Edge Shear Flows and Particle Transport near the Density Limit in the HL-2A Tokamak

Edge shear flow and its effect on regulating turbulent transport have long been suspected to play an important role in plasmas operating near the Greenwald density limit $n_G$. In this study, equilibrium profiles as well as the turbulent particle flux and Reynolds stress across the separatrix in the HL-2A tokamak are examined as $n_G$ is approached in ohmic L-mode discharges. As the normalized line-averaged density $\bar{n}_e/n_G$ is raised, the shearing rate of the mean poloidal flow $\omega_{\rm sh}$ drops, and the turbulent drive for the low-frequency zonal flow (the Reynolds power $\mathcal{P}_{Re}$) collapses. Correspondingly, the turbulent particle transport increases drastically with increasing collision rates. The geodesic acoustic modes (GAMs) gain more energy from the ambient turbulence at higher densities, but have smaller shearing rate than low-frequency zonal flows. The increased density also introduces decreased adiabaticity which not only enhances the particle transport but is also related to a reduction in the eddy-tilting and the Reynolds power. Both effects may lead to the cooling of edge plasmas and therefore the onset of MHD instabilities that limit the plasma density

Vector modulation instability induced by vacuum fluctuations in highly birefringent fibers in the anomalous dispersion regime

We report a detailed experimental study of vector modulation instability in highly birefringent optical fibers in the anomalous dispersion regime. We prove that the observed instability is mainly induced by vacuum fluctuations. The detuning of the spectral peaks agrees with linear perturbation analysis. The exact shape of the spectrum is well reproduced by numerical integration of stochastic nonlinear Schrodinger equations describing quantum propagation.Comment: 11 pages, 4 figures, to be published in Optics Letter

Behavior of bulk high-temperature superconductors of finite thickness subjected to crossed magnetic fields

Crossed magnetic field effects on bulk high-temperature superconductors have been studied both experimentally and numerically. The sample geometry investigated involves finite-size effects along both (crossed) magnetic field directions. The experiments were carried out on bulk melt-processed Y-Ba-Cu-O (YBCO) single domains that had been pre-magnetized with the applied field parallel to their shortest direction (i.e. the c-axis) and then subjected to several cycles of the application of a transverse magnetic field parallel to the sample ab plane. The magnetic properties were measured using orthogonal pick-up coils, a Hall probe placed against the sample surface and Magneto-Optical Imaging (MOI). We show that all principal features of the experimental data can be reproduced qualitatively using a two-dimensional finite-element numerical model based on an E-J power law and in which the current density flows perpendicularly to the plane within which the two components of magnetic field are varied. The results of this study suggest that the suppression of the magnetic moment under the action of a transverse field can be predicted successfully by ignoring the existence of flux-free configurations or flux-cutting effects. These investigations show that the observed decay in magnetization results from the intricate modification of current distribution within the sample cross-section. It is also shown that the model does not predict any saturation of the magnetic induction, even after a large number (~ 100) of transverse field cycles. These features are shown to be consistent with the experimental data.Comment: 41 pages, 9 figures, accepted in Phys. Rev. B Changes : 8 references added, a few precisions added, some typos correcte

Nanocomposite catalysts for soot combustion and propane steam reforming

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, February 2007.Includes bibliographical references.A nanocomposite system, CuO-Ag/CeO 2, has been successfully developed to complete carbon black combustion by 400*C. This novel catalyst has excellent potential for application in the emission control of soot particulates generated by diesel engines. In this work, CuO was coated onto nanocrystalline supports, such as alumina, ceria, zirconia and titania. The best activity and catalyst reducibility were achieved by CuO supported on CeO2, a material known for its redox property and oxygen storage capacity. A CuO loading of 20-40 wt% presented the optimal balance between CuO content and dispersion, providing for 50% carbon black combustion at 390° C. To further improve the catalytic activity, Ag was introduced as an additive to CuO/CeO2. CuO-Ag/CeO 2 demonstrated superior reducibility. The optimal CuO-Ag loading was 40 wt%, with a Cu/Ag molar ratio of 1.50% carbon black conversion was achieved at 304° C over the optimal catalyst, and complete combustion was attained well below 400° C. A nanocomposite system of nickel aluminate (NiAl20 4), Re and V has been synthesized for propane steam reforming. Nickel aluminate nanocrystals with various Ni/Al molar ratios were prepared by chemical co-precipitation. They were pretreated in H2 prior to propane steam reforming to obtain highly dispersed Ni species. The Al in nickel aluminate acted as a spacer to provide uniform Ni dispersion. The catalyst with a Ni/Al molar ratio of 1.10 showed the highest reducibility and the highest active surface area. It gave rise to the best propane conversion and H2 yield. H2 yield was improved with increasing reaction temperature up to 700*C, and more H2 could be produced at higher H2O/C molar ratios. Various metal promoters were introduced by impregnation or vapor grafting to improve the catalytic activity and coke resistance of the optimal nickel aluminate catalyst. Catalytic activity could be improved by introducing metal promoters in the order of Re > Rh > Pt > Ir > Pd > Ru > V. These selected additives provided increased reducibility and active surface area. In particular, I wt% Re-promoted nickel aluminate gave rise to the best catalytic activity and H2 yield. Although Re was the most effective promoter at improving low-temperature activity, it only lowered the coking rate of nickel aluminate slightly. To inhibit coke formation over Re-promoted nickel aluminate, V was introduced as an additive. The optimal nickel aluminate system with 2 wt% Re and 2 wt% V demonstrated the highest catalytic activity and H2 yield at low H2O/C ratios. Coking rate was reduced dramatically with the V additive, which promoted carbon gasification. The excellent activity, coke resistance and thermal/hydrothermal stability of Re,V-promoted nickel aluminate nanocomposite would be attractive towards H2 generation for fuel cell applications, especially in portable devices.by Hong He.Ph.D

