bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamical plasma response of resistive wall modes to changing external magnetic perturbations
Authors: Shilov M. , Cates C. , James R. , Klein A. , Katsuro-Hopkins O. , Liu Y. , Mauel M. E. , Maurer D. A. , Navratil G. A. , Pedersen T. S. , Stillits N. , Fitzpatrick R. , Paul S. F.
Published: April 2004

The plasma response to external resonant magnetic perturbations is measured as a function of stability of the resistive wall mode (RWM). The magnetic perturbations are produced with a flexible, high-speed waveform generator that is preprogrammed to drive an in-vessel array of 30 independent control coils and to produce an m/n=3/1 helical field. Both quasi-static and “phase-flip” magnetic perturbations are applied to time-evolving discharges in order to observe the dynamical response of the plasma as a function of RWM stability. The evolving stability of the RWM is estimated using equilibrium reconstructions and ideal stability computations, facilitating comparison with theory. The plasma resonant response depends upon the evolution of the edge safety factor, q*, and the plasma rotation. For discharges adjusted to maintain relatively constant edge safety factor, q*<3, the amplitude of the plasma response to a quasistatic field perturbation does not vary strongly near marginal stability and is consistent with the Fitzpatrick–Aydemir equations with high viscous dissipation. Applying “phase-flip” magnetic perturbations that rapidly change toroidal phase by 180° allows observation of the time scale for the plasma response to realign with the applied perturbation. This phase realignment time increases at marginal stability, as predicted by theory. This effect is easily measured and suggests that the response to time-varying external field perturbations may be used to detect the approach to RWM instability.
Application of adaptive mesh refinement to particle-in-cell simulations of plasmas and beams
Authors: Vay J.-L. , Colella P. , Kwan J. W. , McCorquodale P. , Serafini D. B. , Friedman A. , Grote D. P. , Westenskow G. , Adam J.-C. , Héron A. , Haber I.
Published: April 2004

Plasma simulations are often rendered challenging by the disparity of scales in time and in space which must be resolved. When these disparities are in distinctive zones of the simulation domain, a method which has proven to be effective in other areas (e.g., fluid dynamics simulations) is the mesh refinement technique. A brief discussion of the challenges posed by coupling this technique with plasma particle-in-cell simulations is given, followed by a presentation of examples of application in heavy ion fusion and related fields which illustrate the effectiveness of the approach. Finally, a report is given on the status of a collaboration under way at Lawrence Berkeley National Laboratory between the Applied Numerical Algorithms Group (ANAG) and the Heavy Ion Fusion group to upgrade ANAG’s mesh refinement library Chombo to include the tools needed by particle-in-cell simulation codes.
The magnetized electron-acoustic instability driven by a warm, field-aligned electron beam
Authors: Sooklal A. , Mace R. L.
Published: April 2004

The electron-acoustic instability in a magnetized plasma having three electron components, one of which is a field-aligned beam of intermediate temperature, is investigated. When the plasma frequency of the cool electrons exceeds the electron gyrofrequency, the electron-acoustic instability “bifurcates” at sufficiently large propagation angles with respect to the magnetic field to yield an obliquely propagating, low-frequency electron-acoustic instability and a higher frequency cyclotron-sound instability. Each of these instabilities retains certain wave features of its progenitor, the quasiparallel electron-acoustic instability, but displays also new magnetic qualities through its dependence on the electron gyrofrequency. The obliquely propagating electron-acoustic instability requires a lower threshold beam speed for its excitation than does the cyclotron-sound instability, and for low to intermediate beam speeds has the higher maximum growth rate. When the plasma is sufficiently strongly magnetized that the plasma frequency of the cool electrons is less than the electron gyrofrequency, the only instability in the electron-acoustic frequency range is the strongly magnetized electron-acoustic instability. Its growth rate and real frequency exhibit a monotonic decrease with wave propagation angle and it grows at small to intermediate wave numbers where its parallel phase speed is approximately constant. The relevance of the results to the interpretation of cusp auroral hiss and auroral broadband electrostatic noise is briefly discussed.
Control of linear and nonlinear resistive wall modes
Authors: Finn J. M. , Chacón L.
Published: April 2004

Control of linear and nonlinear resistive wall modes (RWMs) is studied with a model that assumes: (1) a single Fourier harmonic of the normal component of the magnetic field is measured at the resistive wall; and (2) the control applied consists of that same harmonic at a larger radius, the control surface. For this model, it is shown that real gain Gr (zero phase shift) is exactly equivalent to having a perfectly conducting wall between the actual resistive wall and the control surface. It is also shown that imaginary gain Gi (π/2 phase shift) is exactly equivalent to the rotation of the resistive wall, which is in turn equivalent to plasma rotation. If there are two resistive walls separated by an insulator, Gi is equivalent to the rotation of the outer wall, and this effective differential rotation of the resistive walls can stabilize the modes for arbitrary plasma rotation. Complex gain Gr+iGi is equivalent to a closer conducting wall with rotation of the resistive wall. These equivalences are exact in two-dimensional linear theory (single Fourier harmonic k), and are good approximations when there is a spectrum of k. It is also shown in this slab model that “mode control,” used in DIII–D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)], is equivalent to higher Gr. Two-dimensional nonlinear simulations of control of RWM driven by current and pressure are presented. Investigations are shown of the validity of the feedback equivalences in nonlinear theory, showing that the equivalences hold to a good approximation even when a spectrum of k is present. It is found that the real gain required to give benign saturation of the nonlinear RWMs can be much less than that required for linear stabilization, particularly near the threshold for instability with a perfectly conducting wall.
Dependence of frequency of nonlinear cold plasma cylindrical oscillations on electron density
Authors: Osherovich Vladimir , Fainberg J.
Published: April 2004

