bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of second harmonic generation in plasma crystals
Authors: Avinash K.
Published: April 2004

Second harmonic generation in plasma crystals is studied. Starting from a linear chain model, a set of coupled nonlinear differential equations for second harmonic generation is derived. It is shown that with some modifications these equations are also applicable to a two-dimensional (2D) lattice. The solutions show that the amplitude of the harmonic attains a maximum away from the excitation region. An efficiency factor, which is the ratio of the energy in the pump wave to the maximum energy in the harmonic is defined and estimated using Lindemann’s criterion for melting and phase matching conditions. The efficiency is limited by the damping due to the background gas. Relevance of these results to recent experiments on harmonic generation in 2D plasma crystals is discussed.
Magnetic field perturbations in closed-field-line systems with zero toroidal magnetic field
Authors: Ryutov D. D. , Kesner J. , Mauel M. E.
Published: April 2004

In some plasma confinement systems [e.g., field-reversed configurations (FRC) and levitated dipoles] the confinement is provided by a closed-field-line poloidal magnetic field. The influence of the magnetic field perturbations on the structure of the magnetic field in such systems is considered and it is found that the effect of perturbations is quite different from that in the systems with a substantial toroidal field. In particular, even infinitesimal perturbations can, in principle, lead to large radial excursions of the field lines in FRCs and levitated dipoles. Under such circumstances, particle drifts and particle collisions may give rise to significant neoclassical transport. Introduction of a weak regular toroidal magnetic field reduces radial excursions of the field lines and neoclassical transport.
Twisting-tearing compound mode induced by energetic ions
Authors: Marchenko V. S. , Reznik S. N.
Published: April 2004

It is shown that, during the burst of the high frequency twisting mode induced by energetic ions, significant reconnected flux can be transiently generated due to toroidal coupling with tearing-parity mode. This forced reconnection is accompanied by a sudden jump of the local plasma rotation frequency, induced by electromagnetic torque, which tends to “lock” the twisting mode to the tearing mode at the coupled rational surface. Results are applied to interpret early experiments on the DIII-D tokamak [Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159], where sawtooth inversion has been observed during high frequency (f∼100 kHz), intermediate mode number (m∼10) bursting activity [Heidbrink et al., Nucl. Fusion 31, 1635 (1991)].
Effects of nonresonant hot ions with large orbits on Alfvén cascades and on magnetohydrodynamic instabilities in tokamaks
Authors: Sharapov S. E. , Mikhailovskii A. B. , Huysmans G. T. A.
Published: April 2004

The effects of nonresonating hot ions on the spectrum of magnetohydrodynamic (MHD) waves and instabilities in tokamaks are studied in the limit when the width of the hot ion drift orbits is much larger than the radial scale length of the MHD perturbations. Due to the large magnetic drift velocities the hot ions cannot contribute to the MHD perturbations directly, but two main effects of the hot ions, the hot-ion density-dependent effect and the hot-ion pressure-dependent effect, influence the MHD perturbations indirectly. The physics of both effects is elucidated and it is shown that both these effects can be described in MHD approach. A new code, MISHKA-H (MISHKA including the hot-ion indirect effects), is developed as an extension of the ideal MHD code MISHKA-D [Huysmans et al., Phys. Plasmas 8, 4292 (2002)]. Analytical benchmarks for this code are given. Results of the MISHKA-H code on Alfvén spectrum in a shear-reversed discharges with ion-cyclotron resonance frequency (ICRF) heating are presented. Modeling of Alfvén cascades and their transition into toroidal Alfvén eigenmodes in shear-reversed tokamak equilibrium is considered. The hot-ion effect on the unstable branch of the MHD spectrum is studied for the test case of an n=1 ideal MHD internal kink mode, which is relevant to short-period sawteeth in low-density plasmas observed in Joint European Torus (JET) [Rebut et al., Proceedings of the 10th International Conference, Plasma Physics and Controlled Nuclear Fusion, London (International Atomic Energy Agency, Vienna, 1985), Vol. I, p. 11] experiments with high-power ICRF heating.
Current profile control by alternating current magnetic helicity injection
Authors: Ebrahimi F. , Prager S. C.
Published: April 2004

Current profile control by ac magnetic helicity injection is computationally investigated in a reversed field pinch (RFP). Three-dimensional resistive magnetohydrodynamics (MHD) computation is employed to study the nonlinear MHD. During ac helicity injection, when toroidal and poloidal surface voltages are oscillated out of phase, the parallel electric field is maintained positive over a majority of the cycle leading to current density profile flattening. The total time-averaged magnetic fluctuations are reduced and the magnetic surfaces are nonstochastic for much of the cycle. The steady-state nature of this technique could make it useful for improving RFP performance.
Study of pinholes and nanotubes in AlInGaN films by cathodoluminescence and atomic force microscopy
Authors: Herrera M. , Cremades A. , Piqueras J. , Stutzmann M. , Ambacher O.
Published: April 2004

Cathodoluminescence (CL) in the scanning electron microscope and atomic force microscopy (AFM) have been used to study the formation of pinholes in tensile and compressively strained AlInGaN films grown on Al2O3 substrates by plasma-induced molecular beam epitaxy. Nanotubes, pits, and V-shaped pinholes are observed in a tensile strained sample. CL images show an enhanced emission around the pits and a lower intensity at the V-shaped pinholes. Rounded pinholes appear in compressively strained samples in island-like regions with higher In concentration. The grain structure near the pinholes is resolved by AFM.
The influence of asymmetry on mix in direct-drive inertial confinement fusion experiments
Authors: Christensen C. R. , Wilson D. C. , Barnes Cris W. , Grim G. P. , Morgan G. L. , Wilke M. D. , Marshall F. J. , Glebov V. Yu. , Stoeckl C.
Published: April 2004

