995 research outputs found
Plasma Shape and Current Density Profile Control in Advanced Tokamak Operating Scenarios
The need for new sources of energy is expected to become a critical problem within the next few decades. Nuclear fusion has sufficient energy density to potentially supply the world population with its increasing energy demands. The tokamak is a magnetic confinement device used to achieve controlled fusion reactions. Experimental fusion technology has now reached a level where tokamaks are able to produce about as much energy as is expended in heating the fusion fuel. The next step towards the realization of a nuclear fusion tokamak power plant is ITER, which will be capable of exploring advanced tokamak (AT) modes, characterized by a high fusion gain and plasma stability. The extreme requirements of the advanced modes motivates researchers to improve the modeling of the plasma response as well as the design of feedback controllers. This dissertation focuses on several magnetic and kinetic control problems, including the plasma current, position and shape control, and data-driven and first-principles-driven modeling and control of plasma current density profile and the normalized plasma pressure ratio βN.The plasma is confined within the vacuum vessel by an external electromagnetic field, produced primarily by toroidal and poloidal field coils. The outermost closed plasma surface or plasma boundary is referred to as the shape of the plasma. A central characteristic of AT plasma regimes is an extreme elongated shape. The equilibrium among the electromagnetic forces acting on an elongated plasma is unstable. Moreover, the tokamak performance is improved if the plasma is located in close proximity to the torus wall, which guarantees an efficient use of available volume. As a consequence, feedback control of the plasma position and shape is necessary. In this dissertation, an H∞-based, multi-input-multi-output (MIMO) controller for the National Spherical Torus Experiment (NSTX) is developed, which is used to control the plasma position, shape, and X-point position.Setting up a suitable toroidal current profile is related to both the stability and performance of the plasma. The requirements of ITER motivate the research on plasma current profile control. Currently, physics-based control-oriented modeling techniques of the current profile evolution can be separated into two major classes: data-driven and first-principles-driven. In this dissertation, a two-timescale linear dynamic data-driven model of the rotational transform profile and βN is identified based on experimental data from the DIII-D tokamak. A mixed-sensitivity H∞ controller is developed and tested during DIII-D high-confinement (H-mode) experiments by using the heating and current drive (H&CD) systems to regulate the plasma rotational transform profile and βN around particular target values close to the reference state used for system identification. The preliminary experimental results show good progress towards routine current profile control in DIII-D. As an alternative, a nonlinear dynamic first-principles-driven model is obtained by converting the physics-based model that describes the current profile evolution in H-mode DIII-D discharges into a form suitable for control design. The obtained control-oriented model is validated by comparing the model prediction to experimental data. An H∞ control design problem is formulated to synthesize a stabilizing feedback controller, with the goal of developing a closed-loop controller to drive the current profile in DIII-D to a desirable target evolution. Simulations show that the controller is capable of regulating the system around the target rotational transform profile in the presence of disturbances. When compared to a previously designed data-driven model-based controller, the proposed first-principles-driven model-based controller shows potential for improving the control performance
Nanoporous Oxides and Nanoporous Composites
Nanoporous oxides, such as cupric oxide (CuO), nickelous oxide (NiO), titanium dioxide (TiO2), cobaltosic oxide (Co3O4), and cerium oxide (CeO2), and noble-metal-based nanoporous composites, such as silver (Ag) ligaments loaded with CeO2, TiO2, zirconium dioxide (ZrO2) or NiO and palladium (Pd) ligaments loaded with TiO2 or ZrO2, are described in the chapter. Oxide-based nanoporous composites, such as Au loaded on CuO and CeO2 or platinum (Pt) loaded on TiO2, are also summarized. The structures, microstructures, and microstructure parameters of these materials are reviewed. The performance of the noble-based nanoporous composites is presented, including the catalytic oxidation of methanol and ethanol. Environmental protection applications, such as catalytic oxidation of carbon monoxide (CO) for the oxide-based nanoporous composites, have also been developed. Applications of rare earth elements in nanoporous materials are also reviewed
Yelp Reviews and Food Types: A Comparative Analysis of Ratings, Sentiments, and Topics
This study examines the relationship between Yelp reviews and food types,
investigating how ratings, sentiments, and topics vary across different types
of food. Specifically, we analyze how ratings and sentiments of reviews vary
across food types, cluster food types based on ratings and sentiments, infer
review topics using machine learning models, and compare topic distributions
among different food types. Our analyses reveal that some food types have