For cold plasma, the frequency of small amplitude Langmuir oscillations along one Cartesian coordinate is ωpo=4πe2no/me, where np=no is the constant proton density which is equal to the average electron density ne¯. This formula for ωpo is the basis for measurements of ne¯ in passive and active radio experiments located on spacecraft. We find that for cold plasma nonlinear cylindrical oscillations ne¯>np (i.e., a buildup of negative space charge near the axis of the cylinder). The resulting frequency of oscillations ωpe is greater than ωpo. The relation between ωpe and ne¯ is found to be logarithmic: ωpe=ωpo[1+ln(ne¯/no)/12] with 0.5% accuracy for the range 1⩽ne¯/np<6. For quasi-neutral plasma, when ne¯/np≈1, the logarithmic formula reduces to the linear one: ωpe=ωpo[11/12+(ne¯/np)/12]. For ne¯/np≫6, ωpe approaches an upper limit of 2ωpo. These results are expected to be helpful in diagnostics of ne¯ in the solar wind and in magnetospheric plasmas as well as in laboratory plasmas where cylindrical symmetry is present.
The generation of micro-fiducials in laser-accelerated proton flows, their imaging property of surface structures and application for the characterization of the flow
Authors: Ruhl H. , Cowan T. , Fuchs J.
Published: April 2004

A mechanism that has gained great attention in the field of laser-ion acceleration due to remarkable properties of the accelerated beam is laser acceleration of protons from the rear surface of solid targets. It is shown that these protons are capable of transporting structure information from the rear surface of the target into the detector, which can be used to measure essential properties of the flow like an upper limit of the lateral beam emittance and the source area of the laser accelerated protons for a particular energy. In this Letter the essential mechanism responsible for the generation of images of surface structures by laser-accelerated protons is addressed and used to explain the experiment is described in the Letter.
Characteristics of THz waves and carrier scattering in boron-doped epitaxial Si and Si1−xGex films
Authors: Ray S. K. , Adam T. N. , Troeger R. T. , Kolodzey J. , Looney G. , Rosen A.
Published: April 2004

The absorption and reflection characteristics of boron-doped silicon and silicon-germanium alloys have been investigated in the frequency range from 1.6 to 60 THz. The absorption increases with doping concentration, in agreement with free carrier effects, but saturates for wavelengths longer than about 20 μm. As compared to silicon, the attenuation increases with the Ge fraction in the alloy. Terahertz reflectance data has been analyzed to study the doping dependent plasma-edge frequency, which may play an important role for the design of emitters, detectors, and plasmon waveguides. The best fitting of the experimental data with Drude theory has been used to extract the hole scattering relaxation time in doped silicon. The results have been utilized to explain the doping-dependent attenuation characteristics of the THz radiation.
Slow nonlinear waves in magnetic flux tubes
Authors: Zhugzhda Y. D.
Published: April 2004

The theory of nonlinear slow body and surface waves is developed. The conventional long-wavelength approximation is lifted. A new kind of nonlinear evolution equations is obtained. Two complementary equations are obtained. One of them is of the highest accuracy. Another one has less accuracy but its analytical solutions are found. Two analytical solutions of a distinct kind are found. It is revealed that both dark and bright solitons are the solutions of the evolutionary equation for slow waves. One of these two solutions is not singular for arbitrary amplitudes. Another one is not singular only when the critical amplitude is exceeded. The canonic form of the new evolutionary equation is presented. As a by-product of the exploration, the equation of the highest accuracy for body waves in the long-wavelength approximation is obtained. The interplay of shocks and solitons in flux tubes is discussed.
Temperature-dependent contact resistivity of the nonalloyed ohmic contacts to p-GaN
Authors: Kwak Joon Seop , Nam Ok-Hyun , Park Yongjo
Published: April 2004

Temperature-dependent contact resistivity of nonalloyed Pd/Pt/Au contacts to p-GaN films as well as temperature-dependent sheet resistivity of p-GaN films has been investigated in order to understand anomalously low contact resistivity (∼10−4 Ω cm2) considering the large work-function difference between the Pd and p-GaN. As the measured temperature decreases from 300 to 100 K, the contact resistivity increases by more than one order of magnitude. In addition, the sheet resistivity increases linearly with exp(To/T)1/4, implying variable-range hopping conduction via deep-level defects (DLDs). The density of the DLDs in the p-GaN films is estimated to be over 1019 cm−3, which suggests that the carriers may flow from the Pd directly to the dense DLDs, resulting in the anomalously low contact resistivity.
Development of global magnetohydrodynamic instabilities in Z-pinch plasmas in the presence of nonideal effects
Authors: Sotnikov V. I. , Bauer B. S. , Leboeuf J. N. , Hellinger P. , Trávnı́ček P. , Fiala V.
Published: April 2004

