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At least 1,711 records · Page 95
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DOE OSTI2011

A study of the role of various reactions on the density distribution of hydrogen, silylene, and silyl in SiH{sub 4}/H{sub 2} plasma discharges

Fluid model has been used to study the effect of pressure on the distribution of hydrogen (H), silylene (SiH{sub 2}), and silyl (SiH{sub 3}) in hydrogen silane plasma discharges used for deposition of hydrogenated microcrystalline silicon ({mu}c-Si:H) or hydrogenated amorphous silicon (a-Si:H) thin films for solar cells. Time averaged reaction rates have been calculated to study the influence of various reactions on the density distributions of hydrogen, silylene, and silyl. Change in the distributions of hydrogen and silylene from bell shaped distribution at low pressure (1 Torr) to double humped distribution at high pressure (5 Torr) is explained with the help of time averaged reaction rates. Important reactions have been identified that contribute to the production and consumption of hydrogen (H), silylene (SiH{sub 2}), and silyl (SiH{sub 3}). The hydrogen consumption reactions SiH{sub 4} + H {yields} SiH{sub 3} + H{sub 2} and SiH{sub 3} + H {yields} SiH{sub 2} + H{sub 2} are found to play a central role in deciding the distribution of hydrogen. On the other hand, silylene consumption reaction SiH{sub 2} + H{sub 2}{yields} SiH{sub 4} is found to play a central role in determining the distribution of silylene. The distribution of these species at high pressure has been explained by using time averaged continuity equation. The code has been optimized by identifying 33 reactions (out of 53 reactions which contribute in the production and consumption of H, SiH{sub 2}, and SiH{sub 3}) that have no net effect on the density and distribution of these species. It is observed that dropping of 33 reactions has insignificant effect on the density of all the thin film deposition precursors such as Si, SiH, SiH{sub 2}, SiH{sub 3}, and Si{sub 2}H{sub 5}. This reduced set of 20 reactions can be used instead of 53 reactions to calculate the density and distribution of H, SiH{sub 2}, and SiH{sub 3} in the fluid simulation of SiH{sub 4}/H{sub 2} plasma discharges.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Coherent photoproduction of {pi}{sup +} from {sup 3}He with CLAS at Jefferson Lab

We have measured the differential cross section for the {gamma}{sup 3}He{yields}{pi}{sup +}t reaction. This reaction was studied using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. Real photons produced with the Hall-B bremsstrahlung tagging system in the energy range from 0.5 to 1.55 GeV were incident on a cryogenic liquid {sup 3}He target. The differential cross sections for the {gamma}{sup 3}He{yields}{pi}{sup +}t reaction were measured as a function of photon-beam energy and pion-scattering angle. Theoretical predictions to date cannot explain the large cross sections except at backward angles, showing that additional components must be added to the model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Lattice Simulations in MOM v.s. Schroedinger Functional Scheme and Triality

The QCD beta function extracted from polarized electron proton scattering data obtained at JLab and the lattice simulation in the MOM scheme suggest that the critical flavor number for the presence of IR fixed point is about three. In analyses of Schroedinger functional scheme, however, critical flavor number for the presence of IR fixed point and the conformality is larger than nine.In the QCD analysis, when quarks are expressed in quaternion basis, the product of quaternions are expressed by octonions and the octonion posesses the triality symmetry. Since the triality has the effect of multiplying the falvor number, it could explain the apparent large critical flavor number in the Schroedinger functinal scheme. In this scheme, larger degrees of freedom in adjusting data of different scales on the boundary are necessary than in the MOM scheme.In weak interaction, there is no clear lepton-flavor violation except in the neutrino oscillation. If the triality is assigned to the lepton flavors(e,{mu} and {tau}) and they are assumed to be exact symmetry, or the electro-magnetic interaction preserves tiality, but the strong interaction is triality blind, there is a possibility of explaining the neutrino oscillation through triality mixing of the matter field.The self energy of gluons, ghost and gauge bosons due to self-dual gauge fields and leptonic decays of B,D and D{sub s} mesons are discussed.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Isotropic Hall effect and ''freeze-in'' of carriers in the InGaAs self-assembled quantum wires