Single-walled carbon nanotubes as near infrared fluorescent sensors : characterization, biological and analytical applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, February 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 118-126).Reactive oxygen species (ROS) have emerged as biological signaling molecules, participating in newly discovered cascades that govern cell proliferation, migration, and pathogenesis. A major challenge in understanding these pathways is the lack of detection technologies that allow for spatial and temporal resolution of specific ROS at the cellular level. The goal of this thesis is to design a nanotube sensor platform able to detect and study H2 0 2 signaling fluxes at the cellular level in order to elucidate their role in biological processes. Understanding this role may lead to new therapeutic targets, and improve understanding of biological signaling. Single-walled carbon nanotubes (SWNT) are rolled sheets of graphene and can be either semiconducting or metallic depending on the angle of rolling and the diameter of the tube. Semi-conducting SWNT are one of only a few types of molecules that exhibit band gap photoluminescence (PL) in the near infrared (nIR), making them ideal for detection in biologically relevant media since it avoids biological auto-fluorescence. SWNT are also completely photostable even at high fluence, unlike conventional fluorophores and quantum dot systems, allowing them to serve as nIR single molecule optical sensors capable of long term and stable operations in vitro and in vivo. In this thesis, we show that the 1D quantum confinement of photogenerated excitons in SWNT can amplify the detection of molecular adsorption to where single-molecule discrimination is realizable, even from within living cells and tissues.(cont.) We have developed a type I collagen film, similar to those used as 3D cell scaffolds for tissue engineering, containing embedded SWNT capable of reporting single-molecule adsorption of quenching molecules such as H₂0₂ . A Hidden Markov Modeling algorithm is utilized to link single-molecule adsorption events detected on the nanotube to forward and reverse kinetic rate constants for many different analytes. The collagen matrix is shown to impart selectivity to H₂0₂ over other ROS and common interferents. We utilized these new single-molecule sensors to study the fluxes of H₂0₂ from A431 human skin carcinoma cells and particularly the local generation rate from Epidermal Growth Factor Receptor (EGFR), a membrane protein and tyrosine kinase that controls cell proliferation among other functions. We show that an array of nIR fluorescent SWNT is capable of recording the discrete, stochastic quenching events that occur as H₂0₂ molecules are emitted from individual A431 and murine 3T3 fibroblasts cells in response to epidermal growth factor (EGF). We also show mathematically that such single molecule detection arrays have the unique property of distinguishing between "near field" and "far field" molecular generation, allowing one to isolate the flux originating from only the membrane protein. Corresponding inhibition experiments suggest a mechanism whereby water oxidizes singlet oxygen at a catalytic site on the receptor itself, generating H₂0₂ in response to receptor binding. An EGFR-mediated H₂0₂ generation pathway that is consistent with all current and previous literature findings has been proposed for the first time and numerically tested for consistency.(cont.) In an effort to extend this detection to in vivo systems, we investigated how SWNT are uptaken and localized within living cells and as well as their potential cytotoxicity. To this end, we have developed a novel method of studying this problem by tracking the non-photobleaching SWNT in real time by using a single particle tracking method. Over 10,000 individual trajectories of SWNT were tracked as they are incorporated into and expelled from NIH-3T3 cells in real time on a perfusion microscope stage. An analysis of mean square displacement allows the complete construction of the mechanistic steps involved from single duration experiments. We observe the first conclusive evidence of SWNT exocytosis and show that the rate closely matches the endocytosis rate with negligible temporal offset, thus explains why SWNT are non-cytotoxic for various cell types at a concentration up to 5 mg/L, as observed from our live-dead assay experimental results. Further, we studied the cellular uptake and expulsion rates of length-fractionated SWNT from 130 to 660 nm in NIH-3T3 cells using this method. We developed a quantitative model to correlate endocytosis rate with nanoparticle geometry that accurately describes our data set and also literature results for Au nanoparticles. The model asserts that nanoparticles cluster on the cell membrane to form a size sufficient to generate a large enough enthalpic contribution via receptor ligand interations to overcome the elastic energy and entropic barriers associated with vesicle formation.(cont.) The total uptake of both SWNT and Au nanoparticles is maximal at a common radius of 25 nm when scaled using an effective capture dimension for membrane diffusion. The ability to understand and predict the cellular uptake of nanoparticles quantitatively should find utility in designing nanosystems with controlled toxicity, efficacy and functionality. The development of such single molecule detection technologies for ROS motivates their application to many other unexplored signaling pathways both in vitro and in vivo.by Hong Jin.Ph.D