The mix of shell material into the fuel of inertial confinement fusion (ICF) implosions is thought to be a major cause of the failure of most ICF experiments to achieve the fusion yield predicted by computer codes. Implosion asymmetry is a simple measurable quantity that is expected to affect the mix. In order to measure the coupling of asymmetry to mix in ICF implosions, we have performed experiments on the OMEGA laser [T. R. Boehly et al., Rev. Sci. Instrum. 66, 508 (1995)] that vary the energy of each of the sixty beams individually to achieve a given fraction of L2, the second-order Legendre polynomial. Prolate, symmetric, and oblate implosions resulted. Three different fill pressures were used. Simultaneous x-ray and neutron images were obtained. The experiments were modeled with a radiation/hydrodynamics code using the multi-fluid interpenetration mix model of Scannapieco and Cheng. It fits the data well with a single value of its one adjustable parameter (0.07±0.01). This agreement is demonstrated by neutron yield, x-ray images, neutron images, and ion temperatures. The degree of decline of the neutron yield with asymmetry at different fill pressures provides a hard constraint on ICF mix modeling.
Electron transport and the critical temperature gradient
Authors: Horton W. , Hoang G. T. , Bourdelle C. , Garbet X. , Ottaviani M. , Colas L.
Published: April 2004

Tore Supra electron thermal fluxes, analyzed over a range of heating powers and plasma densities, are shown to vary parametrically according to the small-scale electron temperature gradient (ETG) model, rather than the ion inertial scale electrostatic gyro-Bohm model. Steady-state power balance analysis and time-varying interpretative transport simulations, performed on the Tore Supra Fast Wave Electron Heating database, validate the ETG thermal flux-versus-gradient relation and the existence of a critical electron temperature gradient. The critical gradient length R/Lc and the parametric dependence of the thermal flux, qe(ne,Te,∇Te,q,s), agree well with the ETG model.
Theory and experiment of a 94 GHz gyrotron traveling-wave amplifier
Authors: Song H. H. , McDermott D. B. , Hirata Y. , Barnett L. R. , Domier C. W. , Hsu H. L. , Chang T. H. , Tsai W. C. , Chu K. R. , Luhmann N. C.
Published: April 2004

Experimental results are presented on the first W-band gyrotron Traveling-Wave Tube (gyro-TWT) developed to exploit the 94 GHz atmospheric window for long-range, high-resolution radar applications. The gyro-TWT is designed to operate in the higher order TE01 mode and is driven by a 100 kV, 5 A electron beam with a pitch angle of v⊥/vz=1 and velocity spread of Δvz/vz=5%. Large-signal simulations predict 140 kW output power at 92 GHz with 28% efficiency, 50 dB saturated gain, and 5% bandwidth. The stability of the amplifier against spurious oscillations has been checked with linear codes. To suppress the potential gyro-BWO interactions involving the TE02, TE11, and TE21 modes, the interaction circuit with a cutoff frequency of 91 GHz has been loaded with loss so that the single-path, cold-circuit attenuation is 90 dB at 93 GHz. A coaxial input coupler with 3% bandwidth is employed with a predicted and measured coupling of 1 dB and 2 dB, respectively. The operating voltage is limited to below 75 kV because of oscillations encountered at higher voltages in this initial embodiment. Preliminary test at Vb=60 kV and Ib=3.7 A yielded 59 kW saturated output power at 92.2 GHz with 42 dB gain, 26.6% efficiency, and a 3 dB bandwidth of 1.2 GHz (1.3%).
Electrostatic waves in bounded dusty magnetoplasma
Authors: Vranjes J. , Poedts S.
Published: April 2004

Electrostatic perturbations propagating obliquely to the magnetic field lines are studied in a cylindric magnetized dusty plasma configuration. A set of two coupled equations describing the perturbed electrostatic and gravity potentials in a radially nonuniform plasma is derived. The equations are discussed and solved in two limits, without and with the self-gravity effects included. Without the gravity the corresponding equation for the electrostatic potential is solved analytically for a Gaussian-type equilibrium density profile. The general solution, which can be applied to various plasma situations, is expressed in terms of the Kummer confluent hypergeometric functions. In the self-gravitating case the equations are solved analytically for a homogeneous plasma and the general solutions are written in terms of the Bessel functions, describing well-behaving radially localized wave amplitudes.
Measurement and analysis of effective thermal conductivities of lotus-type porous copper
Authors: Ogushi T. , Chiba H. , Nakajima H. , Ikeda T.
Published: April 2004

Lotus-type porous copper is a porous medium made of copper that contains many straight pores. To effectively employ lotus-type porous copper as a heat sink, it is necessary to clarify the pore effect on the thermal conductivity of lotus copper. This article describes an experimental and analytical investigation on the effective thermal conductivities of lotus copper parallel and perpendicular to the pores. The lotus copper displayed anisotropy of the effective thermal conductivity. The effective thermal conductivity keff⊥ perpendicular to the pores was lower than that of the parallel ones keff∥ and was 40% that of lotus copper material ks with porosity ε of 0.4. Experimental data for keff∥ showed good agreement with analytical results derived from the assumption that heat flow through the cross-sectional area parallel to the pore axis is proportional to (1−ε). Experimental data for keff⊥ showed good agreement with the analytical results derived from the assumption of orthorhombic symmetry and with the numerical results under a uniform staggered array with a nonuniform pore diameter.
Hard transition to chaotic dynamics in Alfvén wave fronts
Authors: Sanmartin J. R. , Lopez-Rebollal O. , del Rio E. , Elaskar S.
Published: April 2004

