bluuebunny/crossref_metadata_2025 huggingface dataset:
Wave emission by resonance crossing
Authors: Tracy E. R. , Kaufman A. N. , Liang Y.-M.
Published: July 2002

The emission of collective waves by a moving charged particle in a nonuniform medium is discussed. Emission occurs in a nonuniform medium when the local dispersion relation of the collective wave is satisfied. This is a form of resonance crossing. Using the Weyl symbol calculus, a local expansion of the collective wave equation driven by the particle source is derived in the neighborhood of the crossing. The collective wave dispersion manifold and the gyroballistic wave dispersion manifold can be used as a pair of local coordinates in the neighborhood of the resonance crossing, which greatly simplifies the analysis. This change of representation is carried out using a metaplectic transform (a generalization of the fourier transform). The Wigner function of the emitted wave field is then computed in the new coordinates. The Wigner function is a phase space scalar, hence the numerical value is invariant under linear canonical transformations. This invariance is invoked to finally arrive at the Wigner function in the original (physical) coordinates. The wave-action and -energy emission rates are then computed from the Wigner function.
Observation of spontaneously excited chaos-like ion plasma oscillations
Authors: Saitou Y. , Honzawa T. , Hada T.
Published: July 2002

An oscillation which behaves quite similarly to chaos under some conditions is observed to exist among ion plasma oscillations spontaneously excited in an ion beam–plasma system. There are two different states of the system, the ‘‘silent’’ and ‘‘chaotic’’ states, sensitively depending on the value of a direct current (DC) voltage VB, which determines the beam energy and is adopted as a control parameter here. In the chaotic state, a spontaneously excited ion plasma oscillation is observed to become chaotic. Here, the correlation dimension for the oscillation in the chaotic state is calculated to be 1.64±0.22. The result shows that an attractor for the oscillation has a low degree of freedom and a noninteger dimension.
Stabilization of ideal modes by resistive walls in tokamaks with plasma rotation and its effect on the beta limit
Authors: Ward D. J. , Bondeson A.
Published: July 2002

It is shown that pressure-driven, ideal external modes in tokamaks can be fully stabilized by resistive walls with plasma rotation. For wall stabilized plasmas, there are two types of potentially unstable external modes: (i) the ‘‘resistive wall modes’’ that penetrate, and are nearly locked to the wall, and (ii) modes that rotate with the plasma and for which the wall acts as a good conductor. For the quickly rotating modes, the stabilizing effect of the wall increases when the wall is brought closer to the plasma, while for the resistive wall modes, the stabilization improves with increasing wall distance. When the plasma rotates at some fraction of the sound speed, there is a window of stability to both the wall-locked and the rotating mode. The stabilization depends principally on the toroidal coupling to sound waves and is affected by ion Landau damping. Two-dimensional stability calculations are presented to evaluate the gains in beta limit resulting from this wall stabilization for different equilibria and rotation speeds. Results are shown for advanced tokamak configurations with bootstrap fractions of ≊100%.
Pulsed currents carried by whistlers. IV. Electric fields and radiation excited by an electrode
Authors: Stenzel R. L. , Urrutia J. M. , Rousculp C. L.
Published: July 2002

Electromagnetic properties of current pulses carried by whistler wave packets are obtained from a basic laboratory experiment. While the magnetic field and current density are described in the preceding companion paper (Part III), the present analysis starts with the electric field. The inductive and space charge electric field contributions are separately calculated in Fourier space from the measured magnetic field and Ohm’s law along B0. Inverse Fourier transformation yields the total electric field in space and time, separated into rotational and divergent contributions. The space-charge density in whistler wave packets is obtained. The cross-field tensor conductivity is determined. The frozen-in condition is nearly satisfied, E+ve×B≂0. The dissipation is obtained from Poynting’s theorem. The waves are collisionally damped; Landau damping is negligible. A radiation resistance for the electrode is determined. Analogous to Poynting’s theorem, the transport of helicity is analyzed. Current helicity is generated by a flow of helicity between pulses traveling in opposite directions which carry opposite signs of helicity. Helicity is dissipated by collisions. These observations complete a detailed description of whistler/current pulses which can occur in various laboratory and space plasmas.
Anomalous particle transport in a low-temperature plasma without magnetic shear
Authors: Eickermann Th. , Spatschek K. H.
Published: July 2002

A simple shearless model for collisional drift waves in low-temperature plasmas is analyzed to point at fluctuation-induced anomalous plasma transport. Spatially coherent structures do play an important role in the particle transport beyond a critical magnetic field strength. The transition from collisional to fluctuation-induced transport is investigated. Special emphasis is put on analytical models for understanding the transition. The results from numerical simulations are interpreted.
On a kinetic theory for strongly coupled dusty plasmas
Authors: Wang Xiaogang , Bhattacharjee A.
Published: July 2002

A kinetic theory for strongly coupled dusty plasmas is developed from the Bogolyubov–Born–Green–Kirkwood–Yvon (BBGKY) hierarchy. In these plasmas, interparticle distances are typically much smaller than the Debye length, and the coupling parameter Γd≡q2d/ddTd is much larger than unity. (Here qd is the dust charge, Td is the temperature, and dd≡[3/(4πnd)]1/3 is the inter-dust-particle distance.) For such quasi-steady-state plasmas, a theoretical argument is given for the validity of the Vlasov equation in the regimes Γd≪1 and Γc≫Γd≫1, where Γc designates the coupling parameter when Coulomb crystallization occurs.
Local transit-time damping in a magnetic field, and the arrest of lower-hybrid wave collapse
Authors: Melatos A. , Robinson P. A.
Published: July 2002

