Characterization of Core and Edge Turbulence in L- and Enhanced D[alpha] H-mode Alcator C-Mod Plasmas PDF Download
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Author: Nils Plesner Basse Publisher: ISBN: Category : Languages : en Pages : 96
Book Description
The recently upgraded phase-contrast imaging (PCI) diagnostic is used to characterize the transition from the low (L) to the enhanced D[alpha] (EDA) high (H) confinement mode in Alcator C-Mod [I. H. Hutchinson, et al., Phys. Plasmas, 1511 (1994)] plasmas. PCI yields information on line integrated density fluctuations along vertical chords. The number of channels has been increased from 12 to 32, the sampling rate from 1 MHz to 10 MHz. This expansion of diagnostic capabilities is used to study broadband turbulence in L- and EDA H-mode and to analyze the quasi-coherent (QC) mode associated with EDA H-mode. Changes in broadband turbulence at the transition from L- to EDA H-mode can be interpreted as an effect of the Doppler rotation of the bulk plasma. Additional fluctuation measurements of D[alpha]-light and the poloidal magnetic field show features correlated with PCI in two different frequency ranges at the transition. The back transition from EDA H- to L-mode, the so-called enhanced neutron (EN) mode, is investigated by new high frequency (132 and 140 GHz) reflectometer channels operating in the ordinary (O) mode. This additional hardware has been installed in an effort to study localized turbulence associated with internal transport barriers (ITBs). The EN-mode is a suitable candidate for this study, since an ITB exists transiently as the outer density decreases much faster than the core density in this mode. The fact that the density decays from the outside inward allows us to study fluctuations progressing towards the plasma core. Our results mark the first localized observation of the QC-mode at medium density: 2.2 x 1020 m-3 (132 GHz). Correlating the reflectometry measurements with other fluctuating quantities provides some insight regarding the causality of the EN-mode development.
Author: Nils Plesner Basse Publisher: ISBN: Category : Languages : en Pages : 96
Book Description
The recently upgraded phase-contrast imaging (PCI) diagnostic is used to characterize the transition from the low (L) to the enhanced D[alpha] (EDA) high (H) confinement mode in Alcator C-Mod [I. H. Hutchinson, et al., Phys. Plasmas, 1511 (1994)] plasmas. PCI yields information on line integrated density fluctuations along vertical chords. The number of channels has been increased from 12 to 32, the sampling rate from 1 MHz to 10 MHz. This expansion of diagnostic capabilities is used to study broadband turbulence in L- and EDA H-mode and to analyze the quasi-coherent (QC) mode associated with EDA H-mode. Changes in broadband turbulence at the transition from L- to EDA H-mode can be interpreted as an effect of the Doppler rotation of the bulk plasma. Additional fluctuation measurements of D[alpha]-light and the poloidal magnetic field show features correlated with PCI in two different frequency ranges at the transition. The back transition from EDA H- to L-mode, the so-called enhanced neutron (EN) mode, is investigated by new high frequency (132 and 140 GHz) reflectometer channels operating in the ordinary (O) mode. This additional hardware has been installed in an effort to study localized turbulence associated with internal transport barriers (ITBs). The EN-mode is a suitable candidate for this study, since an ITB exists transiently as the outer density decreases much faster than the core density in this mode. The fact that the density decays from the outside inward allows us to study fluctuations progressing towards the plasma core. Our results mark the first localized observation of the QC-mode at medium density: 2.2 x 1020 m-3 (132 GHz). Correlating the reflectometry measurements with other fluctuating quantities provides some insight regarding the causality of the EN-mode development.
Author: Joseph Allan Snipes Publisher: ISBN: Category : Languages : en Pages : 10
Book Description
The steady-state H-mode regime found at moderate to high density in Alcator C-Mod, known as Enhanced D alpha H-mode (EDA), appears to be maintained by a continuous quasi-coherent (QC) mode in the edge steep gradient region. Large amplitude density and magnetic fluctuations with typical frequencies of %7E100 kHz are driven by the QC mode. These fluctuations are measured in the steep edge gradient region by inserting a fast scanning probe containing two poloidally separated Langmuir probes and a poloidal field pick-up coil. As the probe approaches the plasma edge, clear magnetic fluctuations were measured within %7E 2 cm of the last closed flux surface (LCFS). The mode amplitude falls off rapidly with distance from the plasma center with an exponential decay length of kr approximately equal to 1.5 cm-1, measured 10 cm above the outboard midplane. The root-mean-square amplitude of the poloidal field fluctuation extrapolated to the LCFS was approximately 5 G. The density fluctuations, on the other hand, were visible on the Langmuir probe only when it was within a few mm of the LCFS. The potential and density fluctuations were sufficiently in phase to enhance particle transport at the QC mode frequency. These results show that the quasi-coherent signature of the EDA H-mode is an electromagnetic mode that appears to be responsible for the enhanced particle transport in the plasma edge.