The derivative nonlinear Schrödinger (DNLS) equation, describing propagation of circularly polarized Alfvén waves of finite amplitude in a cold plasma, is truncated to explore the coherent, weakly nonlinear, cubic coupling of three waves near resonance, one wave being linearly unstable and the other waves damped. In a reduced three-wave model (equal dampings of daughter waves, three-dimensional flow for two wave amplitudes and one relative phase), no matter how small the growth rate of the unstable wave there exists a parametric domain with the flow exhibiting chaotic relaxation oscillations that are absent for zero growth rate. This hard transition in phase-space behavior occurs for left-hand (LH) polarized waves, paralleling the known fact that only LH time-harmonic solutions of the DNLS equation are modulationally unstable, with damping less than about (unstable wave frequency)2/4×ion cyclotron frequency. The structural stability of the transition was explored by going into a fully 3-wave model (different dampings of daughter waves, four-dimensional flow); both models differ in significant phase-space features but keep common features essential for the transition.
Modeling the time evolution of laser-induced plasmas for various pulse durations and fluences
Authors: Laville S. , Vidal F. , Johnston T. W. , Chaker M. , Le Drogoff B. , Barthélemy O. , Margot J. , Sabsabi M.
Published: April 2004

In this paper, the expansion in ambient air of a plasma produced by laser ablation from an aluminum target is studied with the help of a one-dimensional fluid code that includes a consistent description of ablation and of the subsequent plasma expansion. Two limiting cases for the expansion geometry have been considered: Cartesian and hemispheric. The plasma temperature and electron density as a function of the space coordinate and time have been obtained for laser pulse durations of 100 fs, 200 ps, and 5 ns, and laser fluences up to a few tens of J/cm2. Simulation results are in good agreement with experiments. The code shows that for times typically longer than 1 μs, the plasma space-averaged temperature and electron density are nearly independent of the laser parameters and of the chosen expansion geometry. For a given pulse duration the plasma temperature first increases with the fluence and then reaches a saturation value. It is shown that most of the observed plasma behavior can be explained by radiative cooling.
The low MN map for single-null divertor tokamaks
Authors: Ali Halima , Punjabi Alkesh , Boozer Allen , Evans Todd
Published: April 2004

The low MN map is derived from the general theory of maps and the generating function for the low mn perturbation. The unperturbed magnetic topology of a single-null divertor tokamak is represented by the symmetric simple map. The perturbed topology is represented by the low MN map. The method of maps is applied to calculate the effects of low mn perturbation on the stochastic layer and the magnetic footprint. The low mn perturbation organizes the stochastic layer into large scale spatial structures. This is reflected in the phase portraits, safety factor, Liapunov exponents, magnetic footprints, and the semiconnection length. For the expected range of the amplitude of the low mn perturbation, the fraction of magnetic flux escaping the stochastic layer, the width of stochastic layer, the area of the magnetic footprints increase, while the number of hot spots and the fraction of flux going into the hot spots both decrease. The key features of the complex patterns in the heat deposition on the collector plates in divertor tokamaks are quite well recovered from the results of the low MN map.
Operation of ferroelectric plasma sources in a gas discharge mode
Authors: Dunaevsky A. , Fisch N. J.
Published: April 2004

Ferroelectric plasma sources in vacuum are known as sources of ablative plasma, formed due to surface discharge. In this paper, observations of a gas discharge mode of operation of the ferroelectric plasma sources (FPS) are reported. The gas discharge appears at pressures between ∼20 and ∼80 Torr. At pressures of 1–20 Torr, there is a transition from vacuum surface discharge to the gas discharge, when both modes coexist and the surface discharges sustain the gas discharge. At pressures between 20 and 80 Torr, the surface discharges are suppressed, and FPS operates in pure gas discharge mode, with the formation of almost uniform plasma along the entire surface of the ceramics between strips. The density of the expanding plasma is estimated to be about 1013 cm−3 at a distance of 5.5 mm from the surface. The power consumption of the discharge is comparatively low, making it useful for various applications. This paper also presents direct measurements of the yield of secondary electron emission from ferroelectric ceramics, which, at low energies of primary electrons, is high and dependent on the polarization of the ferroelectric material.
Double-kink fishbone instability caused by circulating energetic ions
Authors: Kolesnichenko Ya. I. , Lutsenko V. V. , Marchenko V. S. , White R. B.
Published: April 2004

The destabilization of double kink modes by the circulating energetic ions in tokamaks with the plasma current having an off-axis maximum is studied. It is shown that the high-frequency fishbone instability [energetic particle mode (EPM)] and the low-frequency (diamagnetic) fishbones are possible for such an equilibrium, their poloidal and toroidal mode numbers being not necessarily equal to unity. A new kind of the EPM instability, “doublet fishbones,” is predicted. This instability is characterized by two frequencies; it can occur in a plasma with a nonmonotonic radial profile of the energetic ions when the particle orbit width is less than the width of the region where the mode is localized. It is found that the diamagnetic fishbone branch exists even when the orbit width exceeds the mode width; in this case, however, the instability growth rate is relatively small.
On nonlinear plasma instabilities during the substorm expansive phase onset
Authors: Dobias P. , Voronkov I. O. , Samson J. C.
Published: April 2004

