, 185 (1982)] is reported. This has been achieved by first exciting the slab ITG mode and gradually increasing the magnetic curvature by energizing a magnetic mirror coil. The magnetic curvature can then be adjusted either by moving the mirror coil axially (to change the mirror cell length) or by varying the mirror coil current (to change the mirror ratio). The effects of the modest levels of magnetic curvature are seen to be further destabilization and reduction of the real frequency of the mode. However, with a further increase in the mirror ratio, the mode amplitude decreased. A kinetic model of the mixed slab and ‘‘toroidal’’ ITG mode with trapped particles has been developed to predict the mode behavior. Computational results from this with the plasma parameters in the CLM corroborate the existence of an instability with the observed real frequency and mode amplitude, and which has the appearance of a hybrid ITG–dissipative trapped electron mode.
Oscillating field current drive and nonlinear plasma response in bootstrapped tokamaks
Authors: Weening R. H.
Published: July 2002

The nonlinear response of bootstrapped tokamaks to oscillating field current drive (OFCD) is studied with the aid of a formal mathematical solution. The tokamak plasma response to OFCD is shown to be similar to that of a driven inductor–resistor (LR) circuit, but the effective plasma resistance can be negative. A physical picture is constructed in which the tokamak bootstrap effect is viewed as a nonlinear amplification of the plasma magnetic helicity. The bootstrap amplifier can rectify the toroidal plasma current. The importance of the tokamak thermal instability for peaking the plasma current and creating a plasma dynamo is stressed. Estimates are made which suggest that steady-state tokamak fusion reactors operating with OFCD may be achievable with moderate amplitude toroidal magnetic flux and loop voltage oscillations and oscillation driving frequencies significantly below the audible frequency range.
Global energy confinement under coupling of the transport process with magnetohydrodynamic modes
Authors: Deng X. H. , Huo Y. P. , Zhang C. , Wang J. F. , Wang S.
Published: July 2002

Macroscopic instabilities are governed by transport-determined plasma profiles and transport, in turn, is affected by instabilities. It is clear that neither transport process nor the behavior of instabilities can be treated in isolation. A model is developed for investigating the effects of the instabilities on global energy confinement in tokamaks. The results clearly demonstrate that magnetohydrodynamic (MHD) instabilities coupled with the transport process change not only the value of global energy confinement time, but also, perhaps more importantly, the energy confinement scaling law. While considering MHD effects, global energy confinement becomes sensitively dependent on the boundary transport conditions, and the cases with and without MHD stability can result in quite different parameter behavior of the global energy confinement. Such coupling should be considered when attempts are made to analyze the global energy confinement in tokamaks.
Impurity penetration through the stochastic layer near the separatrix in tokamaks
Authors: Morozov D. Kh. , Herrera J. J. E. , Rantsev-Kartinov V. A.
Published: July 2002

It is shown that a stochastic layer produced by ripple perturbations near the separatrix in tokamaks, leads to anomalous plasma flow out of the bulk plasma along perturbed field lines, which brings out impurities. This suggests that the stochastic layer may play a cleaning role. There is an opposite process of anomalous impurity diffusion into the plasma. The balance of these two processes defines the impurity concentration in the bulk plasma.
Numerical analysis of the trajectories of a single charged particle in a nonadiabatic cusp magnetic field configuration
Authors: Selvarajan V. , Vijayalakshmi K. A.
Published: July 2002

The trajectories of a nonrelativistic single charged particle in a nonadiabatic cusped magnetic field configuration are numerically analyzed. Depth of penetration of the particle into the cusped field, multiple reflections and radial excursion of the particle transmitted by the cusped field are discussed. A significant observation was that a meticulous choice of particle and field parameters could lead to several number of reflections. For radial excursion a plateau region over Φ was exhibited.
Coupling between electron plasma waves in laser–plasma interactions
Authors: Everett M. J. , Lal A. , Clayton C. E. , Mori W. B. , Joshi C. , Johnston T. W.
Published: July 2002