similar ratings, sentiments, and topics distributions, while others have
distinct patterns. We identify four clusters of food types based on ratings and
sentiments and find that reviewers tend to focus on different topics when
reviewing certain food types. These findings have important implications for
understanding user behavior and cultural influence on digital media platforms
and promoting cross-cultural understanding and appreciation
Crystal Structure Manipulation of the Exchange Bias in an Antiferromagnetic Film
Exchange bias is one of the most extensively studied phenomena in magnetism,
since it exerts a unidirectional anisotropy to a ferromagnet (FM) when coupled
to an antiferromagnet (AFM) and the control of the exchange bias is therefore
very important for technological applications, such as magnetic random access
memory and giant magnetoresistance sensors. In this letter, we report the
crystal structure manipulation of the exchange bias in epitaxial hcp Cr2O3
films. By epitaxially growing twined (10-10) oriented Cr2O3 thin films, of
which the c axis and spins of the Cr atoms lie in the film plane, we
demonstrate that the exchange bias between Cr2O3 and an adjacent permalloy
layer is tuned to in-plane from out-of-plane that has been observed in (0001)
oriented Cr2O3 films. This is owing to the collinear exchange coupling between
the spins of the Cr atoms and the adjacent FM layer. Such a highly anisotropic
exchange bias phenomenon is not possible in polycrystalline films.Comment: To be published in Scientific Reports, 12 pages, 6 figure
Biocontrol of bacterial spot diseases of muskmelon using Paenibacillus polymyxa G-14
Paenibacillus polymyxa strain G-14 (PpG14) isolated from the muskmelon rhizosphere, produces antibiotic(s) that are active against Pseudomonas syringae pv. lachrymans and Acidovorax avenae subsp. citrulli (two pathogens that cause bacterial spot diseases). Strain G-14 strongly inhibited the growth of Pseudomonas syringae pv. lachrymans and Acidovorax avenae subsp. citruli in a dual-culture plate assay. The biocontrol activity of PpG14 was examined by pot and field tests. Results show that the strain significantly reduced the development and suppressed the incidence of bacterial spot diseases. Moreover, the prevention treatment was better than the therapy treatment when using this strain. Based on its main bacteriological properties, identification using VITEK 32 and analysis of the 16S rDNA gene sequence, showed that strain G-14 belonged to P. polymyxa. Optimal growth was studied; temperature and pH were 28°C and 7, respectively.Key words: Muskmelon, bacterial spot diseases, Paenibacillus polymyxa, biocontrol
Image-Adaptive YOLO for Object Detection in Adverse Weather Conditions
Though deep learning-based object detection methods have achieved promising
results on the conventional datasets, it is still challenging to locate objects
from the low-quality images captured in adverse weather conditions. The
existing methods either have difficulties in balancing the tasks of image
enhancement and object detection, or often ignore the latent information
beneficial for detection. To alleviate this problem, we propose a novel
Image-Adaptive YOLO (IA-YOLO) framework, where each image can be adaptively
enhanced for better detection performance. Specifically, a differentiable image
processing (DIP) module is presented to take into account the adverse weather
conditions for YOLO detector, whose parameters are predicted by a small
convolutional neural net-work (CNN-PP). We learn CNN-PP and YOLOv3 jointly in
an end-to-end fashion, which ensures that CNN-PP can learn an appropriate DIP
to enhance the image for detection in a weakly supervised manner. Our proposed
IA-YOLO approach can adaptively process images in both normal and adverse
weather conditions. The experimental results are very encouraging,
demonstrating the effectiveness of our proposed IA-YOLO method in both foggy
and low-light scenarios.Comment: AAAI 2022, Preprint version with Appendi
Pressure-induced spin reorientation transition in layered ferromagnetic insulator Cr2Ge2Te6
Anisotropic magnetoresistance (AMR) of Cr2Ge2Te6 (CGT), a layered
ferromagnetic insulator, is investigated under an applied hydrostatic pressure
up to 2 GPa. The easy axis direction of the magnetization is inferred from the
AMR saturation feature in the presence and absence of the applied pressure. At
zero applied pressure, the easy axis is along the c-direction or perpendicular
to the layer. Upon application of a hydrostatic pressure>1 GPa, the uniaxial
anisotropy switches to easy-plane anisotropy which drives the equilibrium
magnetization from the c-axis to the ab-plane at zero magnetic field, which
amounts to a giant magnetic anisotropy energy change (>100%). As the
temperature is increased across the Curie temperature, the characteristic AMR
effect gradually decreases and disappears. Our first-principles calculations
confirm the giant magnetic anisotropy energy change with moderate pressure and
assign its origin to the increased off-site spin-orbit interaction of Te atoms
due to a shorter Cr-Te distance. Such a pressure-induced spin reorientation
transition is very rare in three-dimensional ferromagnets, but it may be common
to other layered ferromagnets with similar crystal structures to CGT, and
therefore offers a unique way to control magnetic anisotropy
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