Modification of argon impurity transport by electron cyclotron heating in KSTAR H-mode plasmas

Experiments with a small amount of Ar gas injection as a trace impurity were conducted in the Korea Superconducting Tokamak Advanced Research (KSTAR) H-mode plasma (BT  = 2.8 T, IP = 0.6 MA, and PNBI = 4.0 MW). 170 GHz electron cyclotron resonance heating (ECH) at 600 and 800 kW was focused along the mid-plane with a fixed major radial position of R  =  1.66 m. The emissivity of the Ar16+ (3.949 Å) and Ar15+ (353.860 Å) spectral lines were measured by x-ray imaging crystal spectroscopy (XICS) and a vacuum UV (VUV) spectrometer, respectively. ECH reduces the peak Ar15+ emission and increases the Ar16+ emission, an effect largest with 800 kW. The ADAS-SANCO impurity transport code was used to evaluate the Ar transport coefficients. It was found that the inward convective velocity found in the plasma core without ECH was decreased with ECH, while diffusion remained approximately constant resulting in a less-peaked Ar density profile. Theoretical results from the NEO code suggest that neoclassical transport is not responsible for the change in transport, while the microstability analysis using GKW predicts a dominant ITG mode during both ECH and non-ECH plasmas

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

Clustering Based Gaussian Process Regression

Gaussian Process (GP) has become a common Bayesian inference framework and has been applied in many tasks, for example, data mining and non-linear transformation, in recent years. After introducing the model of GP regression and its training process, this thesis points out the inefficiency of the training process of GP, and proposes the modification to the original GP to speed up the training process by the clustering data set. Extensive experiments were conducted including model selection experiments and comparison experiments. The results show that the proposed algorithms are about 10 times faster than the original GP, with comparable precision

Magnetoresistance and domain wall tunneling in mesoscopic ferromagnets

We have investigated the magnetoresistance in one and two dimensional Ni structures. The measurements showed that the resistivity of these small structures were very sensitive to the relative orientation between the measurement current and the magnetization. We have explained some of the observations in terms of the two band model. However, we found that the two current model did not always agree with our measurements, especially when the measurement current was parallel to the direction of magnetization. In addition, the magnetoresistance of a thin wire under this measurement conditions showed reproducible discontinuities. We provide experimental evidence indicating that these discontinuities are from pinning and de-pinning of a single domain wall trapped by structural defects. The de-pinning process of various samples showed quantum tunneling at temperatures below 2-6 K and thermal activation at higher temperatures, which is about an order of magnitude higher than predicted by the theory

Effect of microwaves on domain wall motion in thin Ni wires

We report new results on domain wall motion in very narrow ($\sim 400\;{\rm \AA}$ diameter) Ni wires. In previous work we have presented evidence that the magnetoresistance can be used to observe the pinning and depinning of domain walls from what we believe are structural defects. Those results suggest that at low temperatures the depinning can occur via quantum tunneling of a wall through the associated pinning barrier. In the present paper we describe new results for the effects of microwave radiation on this behavior. This radiation alters the tunneling probability in a manner which suggests that the energy levels of a domain wall are quantized, much like those of a particle in a potential well

Giant positive magnetoresistance of Bi nanowire arrays in high magnetic fields

We have studied the magnetoresistance of electrodeposited Bi wires with diameters between 200 nm and 2 μm in magnetic fields up to B=55 T. In zero field, the resistance increases with decreasing temperature, indicating that the mean free path is strongly influenced by the nanowire geometry. The high-field magnetoresistance shows strong dependence on field orientation; typically 200% for B parallel to the wires, and 600%-800% for B perpendicular to the wires. The perpendicular magnetoresistance is well described by a modified two-current model which suggests that the high-field response of the arrays is fairly insensitive to the wire diameter, and is dominated by bulk properties of Bi

Giant positive magnetoresistance of Bi nanowire arrays in high magnetic fields

We have studied the magnetoresistance of electrodeposited Bi wires with diameters between 200 nm and 2 μm in magnetic fields up to B=55 T. In zero field, the resistance increases with decreasing temperature, indicating that the mean free path is strongly influenced by the nanowire geometry. The high-field magnetoresistance shows strong dependence on field orientation; typically 200% for B parallel to the wires, and 600%-800% for B perpendicular to the wires. The perpendicular magnetoresistance is well described by a modified two-current model which suggests that the high-field response of the arrays is fairly insensitive to the wire diameter, and is dominated by bulk properties of Bi

High-Power-Density Skutterudite-Based Thermoelectric Modules with Ultralow Contact Resistivity Using Fe–Ni Metallization Layers

Most reported thermoelectric modules suffer from considerable power loss due to high electrical and thermal resistivity arising at the interface between thermoelectric legs and metallic contacts. Despite increasing complaints on this critical problem, it has been scarcely tackled. Here we report the metallization layer of Fe–Ni alloy seamlessly securing skutterudite materials and metallic electrodes, allowing for a minimal loss of energy transferred from the former. It is applied to an 8-couple thermoelectric module that consists of n-type (Mm,Sm)_<i>y</i>Co₄Sb₁₂ (ZT_max = 0.9) and p-type DD_<i>y</i>Fe₃CoSb₁₂ (ZT_max = 0.7) skutterudite materials. It performs as a diffusion barrier suppressing chemical reactions to produce a secondary phase at the interface. Consequent high thermal stability of the module results in the lowest reported electrical contact resistivity of 2.2–2.5 μΩ cm² and one of the highest thermoelectric power density of 2.1 W cm^–2 for a temperature difference of 570 K. Employing a scanning transmission electron microscope equipped with an energy-dispersive X-ray spectroscope detector, we confirmed that it is negligible for atomic diffusion across the interface and resulting formation of a detrimental secondary phase to energy transfer and thermal stability of the thermoelectric module

High-performance dye-sensitized solar cells using edge-halogenated graphene nanoplatelets as counter electrodes

Edge-selectively halogenated graphene nanoplatelets (XGnPs, X=C1, Br, and I) were prepared by the mechanochemically driven reaction between graphite and diatomic halogen molecules (Cl-2, Br-2 or 12). The contents of halogens (Cl, Br, and I) in XGnPs were 3.18, 1.77, and 0.66 at%, respectively, by X-ray photoelectron spectroscopy. The XGnPs as counter electrodes (CEs) showed remarkably enhanced electrocatalytic activities toward Co(bpy)(3)(3+) reduction reaction in dye-sensitized solar cells (DSSCs) with an excellent electrochemical stability. Amongst XGnPs, IGnP-CE demonstrated the lowest charge-transfer resistance (R-ct of 0.46 Omega cm(2) at the CE/electrolyte interface. This value is much lower than that of Pt-CE (0.81 Omega cm(2)). In addition, the DSSC with IGnP-CE had the highest fill factor (71.3%) and cell efficiency (10.31%), whereas those of DSSCs with Pt-CE were only 70.6% and 9.92%, respectively. (C) 2015 Elsevier Ltd. All rights reservedclose2