A Lagrangian fluid model (cold plasma, fixed ions) is developed for analyzing the coupling between electron plasma waves. This model shows that a small wave number electron plasma wave (ω2,k2) will strongly affect a large wave number electron plasma wave (ω1,k1), transferring its energy into daughter waves or sidebands at (ω1+nω2,k1+nk2) in the lab frame. The accuracy of the model is checked via particle-in-cell simulations, which confirm that the energy in the mode at (ω1,k1) can be completely transferred to the sidebands at (ω1+nω2,k1+nk2) by the presence of the electron plasma mode at (ω2,k2). Conclusive experimental evidence for the generation of daughter waves via this coupling is then presented using time- and wave number-resolved spectra of the light from a probe laser coherently Thomson scattered by the electron plasma waves generated by the interaction of a two-frequency CO2 laser with a plasma.
Magnetohydrodynamic solution for a Z pinch showing the production of a hot spot
Authors: Maxon S. , Hammer J. H. , Eddleman J. L. , Tabak M. , Zimmerman G. B. , Alley W. E. , Estabrook K. G. , Harte J. A. , Nash T. J. , Sanford T. W. L. , De Groot J. S.
Published: July 2002

Two-dimensional LASNEX [National Technical Information Service Document No. DE 81026329 (Zimmerman, Report No. UCRL-74811, 1973)] calculations are made for a Z pinch on Saturn, the low-impedance, low-inductance electron accelerator at the Sandia National Laboratories [D. D. Bloomquist et al. Proceedings of the Sixth IEEE Pulsed Power Conference, Arlington, VA, edited by P. J. Turchi and B. H. Bernstein (Institute of Electronics and Electrical Engineers, New York, 1987), p. 310]. The experiment is characterized by a current of 6 MA with a tungsten wire load (4 mg) at 2 mm. Two-dimensional calculations show the evolution of the Rayleigh–Taylor instability to the bubble and spike phase, causing high-density islands to form in the pinch opposite the bubbles. The two-dimensional energy flow causes a ‘‘hot spot’’ to evolve, which is shown to agree in its size and brightness with pinhole camera measurements. This is the first explicit calculation of a hot spot in two dimensions employing the full magnetohydrodynamic equations.
Reflectivity profiles of phase conjugate waves produced via four-wave mixing in laser plasmas
Authors: Gupta G. P. , Sinha B. K.
Published: July 2002

Theoretical profiles of power reflectivity of a phase conjugate electromagnetic wave generated by nearly degenerate four-wave mixing in a carbon plasma via parametric decay instability (PDI) are studied. The plasma is considered to be produced by irradiating a carbon slab target with an Nd:glass high-power laser pulse at an intensity above the PDI threshold. The plasma refractive index corresponding to the PDI region is taken into account in the wave equations. Two electromagnetic pump waves counterpropagating in the plasma are Nd:glass laser light waves and a weak electromagnetic probe wave incident upon the plasma, which is very slightly frequency upshifted relative to the pump waves. The effects of the frequency and angular detuning between the pump and probe waves, pump wave intensity, and plasma parameters on the reflectivity profiles have been investigated. It is noted that the plasma refractive index significantly affects the reflectivity profiles of the phase conjugate wave.
The influence of dust particles on electromagnetic ion cyclotron waves in a bi-Lorentzian plasma
Authors: Venugopal Chandu , Varughese John K. , Antony S. , Anilkumar C. P. , Renuka G.
Published: July 2002

The influence of dust particles on electromagnetic ion cyclotron (EMIC) waves, propagating parallel to the magnetic field, in a plasma where the hot ions are modelled by a bi-Lorentzian or Kappa distribution has been studied. The electrons and dust particles have been treated as cold. Expressions for the dispersion relations and growth/damping rates in both high- and low-β plasmas have been derived. For the low-β case temperature anisotropy is the source of instability in an electron–ion plasma. This instability is strongly influenced by the temperature anisotropy of the hot ions and the charge and density of the dust particles; the instability increases with these parameters. However, in high-β plasmas, the instability is driven by the dust. The growth rate increases with the charge on the dust; but with increasing dust densities the EMIC wave propagates almost freely.
Magnetic field pulses produced via whistler mode wave decay in the ionosphere
Authors: Yukhimuk Vyacheslav , Roussel-Dupre Robert
Published: July 2002