Using molecular beam epitaxy, we prepared an anisotropic media consisting of InGaAs quantum wires epitaxially grown on GaAs (311)A. Anisotropy is observed in the lateral conductivity and photoluminescence polarization. However, an isotropic Hall effect is observed in the same samples. We show that the Hall effect in this anisotropic heterostructure remains isotropic regardless of the change of the doping in GaAs barriers and regardless of the InGaAs coverage, whereas the conductivity anisotropy experiences a strong change under these actions. In addition, we observed an anomalous increase in carrier density, ''freeze-in,'' at low temperatures. In order to explain this, we generalized the theory of Look [D. C. Look, Phys. Rev B 42, 3578 (1990)] by considering the low field magneto-transport in anisotropic media. This theory confirms that the Hall constant remains isotropic in anisotropic semiconductor heterostructures, agreeing with our experiment and explains the anomalous behavior of carriers as a result of multi-band conductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Dynamical Model for Meson Production off Nucleon and Application to Neutrino-Nucleus Reactions

I explain the Sato-Lee (SL) model and its extension to the neutrino-induced pion production off the nucleon. Then I discuss applications of the SL model to incoherent and coherent pion productions in the neutrino-nucleus scattering. I mention a further extension of this approach with a dynamical coupled-channels model developed in Excited Baryon Analysis Center of JLab.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Cross-section Effects in the Super-Kamiokande Tau Appearance Analysis

In this talk, I explain the search for tau neutrino appearance in the atmospheric neutrino flux at Super-Kamiokande with a particular emphasis on the effect deep inelastic cross section uncertainties have on interpreting the result. In particular, I explain why the normalization of the DIS cross-section also needs to be treated as a parameter in the fit of tau normalization, and show how a neural net based on event parameters can separate various cross-section modes in the background sample.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

Selected topics on parton distribution functions

We report recent studies on structure functions of the nucleon and nuclei. First, clustering effects are investigated in the structure function F{sub 2} of {sup 9}Be for explaining an unusual nuclear correction found in a JLab experiment. We propose that high densities created by formation of clustering structure like 2{alpha}+neutron in {sup 9}Be is the origin of the unexpected JLab result by using the antisymmetrized molecular dynamics (AMD). There is an approved proposal at JLab to investigate the structure functions of light nuclei including the cluster structure, so that much details will become clear in a few years. Second, tensor-polarized quark and antiquark distributions are obtained by analyzing HERMES measurements on the structure function b{sub 1} for the deuteron. The result suggests a finite tensor polarization for antiquark distributions, which is an interesting topic for further theoretical and experimental investigations. An experimental proposal exists at JLab for measuring b{sub 1} of the deuteron as a new tensor-structure study in 2010's. Furthermore, the antiquark tensor polarization could be measured by polarized deuteron Drell-Yan processes at hadron facilities such as J-PARC and GSI-FAIR. Third, the recent CDF dijet anomaly is investigated within the standard model by considering possible modifications of the strange-quark distribution. We find that the shape of a dijet-mass spectrum changes depending on the strange-quark distribution. It indicates that the CDF excess could be partially explained as a PDF effect, particularly by the strangeness in the nucleon, within the standard model if the excess at m{sub jj}{approx_equal}140 GeV is not a sharp peak.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Investigation of ISIS and Brookhaven National Laboratory ion source electrodes after extended operation

Linac4 accelerator of Centre Europeen de Recherches Nucleaires is under construction and a RF-driven H{sup -} ion source is being developed. The beam current requirement for Linac4 is very challenging: 80 mA must be provided. Cesiated plasma discharge ion sources such as Penning or magnetron sources are also potential candidates. Accelerator ion sources must achieve typical reliability figures of 95% and above. Investigating and understanding the underlying mechanisms involved with source failure or ageing is critical when selecting the ion source technology. Plasma discharge driven surface ion sources rely on molybdenum cathodes. Deformation of the cathode surfaces is visible after extended operation periods. A metallurgical investigation of an ISIS ion source is presented. The origin of the deformation is twofold: Molybdenum sputtering by cesium ions digs few tenths of mm cavities while a growth of molybdenum is observed in the immediate vicinity. The molybdenum growth under hydrogen atmosphere is hard and loosely bound to the bulk. It is, therefore, likely to peel off and be transported within the plasma volume. The observation of the cathode, anode, and extraction electrodes of the magnetron source operated at BNL for two years are presented. A beam simulation of H{sup -}, electrons, and Cs{sup -} ions was performed with the IBSimu code package to qualitatively explain the observations. This paper describes the operation conditions of the ion sources and discusses the metallurgical analysis and beam simulation results.