Author: Paul Chappell Ennever Publisher: ISBN: Category : Languages : en Pages : 139
Book Description
Turbulence in tokamak plasmas is the primary means by which energy is transported from the core of the plasma to the edge, where it is lost, and is therefore the main limitation of tokamak plasma performance. Dilution of the main-ion species was found to have a stabilizing effect on ion gyroradius scale turbulence in tokamak plasmas. Dilution of deuterium tokamak plasmas is the reduction of the ratio of the deuterium ion density to the electron density, nD=ne, to less than 1.0 through the introduction of low-Z impurity species into the plasma. Controlled dilution experiments were performed on Alcator C-Mod wherein plasmas at a range of electron density and plasma current were seeded with nitrogen while a cryopump held the electron density fixed. The electron density fluctuations due to turbulence were monitored using a phase contrast imaging (PCI) diagnostic, an absolutely calibrated diagnostic that measures the line-integral of the electron density fluctuations along 32 vertical chords. In these experiments the seeding reduced the PCI density fluctuations, and had a stabilizing effect on the ion energy transport. The seeding also reversed the direction of intrinsic rotation in certain cases. Nonlinear simulations using the gyrokinetic turbulence code GYRO were performed using measured kinetic profiles from the dilution experiments both before and after the nitrogen seeding. The GYRO simulations reproduced the observed reduction in the turbulent ion energy transport with the nitrogen seeding. The GYRO simulated turbulent density fluctuations were compared to the PCI measurements using a synthetic diagnostic, and they were found to be consistent. GYRO simulations were also performed varying only the main ion dilution to explore the theoretical effects of the dilution on energy transport. Through this it was found that the dilution reduced the turbulent ion energy transport in a wide variety of cases, but primarily increased the critical gradient at low densities, and primarily reduced the stiffness of the transport at high densities. This dilution effect is related to observations of reductions in energy transport from seeding on other tokamaks, and will likely have an impact on ITER and future fusion reactors.
Author: James L. Terry Publisher: ISBN: Category : Languages : en Pages : 44
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: Yan Yang Publisher: Springer ISBN: 9811381496 Category : Science Languages : en Pages : 144
Book Description
This book revisits the long-standing puzzle of cross-scale energy transfer and dissipation in plasma turbulence and introduces new perspectives based on both magnetohydrodynamic (MHD) and Vlasov models. The classical energy cascade scenario is key in explaining the heating of corona and solar wind. By employing a high-resolution hybrid (compact finite difference & WENO) scheme, the book studies the features of compressible MHD cascade in detail, for example, in order to approximate a real plasma cascade as “Kolmogorov-like” and to understand features that go beyond the usual simplified theories based on incompressible models. When approaching kinetic scales where plasma effects must be considered, it uses an elementary analysis of the Vlasov–Maxwell equations to help identify the channels through which energy transfer must be dissipated. In addition, it shows that the pressure–strain interaction is of great significance in producing internal energy. This analysis, in contrast to many other recent studies, does not make assumptions about wave-modes, instability or other specific mechanisms responsible for the dynamics – the results are direct consequences of the Vlasov–Maxwell system of equations. This is an important step toward understanding dissipation in turbulent collisionless plasma in space and astrophysics.
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Author: Nils Plesner Basse![Characterization of Core and Edge Turbulence in L- and Enhanced D[alpha] H-mode Alcator C-Mod Plasmas PDF](https://books.google.com/books/content/images/frontcover/F0UhcgAACAAJ?fife=w80-h100)
Author: Nils Plesner Basse
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Author: Joseph Allan Snipes
Author: M. H. Redi
Author: Amanda E. Hubbard
Author: Paul Chappell Ennever
Author: Miklos Porkolab![Observations of the Turbulence in the Scrape-off-layers of Alcator C-Mod and NSTX and Comparisons with Simualtion [sic] PDF](https://books.google.com/books/content/images/frontcover/lcIttwAACAAJ?fife=w80-h100)
Author: James L. Terry
Author: Yan Yang