The nonlinear ionospheric effects of very low-frequency (VLF) electromagnetic radiation produced by lightning discharges are considered. Electric and magnetic field measurements by a rocket in the ionosphere reported by Kelly et al. [Geophys. Res. Lett. 17, 2221 (1990)] show the simultaneous presence of powerful electromagnetic whistler wave bursts along with electrostatic waves and a magnetic pulse with duration about 20 ms. The parametric decay of whistler mode waves into electrostatic waves and ultralow-frequency (ULF) electromagnetic waves is proposed as a possible explanation for the observed phenomenon. The evolution of whistler mode wave decay instabilities in time and two spatial dimensions is studied. The nonlinear dispersion equation is obtained, and solved numerically. Two-fluid magnetohydrodynamics is used to describe the three-wave interaction. The theoretical results are compared with observational data.
Stability thresholds for ballooning modes driven by high β internal kinks
Authors: Lütjens Hinrich , Luciani Jean-François
Published: July 2002

The nonlinear destabilization of pressure driven (ballooning) modes by internal kink modes during sawtooth cycles was found in the Tokamak Fusion Test Reactor (TFTR) [Park et al., Phys. Rev. Lett. 75, 1763 (1995)]. It is shown by a numerical parameter study, including aspect ratio and magnetic shear profile effects, that in ideal magneto-hydrodynamics (MHD), this nonlinear destabilization occurs in plasma equilibria close to marginal stability to linear sub-harmonics of the internal kink with toroidal mode number n⩾3.
Relativistic laser-plasma interaction by multi-dimensional particle-in-cell simulations
Authors: Pukhov A. , Meyer-ter-Vehn J.
Published: July 2002

Interaction of relativistically strong laser pulses with plasmas is investigated by a multi-dimensional particle-in-cell (PIC) code VLPL (Virtual Laser Plasma Laboratory) [Bull. Am. Phys. Soc. 41, 1502 (1996)]. Acceleration of background electrons to multi-MeV energies, generation of 100 MG magnetic fields, and dynamics of ion channel boring are studied. It is shown that direct v×B push by the laser pulse in the presence of an azimuthal dc magnetic field effectively accelerates background plasma electrons to energies significantly higher than the ponderomotive potential. The authors call this novel effect “B-loop” acceleration mechanism. It is dominant in near-critical plasma, or when plasma waves disappear due to wavebreaking. Laser channeling in under- and overdense plasmas is also studied. Energy spectra of the accelerated electrons and ions and the laser energy conversion efficiency at the critical surface are presented. It is shown that the accelerated electrons propagate in the form of magnetized jets. This physics is crucial for the fast ignitor concept in inertial confinement fusion.
Multipactor discharge on metals and dielectrics: Historical review and recent theories
Authors: Kishek R. A. , Lau Y. Y. , Ang L. K. , Valfells A. , Gilgenbach R. M.
Published: July 2002

This paper reviews the history of multipactor discharge theory, focusing on recent models of multipactor accessibility and saturation. Two cases are treated in detail: That of a first-order, two-surface multipactor, and that of a single-surface multipactor on a dielectric. In both cases, susceptibility curves are constructed to indicate the regions of external parameter space where multipactor is likely to occur, taking into account the dependence on surface materials, and the effects of space charge and cavity loading. In the case of a dielectric, multipactor is found to deliver about 1% of the rf power to the surface. The two cases are contrasted in light of experimental observations.
Effect of rotation on the Rayleigh–Taylor instability of two superposed magnetized conducting plasma
Authors: Sharma P. K. , Chhajlani R. K.
Published: July 2002

