The Experts below are selected from a list of 44943 Experts worldwide ranked by ideXlab platform
Baining Guo - One of the best experts on this subject based on the ideXlab platform.
-
Pocket Reflectometry
ACM Transactions on Graphics, 2011Co-Authors: Peiran Ren, John Snyder, Xin Tong, Jiaping Wang, Baining GuoAbstract:We present a simple, fast solution for reflectance acquisition using tools that fit into a pocket. Our method captures video of a flat target surface from a fixed video camera lit by a hand-held, moving, linear light source. After processing, we obtain an SVBRDF. We introduce a BRDF chart, analogous to a color "checker" chart, which arranges a set of known-BRDF reference tiles over a small card. A sequence of light responses from the chart tiles as well as from points on the target is captured and matched to reconstruct the target's appearance. We develop a new algorithm for BRDF reconstruction which works directly on these LDR responses, without knowing the light or camera position, or acquiring HDR lighting. It compensates for spatial variation caused by the local (finite distance) camera and light position by warping responses over time to align them to a specular reference. After alignment, we find an optimal linear combination of the Lambertian and purely specular reference responses to match each target point's response. The same weights are then applied to the corresponding (known) reference BRDFs to reconstruct the target point's BRDF. We extend the basic algorithm to also recover varying surface normals by adding two spherical caps for diffuse and specular references to the BRDF chart. We demonstrate convincing results obtained after less than 30 seconds of data capture, using commercial mobile phone cameras in a casual environment.
Nhan V. Nguyen - One of the best experts on this subject based on the ideXlab platform.
-
Neutron Reflectometry, x-ray Reflectometry, and spectroscopic ellipsometry characterization of thin SiO2 on Si
Applied Physics Letters, 1998Co-Authors: Joseph A. Dura, Curt A. Richter, Charles F. Majkrzak, Nhan V. NguyenAbstract:We present here a comparison of neutron Reflectometry, x-ray Reflectometry, and spectroscopic ellipsometry on a thin oxide film. These three probes each independently determine the structure of the film as a function of depth. We find an excellent agreement between the three techniques for measurements of thicknesses and interfacial roughnesses for both the SiO2 and surface contamination layers found in the sample. Realistic models based on interface parameters measured herein indicate that as the SiO2 layers decrease to sizes projected for future generations of electronic devices, both spectroscopic ellipsometry and neutron Reflectometry can easily measure SiO2 films to 2 nm thick or less.
Stéphane Heuraux - One of the best experts on this subject based on the ideXlab platform.
-
Synthetic conventional Reflectometry probing of edge and scrape-o layer plasma turbulence
Journal of Instrumentation, 2019Co-Authors: Jose Vicente, Stéphane Heuraux, Garrard Conway, Filipe Da Silva, T. Ribeiro, Bruce Scott, Luis Guimarãis, L. Gil, Egor Seliunin, P. ManzAbstract:Microwave Reflectometry techniques have been successfully applied to measure electron density fluctuations in magnetic confined fusion devices with good spatial and time resolutions. However, quantitative interpretation of turbulence measurements has driven continuous development of both analytical theory and sophisticated numerical codes in support of Reflectometry. Comparisons between experimental and synthetic Reflectometry have been performed previously while only recently realistic gyro-fluid simulations have been employed together with a full-wave code to simulate measured turbulence properties with Reflectometry. In this work, we report on recent e orts to employ the two-dimensional full-wave code REFMUL to implement a synthetic Reflectometry diagnostic to model plasma turbulence measurements on the edge and scrape-o layer (SOL) peripheral regions of fusion devices. Numerical descriptions of microscopic turbulence were obtained from both an analytical model (following a Kolgomorov-like wavenumber spectrum) and from the GEMR turbulence code based on gyro-fluid theory. Simulations of fixed frequency conventional Reflectometry with ordinary mode (O-mode) wave propagation were carried out for both turbulence cases separately. Preliminary comparisons between synthetic measurements, numerical plasma characteristics, and experimental data from ASDEX Upgrade tokamak are made. As previously described in literature, regimes of linear and non-linear response occurring at low and high turbulence levels, respectively, are observed and characterized. Synthetic spectra across moderate to high turbulence levels display qualitatively good agreement with experimental data across the edge region. K : Nuclear instruments and methods for hot plasma diagnostics; Simulation methods and programs 1Corresponding author.
-
Modelling Reflectometry diagnostics: finite-difference time-domain simulation of Reflectometry in fusion plasmas
Journal of Instrumentation, 2019Co-Authors: F Silva, Stéphane Heuraux, A. Silva, E. Ricardo, T. T. RibeiroAbstract:Reflectometry simulations are particularly important since they allow to assess the measuring capabilities in experimental fusion devices and to predict the performance of future ones. We present a brief overview of Reflectometry and introduce the family of REFMUL* codes for time-dependent Reflectometry simulation. REFMUL* codes are Finite Di erence Time Domain (FDTD) that allow to set up synthetic diagnostics to assess the behaviour of Reflectometry diagnostics. This is illustrated in the current manuscript using the example of the Plasma Position Reflectometers of DEMO. K : Nuclear instruments and methods for hot plasma diagnostics; Simulation methods and programs; Plasma diagnostics-interferometry, spectroscopy and imaging
-
IST CONTRIBUTIONS TO THE ASDEX UPGRADE EDGE AND DIVERTOR PHYSICS USING MICROWAVE Reflectometry
2018Co-Authors: C. Silva, Stéphane Heuraux, L. Gil, D Aguiam, M Bernert, G Conway, B Gonçalves, L Guimarais, T Happel, P. ManzAbstract:Information of the plasma density such as provided by Reflectometry is essential for the study and operation of magnetically confined fusion devices. The microwave Reflectometry systems developed by IST for ASDEX Upgrade consist of: (i) a multi-band frequency modulated continuous wave O-mode reflectometer with the unique capability of providing profile and fluctuations measurements on the high-field side and low-field side (LFS) simultaneously, making it a relevant diagnostic for poloidal asymmetry studies; (ii) a fast frequency hopping O-mode reflectometer used to obtain more detailed information on density fluctuations at the LFS; and (iii) a multichannel X-mode Reflectometry diagnostic recently installed to measure the edge density profile in front of the ICRF antenna. This contribution presents an overview of the scientific results obtained with the different Reflectometry systems used in a complementary way in order to address some of the key issues under investigation at ASDEX Upgrade. The experimental results obtained demonstrate that the IST Reflectometry systems provide a valuable contribution to a better understanding of important physics topics such as connection between midplane and divertor conditions, dynamics of the density profiles, SOL turbulence, pedestal instabilities, RF sheath effects and real-time control.