The transit-time power dissipated locally within a coherent wave packet in the presence of ambient and induced magnetic fields is calculated analytically as a function of position via a perturbed-orbit approach, generalizing earlier results for unmagnetized interactions. The theory is used to investigate local damping in a nonlinearly-collapsing lower-hybrid (LH) wave packet, and hence to estimate the arrest scale of LH wave collapse in a thermal electron-ion plasma. It is shown that either electrons or ions can dominate damping, depending on the strength of the magnetic field and the electron and ion temperatures.
Modes of spheroidal ion plasmas at the Brillouin limit
Authors: Tinkle M. D. , Greaves R. G. , Surko C. M.
Published: July 2002

The confinement properties and collective modes of single-component plasmas are investigated in a quadrupole Penning trap. Brillouin-density pure ion plasmas are generated by electron-beam ionization of a low-pressure gas. Large, spheroidal, steady-state plasmas are produced, extending out to contact one or more of the trap electrodes. With the density fixed at the Brillouin limit by the high ion production rate, the electrode potentials determine the plasma shape. The frequencies of azimuthally propagating cyclotron and diocotron modes are found to vary significantly with the plasma aspect ratio. For oblate plasmas, the frequencies are in good agreement with a simple fluid model.
High-frequency core localized modes in neutral beam heated plasmas on TFTR
Authors: Nazikian R. , Chang Z. , Fredrickson E. D. , Mazzucato E. , Batha S. H. , Bell R. , Budny R. , Bush C. E. , Cheng C. Z. , Janos A. , Levinton F. , Manickam J. , Mansfield D. K. , Park H. K. , Rewoldt G. , Sabbagh S. , Synakowski E. J. , Tang W. , Taylor G. , Zakharov L. E.
Published: July 2002

A band of high-frequency modes in the range 50–150 kHz with intermediate toroidal mode numbers 4<n<10 are commonly observed in the core of supershot plasmas on TFTR [R. Hawryluk, Plasma Phys. Controlled Fusion 33, 1509 (1991)]. Two distinct varieties of magnetohydrodynamic (MHD) modes are identified, corresponding to a flute-like mode predominantly appearing around the q=1 surface and an outward ballooning mode for q≳1. The flute-like modes have nearly equal amplitude on the high-field and low-field side of the magnetic axis, and are mostly observed in moderate performance supershot plasmas with τE<2τL, while the ballooning-like modes have enhanced amplitude on the low-field side of the magnetic axis and tend to appear in higher performance supershot plasmas with τE≳2τL, where τL is the equivalent L-mode confinement time. Both modes appear to propagate in the ion diamagnetic drift direction and are highly localized with radial widths Δr∼5–10 cm, fluctuation levels ñ/n, T̃e/Te<0.01, and radial displacements ξr∼0.1 cm. Unlike the toroidally localized high-n activity observed just prior to major and minor disruptions on TFTR [E. D. Fredrickson et al., Proceedings of the 15th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville, Spain (International Atomic Energy Agency, Vienna, 1995), No. IAEA-CN-60/A-2-II-5], these modes are typically more benign and may be indicative of MHD activity excited by resonant circulating beam ions.
Tunneling versus absorption in X-mode transmission through cyclotron layers
Authors: Swanson D. G.
Published: July 2002

Although proven analytically from mode conversion theory that the transmission coefficient through a cyclotron harmonic layer is independent of absorption, the result seems so nonintuitive that another way of seeing this result is given. In this paper, the transmission coefficient is deduced from the usual integral of the imaginary part of k over the layer, both with and without absorption. The fact that these both give exactly the same result, which agrees with the analytical result from mode conversion theory, shows that the absorption plays no role whatsoever in the transmission coefficient.
Stellarators with the magnetic symmetry of a tokamak
Authors: Garabedian P. R.
Published: July 2002

To meet the requirements of the U.S. Stellarator Power Plant Study, a Modular Helias-like Heliac (MHH) configuration was developed that has features in common with a quasihelically symmetric experiment proposed at the University of Wisconsin [D. Anderson and P. Garabedian, Nucl. Fusion 34, 881 (1994)]. Improvements have been made in the design of the MHH that raise the β limit, increase the confinement time, and simplify the geometry of the coils. In particular, specifications have been found for a stellarator with just two field periods that has a magnetic field spectrum Bmn close to axial symmetry. This configuration has physical properties that make it an interesting candidate for a follow-on experiment to assess the equilibrium, stability and transport of advanced stellarators.
Evolution of toroidal Alfvén eigenmode instability in Tokamak Fusion Test Reactor
Authors: Wong K. L. , Majeski R. , Petrov M. , Rogers J. H. , Schilling G. , Wilson J. R. , Berk H. L. , Breizman B. N. , Pekker M. , Wong H. V.
Published: July 2002

The nonlinear behavior of the toroidal Alfvén eigenmode (TAE) driven unstable by energetic ions in the Tokamak Fusion Test Reactor (TFTR) [Phys. Plasmas 1, 1560 (1994)] is studied. The evolution of instabilities can take on several scenarios: a single mode or several modes can be driven unstable at the same time, the spectrum can be steady or pulsating, and there can be negligible or anomalous loss associated with the instability. This paper presents a comparison between experimental results and recently developed nonlinear theory. Many features observed in experiment are compatible with the consequences of the nonlinear theory. Examples include the structure of the saturated pulse that emerges from the onset of instability of a single mode, and the decrease, but persistence, of TAE signals when the applied rf power is reduced or shut off.
Validity of the impurity entrainment assumption in the thermal stability analysis of multifaceted asymmetric radiation from the edge (MARFEs) in tokamak plasmas
Authors: Stacey Weston M.
Published: July 2002