The study of the Rayleigh–Taylor (RT) instability of two superposed conducting fluids acted upon by a uniform horizontal magnetic field is discussed. The usual magnetohydrodynamic (MHD) equations, with modifications to include the effect of finite Larmor radius corrections (FLR) and rotation, are taken for the description of the problem. The fluid is assumed to be dusty, and dust in the form of suspended particles is considered. The equations of the problem are linearized, and using necessary boundary conditions, a general dispersion relation for the RT instability of two superposed plasma is obtained. The dispersion relation shows no contribution of the magnetic field on the condition of the RT instability. It is also found that rotation, FLR, and suspended particles do not affect the condition of the RT instability. The different special cases are discussed to find the effect of different parameters on the growth and condition of the RT instability.
Phase conjugation in the low reflectivity regime by nearly degenerate four-wave mixing in a homogeneous plasma
Authors: Gupta G. P. , Sinha B. K.
Published: July 2002

Accounting for both the long- and short-wavelength density gratings by considering the polarizations of all four electromagnetic waves parallel to each other and a finite tilt angle between the signal and pump waves, the steady-state phase conjugation in the low reflectivity regime by nearly degenerate four-wave mixing in a homogeneous plasma is investigated. The response of the density gratings caused by the beating of the signal and pump waves is considered from the theory of stimulated Brillouin scattering in a two-component plasma. The expression for the power reflectivity of the conjugate wave for the arbitrary difference frequency between the signal and pump waves and that for resonant four-wave mixing are obtained. Numerical results have been discussed in light of the experiment reported in the literature. It is noted that the measured values of the difference frequency corresponding to either density grating resonance, and the corresponding maximum reflectivity of the conjugate wave, provide a diagnostic tool for the electron temperature and the electron–ion temperature ratio, respectively.
Predictions of finite Larmor radius reduced magnetohydrodynamics for Rayleigh–Taylor unstable plasmas
Authors: Berning M. , Spatschek K. H.
Published: July 2002

Finite Larmor radius magnetohydrodynamics (FLR MHD), being valid when the ion Larmor radius is of the order of the characteristic length scale, predicts a Rayleigh–Taylor instability in an inhomogeneous plasma with unfavorable field line curvature. It is shown that the FLR MHD equations can be reduced to a set of two scalar field equations for the pressure and the electrostatic potential in the flute approximation and the large aspect ratio limit. Using a Galerkin approximation with a few dominating modes, the simple model reproduces previous numerical simulations based on the full FLR MHD equations.
Collisional amplification of test electromagnetic waves in a plasma subject to an intense high-frequency laser field
Authors: Das Amita , Kaw Predhiman
Published: July 2002

Recently, calculations based on Fokker–Planck and Landau collision operators have been used to demonstrate that test waves with electric fields in the plane of polarization of external superintense laser fields can get amplified by nonlinear collisional processes. The calculations are similar in spirit to parametric instability calculations and make use of an averaging process for the collision operator whose range of validity is not clear. The Dawson–Oberman model of high-frequency plasma conductivity is used to reinvestigate this problem. It is shown that in the limit of small test wave frequency, ω1/ω0=ε≪1, no amplification is possible. This result contradicts the one obtained by earlier workers with averaged collision operators. At higher test wave frequencies earlier results are recovered.
Long-range time dependence in the cross-correlation function
Authors: Carreras B. A. , Newman D. E. , van Milligen B. Ph. , Hidalgo C.
Published: July 2002

Detection of a long-range time dependence in the radial cross-correlation function is normally difficult because of the oscillatory behavior of the cross-correlation tail, its low level of coherence, and noise contamination. This problem persists, even with large statistical samples. In this paper, a method for investigating long-range dependence in a single time series is extended to the calculation of the cross-correlation function. With this method and for time series with long-range time correlations, the accuracy of the determination of the cross-correlation function for long time lags is improved. The method is tested by applying it to fractional Gaussian noise and to the fluxes in a running sandpile model. This analysis technique can be applied to the detection of avalanche-type transport in magnetic confinement devices.
Renormalization-group analysis on the scaling of the randomly-stirred magnetohydrodynamic plasmas
Authors: Kim Chang-Bae , Yang Taek-Jin
Published: July 2002