The development of global magnetohydrodynamic (MHD) instabilities in Z-pinch plasmas has been studied with a three-dimensional hybrid simulation model. Plasma equilibria without and with axial sheared flow, and with different values of the parameter εH, which appears as a coefficient before the Hall term in dimensionless nonideal MHD equations, have been considered. Increasing the parameter εH leads to larger simulation growth rates for both m=0 sausage and m=1 kink modes. The hybrid simulations do however show that axial sheared flow severely curtails the linear and nonlinear development of both sausage and kink instabilities. In these respects, the hybrid simulations are in qualitative agreement with linear Hall MHD results. Moreover, in the nonlinear stage, long wavelength modes dominate the excited wave spectrum when the parameter εH is small. For the larger value of the parameter εH, small-scale structures do however develop nonlinearly in the excited wave spectrum at late times.
Organized two-dimensional Ti–SiO2 metal quantum dot composites induced by subplantation
Authors: Zhao J. P. , Huang D. X. , Jacobson A. J. , Rabalais J. W.
Published: April 2004

Two-dimensional (2D) titanium nanodots were formed in the subsurface layer of single crystal SiO2, i.e., a 2D Ti metal quantum dot composite, by subplantation of 9 keV Ti+ ions. Transmission electron microscopy images show that the Ti nanodots have a uniform size distribution of ∼2 nm with very little deviation, almost constant edge-to-edge spacing (∼1 nm) between neighboring nanodots in the lateral direction, and a very narrow depth distribution. These nanodots exhibit no crystallinity and are smaller than our previously reported single crystalline Ti nanodots with body-centered-cubic β phase, indicating that the size and crystallinity of metal nanodots can be controlled through subplantation. A preliminary mechanism for the formation of 2D nanodots during subplantation is discussed.
Direct-drive cryogenic target implosion performance on OMEGA
Authors: McKenty P. W. , Sangster T. C. , Alexander M. , Betti R. , Craxton R. S. , Delettrez J. A. , Elasky L. , Epstein R. , Frank A. , Glebov V. Yu. , Goncharov V. N. , Harding D. R. , Jin S. , Knauer J. P. , Keck R. L. , Loucks S. J. , Lund L. D. , McCrory R. L. , Marshall F. J. , Meyerhofer D. D. , Regan S. P. , Radha P. B. , Roberts S. , Seka W. , Skupsky S. , Smalyuk V. A. , Soures J. M. , Thorp K. A. , Wozniak M. , Frenje J. A. , Li C. K. , Petrasso R. D. , Séguin F. H. , Fletcher K. A. , Padalino S. , Freeman C. , Izumi N. , Koch J. A. , Lerche R. A. , Moran M. J. , Phillips T. W. , Schmid G. J. , Sorce C.
Published: April 2004

Layered and characterized cryogenic D2 capsules have been imploded using high-contrast pulse shapes on the 60-beam OMEGA laser at the Laboratory for Laser Energetics [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. These experiments measure the sensitivity of the direct-drive implosion performance to parameters such as the inner-ice-surface roughness, the adiabat of the fuel during the implosion, and the laser power balance. The goal is to demonstrate a high neutron-averaged fuel ρR with low angular variance using a scaled, α∼3 ignition pulse shape driving a scaled all-DT ignition capsule. Results are reported with improvements in target layering and characterization and in laser pointing and target positioning on the OMEGA laser over previous experiments [T. C. Sangster et al., Phys. Plasmas 10, 1937 (2003)]. These capsules have been imploded using up to 23 kJ of 351-nm laser light with an on-target energy imbalance of less than 2% rms, full beam smoothing (1-THz bandwidth, two-dimensional smoothing by spectral dispersion, and polarization smoothing), and new, optimized, distributed phase plates. Pulse shapes include high-adiabat (∼25) square pulses and low-adiabat (<5) shaped pulses. The data from neutron and charged-particle diagnostics, as well as static and time-resolved x-ray images of the imploding core, are compared with one- and two-dimensional numerical simulations. Scaling of target performance to a weighted quadrature of inner-ice roughness at the end of the acceleration phase is investigated.
Collisional multipactor inside ambient gas
Authors: Riyopoulos Spilios
Published: April 2004

The influence of collisions during rf-induced multipactors in the presence of ambient gas is studied in some detail. The effect of the randomization of the electron rf-interaction phases due to collisional kicks is given particular emphasis, since phase locking among successive generations of rf-driven electrons is essential for avalanching. The systematic collisional friction shifts the parameter region for multipactor eruption to higher rf-fields compared to vacuum. Random velocity fluctuations prevent convergence to exact phase locking, causing the impact phase to wander around its nominal resonant value. The rms width of the phase spread is determined by the balance between the rf-induced dynamical phase focusing and random phase diffusion. For low collision frequencies phase wandering in the unfavorable for secondary emission regime reduces the effective secondary yield and may eventually cause multipactor quenching. For high collision frequencies multipactor is completely suppressed due to loss of coherence with the rf.
Efficient generation of extended plasma waveguides with the axicon ignitor-heater scheme
Authors: Xiao Y.-F. , Chu H.-H. , Tsai H.-E. , Lee C.-H. , Lin J.-Y. , Wang J. , Chen S.-Y.
Published: April 2004