The assumption that impurity ion perturbations are entrained in phase and in fixed proportion with the main plasma ion perturbations is commonly made in analyses of radiative thermal instabilities in the edge of tokamak plasmas, leading to multifaceted asymmetric radiation from the edge (MARFE). An analysis in which the impurity ion perturbations are treated separately from the plasma ion perturbations demonstrates that this assumption does not alter the conclusion of the analysis regarding stability limits or edge density limits for the (m=0, k‖=0) mode, but that inclusion of a separate impurity ion perturbation in the analysis stabilizes the (m>0, k‖≠0) modes.
Neoclassical transport in an annular penning trap
Authors: Robertson Scott
Published: July 2002

A modified Penning trap is described with an annular confinement region and a toroidal magnetic field. A non-neutral electron plasma is confined axially by an electrostatic field and, in the radial direction, particles are constrained to lie within a small drift distance of a cylindrical flux surface. Drift orbits of all particles are banana-shaped and collisions cause neoclassical transport.
Density limits in tokamaks
Authors: Stacey W. M.
Published: July 2002

Analysis of the stability of the radial temperature and density profiles against radial collapse leads to predictive algorithms for plasma and neutral fueling density limits in tokamaks. The predicted plasma density limit agrees with the Greenwald limit in magnitude and parameter scaling for Ohmic plasmas. Auxiliary-heated plasmas are predicted to have a higher plasma density limit than comparable Ohmic-heated plasmas, but fusion alpha-heated plasmas are predicted to have a lower plasma density limit than comparable Ohmic-heated plasmas. Model problem calculations indicate a rather complex dependence of the predicted density limit on the plasma parameters; under certain conditions the weak sensitivity seen in experiments to edge temperature, impurity concentration, and heating power is found, but under other conditions a stronger sensitivity is predicted than is seen in experiments.
Millimeter wave generation in a plasma-filled slow wave device
Authors: Jaiman N. K. , Tripathi V. K. , Srivastava M. P.
Published: July 2002

A plasma-filled slow wave device, e.g., a backward wave oscillator (BWO), offers high efficiency for the generation of TM waves around 10 GHz, by a relativistic electron beam. A large-amplitude TM mode can parametrically drive a millimeter wave unstable by coupling it to a negative energy beam space charge mode. The frequency of radiation is maximum when it propagates in the TM01 mode. In the Raman regime of operation the growth rate of instability scales directly as one-fourth power of beam density and inversely as 21/4 power of relativistic gamma factor. The nonlocal effects reduce the growth rate.
High-intensity illumination of an exploding foil
Authors: Cobble J. A. , Johnson R. P. , Mason R. J.
Published: July 2002

In an experiment relevant to the fast ignitor fusion concept, a preformed plasma simulating the corona of a fusion target has been probed with laser intensities exceeding 1018 W/cm2. It is seen that the f number of the probe beam is increased as it propagates through the target plasma—a channeling of the power. The transmission and backscatter of the probe beam have been examined, and the analysis suggests anomalous absorption of laser light in shorter scale length plasmas.
An analytical theory of corona discharge plasmas
Authors: Uhm Han S. , Lee Woong M.
Published: July 2002

In this paper we describe an analytical investigation of corona discharge systems. Electrical charge and the energy transfer mechanism are investigated based on the circuit analysis. Efficient delivery of electrical energy from the external circuit to the reactor chamber is a major issue in design studies. The optimum condition obtained in this paper ensures 100% energy transfer. Second-order coupled differential equations are numerically solved. All the analytical results agree remarkably well with numerical data. The reactor capacitor plays a pivotal role in circuit performance. The voltage profile is dominated by the reactor capacitor. Corona discharge properties in the reactor chamber are also investigated, assuming that a specified voltage profile V(t) is fed through the inner conductor. The analytical description is based on the electron moment equation. Defining the plasma breakdown parameter u=V/Rcp, plasma is generated for a high-voltage pulse satisfying u>uc, where uc is the critical breakdown parameter defined by geometrical configuration. Here, u is in units of a million volts per m per atm, and Rc is the outer conductor radius. It is found that the plasma density profile generated inside the reactor chamber depends very sensitively on the system parameters. A small change of a physical parameter can easily lead to a density change in one order of magnitude.
Ion acceleration and anomalous transport in the near wake of a plasma limiter
Authors: Sheehan D. P. , Bowles J. , McWilliams R.
Published: July 2002

Ion acceleration and anomalous transport were studied experimentally in the near wake region of an electrically floating disk limiter immersed in two different types of collisionless, supersonically flowing, magnetized plasmas: the first initially quiescent, the second initially turbulent. Ion densities and velocity distributions were obtained using a nonperturbing laser induced fluorescence diagnostic. Large-amplitude, low-frequency turbulence was observed at the obstacle edge and in the wake. Rapid ion and electron configuration space transport and ion velocity space transport were observed. Configuration space and velocity space transport were similar for both quiescent and turbulent plasma-obstacle systems, suggesting that plasma-obstacle effects outweigh the effects of initial plasma turbulence levels.
Feedback control of tearing modes via modulated neutral beams
Authors: Sen A. K. , Singh R. , Sen A. , Kaw P. K.
Published: July 2002