08 HYDROGEN↗
DOE OSTI2012

Formation of carriers in Ti-oxide thin films by substitution reactions

Conductive Ti-oxide thin films are produced using a reactive sputtering and post-annealing process. The lowest resistivity of Ti-oxide thin films (2.30 x 10{sup -2}{Omega}-cm) can be achieved after annealing for 1 h at 400 deg. C in ambient O{sub 2}. Additionally, the Hall measurement results indicate that the carrier concentration increases during the initial 1-h annealing process before decreasing during subsequent annealing. By curve fitting the O{sub ls} core-level peaks in the x ray photoelectron spectroscopy (XPS) spectrum of the annealed Ti-oxide thin films, we found that the oxygen (O) vacancy concentration monotonically increases with annealing time, which differs from the behavior of the carrier concentration regarding annealing time. This means that the O-vacancy mechanism alone cannot explain the formation of carriers in Ti-oxide thin films. By curve-fitting core-level Ti peaks in the XPS spectrum of annealed Ti-oxide thin films, a Ti{sup 3+}-to-Ti{sup 4+} substitution reaction in the TiO{sub 2} phase of the Ti-oxide thin film after annealing plays the dominant role in the formation of conduction carriers. Instead of the O-vacancy mechanism, the Ti{sup 3+}-to-Ti{sup 4+} substitution mechanism can explain the concentration of carriers in Ti-oxide thin films following annealing.

36 MATERIALS SCIENCE↗
DOE OSTI2012

The interaction of 193-nm excimer laser irradiation with single-crystal zinc oxide: Positive ion emission

We examine UV laser-induced ion emission from a wide bandgap semiconductor, single-crystal ZnO, at fluences well below both the damage threshold and plasma formation. At fluences below 200 mJ/cm{sup 2}, we observe only Zn{sup +}, and the Zn{sup +} intensity decreases monotonically during exposure. At higher fluences, after an initial decrease, the emission is sustained; in addition O{sup +} and O{sub 2}{sup +} are observed. We explain: how Zn ions of several eV in energy can be produced on the surface of a semiconductor, how sustained emission can be maintained, and the origin of an anomalous emission of slow Zn{sup +} ions -- the latter is shown to arise from photoionization of atomic Zn, also emitted by this radiation.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Etching properties and electrical characterization of surfaces of silicon-on-insulator substrates in presence of halogens

We have studied the etching properties of silicon-on-insulator (SOI) substrates in recently developed chromium-free solutions containing halogens. We have shown that the presence of halogen compounds X (I{sup -}, Br{sup -}...) in HF/HNO{sub 3}/CH{sub 3}COOH solutions is required for a selective and preferential etching on SOI. The etching rate of such solutions increases with the dissolved halogen concentrations. The chemical reactivity of Si-X (X = Br{sup -}, I{sup -}..) bonds has been analyzed by X-ray Photoelectron Spectroscopy (XPS), Pseudo-MOS (flatband potential) and Kelvin Force Microscopy (KFM) measurements. A negative shift of flatband potential values is explained by an increasing concentration of halogen compounds in the solution and a substitution of Si-H (F) bonds by Si-X bonds during the reaction. Though Si-X bonds, and more particularly Si-I bonds, have been confirmed only at trace levels using XPS, we believe that the formation of Si-X bonds is supported by a mechanism of surface dipoles. Unexpectedly, no significant change in work function could be detected using KFM measurements. Some suggestions, based on KFM technique improvements, are made to explain such results. Finally, though the interaction mechanism between silicon, fluoride, iodide, and nitric acid is not clearly elucidated by our experimental results, the formation of Si-halogen bonds is crucial for etching and defect decoration capability.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Understanding disruptions in tokamaks