Similar to the neutral fluids [M. Lesieur, Turbulence in Fluids (Kluwer, Dordrecht, 1990), p. 137], two-dimensional magnetohydrodynamic (MHD) plasmas appear to obey the power-law behavior in terms of the size of the eddy. Scaling behavior of the randomly stirred MHD plasma is studied through the renormalization-group (RG) method. Generating functional is constructed and, from the power counting, the primitively divergent vertex functions are identified up to one-loop order. They are regularized by proper renormalizations of the viscosity, the resistivity and the size of the fluctuation. Scaling solution is shown to exist at the fixed point of the RG equation. The dependence of the power exponent of the energy spectrum on the driving Gaussian noise is derived.
Lower hybrid wave coupling and propagation in the presence of nonthermal edge electrons
Authors: Jacquet P. , Demers Y. , Fuchs V. , Glaude V.
Published: July 2002

In order to improve our understanding of lower hybrid current drive experiments in high power regimes, where fast electrons (up to few keV) are generated in front of the lower hybrid (LH) grill, an analysis of LH wave coupling and propagation is carried out using a formulation of the dielectric tensor that incorporates the fast electron population (Thot≲5 keV). It is found that both the real and imaginary parts of the dielectric tensor are modified for waves with |N∥|≳10, which can interact with the fast electrons through Landau damping. Consequently high N∥ waves can be absorbed within less than 1 cm, or even cut-off at the grill–plasma interface. Antenna–plasma coupling code results show that the power available to accelerate electrons in front of the grill (launched power with |N∥|>10) decreases in the presence of fast electrons. Also, despite strong modifications of the dielectric tensor, and of the power transfer to high N∥ waves, the presence of fast electrons (Thot≲5 keV) leads to only moderate changes of antenna reflectivity and directivity.
Penetration of a transverse magnetic field by an accelerated field-reversed configuration
Authors: Slough J. T. , Hoffman A. L.
Published: July 2002

The field-reversed configuration (FRC) is a compact toroid with near unity β that is an ideal candidate to provide central fueling for large tokamaks. The study of the acceleration and penetration physics of the FRC into a transverse magnetic field gradient is necessary in order to evaluate the fueling efficiency of the FRC. To this end, experiments were conducted on the LSX/mod facility [J. T. Slough and A. L. Hoffman, 16th IAEA Fusion Energy Conference 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. II, p. 237], where large mass (0.8 mg) FRCs were accelerated to high velocity (2×105 m/s), and then guided into a transverse magnetic field. Various diagnostics were employed to characterize the penetration process. These included thermocouple probes, magnetic probes, and emission arrays. A simple analytical model is developed that explains the basic features of the penetration process. Further modeling with two-dimensional numerical calculations provided for scaling laws to reach the conditions necessary to penetrate a large fusion tokamak.
The gyro-radius scaling of ion thermal transport from global numerical simulations of ion temperature gradient driven turbulence
Authors: Ottaviani M. , Manfredi G.
Published: July 2002

A three-dimensional, fluid code is used to study the scaling of ion thermal transport caused by ion-temperature-gradient-driven (ITG) turbulence. The code includes toroidal effects and is capable of simulating the whole torus. It is found that both close to the ITG threshold and well above threshold, the thermal transport and the turbulence structures exhibit a gyro-Bohm scaling, at least for plasmas with moderate poloidal flow.
Runaway discharge in the small Brazilian Tokamak TBR-1
Authors: Kuznetsov Yu. K. , El Chamaa Neto I. , Nascimento I. C. , Galvão R. M. O.
Published: July 2002