An efficient method for generating extended plasma waveguides is developed by using the axicon lens in conjunction with the ignitor-heater scheme. The short-pulse ignitor generates the seed electrons by multiphoton ionization, and the long-pulse heater expands the plasma by inverse bremsstrahlung heating and builds up the plasma density barrier by collisional ionization. A 1.2-cm-long plasma waveguide is generated in pure Ar gas with a total energy of only 100 mJ. Evolution of the plasma density profile is measured by time-resolved interferometry to show the waveguide forming process and how it can be optimized.
A study of charge dependence of particle transport using impurity pellet injection and high-spatial resolution bremsstrahlung measurement on the Large Helical Device
Authors: Nozato H. , Morita S. , Goto M. , Takase Y. , Ejiri A. , Amano T. , Tanaka K. , Inagaki S. ,   LHD Experimental Group
Published: April 2004

The spatial structure and Z dependence of transport coefficient were studied by injecting impurity pellets (C, Al, and Ti) and measuring high-spatial resolution bremsstrahlung in steady-state neutral beam heated plasmas on the Large Helical Device [A. Iiyoshi et al., Fusion Techol. 17, 169 (1990)]. The D and V were evaluated from a diffusive/convective model with minimizing the residual error between the experimental and computational bremsstrahlung intensities taking the recycling rate into account. As a result, the existence of an inward convection was observed at ρ>0.6, and no convection was required at ρ<0.6. Furthermore, it is found that the dependence of D on the electron density, the species and charge state of impurity is weak [typically D=0.15–0.25 (m2/s) in the range of ne=1.4–5.2×1019 (m−3)], but the inward convection has a strong dependence not only on the electron density but also the charge state. The spatial structure and Z dependence of the convective velocity for carbon and titanium ions were evaluated.
The local limit of global gyrokinetic simulations
Authors: Candy J. , Waltz R. E. , Dorland W.
Published: April 2004

Global gyrokinetic simulations of turbulence include physical effects that are not retained in local flux-tube simulations. Nevertheless, in the limit of sufficiently small ρ* (gyroradius compared to system size) it is expected that a local simulation should agree with a global one (at the local simulation radius) since all effects that are dropped in the local simulations are expected to vanish as ρ*→0. In this Letter, global simulations of a well-established test case are indeed shown to recover the flux-tube limit at each radius.
Measuring shock-bang timing and ρR evolution of D3He implosions at OMEGA
Authors: Frenje J. A. , Li C. K. , Séguin F. H. , Deciantis J. , Kurebayashi S. , Rygg J. R. , Petrasso R. D. , Delettrez J. , Glebov V. Yu. , Stoeckl C. , Marshall F. J. , Meyerhofer D. D. , Sangster T. C. , Smalyuk V. A. , Soures J. M.
Published: April 2004

New experimental results describing the dynamics of D3He capsule implosions, performed at the 60 beam direct-drive OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], are presented. The capsules, nominally 940 μm in diameter and with 20–27 μm thick CH shells, were filled with 18 atm D3He gas and irradiated with 23 kJ of UV light. Simultaneous measurements of D3He burn history, DD burn history, and several time-integrated D3He proton energy spectra provided new results, such as shock-bang timing, shock-burn duration, evolution of the ion temperature, and evolution of ρR and ρR asymmetries. The shock-bang time measurements, when compared to calculation using the 1D LILAC code [J. Delettrez et al., Phys. Rev. A 36, 3926 (1987)], indicate that a varying flux limiter is required to explain the data, while the measured shock-burn duration is significantly shorter than 1D calculations, irrespective of flux limiter. The time evolution of ion temperature [Ti(t)] has been inferred from the ratio of the DD and the D3He burn histories, and a constant temperature is observed during the compression phase. The discrepancy between experimental data and 1D simulations during the final stages of the compression burn indicates that mix is significant, especially for the 20 and 24-μm capsule implosions. Evolution of ρR and ρR asymmetries show that the average ρR grows by a factor of ∼4–5 from shock-bang to compression-bang time, and that ρR asymmetries (l=1), primarily driven by capsule convergence, grows ∼2 times faster than the average ρR growth.
Calculations of band-filling optical nonlinearities in extrinsic semiconductors beyond the low injection limit
Authors: Lettieri S.
Published: April 2004

Theoretical calculations of room temperature band-filling optical nonlinearities in a bulk semiconductor are reported. The approach, based on self-consistent determination of pseudochemical potentials, improves on other existing calculations as it correctly takes into account an arbitrary optical injection level. The self-consistent approach also enables direct calculation of the nonlinear optical susceptibility without fixing the photogenerated electron-hole density as a parameter, as required in existing phenomenological models. Good agreement with experimental results is obtained, in the case of GaAs, when photoexcited carrier densities above the Mott transition are considered. Calculations of pair density and nonlinear refractive index are shown as a function of the doping level and of the pump intensity for bulk GaAs. Finally, the limits of the model and applications to calculations of optical behavior in more complex structures are discussed.
The evolution of magnetic helicity in driven reconnection processes and the flux rope structures at the dayside magnetopause
Authors: Jin S. P. , Cui H. L.
Published: April 2004

