Observations of the Turbulence in the Scrape-off-layers of Alcator C-Mod and NSTX and Comparisons with Simualtion [sic] PDF Download
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Author: James L. Terry
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Languages : en
Pages : 44
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The intermittent turbulent transport in the scrape-off-layer of Alcator C-Mod and NSTX is studied experimentally by imaging with a very high density of spatial measurements. The 2-D structure and dynamics of emission from a localized gas puff are observed, and intermittent features (also sometimes called "filaments" or "blobs") are typically seen. The characteristics of the spatial structure of the turbulence and their relationship to the time-averaged SOL profiles are discussed. The experimental observations are compared with 3-D non-linear numerical simulations of edge turbulence. Radial profiles of the poloidal wavenumber spectra and the poloidal scale length from the simulations are in reasonable agreement with those obtained from the experimental images, once the response of the optical system is accounted for. The resistive ballooning mode is the dominant linear instability in the simulations. The ballooning character of the turbulence is also consistent with fluctuation measurements made at the inboard and outboard midplane, where normalized fluctuation levels are found to be about ten times smaller on the inboard side. For discharges near the density limit, turbulent structures are seen on closed flux surfaces. Keywords: turbulence; edge plasma; fluctuations; transport phenomena; resistive instabilities.
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Author: James L. Terry
Publisher:
ISBN:
Category :
Languages : en
Pages : 44
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Book Description
The intermittent turbulent transport in the scrape-off-layer of Alcator C-Mod and NSTX is studied experimentally by imaging with a very high density of spatial measurements. The 2-D structure and dynamics of emission from a localized gas puff are observed, and intermittent features (also sometimes called "filaments" or "blobs") are typically seen. The characteristics of the spatial structure of the turbulence and their relationship to the time-averaged SOL profiles are discussed. The experimental observations are compared with 3-D non-linear numerical simulations of edge turbulence. Radial profiles of the poloidal wavenumber spectra and the poloidal scale length from the simulations are in reasonable agreement with those obtained from the experimental images, once the response of the optical system is accounted for. The resistive ballooning mode is the dominant linear instability in the simulations. The ballooning character of the turbulence is also consistent with fluctuation measurements made at the inboard and outboard midplane, where normalized fluctuation levels are found to be about ten times smaller on the inboard side. For discharges near the density limit, turbulent structures are seen on closed flux surfaces. Keywords: turbulence; edge plasma; fluctuations; transport phenomena; resistive instabilities.
Author: James L. Terry
Publisher:
ISBN:
Category :
Languages : en
Pages : 12
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Book Description
The intermittent turbulent transport in the scrape-off-layers of Alcator C-Mod, DIII-D, and NSTX is studied experimentally. On DIII-D the fluctuations of both density and temperature have strongly non-Gaussian statistics, and events with amplitudes above 10 times the mean level are responsible for large fractions of the net particle and heat transport, indicating the importance of turbulence on the transport. In C-Mod and NSTX the turbulence is imaged with a very high density of spatial measurements. The 2-D structure and dynamics of emission from a localized gas puff are observed, and intermittent features (also sometimes called 3blobs4) are typically seen. On DIII-D the turbulence is imaged using BES and similar intermittent features are seen. The dynamics of these intermittent features are discussed. The experimental observations are compared with numerical simulations of edge turbulence. The electromagnetic turbulence in a 3-D geometry is computed using non-linear plasma fluid equations. The wavenumber spectra in the poloidal dimension of the simulations are in reasonable agreement with those of the C-Mod experimental images once the response of the optical system is accounted for. The resistive ballooning mode is the dominant linear instability in the simulations.
Author:
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Category :
Languages : en
Pages : 289
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This paper describes quantitative comparisons between turbulence measured in the scrape-off layer (SOL) of Alcator C-Mod [S. Scott, A. Bader, M. Bakhtiari et al., Nucl. Fusion 47, S598 (2007)] and three dimensional computations using electromagnetic gyrofluid equations in a two-dimensional tokamak geometry. These comparisons were made for the outer midplane SOL for a set of inner-wall limited, near-circular Ohmic plasmas. The B field and plasma density were varied to assess gyroradius and collisionality scaling. The poloidal and radial correlation lengths in the experiment and computation agreed to within a factor of 2 and did not vary significantly with either B or density. The radial and poloidal propagation speeds and the frequency spectra and poloidal k-spectra also agreed fairly well. However, the autocorrelation times and relative Da fluctuation levels were higher in the experiment by more than a factor of 2. Possible causes for these disagreements are discussed. 2009 American Institute of Physics.