Tearing modes are suspected to be a principal cause of both minor and major disruptions in a tokamak. Suppression of these modes via a neutral beam which supplies momentum, energy and particles of appropriate phase and amplitude is considered. Model theoretical calculations are presented for the stabilization of drift tearing modes and it is found that for ADITYA [S.B. Bhatt et al., Ind. J. Pure Appl. Phys. 27, 710 (1989)] and other present-day tokamak parameters, stabilization is achievable using modest levels of beam power, current and density.
Test particle simulations for transport in toroidal plasmas
Authors: Park Hyoung-Bin , Heo Eun-Gi , Horton Wendell , Choi Duk-In
Published: July 2002

The authors derive the guiding center equations of motion from the phase space Euler–Lagrange formulation for the motion of a charged particle in toroidal magnetic confinement geometry. The guiding center equations are numerically solved together with the Monte Carlo Coulomb collisional pitch angle scattering. The numerically calculated microscopic diffusion coefficients for various values of collisionality ν* in the case of no electrostatic potential agree well with the results of neoclassical theory. The diffusion coefficient is then measured in the presence of a model electrostatic drift wave fluctuation and an equilibrium potential. The diffusion coefficient increases with increasing fluctuation amplitude while the equilibrium potential diminishes the diffusion processes through both the orbit squeezing and the Er×B shear flow of the poloidal velocity.
Stimulated scattering of relativistically strong radiation from an underdense plasma at high-frequency harmonics
Authors: Sakharov A. S. , Kirsanov V. I.
Published: July 2002

The theory of stimulated scattering of relativistically strong, arbitrarily polarized laser radiation from an underdense plasma is developed. It is shown that, in addition to the scattering at the near-carrier frequency, the scattering at the odd and even high-frequency harmonics can occur in a wide range of angles. The growth rates of the instability associated with the harmonic generation are obtained as functions of the scattering angle. It is shown that, at certain directions, the growth rates for higher harmonics can exceed the growth rate for the fundamental harmonic.
Theory of the inverted gyrotwystron
Authors: Nusinovich G. S. , Walter M.
Published: July 2002

The theory of the inverted gyrotwystron, a gyrodevice consisting of the input waveguide and output cavity separated by the drift section, is developed. The case of frequency doubling when the input waveguide operates at the fundamental cyclotron resonance and the output cavity operates at the second cyclotron harmonic is analyzed. It is shown that the electron prebunching in the input waveguide provides a better harmonic content in the electron current density than a standard ballistic bunching occurring in gyroklystrons with a short input cavity followed by a long drift section. The dependence of the efficiency and locking bandwidth on the input power and the lengths of input waveguide and drift section is studied. The effect of the electron velocity spread on the locking bandwidth and the electronic efficiency is analyzed.
Hamiltonian description of toroidal magnetic fields in vacuum
Authors: Bates Jason W. , Lewis H. Ralph
Published: July 2002

A Hamiltonian description of vacuum magnetic fields in toroidal geometry is presented. Using a magnetic scalar potential in conjunction with Boozer’s canonical representation of a magnetic field [A. Boozer, Phys. Fluids 26, 1288 (1983)], a field-line Hamiltonian for general nonaxisymmetric vacuum fields has been derived.
Plasma equations in general relativity
Authors: Elsässer Klaus , Popel Sergey
Published: July 2002

Vlasov’s equation and the ideal multifluid equations are considered in manifestly covariant form. In the latter case, a thermodynamic closure (locally the first law of thermodynamics) leads to a generalized Kelvin/Helmholtz theorem. In the former case, the local dispersion relation for Langmuir waves in a strong gravitational field is derived and solved.
Strong frequency up-conversion by nonlinear Thomson scattering from relativistic electrons
Authors: Yu Wei , Yu M. Y. , Ma J. X. , Xu Z.
Published: July 2002

The scattering of laser light by electrons is considered. When the electron is counterstreaming and highly relativistic, and the laser light ultraintense, the scattered light is mainly in the higher harmonics, and the frequency upshift is further enhanced by the Doppler effect. This provides a mechanism for very large frequency up-conversion.
Notched velocity profiles and the radial electric field in high ion temperature plasmas in the Tokamak Fusion Test Reactor
Authors: Ernst D. R. , Bell M. G. , Bell R. E. , Bush C. E. , Chang Z. , Fredrickson E. , Grisham L. R. , Hill K. W. , Jassby D. L. , Mansfield D. K. , McCune D. C. , Park H. K. , Ramsey A. T. , Scott S. D. , Strachan J. D. , Synakowski E. J. , Taylor G. , Thompson M. , Wieland R. M.
Published: July 2002

A large “notch,” or non-monotonic feature, appears in measured toroidal velocity profiles of the carbon impurity in the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 26, 11 (1984)], centered near the radius of strongest ion temperature gradient. This is explained as a consequence of radial momentum transport dominated by anomalous diffusion together with parallel heat friction on the impurity ions arising from the hydrogenic neoclassical parallel heat flow. The toroidal velocity profile of the hydrogenic species is predicted to be monotonic, from measurements of the impurity toroidal velocity, consistent with the anomalous radial diffusion of toroidal momentum. This supports a neoclassical calculation of the radial electric field for near-balanced beam injection. In supershot plasmas [Phys. Rev. Lett. 58, 1004 (1987)], a well structure in the radial electric field profile is found in the enhanced confinement region. An associated shear layer separates the core, where the local confinement trends are favorable, from the degraded outer region. This provides a mechanism for the nonlinear coupling of the ion temperature gradient, ion thermal confinement, and the radial electric field, which may help explain the favorable core confinement trends of very high temperature supershot plasmas.
Improved large diameter wire array implosions from increased wire array symmetry and on-axis mass participation
Authors: Deeney C. , Nash T. J. , Spielman R. B. , Seaman J. F. , McGurn J. S. , Jobe D. O. , Vargas M. F. , Gilliland T. L. , Mock R. C. , Struve K. W. , Whitney K. G. , Pulsifer P. E. , Apruzese J. P. , Thornhill J. W. , Davis J.
Published: July 2002