Using a stability analysis based on a Lagrangian–Hamiltonian approach to magnetohydrodynamics, this paper addresses the stability of the Earth’s magnetotail during the later stages of the growth phase of the substorm. The analysis shows that the magnetotail at about 10–12RE is the likely region for instability. Furthermore, the analysis helps to explain why the substorm intensification begins on field line mapping from the Earthward edge of the substorm plasma sheet to auroral arcs near the equatorward edge of the auroral region. Our model and analysis provides a possible explanation for pseudobreakups as well as the initiation of more global substorm intensifications. A stability analysis of the evolving stretched field configurations of the growth phase shows that initially, at a certain level of stretching and in regions where the plasma β is 5–10, localized instabilities are possible. These localized instabilities saturate before producing a more global instability and the reconfiguration of the magnetic field topology of the magnetotail. Eventually the threshold for nonlinear instabilities is reached and a global, explosive instability is possible. Even though this paper looks only at the possibility of explosive instabilities and the initial stages of the substorm intensification, it provides an important first step in forming a complete picture of the substorm instabity and the subsequent return to a lower energy, metastable state through dipolarization of field lines in the magnetotail. A number of important features in the initiation of the substorm intensification can be explained by considering an explosive ballooning mode. Nevertheless, a more complete analysis of the plasma dynamics associated with the instability initiating the substorm intensification and the subsequent evolution of the substorm, including kinetic effects and reconnection, is needed for a more complete model of the substorm process.
Experimental investigation of low-frequency waves propagating in a direct current planar magnetron plasma
Authors: Martines E. , Zuin M. , Antoni V. , Cavazzana R. , Serianni G. , Spolaore M. , Nakashima C.
Published: April 2004

Experimental observations of low frequency waves propagating azimuthally in a direct current planar magnetron sputtering plasma are presented. The measurements, performed with a circular array of electric probes measuring the floating potential fluctuations, show the presence of several Fourier modes with azimuthal periodicity m=3–7, at frequencies of the order of 100 kHz. The modes appear when the discharge current is above a threshold value in a wide range of neutral gas pressures. The wave frequency decreases with increasing pressure. The modes have been interpreted as drift waves destabilized by the combined effect of density gradient and electric field. A nonregular amplitude modulation of the modes, similar to the behavior of coupled oscillators, is observed.
Electron-acoustic solitary waves and double layers in a relativistic electron-beam plasma system
Authors: Sahu Biswajit , Roychoudhury Rajkumar
Published: April 2004

The exact Sagdeev’s pseudopotential is derived for electron-acoustic waves in a two electron temperature plasma (“cold” and “hot” electrons) in the presence of relativistic electron beam plasma using a vortex-like distribution for trapped electrons. It is found that the relativistic effect restricts the region of existence for solitary waves. It is also seen that different ordering in φ, the electric potential, yields different solitary wave solutions. It is also observed that a double layer solution exists even if one keeps terms up to order φ5/2.
Manipulation of two-dimensional arrays of Si nanocrystals embedded in thin SiO2 layers by low energy ion implantation
Authors: Bonafos C. , Carrada M. , Cherkashin N. , Coffin H. , Chassaing D. , Assayag G. Ben , Claverie A. , Müller T. , Heinig K. H. , Perego M. , Fanciulli M. , Dimitrakis P. , Normand P.
Published: April 2004

In silicon nanocrystal based metal–oxide–semiconductor memory structures, tuning of the electron tunneling distance between the Si substrate and Si nanocrystals located in the gate oxide is a crucial requirement for the pinpointing of optimal device architectures. In this work it is demonstrated that this tuning of the “injection distance” can be achieved by varying the Si+ ion energy or the oxide thickness during the fabrication of Si nanocrystals by ultralow-energy silicon implantation. Using an accurate cross-section transmission electron microscopy (XTEM) method, it is demonstrated that two-dimensional arrays of Si nanocrystals cannot be positioned closer than 5 nm to the channel by increasing the implantation energy. It is shown that injection distances down to much smaller values (2 nm) can be achieved only by decreasing the nominal thickness of the gate oxide. Depth profiles of excess silicon measured by time-of-flight secondary ion mass spectroscopy and Si nanocrystal locations determined by XTEM are compared with Monte-Carlo simulations of the implanted Si profiles taking into account dynamic target changes due to ion implantation, ion erosion, and ion beam mixing. This combination of experimental and theoretical studies gives a safe explanation regarding the unique technological route of obtaining Si nanocrystals at distances smaller than 5 nm from the channel: the formation of nanocrystals requires that the interface mixing due to collisional damage does not overlap with the range profile to the extent that there is no more a local maximum of Si excess buried in the SiO2 layer.
Site-selective spectroscopy of Er in GaN
Authors: Dierolf V. , Sandmann C. , Zavada J. , Chow P. , Hertog B.
Published: April 2004