A two-dimensional magnetohydrodynamics simulation with three components is carried out to explore the driven reconnection processes at the dayside magnetopause. The numerical results show significant enhancements of the magnetic field strength B and the By component at the centers of the recurrent plasmoid-like structures. Such features are in line with the signatures of flux transfer events which are envisaged as passages of magnetic flux tubes. The evolution of the topological structure in reconnected flux tubes is investigated in terms of the principle of magnetic helicity. It is found that the total magnetic helicity in the domain increases as time elapses due to the transportation of magnetic helicity flux through the system boundaries and a peak value of the magnetic helicity density h always occurs in the central region of the reconnected flux tube. Furthermore, the gradual accumulation of h in the current sheet is associated with the accumulated magnetic flux which is carried into this region by the plasma inflow. The concentration of the helicity density h in the core of a flux tube arises from the enhancement of the axial field By, which is associated with the continuous addition of By flux to the domain. The present results indicate that the occurrence of the flux rope structure in the vicinity of the magnetopause might be related to the solar wind plasma flow.
Raman and absorption spectra and thermal conductance of Er:Yb:GdCa4O(BO3)3 crystals
Authors: Xia H. R. , Zhao P. , Cheng X. F. , Liu W. L. , Zhang S. J. , Cheng Z. X. , Yang Z. H.
Published: April 2004

Raman data and thermal expansion of Er:Yb:GdCa4O(BO3)3 (EYGCOB) crystals reveal that the crystal thermal conductance and thermomechanical properties are outstanding especially along the b direction. The Z-direction polarized absorbance of the b-direction wafer exhibits that the Yb3+ ion is very effectual as a sensitizer. These indicate that the EYGCOB crystals are suitable as an infrared radiation laser at about 1500 nm by using a pump wavelength of about 1000 nm from an InGaAs diode laser.
Cation distribution in nanosized Ni–Zn ferrites
Authors: Upadhyay Chandan , Verma H. C. , Anand S.
Published: April 2004

Nanoparticles of Ni1−xZnxFe2O4 (x=0.0, 0.25, 0.50, 0.75, and 1.0) in the size range of 6–12 nm have been synthesized by chemical precipitation followed by hydrothermal treatment. A strong correlation between the particle size and the zinc concentration has been identified. Mössbauer studies on these systems show that the cation distribution not only depends on the particle size but also on the preparation route. There are indications that in the present nanophase samples Fe occupies more tetrahedral sites as compared to the normal occupancy in the spinel ferrite structure. The occupancy returns to normal values after heat treatment at 1000 °C. Low-temperature Mössbauer studies indicate a significant amount of deviation of cation distribution from their bulk preferences.
Influence of AlGaN/GaN interface polarization fields on the properties of photoconductive detectors
Authors: Zhou J. J. , Jiang R. L. , Wen B. , Liang L. Y. , Ji X. L. , Shen B. , Zhang R. , Zheng Y. D.
Published: April 2004

The Al0.22Ga0.78N/GaN/Al0.22Ga0.78N multilayer heterostructure photoconductive detectors were designed and fabricated. The influence of AlGaN/GaN interface polarization fields on the properties of the photodetector was investigated. The energy band profile of the heterostructure was approximately calculated. Results indicate that electron and hole wells exist on the GaN sides of the Al0.22Ga0.78N/GaN/Al0.22Ga0.78N interfaces. The existence of two-dimensional electron gas was proved by variable temperature Hall measurements. The spectral response shows that the responsivity of the heterostructure photodetector was as high as 4300 A/W at 355 nm under 3 V bias, which is nearly ten times higher than that of a GaN monolayer structure photodetector. The response time and dark resistance of the photodetectors were also discussed.
Dielectric relaxation in interfacial amorphous phase of copper core–copper oxide shell nanocomposites in gel-derived silica glasses
Authors: Basu S. , Das D. , Chakravorty D.
Published: April 2004

Composites of copper core–copper oxide shell nanoparticles in a gel-derived silica glass were prepared by a chemical method. The core–shell structure formed a percolative system within the glass. The copper core had diameters in the range 3.5–4.8 nm with the shell thickness varying from 1.0 to 1.6 nm. The dielectric modulus for these nanocomposites was measured over the frequency range 100 Hz–6 MHz at temperatures varying from 330 to 535 K. The modulus spectra were analyzed by a Kohlrausch–Williams–Watts stretched exponential relaxation function. The exponent β for the nanocomposites was found to have values ∼0.47 whereas the value obtained in the case of the precursor glass was 0.67. This clearly brought out the difference between the glass medium and the interfacial amorphous phase generated in the nanocomposites. The activation energies for the dielectric relaxation were found to be in reasonable agreement with those obtained earlier for dc conductivity.
Fractal topography of surfaces exposed to gas-cluster ion beams and modeling simulations
Authors: Fenner D. B.
Published: April 2004

Surface topography measured by atomic force microscope is reported before and after various gas-cluster ion beam (GCIB) treatments along with modeling simulations of topography changes. Height correlation and spectral distributions of these surfaces show characteristics of random fractals with Hurst exponent H typically 0.5<H<1.0 and correlation lengths from ∼10 to over 500 nm. The roughness increases or decreases depending on the initial surface and the nature of the GCIB. The power spectral density distributions observed have a broad power-law roll off from a low-frequency plateau to a high-frequency one. This occurs over an approximate range of spatial frequencies from 5×10−3 to 5×10−1 nm−1. Data from several example surfaces are given. Roughening is shown to be a statistical accumulation of individual cluster impacts and the process is modeled by Monte Carlo simulations resulting in fractal surfaces. A continuum model that incorporates surface mobility is used to simulate the smoothing, and methods to combine this with the Monte Carlo model are presented. The behavior of surfaces under exposure to GCIB is satisfactorily simulated by this combined model. Accurate simulation of the surface smoothing requires that the surface-mobility model be independent of the spatial frequency over the bandwidth of observation, unlike Fick’s law of diffusion. The nonphysical prediction of previous simulations that the topography trends toward the complete absence of roughness is also corrected.
Filamentation of Alfvén waves associated with transverse perturbation
Authors: Shukla Anita , Sharma R. P. , Malik Monika
Published: April 2004