Aluminum wire array, Z-pinch experiments have been performed on an 8 MA generator using arrays consisting of 24, 30, and 42 wires. These experiments were designed to scan through a region of (array mass, implosion velocity) space in which greater than 30% conversion of the implosion kinetic energy into K-shell x rays was predicted to occur [Thornhill et al., Phys. Plasmas 1, 321 (1994)]. Array masses between 120 and 2050 μg/cm were used in these experiments. An analysis of the x-ray data taken using 24 wire arrays, shows a one-to-one correspondence between the observed kilo-electron-volt yields (5–64 kJ) and the fraction of initial array mass (0.3%–60%) that is radiating from the K shell. The 30 and 42 wire experiments demonstrated that tighter pinches with increased radiated powers were achieved with these larger wire number, improved symmetry arrays. In addition, increases in the implosion mass and array diameter in the 30 and 42 wire number cases resulted in increases in the radiated yield over the corresponding 24 wire shots, up to 88 kJ, which can be interpreted as due to improved coupling and thermalization of the kinetic energy. Moreover, spectroscopic analyses of the 30 and 42 wire experiments have shown that the increased wire numbers also resulted in K-shell radiating mass fractions of greater than 50%.
K-shell radiation physics in the ultrahigh optical depth pinches of the Z generator
Authors: Apruzese J. P. , Pulsifer P. E. , Davis J. , Clark R. W. , Whitney K. G. , Thornhill J. W. , Sanford T. W. L. , Chandler G. A. , Deeney C. , Fehl D. L. , Nash T. J. , Spielman R. B. , Stygar W. A. , Struve K. W. , Mock R. C. , Gilliland T. L. , Jobe D. O. , McGurn J. S. , Seamen J. F. , Torres J. A. , Vargas M.
Published: July 2002

Al:Mg alloy wire arrays of mass loads 1.3–3.6 mg/cm have been imploded with peak currents of 19 MA on the 60 TW Z generator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories. The large mass loads have resulted in the highest K-shell x-ray line optical depths (∼103) produced to date in Z-pinches. Analysis of the time-resolved spectrum of a 2.1 mg/cm shot near the time of peak compression has yielded a temperature–density profile of the pinch that approximately reproduces all features of the x-ray data except the continuum above 5 keV, which is underpredicted. The Ly α/He α ratio for Al is shown to be enhanced relative to that of Mg by two mechanisms: photopumped ladder ionization and absorption of the Al He-like line in a cool outer halo. This analysis and comparisons to some Ti shots demonstrates that the K-shell yield of Al is significantly reduced by line and continuum self-absorption, but that of Ti is not.
Compressional Alfvén cross–field surface waves in inhomogeneous dusty plasmas
Authors: Cramer N. F. , Vladimirov S. V. , Ostrikov K. N. , Yu M. Y.
Published: July 2002

Compressional Alfvén surface waves in an inhomogeneous dusty plasma are studied. The inhomogeneity is modeled by two distinct regions of dusty plasmas with different ion densities. The stationary external magnetic field is along the interface between the two plasmas. The dispersion properties of cross-field surface waves, impossible in dust-free plasmas, are obtained for the constant dust charge case. The existence of the surface waves is due to an imbalance in the electron and ion Hall currents in a dusty plasma.
Self-gravitational magnetosonic modes in dusty plasmas with quasi-inertialess plasma constituents
Authors: Verheest Frank , Hellberg Manfred A. , Mace Richard L.
Published: July 2002

Self-gravitation could become important in dusty plasmas, due to the presence of very heavy charged dust grains, and hence could modify traditional Jeans collapse criteria known from neutral gases. Perpendicular, low-frequency magnetosonic modes can play a role in resisting gravitational collapse across the magnetic field. By treating the lighter plasma species as effectively inertialess, a unified dispersion law is obtained for Alfvén–Jeans modes, valid at all frequencies smaller than the ordinary ion gyrofrequency. Its possible relevance for interstellar space is discussed, although estimates are only indicative because of the lack of reliable data. Nevertheless, this clearly points to an important stabilizing role for the interstellar magnetic field in the perpendicular direction if all the dust is charged.
External kink modes in a Large Helical Device (LHD) equilibrium with self-consistent bootstrap current
Authors: Johnson J. L. , Ichiguchi Katsuji , Nakamura Yuji , Okamoto Masao , Wakatani Masahiro , Nakajima Noriyoshi
Published: July 2002