Modeling of pattern dependencies in the fabrication of multilevel copper metallization

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.Includes bibliographical references (p. 295-303).Multilevel copper metallization for Ultra-Large-Scale-Integrated (ULSI) circuits is a critical technology needed to meet performance requirements for advanced interconnect technologies with sub-micron dimensions. It is well known that multilevel topography resulting from pattern dependencies in various processes, especially copper Electrochemical Deposition (ECD) and Chemical-Mechanical Planarization (CMP), is a major problem in interconnects. An integrated pattern dependent chip-scale model for multilevel copper metallization is contributed to help understand and meet dishing and erosion requirements, to optimize the combined plating and polishing process to achieve minimal environmental impact, higher yield and performance, and to enable optimization of layout and dummy fill designs. First, a physics-based chip-scale copper ECD model is developed. By considering copper ion depletion effects, and surface additive adsorption and desorption, the plating model is able to predict the initial topography for subsequent CMP modeling with sufficient accuracy and computational efficiency. Second, a compatible chip-scale CMP modeling is developed.(cont.) The CMP model integrates contact wear and density-step-height approaches, so that a consistent and coherent chip-scale model framework can be used for copper bulk polishing, copper over-polishing, and barrier layer polishing stages. A variant of this CMP model is developed which explicitly considers the pad topography properties. Finally, ECD and CMP parts are combined into an integrated model applicable to single level and multilevel metallization cases. The integrated multilevel copper metallization model is applied to the co-optimization of the plating and CMP processes. An alternative in-pattern (rather than between-pattern) dummy fill strategy is proposed. The integrated ECD/CMP model is applied to the optimization of the in-pattern fill, to achieve improved ECD uniformity and final post-CMP topography.by Hong Cai.Ph.D

Structural reliability analysis of multiple limit state functions using multi-input multi-output support vector machine

Selecting and using an appropriate structural reliability method is critical for the success of structural reliability analysis and reliability-based design optimization. However, most of existing structural reliability methods are developed and designed for a single limit state function and few methods can be used to simultaneously handle multiple limit state functions in a structural system when the failure probability of each limit state function is of interest, for example, in a reliability-based design optimization loop. This article presents a new method for structural reliability analysis with multiple limit state functions using support vector machine technique. A sole support vector machine surrogate model for all limit state functions is constructed by a multi-input multi-output support vector machine algorithm. Furthermore, this multi-input multi-output support vector machine surrogate model for all limit state functions is only trained from one data set with one calculation process, instead of constructing a series of standard support vector machine models which has one output only. Combining the multi-input multi-output support vector machine surrogate model with direct Monte Carlo simulation, the failure probability of the structural system as well as the failure probability of each limit state function corresponding to a failure mode in the structural system can be estimated. Two examples are used to demonstrate the accuracy and efficiency of the presented method

Electronic structure and spectroscopy of the quaternary Heusler alloy Co2_2Cr1−x_{1-x}Fex_{x}Al

Quaternary Heusler alloys Co$_2$Cr$_{1-x}$Fe$_{x}$Al with varying Cr to Fe ratio $x$ were investigated experimentally and theoretically. The electronic structure and spectroscopic properties were calculated using the full relativistic Korringa-Kohn-Rostocker method with coherent potential approximation to account for the random distribution of Cr and Fe atoms as well as random disorder. Magnetic effects are included by the use of spin dependent potentials in the local spin density approximation. Magnetic circular dichroism in X-ray absorption was measured at the $L_{2,3}$ edges of Co, Fe, and Cr of the pure compounds and the $x=0.4$ alloy in order to determine element specific magnetic moments. Calculations and measurements show an increase of the magnetic moments with increasing iron content. Resonant (560eV - 800eV) soft X-ray as well as high resolution - high energy ($\geq 3.5$keV) hard X-ray photo emission was used to probe the density of the occupied states in Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al.Comment: J.Phys.D_Appl.Phys. accepte

Spin dependent scattering of a domain-wall of controlled size

Magnetoresistance measurements in the CPP geometry have been performed on single electrodeposited Co nanowires exchange biased on one side by a sputtered amorphous GdCo layer. This geometry allows the stabilization of a single domain wall in the Co wire, the thickness of which can be controlled by an external magnetic field. Comparing magnetization, resistivity, and magnetoresistance studies of single Co nanowires, of GdCo layers, and of the coupled system, gives evidence for an additional contribution to the magnetoresistance when the domain wall is compressed by a magnetic field. This contribution is interpreted as the spin dependent scattering within the domain wall when the wall thickness becomes smaller than the spin diffusion length.Comment: 9 pages, 13 figure

Sondeo arqueológico Cueva Pintada corte 11,12 cierre sur [Material gráfico]

Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte. Subdirección General de Coordinación Bibliotecaria, 201