We investigated different Er3+ defect sites found in Er-doped GaN layers by site-selective combined excitation-emission spectroscopy and studied the role of these sites in different direct and multistep excitation schemes. The layers were grown by molecular beam epitaxy and were 200 nm thick. Two majority sites were found along with several minority sites. The sites strongly differ in excitation and energy transfer efficiencies as well as branching ratios during relaxation. For this reason, relative emission intensities from these sites depend strongly on emission and excitation. The sites were identified for several transitions and a comprehensive list of energy levels has been compiled. One of the minority sites appears strongly under ultraviolet excitation above the GaN band gap suggesting that this site is an excellent trap for excitation energy of electron-hole pairs.
In situ cleaning and characterization of oxygen- and zinc-terminated, n-type, ZnO{0001} surfaces
Authors: Coppa B. J. , Fulton C. C. , Hartlieb P. J. , Davis R. F. , Rodriguez B. J. , Shields B. J. , Nemanich R. J.
Published: April 2004

A layer containing an average of 1.0 monolayer (ML) of adventitious carbon and averages of 1.5 ML and 1.9 ML of hydroxide was determined to be present on the respective O-terminated (0001̄) and Zn-terminated (0001) surfaces of ZnO. A diffuse low-energy electron diffraction pattern was obtained from both surfaces. In situ cleaning procedures were developed and their efficacy evaluated in terms of the concentrations of residual hydrocarbons and hydroxide and the crystallography, microstructure, and electronic structure of these surfaces. Annealing ZnO(0001̄) in pure oxygen at 600–650 °C±20 °C reduced but did not eliminate all of the detectable hydrocarbon contamination. Annealing for 15 min in pure O2 at 700 °C and 0.100±0.001 Torr caused desorption of both the hydrocarbons and the hydroxide constituents to concentrations below the detection limits (∼0.03 ML=∼0.3 at. %) of our x-ray photoelectron spectroscopy instrument. However, thermal decomposition degraded the surface microstructure. Exposure of the ZnO(0001̄) surface to a remote plasma having an optimized 20% O2/80% He mixture for the optimized time, temperature, and pressure of 30 min, 525 °C, and 0.050 Torr, respectively, resulted in the desorption of all detectable hydrocarbon species. Approximately 0.4 ML of hydroxide remained. The plasma-cleaned surface possessed an ordered crystallography and a step-and-terrace microstructure and was stoichiometric with nearly flat electronic bands. A 0.5 eV change in band bending was attributed to the significant reduction in the thickness of an accumulation layer associated with the hydroxide. The hydroxide was more tightly bound to the ZnO(0001) surface; this effect increased the optimal temperature and time of the plasma cleaning process for this surface to 550 °C and 60 min, respectively, at 0.050 Torr. Similar changes were achieved in the structural, chemical, and electronic properties of this surface; however, the microstructure only increased slightly in roughness and was without distinctive features.
Cobalt valence states and origins of ferromagnetism in Co doped TiO2 rutile thin films
Authors: Murakami Makoto , Matsumoto Yuji , Hasegawa Tetsuya , Ahmet Parhat , Nakajima Kiyomi , Chikyow Toyohiro , Ofuchi Hironori , Nakai Izumi , Koinuma Hideomi
Published: April 2004

Co doped rutile thin films were fabricated on α-Al2O3 (10-12) substrates by laser molecular beam epitaxy. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy indicated that the rutile thin films are (101) oriented and have smooth surfaces with no impurity phases. Co K-edge x-ray absorption near-edge structure, extended x-ray absorption fine structure, and x-ray photoelectron spectroscopy revealed the coexistence of different valence states of Co in the film. Magnetic circular dichroism studies suggest that the observed ferromagnetism is uniform and is related to the electron band structure of TiO2 rutile. Contribution of oxidized (Co2+) and metallic (Co0) cobalt to the ferromagnetism is discussed.
Long implosion time (240 ns) Z-pinch experiments with a large diameter (12 cm) double-shell nozzle
Authors: Levine J. S. , Banister J. W. , Failor B. H. , Qi N. , Song Y. , Sze H. M. , Fisher A.
Published: April 2004

Recently, an 8 cm diameter double-shell nozzle has produced argon Z pinches with high K-shell yields with implosion time of 210 ns. To produce even longer implosion time Z pinches for facilities such as Decade Quad [D. Price, et al., “Electrical and Mechanical Design of the Decade Quad in PRS Mode,” in Proceedings of the 12th IEEE Pulsed Power Conference, Monterey, CA, edited by C. Stallings and H. Kirbie (IEEE, New York, 1999), p. 489] (9 MA short circuit current at 300 ns), a larger nozzle (12 cm outer diameter) was designed and fabricated. During initial testing on Double-EAGLE [P. Sincerny et al., Proceedings of the 5th IEEE Pulsed Power Conference, Arlington, VA, edited by M. F. Rose and P. J. Turchi (IEEE, New York, 1985), p. 151], 9 kJ of argon K-shell radiation in a 6 ns full width at half maximum pulse was produced with a 240 ns implosion. The initial gas distributions produced by various nozzle configurations have been measured and their impact on the final radiative characteristics of the pinch are presented. The addition of a central jet to increase the initial gas density near the axis is observed to enhance the pinch quality, increasing K-shell yield by 17% and power by 40% in the best configuration tested.
Near-edge x-ray absorption fine structure and x-ray photoemission spectroscopy study of the InN epilayers on sapphire (0001) substrate
Authors: Lee Ik Jae , Kim Jae-Yong , Shin Hyun-Joon , Kim Hyung-Kook
Published: April 2004