Magnetohydrodynamic (MHD) stability studies of low-mode-number free-boundary kink modes in a finite-β Large Helical Device (LHD) [A. Ilyoshi, et al. Fusion Technology 17, 169 (1990)] equilibrium series with self-consistent bootstrap current find some serious free-boundary modes. They indicate that care must be taken in the design of high-β experiments. Since the LHD configuration is flexible, with the possibility of introducing or modifying dipole and quadrapole fields, unbalancing the currents in the helical coils, operating with a high-temperature divertor, and changing the collisionality regime by working with different values of temperature and density, paths to high-β operation should exist. Comparison of the experimental results with these theoretical predictions for the studied equilibrium sequence will provide understanding of the MHD stability properties of LHD.
Crystalline order in laser-cooled, non-neutral ion plasmas
Authors: Bollinger J. J. , Mitchell T. B. , Huang X.-P. , Itano W. M. , Tan J. N. , Jelenković B. M. , Wineland D. J.
Published: July 2002

Laser-cooled trapped ions can be strongly coupled and form crystalline states. In this paper we review experimental studies that measure the spatial correlations of Be+ ion crystals formed in Penning traps. Both Bragg scattering of the cooling-laser light and spatial imaging of the laser-induced ion fluorescence are used to measure these correlations. In spherical plasmas with more than 2×105 ions, body-centered-cubic (bcc) crystals, the predicted bulk structure, are the only type of crystals observed. The orientation of the ion crystals can be phase locked to a rotating electric-field perturbation. With this “rotating wall” technique and stroboscopic detection, images of individual ions in a Penning trap are obtained. The rotating wall technique also provides a precise control of the time-dilation shift due to the plasma rotation, which is important for Penning trap frequency standards.
Intermittency and self-similarity in plasma edge fluctuations
Authors: Carbone V. , Regnoli G. , Martines E. , Antoni V.
Published: July 2002

The intermittency of the floating potential fluctuations as measured at the edge of a plasma of interest for controlled thermonuclear fusion research is investigated. The probability distribution functions of fluctuations are not scale invariant, that is the wings of these functions are more important at the smallest scales, a classical signature of intermittency. Self-similarity is recovered at scales greater than about 20 μs.
Nonlinear time evolution of thermal structures
Authors: Trevisan Marı́a Cristina , Ibáñez S. Miguel H.
Published: July 2002

The nonlinear stability and time evolution of thermal structures constituted by optically thin plasmas are analyzed. The structure has been assumed to be heated at a rate ∼Tm, cooled by the standard cooling function for plasmas with solar abundances and with an anisotropic thermal conduction coefficient. Second-order analytical results are obtained for the thermal equilibrium solutions. For nonhomogeneous solutions and strong disturbances, a numerical analysis is carried out. The angle between the temperature gradient and the magnetic field is a crucial parameter in determining the stability of structures close to the marginal state. A central overheating may occur for large enough amplitudes of the initial disturbance imposed on stable steady-state thermal structures. Implications of the above results in the formation of cool structures in the solar atmosphere and in the interstellar medium are outlined.
On the exact amplitude, speed and shape of ion-acoustic waves
Authors: Johnston Clifton R. , Epstein Marcelo
Published: July 2002

Nonlinear ion-acoustic solitary waves in cold collisionless plasma are investigated by a direct analysis of the field equations. The exact amplitude is obtained by simply solving an algebraic equation. The results are compared to those of a third-order perturbation approach and the error associated with using the perturbation technique is determined.
Observation of reduced beam deflection using smoothed beams in gas-filled hohlraum symmetry experiments at Nova
Authors: Delamater N. D. , Lindman E. L. , Magelssen G. R. , Failor B. H. , Murphy T. J. , Hauer A. A. , Gobby P. , Moore J. B. , Gomez V. , Gifford K. , Kauffman R. L. , Landen O. L. , Hammel B. A. , Glendinning G. , Powers L. V. , Suter L. J. , Dixit S. , Peterson R. R. , Richard A. L.
Published: July 2002

Execution and modeling of drive symmetry experiments in gas-filled hohlraums have been pursued to provide both a better understanding of radiation symmetry in such hohlraums and to verify the accuracy of the design tools which are used to predict target performance for the National Ignition Facility (NIF) [J. Lindl, Phys. Plasmas 2, 3933 (1995)]. In this paper, the results of a series of drive symmetry experiments using gas-filled hohlraums at the Nova laser facility [C. Bibeau et al., Appl. Opt. 31, 5799 (1992)] at Lawrence Livermore National Laboratory are presented. A very important element of these experiments was the use of kineform phase plates (KPP) to smooth the Nova beams. The effect of smoothing the ten Nova beams with KPP phase plates is to remove most of the beam bending which had been observed previously, leaving a residual bending of only 1.5°, equivalent to a 35 μm pointing offset at the hohlraum wall. The results show that the symmetry variation with pointing of implosions in gas-filled hohlraums is consistent with time integrated modeling.
Stable high beta spheromak equilibria using concave flux conservers
Authors: Shumlak U. , Jarboe T. R.
Published: July 2002

Ideal magnetohydrodynamic stability to the n=1 and n=2 modes are calculated for spheromak equilibria in flux conserver shapes, which include a midplane gap, conforming outer wall, and concave side walls. The equilibria are force-free (∇×B=λ(ψ)B) but not minimum energy states and, therefore, have nonuniform λ(ψ) profiles. For each flux conserver shape, the equilibrium with the most hollow linear λ(ψ) profile is found that is stable to the n=1 and n=2 modes. The Mercier β limit is calculated for each flux conserver shape using the most hollow current profile constrained by the stability boundary. The results show that a stable high 〈β〉 (>10%) spheromak equilibrium can be produced using a concave flux conserver, improving energy confinement times and plasma performance.
UV laser-generated fluorine atom etching of polycrystalline Si, Mo, and Ti
Authors: Loper G. L. , Tabat M. D.
Published: July 2002