-
A numerical study of fixed frequency Reflectometry measurements of plasma filaments with radial and poloidal velocity components
Review of Scientific Instruments, 2014Co-Authors: Jose Vicente, M. E. Manso, Stéphane Heuraux, G D Conway, F Silva, C. SilvaAbstract:A 2D finite-differences time-domain full-wave code is used to simulate the measurements of plasma filaments with fixed frequency O-mode Reflectometry. The plasma is modeled by a linear slab plasma plus a Gaussian perturbation propagating in a direction that can vary from poloidal to radial. The plasma background density gradient is chosen in agreement with the steep edge transport barrier of H-modes in the ASDEX Upgrade (AUG) tokamak. Illustrative results are presented and different types of Reflectometry responses are observed depending on filament sizes and propagation directions. The Reflectometry signatures obtained here with numerical simulations support previous experimental findings on filament measurements.
-
On turbulence-correlation analysis based on correlation Reflectometry
Physica Scripta, 2013Co-Authors: Natalia Teplova (kosolapova), Stéphane Heuraux, K. Itoh, S-i Itoh, E. Gusakov, S. Inagaki, M. Sasaki, T. Kobayashi, Y. Nagashima, S. OldenbürgerAbstract:Drift wave micro-turbulence is the main source of anomalous transport in a tokamak. Correlation Reflectometry is a powerful diagnostic tool which provides information on plasma turbulence and subsequently on underlying instability. In this paper, theoretical expressions for the analysis of radial correlation Reflectometry (RCR) data are derived. Integral kernels, which convert the correlation function of two microwave Reflectometry signals into a correlation function of plasma turbulence and inverse, are discussed. The analytical expression and the method of combining the RCR diagnostic and another local density fluctuation (e.g. Doppler enhanced scattering or heavy ion beam probe) is proposed. The correlation between Reflectometry signals and those from other local fluctuation measurements is analysed. The long-range tail of the correlation decays much more gradually than the turbulence correlation, however, it decays faster than that of two microwave Reflectometry signals. The way to calculate turbulence wave number spectrum for this case is also proposed.
Peiran Ren - One of the best experts on this subject based on the ideXlab platform.
-
Pocket Reflectometry
ACM Transactions on Graphics, 2011Co-Authors: Peiran Ren, John Snyder, Xin Tong, Jiaping Wang, Baining GuoAbstract:We present a simple, fast solution for reflectance acquisition using tools that fit into a pocket. Our method captures video of a flat target surface from a fixed video camera lit by a hand-held, moving, linear light source. After processing, we obtain an SVBRDF. We introduce a BRDF chart, analogous to a color "checker" chart, which arranges a set of known-BRDF reference tiles over a small card. A sequence of light responses from the chart tiles as well as from points on the target is captured and matched to reconstruct the target's appearance. We develop a new algorithm for BRDF reconstruction which works directly on these LDR responses, without knowing the light or camera position, or acquiring HDR lighting. It compensates for spatial variation caused by the local (finite distance) camera and light position by warping responses over time to align them to a specular reference. After alignment, we find an optimal linear combination of the Lambertian and purely specular reference responses to match each target point's response. The same weights are then applied to the corresponding (known) reference BRDFs to reconstruct the target point's BRDF. We extend the basic algorithm to also recover varying surface normals by adding two spherical caps for diffuse and specular references to the BRDF chart. We demonstrate convincing results obtained after less than 30 seconds of data capture, using commercial mobile phone cameras in a casual environment.
Joseph A. Dura - One of the best experts on this subject based on the ideXlab platform.
-
Neutron Reflectometry, x-ray Reflectometry, and spectroscopic ellipsometry characterization of thin SiO2 on Si
Applied Physics Letters, 1998Co-Authors: Joseph A. Dura, Curt A. Richter, Charles F. Majkrzak, Nhan V. NguyenAbstract:We present here a comparison of neutron Reflectometry, x-ray Reflectometry, and spectroscopic ellipsometry on a thin oxide film. These three probes each independently determine the structure of the film as a function of depth. We find an excellent agreement between the three techniques for measurements of thicknesses and interfacial roughnesses for both the SiO2 and surface contamination layers found in the sample. Realistic models based on interface parameters measured herein indicate that as the SiO2 layers decrease to sizes projected for future generations of electronic devices, both spectroscopic ellipsometry and neutron Reflectometry can easily measure SiO2 films to 2 nm thick or less.