A polarization-dependent near-edge x-ray absorption fine structure (NEXAFS) study was performed on InN films epitaxially grown on sapphire (0001) substrates, in order to investigate the change in structure and crystallographic orientation of the film as a function of film thickness. For thin films, the N K-edge NEXAFS spectra showed a strong polarization-dependent spectral feature. The polarization dependence decreased with increasing film thickness and disappeared at a thickness of about 3000 Å. Chemical configuration in InN films was investigated using high-resolution x-ray photoemission spectroscopy (XPS). XPS analysis on the In 3d peak and the N 1s main peak at 396.4 eV suggested that indium and nitrogen are bound in the form of InN in all of the samples. An additional peak observed at 397.4 eV in the N 1s photoelectrons is believed to originate from the formation of oxynitrides at the topmost region of the film.
Influence of high-temperature annealing on the properties of Fe doped semi-insulating GaN structures
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Shlensky A. A. , Pearton S. J.
Published: April 2004

The properties of semi-insulating GaN films doped with Fe were studied after rapid thermal annealing (RTA) at temperatures 750–1050 °C and furnace annealing in hydrogen at temperatures up to 850 °C. It was shown that the Fe in such samples is distributed nonuniformly, showing a clear minimum near 0.5–1 μm from the surface. The Fe concentration increases toward both interfaces. The Fermi level in the as-grown state is pinned by deep levels near Ec–0.5 eV whose concentration is ∼3×1016 cm−3. The room temperature resistivity is on the order of 2×1010 Ω/square. RTA at temperatures higher than 850 °C leads to increasing roughness of the surface and decreases of the sheet resistivity and the cathodoluminescence intensity. The density of deep traps also greatly decreases. The results are explained by partial evaporation of nitrogen from the surface upon annealing. The effect becomes much more pronounced for furnace annealing in hydrogen for times on the order of 15 min and in fact the annealing temperature of 850 °C seems to be the highest practicable under these conditions without destroying the surface morphology.
Tunable optics derived from nonlinear acoustic effects
Authors: Higginson Keith A. , Costolo Michael A. , Rietman Edward A. , Ritter Joseph M. , Lipkens Bart
Published: April 2004

Gradient index lenses were formed in a liquid-filled cavity supporting an ultrasonic standing wave. The constructed devices acted as diverging lenses or axicon lenses, depending on whether the center or edge region is interrogated. The focal length of the diverging lens was controllable with the frequency and amplitude of applied ultrasound from −100 mm to negative infinity. Experiments and models suggest that the primary process contributing to lensing is the steady-state density component of the finite-amplitude standing wave; sound amplitudes up to 150 MPa were calculated in glycerin, corresponding to a maximum contrast in the refractive on the order of 0.1%. This amplitude was also sufficient to move high index nanometer-scale particles via an acoustic radiation force and thereby create larger refractive index gradients. The segregation of suspended nanoparticles was found to enhance the lensing effects that occurred in the pure fluids. Concepts are also explored to manipulate the particle distribution in order to create converging lenses and/or other desirable optical components.
The role of thermal plasma density gradient in the problem of Alfvén cascades in tokamaks
Authors: Konovalov S. V. , Mikhailovskii A. B. , Shirokov M. S. , Ozeki T.
Published: April 2004

The localization effect of thermal plasma density gradient on the Alfvén cascades in tokamaks is studied. It is shown that this effect can be stronger than the toroidal magnetohydrodynamic effect considered by Breizman et al. [Phys. Plasmas 10, 3649 (2003)] as squared aspect ratio. Thus, the Alfvén cascade modes can be theoretically shown in cylindrical geometry approximation. Then the role of thermal plasma density gradient can be dominant if the localization effect of density gradient of large-orbit hot ions [Berk et al., Phys. Rev. Lett. 87, 185002 (2001)] is sufficiently weak.
Nonideal dusty plasma expansion
Authors: Djebli M. , Annou R. , Zerguini T. H.
Published: April 2004

The expansion of a nonideal dusty plasma is investigated. The nonideal effects are modelled by a van der Waals equation of state, where the coefficients are calculated for each intergrain potential. It is found that the nonideal character of the expansion is still relevant for submicron sized grains. When a realistic intergrain potential that takes into account the openness of the system is considered, the expansion shows insignificant deviation from the ideal case. The ideal equation of state for the dust fluid is then justified.
Strain relaxation and threading dislocation density in helium-implanted and annealed Si1−xGex/Si(100) heterostructures
Authors: Cai J. , Mooney P. M. , Christiansen S. H. , Chen H. , Chu J. O. , Ott J. A.
Published: April 2004

Strain relaxation and threading dislocation densities in Si1−xGex (0.15<x<0.30) produced by He implantation and annealing have been investigated using x-ray diffraction and transmission electron microscopy. The degree of strain relaxation is very sensitive to the SiGe layer thickness; only small differences in strain relaxation are obtained when the helium dose and energy are varied over a relatively wide range. In contrast, the threading dislocation density is strongly influenced by the implantation dose and depth. A composite parameter, the He dose in the SiGe layer (He(SiGe)), calculated from He profiles simulated using the program Stopping and Range of Ions in Matter (SRIM2000), correlates well with the threading dislocation density. We have found that to achieve a low threading dislocation density, <5×107 cm−2, He(SiGe) must be less than 1×1015 cm−2.
Modified frequency-domain method for simulating the electromagnetic properties in periodic magnetoactive systems
Authors: Drikis I. , Yang S. Y. , Horng H. E. , Hong Chin-Yih , Yang H. C.
Published: April 2004