Fluorine atoms, generated by the 193-nm ArF laser photolysis of carbonyl difluoride, have been shown to be capable of efficiently and selectively etching polycrystalline silicon deposited on silicon dioxide. The highly specific and rapid removal of either molybdenum or titanium deposited on silicon dioxide has been demonstrated with fluorine atoms produced by the ArF laser photolysis of nitrogen trifluoride.
Determination of interfacial quality of GaAs-GaAlAs multi-quantum well structures using photoluminescence spectroscopy
Authors: Reynolds D. C. , Bajaj K. K. , Litton C. W. , Yu P. W. , Singh Jasprit , Masselink W. T. , Fischer R. , Morkoç H.
Published: July 2002

Well size fluctuations have been observed in high quality GaAs-Ga0.75Al0.25As multi-quantum well structures having very sharp photoluminescence transitions. The effect of well size fluctuations appears as multiple peaks in both the heavy hole free exciton and the heavy hole donor bound exciton transitions. The observed energy separation of the peaks corresponds to what would be expected for a change in well thickness of 1/2 monolayer. The observed linewidths and the well size fluctuations suggest that these are interlayer rather than intralayer fluctuations. These results are different from earlier reported work. The very narrow emission lines reflect the excellent structural quality of the layers. A model to explain the effective half-monolayer well size fluctuations is proposed.
Investigation of the surface photochemical basis for metal film nucleation in laser chemical vapor deposition
Authors: Higashi G. S. , Rothberg L. J.
Published: July 2002

We report the results of ultraviolet irradiation of monolayers of organoaluminum complexes adsorbed to sapphire. Using surface vibrational spectroscopy, the compositional and structural changes caused by 248 and 193 nm surface illumination are documented. Electronic photochemistry of adsorbed trimethylaluminum is observed and can lead to nucleation of aluminum on the surface. A strong wavelength dependence to this process is observed which cannot be explained by assuming photochemical yields to be proportional to previously measured surface absorption cross sections. The implications for laser chemical vapor deposition of spatially localized metallic features are discussed.
cw double cross pumping of the 5I7–5I8 laser transition in Ho3+-doped garnets
Authors: Duczynski E. W. , Huber G. , Ostroumov V. G. , Shcherbakov I. A.
Published: July 2002

We obtained for the first time efficient room-temperature cw lasing at 2.086 μm in Cr,Tm,Ho:YScAl-garnet and Cr,Tm,Ho:YScGa-garnet crystals. Double cross pumping via Cr3+ and Tm3+ with a krypton pump laser yields threshold pump powers of 25 mW and slope efficiencies up to 13%. Broad band pumping can also be expected.
Picosecond time-of-flight measurements of minority electrons in GaAs/AlGaAs quantum well structures
Authors: Höpfel R. A. , Shah J. , Block D. , Gossard A. C.
Published: July 2002

We report direct measurements of drift velocities of photoexcited minority electrons in p-doped quantum well structures of GaAs/Al0.48Ga0.52As, using picosecond time-of-flight techniques. At low electric fields the electron mobility is strongly reduced by electron-hole scattering. At high fields (>8 kV/cm) negative differential mobility is observed, which we interpret as real-space and valley transfer of hot electrons into the X valley of Al0.48Ga0.52As. From the photoluminescence spectra the carrier temperatures at which the transfer effects occur are determined.
Fabrication of Nb3Al and Nb3(Al,Ge) superconducting composite tapes by electron beam irradiation
Authors: Kumakura H. , Togano K. , Tachikawa K. , Tsukamoto S. , Irie H.
Published: July 2002

Nb3Al and Nb3(Al,Ge) superconductors were synthesized using continuous electron beam irradiation on powder metallurgy processed Nb-Al and Nb-Al-Ge composite tapes. Transition temperatures Tc(onset) of 18.5 and 20.2 K were obtained for Nb-Al and Nb-Al-Ge tapes respectively, which indicated that nearly stoichiometric Al5 compounds were formed. Critical current density Jc (for reacted area) is nearly constant at fields above 15 T, with excellent values of ∼2×104 A/cm2 at 23 T and 4.2 K for both tapes. The excellent Jc’s as well as the easy scale-up procedure indicate that electron beam irradiation is promising for the fabrication of practical Nb3Al and Nb3(Al,Ge) superconductors, which can generate extremely high fields of ∼25 T.
Injected current-related distortion of photothermal signals from a photovoltaic cell
Authors: Cahen David , Nordal Per-Erik , Kanstad Svein Otto
Published: July 2002

When power dissipation in a photovoltaic cell is measured as a function of load resistance via photothermal techniques which sense only part of the photovoltaic cell, significant deviations from previously reported photoacoustic measurements can be obtained. These deviations occur under nonuniform illumination. They are due to the fact that, under these experimental conditions, the measured signal is proportional to only part of the power dissipation by injected carriers, while all of the photogenerated carriers contribute to the signal. Results from photothermal radiometry experiments are presented and explained in this way.
Mechanism of electroluminescence in alkaline-earth sulfides
Authors: Crandall Richard S.
Published: July 2002

A novel use of a staircase voltage waveform shows that three light emission peaks can be produced with a unipolarity voltage pulse in alkaline-earth-sulfide electroluminescent devices. Two peaks following the voltage decrease demonstrate the following: Light can be produced at fields much below those required for hot-electron effects. Activators are ionized when a field is applied. Recombination controls electroluminescence.
Drude parameters of liquid silicon at the melting temperature
Authors: Li K. D. , Fauchet P. M.
Published: July 2002