This paper presents an investigation of the growth of a Gaussian perturbation on a uniform plane wave front of an Alfvén wave (AW). The effect of the nonlinear coupling between the main AW and the perturbation has been studied. The dynamical equation for the field of the perturbation has been established and its semianalytical solution has been obtained in low (β≪me/mi) and high (β≫me/mi) β cases (β≪1). The critical field of the perturbation has been evaluated for the given size of the perturbation for its filamentation process (hot spot formation). Nonlinear evolution of the perturbation into filamentary structures and its dependence on various parameters of the solar wind and corona has been investigated in detail.
Structural study of (CdS/ZnSe)/BeTe superlattices for λ=1.55 μm intersubband transition
Authors: Li B. S. , Akimoto R. , Akita K. , Hasama H.
Published: April 2004

A (CdS/ZnSe)/BeTe superlattice (SL), based on wide band gap II–VI compounds, with a large band offset of 3.1 eV was grown on a GaAs (001) substrate using molecular-bean epitaxy and an intersubband transition (ISB-T) of 0.78 eV (λ=1.58 μm) with a full width at half maximum (FWHM) of 96 meV observed. We studied structural properties using high-resolution x-ray diffraction combined with dynamic simulation and found through the strain state in samples that a ZnSe/BeTe interface having a quaternary interface layer (ZnTe)0.45(BeSe)0.55 is preferentially formed despite the promotion of one molecular layer (ML) ZnTe interface formation. Be–Se bonds thus replace the Zn–Te bond in the transition region. For the CdS/ZnSe interface, an approximately 1 ML Zn0.75Cd0.25S ternary layer accompanied by ∼1 ML Zn0.85Cd0.15Se forms at the transition region due to Cd diffusion. X-ray (002) ω/2θ scan curves for (CdS/ZnSe)/BeTe SLs show sharp, intense satellite peaks exceeding ten orders, indicating high structure quality. We obtained excellent agreement between experimental diffraction patterns and the calculated curve via dynamic simulation for (CdS/ZnSe)/BeTe SLs. The good fits allows us to identify structure parameters in (CdS/ZnSe)/BeTe SLs, which are consistent with results of high-resolution transmission electron microscopy measurement. Based on dynamic simulated results, we obtained a structure of (CdS/ZnSe)/Be1−xMgxTe (x=1.2%) with an average lattice constant aSL matching the GaAs substrate. An ISB-T located at wavelength λ=1.55 μm with a narrow FWHM of 90 meV was thus realized at room temperature.
High performance stationary discharges in the DIII-D tokamak
Authors: Luce T. C. , Wade M. R. , Ferron J. R. , Politzer P. A. , Hyatt A. W. , Sips A. C. C. , Murakami M.
Published: April 2004

Recent experiments in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] have demonstrated high β with good confinement quality under stationary conditions. Two classes of stationary discharges are observed—low q95 discharges with sawteeth and higher q95 without sawteeth. The discharges are deemed stationary when the plasma conditions are maintained for times greater than the current profile relaxation time. In both cases the normalized fusion performance (βNH89P/q952) reaches or exceeds the value of this parameter projected for Qfus=10 in the International Thermonuclear Experimental Reactor (ITER) design [R. Aymar et al., Plasma Phys. Controlled Fusion 44, 519 (2002)]. The presence of sawteeth reduces the maximum achievable normalized β, while confinement quality (confinement time relative to scalings) is largely independent of q95. Even with the reduced β limit, the normalized fusion performance maximizes at the lowest q95. Projections to burning plasma conditions are discussed, including the methodology of the projection and the key physics issues which still require investigation.
Correlation of lattice distortion with optical and electrical properties of In2O3:Sn films
Authors: Mergel D. , Qiao Z.
Published: April 2004

Polycrystalline In2O3:Sn (ITO) films were prepared by direct current magnetron sputtering at substrate temperatures between 150 and 400 °C and at various oxygen admixtures to the sputter gas. X-ray diffraction peaks (position and width) were evaluated with respect to lattice distortion (net increase and fluctuations of the lattice constant). The characteristic optical parameters of the films were obtained by dielectric modeling of experimental transmittance and reflectance spectra. The plasma edge was evaluated with respect to electron density and mobility. The electron density decreases linearly with the lattice distortion. This confirms a model of oxygen incorporation into the lattice during sputtering and allows an estimation of the density of oxygen interstitials from x-ray spectra. The electron mobility inside the grains decreases with decreasing electron density. This is well explained by a model based on scattering at ionized Sn donors that are partially compensated by interstitial oxygen ions. The direct current conductivity is smaller than the conductivity derived from the optically determined parameters because it is hampered by badly conducting grain boundaries. The refractive index decreases linearly with the free electron density which is explained, via the Kramers–Kronig relation, by increasing absorption in the plasma edge and the Burstein–Moss shift of the band edge.
Energy deposition model for low-energy electrons (10–10 000 eV) in air
Authors: Roldán A. , Pérez J. M. , Williart A. , Blanco F. , Garcı́a G.
Published: April 2004