Runaway discharges are studied in the small Brazilian Tokamak TBR-1 (R=30 cm, a=8 cm, Bt=0.5 T, It=12 kA). Low plasma density, up to 1012 cm−3, provides reproducible runaway discharges with the runaway current more than 95% of the total current, the Shafranov parameter up to 1.8, and the effective kinetic energy of the runaway electrons about 300–360 keV. The runaway-related relaxation phenomenon is triggered by the biased electrode technique and the plasma resistance is determined from the time decay of the loop voltage spikes. The experimental data are analyzed by comparison with the theory of runaway generation and equilibrium of the toroidal relativistic beam-plasma system.
Soft x-ray emission in the neon gas puff plasma focus
Authors: Yanagidaira Takeshi , Shimoda Katsuji , Ono Yasushi , Hirano Katsumi
Published: July 2002

Correlation between soft x-ray emission and dynamic plasma behavior is discussed for a plasma focus device with injected neon. Intense H-like and He-like lines from neon are radiated during an interval when the plasma appears from a visible-light streak camera to be stable. However, data from a multiframing soft x-ray camera indicate the growth of m=0 macroscopic instabilities during this period. The peak of the soft x-ray signal coincides with the micropinch generated in the plasma column, and the emission ends simultaneously with the extinction of the visible light. The electron temperature and density, which are obtained from spectroscopic analysis, are ∼0.5 keV and ∼1022 cm−3, respectively.
Comparisons and physics basis of tokamak transport models and turbulence simulations
Authors: Dimits A. M. , Bateman G. , Beer M. A. , Cohen B. I. , Dorland W. , Hammett G. W. , Kim C. , Kinsey J. E. , Kotschenreuther M. , Kritz A. H. , Lao L. L. , Mandrekas J. , Nevins W. M. , Parker S. E. , Redd A. J. , Shumaker D. E. , Sydora R. , Weiland J.
Published: July 2002

The predictions of gyrokinetic and gyrofluid simulations of ion-temperature-gradient (ITG) instability and turbulence in tokamak plasmas as well as some tokamak plasma thermal transport models, which have been widely used for predicting the performance of the proposed International Thermonuclear Experimental Reactor (ITER) tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 3], are compared. These comparisons provide information on effects of differences in the physics content of the various models and on the fusion-relevant figures of merit of plasma performance predicted by the models. Many of the comparisons are undertaken for a simplified plasma model and geometry which is an idealization of the plasma conditions and geometry in a Doublet III-D [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] high confinement (H-mode) experiment. Most of the models show good agreements in their predictions and assumptions for the linear growth rates and frequencies. There are some differences associated with different equilibria. However, there are significant differences in the transport levels between the models. The causes of some of the differences are examined in some detail, with particular attention to numerical convergence in the turbulence simulations (with respect to simulation mesh size, system size and, for particle-based simulations, the particle number). The implications for predictions of fusion plasma performance are also discussed.
Nonlinear dynamics of electron flows with density gradient in spherical diodes
Authors: Alves M. Virgı́nia , Gnavi Graciela , Gratton Fausto T. , Ferro Sergio , Moreno César H.
Published: July 2002

A variety of Pierce diodes formed by a pair of grounded concentric spheres with electrons streaming radially are investigated with particle-in-cell simulations and analytical models. The electrons flow parallel or antiparallel to a density gradient in convergent or divergent diodes, respectively. The existence and stability (or instability) of laminar steady states with space charge and nonuniform velocity that are possible for a given set of diode parameters, are crucial to determine the outcome of the numerical experiments. The dispersion relation for the stability of the initial constant velocity states and the concomitant energy flow balance are examined in relation to the nonlinear dynamical evolution. Spherical diodes have properties that are common to a dual pair of diodes with reciprocal ratios of collector to emitter radii. Stable oscillatory electrostatic states, associated with Hopf bifurcations, are observed in the simulations. The breakdown of the laminar electron flow has completely different physical characteristics in divergent and convergent diodes.
Submicron-gap high-mobility silicon picosecond photodetectors
Authors: Shahidi Ghavam G. , Ippen E. P. , Melngailis J.
Published: July 2002