This paper describes progress achieved since 2007 in understanding disruptions in tokamaks, when the effect of plasma current sharing with the wall was introduced into theory. As a result, the toroidal asymmetry of the plasma current measurements during vertical disruption event (VDE) on the Joint European Torus was explained. A new kind of plasma equilibria and mode coupling was introduced into theory, which can explain the duration of the external kink 1/1 mode during VDE. The paper presents first results of numerical simulations using a free boundary plasma model, relevant to disruptions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Continuum limit of electrostatic gyrokinetic absolute equilibrium

Electrostatic gyrokinetic absolute equilibria with continuum velocity field are obtained through the partition function and through the Green function of the functional integral. The new results justify and explain the prescription for quantization/discretization or taking the continuum limit of velocity. The mistakes in the Appendix D of our earlier work [J.-Z. Zhu and G. W. Hammett, Phys. Plasmas 17, 122307 (2010)] are explained and corrected. If the lattice spacing for discretizing velocity is big enough, all the invariants could concentrate at the lowest Fourier modes in a negative-temperature state, which might indicate a possible variation of the dual cascade picture in 2D plasma turbulence.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Electron energy transport in the thin foil driven by high contrast high intensity laser pulse

The fast electron spectrum was measured for the interaction between the thin foil and the relativistic optical high field. There were two components in the electron spectrum. The higher temperature component was almost explained by the ponderomotive formula T{sub e}[MeV] = 0.511({radical}(1+I{sub 18}{lambda}{sup 2}{sub {mu}m}/1.37)-1). The total electron spectrum including the lower temperature component was also explained by a simple model.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

High-order harmonics from bow wave caustics driven by a high-intensity laser

We propose a new mechanism of high-order harmonic generation during an interaction of a high-intensity laser pulse with underdense plasma. A tightly focused laser pulse creates a cavity in plasma pushing electrons aside and exciting the wake wave and the bow wave. At the joint of the cavity wall and the bow wave boundary, an annular spike of electron density is formed. This spike surrounds the cavity and moves together with the laser pulse. Collective motion of electrons in the spike driven by the laser field generates high-order harmonics. A strong localization of the electron spike, its robustness to oscillations imposed by the laser field and, consequently, its ability to produce high-order harmonics is explained by catastrophe theory. The proposed mechanism explains the experimental observations of high-order harmonics with the 9 TW J-KAREN laser (JAEA, Japan) and the 120 TW Astra Gemini laser (CLF RAL, UK) [A. S. Pirozhkov, et al., arXiv:1004.4514 (2010); A. S. Pirozhkov et al, AIP Proceedings, this volume]. The theory is corroborated by high-resolution two-and three-dimensional particle-in-cell simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Unification and D3-branes in F-theory

A central ingredient in many string based Grand Unified Theories (GUTs) is p-branes filling our spacetime and wrapping some number of internal directions. In this talk we discuss the potential role of D3-branes sitting at a point of the internal geometry in an F-theory GUT. These D3-branes can naturally realize additional superconformal sectors which can couple to the states of the Standard Model. We explain how detailed features of these sectors and their impact on the visible sector can be extracted. Additionally, we explain how in scenarios where the scale of conformal symmetry breaking is close to the weak scale, this extra sector can modify the physics of the Higgs.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Multi-keV x-ray sources from metal-lined cylindrical hohlraums

As multi-keV x-ray sources, plastic hohlraums with inner walls coated with titanium, copper, and germanium have been fired on Omega in September 2009. For all the targets, the measured and calculated multi-keV x-ray power time histories are in a good qualitative agreement. In the same irradiation conditions, measured multi-keV x-ray conversion rates are {approx}6%-8% for titanium, {approx}2% for copper, and {approx}0.5% for germanium. For titanium and copper hohlraums, the measured conversion rates are about two times higher than those given by hydroradiative computations. Conversely, for the germanium hohlraum, a rather good agreement is found between measured and computed conversion rates. To explain these findings, multi-keV integrated emissivities calculated with RADIOM [M. Busquet, Phys. Fluids 85, 4191 (1993)], the nonlocal-thermal-equilibrium atomic physics model used in our computations, have been compared to emissivities obtained from different other models. These comparisons provide an attractive way to explain the discrepancies between experimental and calculated quantitative results.

36 MATERIALS SCIENCE↗