In this work, we develop a numerical simulation method to characterize the photonic properties of photonic crystals made of either dielectric or magnetic materials. Due to the magnetic materials in the photonic crystals, not only the dielectric but also the magnetic permeability functions vary periodically with position. Thus, a master equation is needed. The simulation method based on the existing algorithm developed for periodic media with uniform magnetic permeability [Johnson and Joannopoulos, Opt. Express 8, 173 (2001)] is further modified for the present case. We then use the modified numerical simulation method to investigate the photonic properties, such as the dispersion relation, photonic band gap, and electromagnetic field distribution, of some typical photonic crystals which possess analytic expression or well-known photonic properties. A good match between the simulated and the analytic results is obtained. This fact shows that the modified numerical simulation method can be used to explore electromagnetic waves in both dielectrically and the magnetically periodic media.
Self-assembling nanoparticles into holographic nanopatterns
Authors: Lee Seung-Heon , Diana Frédéric S. , Badolato Antonio , Petroff Pierre M. , Kramer Edward J.
Published: April 2004

We demonstrate a method to self-assemble metal nanoparticles into two-dimensional lattices. Monodisperse cobalt nanoparticles were synthesized within inverse micelles of polystyrene-block-poly(2-vinylpyridine) copolymer in toluene. A periodic hole pattern of photoresist (PR) was fabricated on a GaAs substrate by holographic lithography. The nanoparticles as prepared above were self-assembled onto the PR nanopatterns by dip or spin casting. They were selectively positioned in the holes due to the capillary forces related to the pattern geometry. Our study reveals that self-assembled nanoparticles in two-dimensional lattices can be obtained with a controllable number of particles per lattice point.
Vibrational spectroscopic study of the site occupancy distribution of cations in nickel cobalt oxides
Authors: Windisch Charles F. , Exarhos Gregory J. , Owings Robert R.
Published: April 2004

Mid-infrared, far-infrared, and Raman spectra for Ni1−xCoxOy indicated the systematic variation from a spinel-type (x=1.0) to an inverse spinel-type (x=0.67) lattice, through a multiphase transition region, to an NaCl-type (x<0.25) structure. Further analysis reveals that a decrease in x in this system is accompanied by the formation of octahedral/Ni2+, which, in the case of spinel compositions Ni1−xCoxO4/3 (x=0.67 to 1.0), results in substitution of Co3+ into tetrahedral sites and/or the redistribution of charge around some of the oxygen ions. The proposed transition region explains the observed break in continuity, at around x=0.5, in the previously reported resistivity data for nickel cobalt oxide films.
Reduced pressure–chemical vapor deposition of Ge thick layers on Si(001) for 1.3–1.55-μm photodetection
Authors: Hartmann J. M. , Abbadie A. , Papon A. M. , Holliger P. , Rolland G. , Billon T. , Fédéli J. M. , Rouvière M. , Vivien L. , Laval S.
Published: April 2004

Ge-based photodetectors operating in the low loss windows (1.3–1.6 μm) of silica fibers are highly desirable for the development of optical interconnections on silicon-on-insulator substrates. We have therefore investigated the structural and optical properties of Ge thick films grown directly onto Si(001) substrates using a production-compatible reduced pressure chemical vapor deposition system. We have first of all evidenced a Ge growth regime which is akin to a supply-limited one in the 400–750 °C temperature range (Ea=6.9 kcal mol−1). The thick Ge layers grown using a low-temperature/high-temperature approach are in a definite tensile-strain configuration, with a threading dislocation density for as-grown layers of the order of 9×108 cm−2 (annealed: <2×108 cm−2). The surface of those Ge thick layers is rather smooth, especially when considering the large lattice mismatch between Ge and Si. The root-mean-square roughness is indeed of the order of 0.6 nm (2 nm) only for as-grown (annealed) layers. A chemical mechanical polishing step followed by some Ge re-epitaxy can help in bringing the surface roughness of annealed layers down, however (0.5 nm). The Ge layers produced are of high optical quality. An absorption coefficient alpha equal to 4300 cm−1 (3400 cm−1) has indeed been found at room temperature and for a 1.55-μm wavelength for as-grown (annealed) layers. A 20-meV band-gap shrinkage with respect to bulk Ge (0.78 eV⇔0.80 eV) is observed as well in those tensile-strained Ge epilayers.
Probabilistic finite-size transport models for fusion: Anomalous transport and scaling laws
Authors: van Milligen B. Ph. , Sánchez R. , Carreras B. A.
Published: April 2004

Transport in fusion plasmas in the low confinement mode is characterized by several remarkable properties: the anomalous scaling of transport with system size, stiff (or “canonical”) profiles, power degradation, and rapid transport phenomena. The present article explores the possibilities of constructing a unified transport model, based on the continuous-time random walk, in which all these phenomena are handled adequately. The resulting formalism appears to be sufficiently general to provide a sound starting point for the development of a full-blown plasma transport code, capable of incorporating the relevant microscopic transport mechanisms, and allowing predictions of confinement properties.
Effect of the tilt angle on antimony in silicon implanted wafers
Authors: Claudio G. , Kirkby K. J. , Bersani M. , Low R. , Sealy B. J. , Gwilliam R.
Published: April 2004