The Drude parameters of liquid silicon at the melting temperature have been obtained from time-resolved reflectivity measurements at 1064, 532, and 355 nm following melting of an optically thick layer by a picosecond visible laser pulse. The ratio of the electron density N to the electron mass m is found to be equal to 2.17×1059 m−3 kg−1 and the relaxation time τ to be equal to 212 as. These values are compared to previous results obtained by cw ellipsometry between 1 and 0.4 μm and by nanosecond time-resolved ellipsometry of 632.8 nm.
Perpendicular electronic transport in doping superlattices
Authors: Schubert E. F. , Cunningham J. E. , Tsang W. T.
Published: July 2002

Electronic transport measurements are performed at 77 and 300 K on doping superlattices in perpendicular direction to the quantum well planes. The periods of the doping superlattices range from 150 to 600 Å. The current-voltage characteristics in long period doping superlattices (type B) are strongly nonlinear and exhibit a new type of negative differential conductivity. The occurrence of negative differential conductivity is caused by avalanche multiplication in the high-field regions of the superlattice and accumulation of holes in the valence-band maxima. In short period doping superlattices (type A) the current-voltage characteristics are linear at temperatures of 300 and 77 K showing that tunneling through thin barriers dominates perpendicular transport. The calculation of the tunneling current yields a strong dependence on the period (exp−z3/2p).
Faraday effect magnetometry utilizing high Verdet constant glass
Authors: Tatam R. P. , Berwick M. , Jones J. D. C. , Jackson D. A.
Published: July 2002

A magnetometer based on the Faraday effect is described, in which the sensing element is a rod of high Verdet constant glass. The instrument is capable of remote operation, and may be addressed via a monomode optical fiber. A pseudoheterodyne signal processing technique is used, which utilizes an input beam of rotating linear polarization state. A fiber optic modulator for producing the rotating state is described. Using a high Verdet constant sensing element gives higher sensitivity than can be achieved with a conventional fiber sensing element, increasing the sensitivity by multipassing is also discussed. The instrument has been demonstrated with both static and periodic fields.
Photoemission study of sputter-etched InP surfaces
Authors: Lau W. M. , Sodhi R. N. S. , Flinn B. J. , Tan K. H. , Bancroft G. M.
Published: July 2002

The effects of sputter etching and subsequent surface treatments including thermal annealing and oxygen exposure on n-InP surfaces have been studied by measuring the photoemission spectra using both Al Kα and synchrotron radiation. Results indicate that sputtering with Ar+ at 1–3 keV and low current density leaves an In-rich surface (In/P=2) and, from the movement of the Fermi level relative to the conduction-band minimum, is seen to introduce acceptor-type defects. Annealing of the surface causes dissolution of some excess In into the bulk as well as removal of some of these defects. The behavior of the annealed surface to oxygen exposure is similar to that of the cleaved surface with the creation of donor defects.
Gas source silicon molecular beam epitaxy using disilane
Authors: Hirayama Hiroyuki , Tatsumi Toru , Aizaki Naoaki
Published: July 2002

In gas source silicon molecular beam epitaxy (MBE) using disilane, good crystal quality silicon epitaxial films were obtained at a temperature as low as 630 °C. In comparison with gas source Si-MBE using silane, a growth rate several times higher was attained. Boron doping in the gas source Si-MBE was also studied. Using HBO2 as the B-doping source, the doping level was controlled in the range of 1016–1018 cm−3 by the cell temperature. Because of the low growth temperature and the high growth rate, an abrupt doping profile was obtained.
Undulator generating any kind of elliptically polarized radiation
Authors: Onuki Hideo , Saito Norio , Saito Terubumi
Published: July 2002

A new type of undulator with crossed and retarded magnetic fields which enables us to obtain any kind of elliptically polarized synchrotron radiation from relativistic electrons was developed. This undulator was installed in a straight section of the electron storage ring, TERAS, and its polarization characteristics were investigated. The experimental results proved that this device can generate linearly or circularly polarized synchrotron radiation which would be very useful in polarization modulation spectroscopy.
Nondestructive measurement of layer thicknesses in double heterostructures by x-ray diffraction
Authors: Macrander A. T. , Lau S. , Strege K. , Chu S. N. G.
Published: July 2002

Double-crystal rocking curves for an InP/InGaAsP/InP double heterostructure grown by hydride vapor phase epitaxy have been obtained which reveal fringes corresponding to both the top InP layer and to the buried InGaAsP layer thickness. Fourier transform spectra of the rocking curves are shown to be very useful in extracting the thicknesses. We believe this to be the first report of a measurement of the thickness of a buried layer using only x-ray fringe spacings.
Femtosecond ultraviolet pulse generation in β-BaB2O4
Authors: Edelstein D. C. , Wachman E. S. , Cheng L. K. , Bosenberg W. R. , Tang C. L.
Published: July 2002

Intracavity doubling in β-BaB2 O4 (BBO) of femtosecond pulses into the ultraviolet with high efficiency is reported. Pulse widths down to 43 fs at a 108 Hz repetition rate and outputs as high as 20 mW per arm of the femtosecond laser on a continuous-wave basis can be achieved. The ultraviolet pulse widths were determined through detailed cross-correlation measurements based on sum-frequency mixing to 210 nm in ultrathin BBO crystals.