17 research outputs found
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 ( 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)<sub><i>y</i></sub>Co<sub>4</sub>Sb<sub>12</sub> (ZT<sub>max</sub> = 0.9) and p-type DD<sub><i>y</i></sub>Fe<sub>3</sub>CoSb<sub>12</sub> (ZT<sub>max</sub> = 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 cm<sup>2</sup> and one of the highest thermoelectric
power density of 2.1 W cm<sup>–2</sup> 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