An energy deposition model for electrons in air that can be useful in microdosimetric applications is presented in this study. The model is based on a Monte Carlo simulation of the single electron scattering processes that can take place with the molecular constituents of the air in the energy range 10–10 000 eV. The input parameters for this procedure have been the electron scattering cross sections, both differential and integral. These parameters were calculated using a model potential method which describes the electron scattering with the molecular constituent of air. The reliability of the calculated integral cross section values has been evaluated by comparison with direct total electron scattering cross-section measurements performed by us in a transmission beam experiment. Experimental energy loss spectra for electrons in air have been used as probability distribution functions to define the electron energy loss in single collision events. The resulting model has been applied to simulate the electron transport through a gas cell containing air at different pressures and the results have been compared with those observed in the experiments. Finally, as an example of its applicability to dosimetric issues, the energy deposition of 10 000 eV by means of successive collisions in a free air chamber has been simulated.
Reduced magnetohydrodynamics and parallel spectral transfer
Authors: Oughton S. , Dmitruk P. , Matthaeus W. H.
Published: April 2004

The self-consistency of the reduced magnetohydrodynamics (RMHD) model is explored by examining whether (parallel) spectral transfer might invalidate the assumptions employed in deriving it. Using direct numerical simulations we find that transfer of energy to structures with high parallel wavenumber is in fact limited by ongoing perpendicular transfer. Thus, the dynamics associated with RMHD models remains consistent with the underlying assumptions of RMHD. In particular, in well-resolved simulations it is neither necessary nor correct to introduce additional dissipation terms that (artificially) damp spectral transfer parallel to the mean magnetic field B0.
Suppression of Rayleigh–Taylor instability due to radiative ablation in brominated plastic targets
Authors: Fujioka Shinsuke , Sunahara Atsushi , Ohnishi Naofumi , Tamari Yohei , Nishihara Katsunobu , Azechi Hiroshi , Shiraga Hiroyuki , Nakai Mitsuo , Shigemori Keisuke , Sakaiya Tatsuhiro , Tanaka Motohiro , Otani Kazuto , Okuno Kazuki , Watari Takeshi , Yamada Takeshi , Murakami Masakatsu , Nagai Keiji , Norimatsu Takayoshi , Izawa Yasukazu , Nozaki Shinya , Chen Yen-wei
Published: April 2004

Suppression of hydrodynamic instabilities is very crucial for the ultimate goal of inertial fusion energy (IFE). A high-Z doped plastic of CHBr (brominated polystyrene) ablator is a very promising candidate to suppress the ablative Rayleigh–Taylor (RT) instability in a directly laser-driven IFE target. When a CHBr target is irradiated by intense laser beams, bromine atoms in the corona plasma emit strong radiation. The strong radiation drives the radiative ablation front inside the CHBr targets. This radiative ablation in the high-Z doped plastic target has many advantages for the suppression of the growth of the RT instability in analogy to the indirect-drive approach, i.e., large mass ablation rate, long density scale length and low peak density. Two-dimensional (2D) hydrodynamic simulation shows significant suppression of the RT instability in a CHBr target compared to an undoped polystyrene (CH) target. RT growth rate, calculated theoretically using the Betti–Goncharov procedure with a one-dimensional radiation-hydrodynamic simulation code, is in good agreement with the 2D calculations. Experiments were performed at the GEKKO XII– [C. Yamanaka et al., IEEE J. Quantum Electron. QE-17, 1639 (1981)] HIPER (High Intensity Plasma Experimental Research) laser facility. The trajectory of a laser-driven CHBr target observed in the experiment was reproduced fairly well by the simulation. The radiative ablation front formed inside a directly laser-driven CHBr target was clearly observed for the first time. The strong suppression of the RT instability in the CHBr target was confirmed using the face-on and side-on x-ray backlighting technique. The high-Z doped ablator can be applied to high density cryogenic deuterium–deuterium and deuterium–tritium compression, because the hydrogen-isotopes are nearly transparent to x rays, which are transmitted through the ablator from the laser-irradiation side.
Cavity-enhanced photoluminescence of SiGe/Si multiquantum wells grown on silicon-on-insulator substrate
Authors: Li C. B. , Huang C. J. , Cheng B. W. , Zuo Y. H. , Mao R. W. , Luo L. P. , Yu J. Z. , Wang Q. M.
Published: April 2004

Sharp and strong room-temperature photoluminescence (PL) of the Si0.59Ge0.41/Si multiquantum wells grown on the silicon-on-insulator substrate is investigated. The cavity formed by the mirrors at the surface and the buried SiO2 interface enhances the PL emission and has a wavelength-selective effect on the luminescence. The peak position is consistent with the simulation result and independent of the exciting power, which indicates a strong cavity effect on the room-temperature PL.
InGaAs quantum dots grown with GaP strain compensation layers
Authors: Lever P. , Tan H. H. , Jagadish C.
Published: April 2004