Author:
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Category :
Languages : en
Pages :
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Author: S. J. Zweben
Publisher:
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Category : National Spherical Torus Experiment (Project)
Languages : en
Pages : 4
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Author: Thomas Nicholas
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Category :
Languages : en
Pages :
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Author: M. H. Redi
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Category : Computer simulation
Languages : en
Pages : 4
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Author: Jennifer Marie Sierchio
Publisher:
ISBN:
Category :
Languages : en
Pages : 119
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In the past, two methods for analyzing data from the Gas Puff Imaging diagnostic on Alcator C-Mod have been used. One uses temporal and spatial Fourier analysis to obtain wavenumber-frequency spectra, from which a phase velocity is computed [1, 2]. The other is based on time-delay cross-correlation of successive images used to track the motion of discrete emission structures [3, 4]. Several Gas-Puff-Imaging experiments were conducted to obtain data taken using the GPI Phantom Camera. The analysis of and results from these data are discussed in [3]. The results showed that the tracking time-delay-estimation technique found poloidal velocity magnitudes in the 0.1-1.4 km/sec range. However, independent examination of these data using the Fourier analysis yielded magnitudes up to a factor of 10 larger for the same data, and sometimes even disagreed with the direction of motion found. To understand the reasons for these discrepancies, we designed and generated synthetic data that mimics the real data. The user inputs the velocities, sizes, intensities, and distributions of the synthetic emission structures. We have used the synthetic data to test each code rigorously for strengths, weaknesses, and weighting. We have found that the Fourier analysis perfectly returns the correct poloidal velocity when there is no radial velocity component present. We have found that the tracking TDE analysis weights low frequency, low wavenumber features most heavily since they are typically the most intense, but systematically returns a smaller velocity than expected due to issues associated with averaging. After adjusting for these issues, the tracking TDE code now returns the correct value of the poloidal and radial velocities to within 10% for synthetic data as long as there is only one velocity present in the synthetic simulation. We applied these corrections to the analysis of the real data, and found that the measurements changed little in most cases. We then examined, in detail, the Fourier-analysis-derived "conditional" spectra for each shot, and determined that the likely causes for the discrepancies are due either to multiple velocities with emission structures moving in opposite directions in the same field of view or to non-zero "dispersion" in which lower-frequency/lower-wavenumber features are moving with one phase velocity and higher-frequency/higher-wavenumber features are moving with a different phase velocity. In a couple of cases, there may be a radial component in the actual images that may affect the poloidal velocity measurement for the Fourier analysis. Accounting for these explanations, we believe that we have resolved the discrepancies in many cases, and can explain it in the others.
Author: Symposium on Boundary Layers and Turbulence ; 13 (Dallas) : 1999.01.10-15
Publisher:
ISBN:
Category :
Languages : en
Pages : 4753
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Author: L. J. Reinders
Publisher: Springer Nature
ISBN: 3030643441
Category : Science
Languages : en
Pages : 628
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Book Description
This carefully researched book presents facts and arguments showing, beyond a doubt, that nuclear fusion power will not be technically feasible in time to satisfy the world's urgent need for climate-neutral energy. The author describes the 70-year history of nuclear fusion; the vain attempts to construct an energy-generating nuclear fusion power reactor, and shows that even in the most optimistic scenario nuclear fusion, in spite of the claims of its proponents, will not be able to make a sizable contribution to the energy mix in this century, whatever the outcome of ITER. This implies that fusion power will not be a factor in combating climate change, and that the race to save the climate with carbon-free energy will have been won or lost long before the first nuclear fusion power station comes on line. Aimed at the general public as well as those whose decisions directly affect energy policy, this book will be a valuable resource for informing future debates.