A dose of 5.0×1014 antimony (Sb+) ions cm−2 was implanted into silicon wafers at an energy of 70 keV, at different tilt angles (0°, 15°, 30°, 45°, and 60°). One set of samples was preamorphized with 160 keV germanium (Ge+) ions with a dose of 1×1015 cm−2. The second set consisted of implants into single crystal silicon. After implantation the samples were annealed at 700 °C for 30 s in a pure nitrogen ambient. Secondary ion mass spectroscopy was performed to evaluate the atomic profile and the retained dose as a function of the tilt angle before and after annealing. Rutherford backscattering spectroscopy and ion channeling measurements were performed to quantify the retained dose and the subtitutional fraction of antimony in the preamorphized and crystalline samples. Hall measurements were performed in order to calculate the proportion of the dopant which was electrically active. It was found that the substitutional fraction was a function of the tilt angle for all of the implanted samples. A correlation was also found between the electrical activation and the subtitutional fraction with the highest electrical activation being found at 15° tilt (90% for antimony implants into crystalline silicon) and 0° tilt (94% for antimony implants into amorphous silicon). For all implant angles the substitutional fraction and dopant activation was greater for the implants into amorphized silicon than for those into the crystalline phase.
Structure and magnetotransport properties of Fe3O4–SiO2 composite films reactively sputtered at room temperature
Authors: Liu Hui , Jiang E. Y. , Bai H. L. , Zheng R. K.
Published: April 2004

( Fe 3 O 4 ) 1−x –( SiO 2 ) x composite films have been prepared by reactive sputtering iron and SiO2 targets in Ar+O2 mixture at room temperature. Transmission electron microscopy bright field images show that with the increase of SiO2 addition, uniform Fe3O4 grains are well separated by the amorphous SiO2 matrix, forming a well-defined granular structure. Temperature dependence of resistivity ρ(T) indicates that the electron tunneling mechanism featured by log ρ∝T−1/2 dominates the transport properties of the films, which smears out the Verwey transition intrinsic to Fe3O4. This tunneling transport of electrons causes a spin-dependent magnetoresistance {=(ρH−ρ0)/ρ0} of about −4.7% for Fe3O4 films and −1.8% for (Fe3O4)0.6(SiO2)0.4 composite films under a 46 kOe magnetic field at room temperature. Magnetic and magnetoresistance measurements reveal that the antiferromagnetically coupled Fe3O4 grains are decoupled and show the behavior of superparamagnetism at x⩾0.4.
Nonlinear modulation of transverse dust lattice waves in complex plasma crystals
Authors: Kourakis I. , Shukla P. K.
Published: April 2004

The occurrence of the modulational instability in transverse dust lattice waves propagating in a one-dimensional dusty plasma crystal is investigated. The amplitude modulation mechanism, which is related to the intrinsic nonlinearity of the sheath electric field, is shown to destabilize the carrier wave under certain conditions, possibly leading to the formation of localized envelope excitations. Explicit expressions for the instability growth rate and threshold are presented and discussed.
Lattice damage produced in GaN by swift heavy ions
Authors: Kucheyev S. O. , Timmers H. , Zou J. , Williams J. S. , Jagadish C. , Li G.
Published: April 2004

Wurtzite GaN epilayers bombarded at 300 K with 200 MeV Au16+197 ions are studied by a combination of transmission electron microscopy (TEM) and Rutherford backscattering/channeling spectrometry (RBS/C). Results reveal the formation of near-continuous tracks propagating throughout the entire ∼1.5-μm-thick GaN film. These tracks, ∼100 Å in diameter, exhibit a large degree of structural disordering but do not appear to be amorphous. Throughout the bombarded epilayer, high-resolution TEM reveals planar defects which are parallel to the basal plane of the GaN film. The gross level of lattice disorder, as measured by RBS/C, gradually increases with increasing ion fluence up to ∼1013 cm−2. For larger fluences, delamination of the nitride film from the sapphire substrate occurs. Based on these results, physical mechanisms of the formation of lattice disorder in GaN in such a high electronic stopping power regime are discussed.
Capacitance and tunneling spectroscopy of InAs quantum dots
Authors: Schmidt K. H. , Bock C. , Versen M. , Kunze U. , Reuter D. , Wieck A. D.
Published: April 2004

A Schottky diode type sample (Au/i-GaAs/InAs/i-GaAs/n+-GaAs) with InAs quantum dots (QDs) embedded in the intrinsic GaAs region between the Schottky contact and the n-doped GaAs-back contact was used to investigate the electron transport into and through InAs QDs. According to a simple leverage law the QD ground state resonance shifts to higher gate voltages when the thickness of the tunneling barrier t2 between the QDs and the Schottky contact is reduced from t2=90 nm to t2=10 nm. Additionally, the transition from a pure capacitive to a predominant conductive signal of the QD ground state is observed. The gate voltage offset between the charging and the tunneling signal of the s-shell is explained by QDs of different size dominating the ohmic and the capacitive I–V traces, respectively. This interpretation is confirmed by different Coulomb blockade energies as well as different confinement energies of the QD ground state determined from both types of signal.
Relativistic and ponderomotive effects on laser plasma interaction dynamics
Authors: Jha Pallavi , Wadhwani Navina , Raj Gaurav , Upadhyaya Ajay K.
Published: April 2004

The combined effect of relativistic and ponderomotive nonlinearities on the propagation characteristics and modulation instability of a laser beam propagating through partially stripped plasma is studied. Under the present approach it is seen that the ponderomotive nonlinearity tends to defocus the laser beam as against the nonlinear relativistic self-focusing phenomenon. Also density perturbation arising due to the ponderomotive nonlinearity when combined with relativistic nonlinearity tends to increase the modulation instability of the laser beam. The peak growth rate is enhanced in comparison to the case where ponderomotive nonlinearity is neglected.