A major obstacle in the growth of stacked dot structures with a large number of layers is the high degree of strain in the dot layers. Strain buildup can affect the nucleation of the dots, which may cause defects that are detrimental to device performance. In this work, thin GaP layers are inserted below the quantum dot (QD) layer in single and stacked In0.5Ga0.5As/GaAs QD structures. These layers counterbalance the strain caused by the previous layers. Changes in dot nucleation are observed for dots grown directly on GaP layers. The QDs are found to be smaller in width and height. The luminescence from these dots is blueshifted due to interdiffusion between the dots and the GaP buffer layer. In a single layer of dots, no change in dot formation is seen when a thin GaAs barrier layer is deposited between the GaP layer and the QD layer. However, in stacked structures, the addition of a GaP layer below each QD layer is found to increase the density of the dots in the top layer of the stack. The room-temperature photoluminescence intensity is also increased.
Femtosecond laser-pulse-induced birefringence in optically isotropic glass
Authors: Yang Pin , Burns George R. , Guo Junpeng , Luk Ting Shan , Vawter G. Allen
Published: April 2004

We used a regeneratively amplified Ti:sapphire femtosecond laser to create optical birefringence in an isotropic glass medium. Between two crossed polarizers, regions modified by the femtosecond laser show bright transmission with respect to the dark background of the isotropic glass. This observation immediately suggests that these regions possess optical birefringence. The angular dependence of transmission through the laser-modified region is consistent with that of an optically birefringent material. Laser-induced birefringence is demonstrated in different glasses, including fused silica and borosilicate glass. Experimental results indicate that the optical axes of laser-induced birefringence can be controlled by the polarization direction of the femtosecond laser. The amount of laser-induced birefringence depends on the pulse energy level and number of accumulated pulses.
Influence of β-FeSi2 particle size and Si growth rate on 1.5 μm photoluminescence from Si/β-FeSi2-particles/Si structures grown by molecular-beam epitaxy
Authors: Ozawa Y. , Ohtsuka T. , Li Cheng , Suemasu T. , Hasegawa F.
Published: April 2004

Si /β- FeSi 2 -particles/Si structures have been fabricated by reactive deposition epitaxy for β-FeSi2 and molecular-beam epitaxy (MBE) for Si, and the influence of the size of the β-FeSi2 particle and the MBE-Si growth rate for embedding the β-FeSi2 in Si on 1.5-μm photoluminescence (PL) intensity of β-FeSi2 was investigated. The 1.5-μm PL intensity was observed to increase with the size of the β-FeSi2 particle, but the broad background luminescence, ranging from 1.2 to 1.4 μm, also increased. Transmission electron microscopy observation suggested that the broad luminescence was due to the dislocations induced in the Si matrix when the size of the embedded β-FeSi2 particles was too large. Furthermore, the 1.5-μm PL intensity was observed to be strongly affected by MBE-Si growth rate. This is thought to be due to the strain induced in the β-FeSi2 particles upon being embedded in the Si.
Impact of the k-linear term on nonlinear optical response of the C-exciton manifold in ZnO
Authors: Hazu K. , Torii K. , Sota T. , Adachi S. , Chichibu SF. , Cantwell G. , Reynolds D. C. , Litton C. W.
Published: April 2004

Time-integrated and spectrally resolved four-wave mixing (FWM) has been applied to study the impact of the k-linear term on nonlinear optical response of the C-exciton manifold in ZnO. From the excitation wavelength dependence of the FWM signal, we have found possible evidence that the mixing of the Γ1 and Γ5L states leads to a three-branch excitonic polariton dispersion but the oscillator strength transfer is small. The energy separation between the lower polariton branch and the extra branch due to the k-linear term has been found to be 3.5 meV. In addition, a binding energy of biexcitons consisting of two C excitons of 1.4 meV has been obtained.
Growth mechanism for single-crystalline thin film of InGaO3(ZnO)5 by reactive solid-phase epitaxy
Authors: Nomura Kenji , Ohta Hiromichi , Ueda Kazushige , Kamiya Toshio , Orita Masahiro , Hirano Masahiro , Suzuki Toshiyuki , Honjyo Chizuru , Ikuhara Yuichi , Hosono Hideo
Published: April 2004

The growth mechanism for a single-crystalline film of InGaO3(ZnO)5 (IGZO) on a (111)-oriented yttria-stabilized-zirconia substrate by reactive solid-phase epitaxy was studied by high-resolution x-ray diffraction, transmission electron microscopy, and atomic force microscopy. We focused on the role of the 2-nm-thick ZnO epitaxial layer during the structural evolution of the 200-nm-thick polycrystalline- (poly-) IGZO layer while thermal annealing. When a thin ZnO epitaxial layer was formed before depositing the poly-IGZO layer, thin single-crystalline epitaxial IGZO seeds were initially formed only on the substrate surface or on the thin ZnO epitaxial layer through a solid-phase reaction between the ZnO epitaxial layer and the poly-IGZO layer. A single-crystalline IGZO layer grew from the IGZO seeds toward the top surface of the film along with a much faster lateral grain growth, which formed a large-area single-crystalline IGZO film with an atomically flat terraced and stepped surface. On the other hand, an epitaxial film was not obtained unless a ZnO epitaxial layer was used, demonstrating that the ZnO epitaxial layer plays a crucial role as the seed for subsequent crystallite growth and a template for determining the crystallographic orientation.