High-speed photoconductive switching in silicon is achieved, without a reduction in mobility or lifetime, by carrier sweep-out. Using photoconductive gaps ranging in width from 14 to 0.22 μm, we have studied the variation of switching speed with gap dimension and applied voltage. With submicron gaps and optical energies of less than 1 nJ, we have been able to switch pulses of more than 1 V with durations as short as 27 ps full width at half-maximum.
Reduction of fast interface states and suppression of drift phenomena in arsenic-stabilized metal-insulator-InP structures
Authors: Blanchet R. , Viktorovitch P. , Chave J. , Santinelli C.
Published: July 2002

A significant improvement of the electronical properties of metal-Al2O3-InP structures is obtained after treating InP substrates at 500 °C in an As overpressure (10−6 Torr). The density of fast interface states near the conduction-band edge is markedly reduced and drift phenomena are greatly suppressed, as compared to results obtained with chemically etched reference substrates. An interpretation of these effects is given based on the compensation of phosphorus vacancies by As atoms more strongly attached to the InP surface than P atoms.
Direct molybdenum contacts to silicon
Authors: Cohen S. S. , Kim M. J. , Brown D. M. , Gildenblat G.
Published: July 2002

Stringent requirements on the electrical and metallurgical properties of metallization systems for use in advanced, very large scale integrated circuits have created the need to study new metal-silicon systems. We have conducted a study of the Mo/Si direct contact system. The choice of molybdenum stems from its desirable electrical and metallurgical properties. To date, however, its contact properties to silicon were not examined in detail. Results of the present study show that Mo can result in low resistivities (<5 Ω μm2) for both contacts to heavily doped p+-Si and n+-Si, provided appropriate care is taken in opening the contact windows. Further, we found the Mo/Si contact system to be stable under extended heat treatments at temperatures of up to 650 °C.
Persistent electron redistribution among deep levels in CdTe
Authors: Jantsch W. , Brunthaler G.
Published: July 2002

Photo electron paramagnetic resonance measurements in CdTe show that sub-band-gap illumination causes a transfer of electrons from deep Ge donors to deep Ni acceptor states at a higher energy. This metastable state persists in the dark and can be quenched by illumination with photon energies in the midgap range. The metastability is explained in terms of the strong localization of both the Ni and the Ge states.
Spatial distribution of 0.68-eV emission from undoped semi-insulating gallium arsenide revealed by high resolution luminescence imaging
Authors: Warwick C. A. , Brown G. T.
Published: July 2002

The luminescence properties of undoped semi-insulating liquid encapsulated Czochralski-grown GaAs ingots have been characterized with ∼3-μm spatial resolution using a scanning electron microscope based, cathodoluminescence (CL) system. The use of a cryostat operating at ∼6 K with cooled PbS and Ge detectors for midgap luminescence imaging has revealed for the first time a depletion in the 0.68-eV emission from zones ∼20 μm wide adjacent to individual polygonized dislocation arrays known as cell walls. These midgap luminescence results have been correlated with band edge CL images of cell structures and with dislocation images obtained using double crystal x-ray topography. It is concluded that dislocation arrays attract or getter the defect centers responsible for the 0.68-eV emission in undoped semi-insulating GaAs, as opposed to generating them.
Role of quasiparticle scattering in Gray’s superconducting transistor
Authors: Frank D. J. , Davidson A. , Klapwijk T. M.
Published: July 2002

We report the results of detailed numerical simulations of Gray’s transistor mechanism [Appl. Phys. Lett. 32, 392 (1978)]. These calculations, carried out using two different approaches, show that the energy scattering of quasiparticles is very important in achieving gain such as Gray reported. When this scattering is included, we find that gain is possible, not only in aluminum, but also in niobium and tantalum.