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Author: Publisher: ISBN: Category : Languages : en Pages : 17
Book Description
Results are presented for turbulence simulations with the fluid edge turbulence code BOUT [1]. The present study is focused on turbulence in the divertor leg region and on the role of the X-point in the structure of turbulence. Results of the present calculations indicate that the ballooning effects are important for the divertor fluctuations. The X-point shear leads to weak correlation of turbulence across the X-point regions, in particular for large toroidal wavenumber. For the saturated amplitudes of the divertor region turbulence it is found that amplitudes of density fluctuations are roughly proportional to the local density of the background plasma. The amplitudes of electron temperature and electric potential fluctuations are roughly proportional to the local electron temperature of the background plasma.
Author: Publisher: ISBN: Category : Languages : en Pages : 17
Book Description
Results are presented for turbulence simulations with the fluid edge turbulence code BOUT [1]. The present study is focused on turbulence in the divertor leg region and on the role of the X-point in the structure of turbulence. Results of the present calculations indicate that the ballooning effects are important for the divertor fluctuations. The X-point shear leads to weak correlation of turbulence across the X-point regions, in particular for large toroidal wavenumber. For the saturated amplitudes of the divertor region turbulence it is found that amplitudes of density fluctuations are roughly proportional to the local density of the background plasma. The amplitudes of electron temperature and electric potential fluctuations are roughly proportional to the local electron temperature of the background plasma.
Author: R. H. Cohen Publisher: ISBN: Category : Languages : en Pages :
Book Description
Results of recent modeling of tokamak edge plasma with the turbulence code BOUT are presented. In previous studies with BOUT the background profiles of plasma density and temperature were set as flux surface functions. However in the divertor region of a tokamak the temperature is typically lower and density is higher than those at the mid-plane. To account for this in the present study a poloidal variation of background plasma density and temperature is included to provide a more realistic model. For poloidally uniform profiles of the background plasma the calculated turbulence amplitude peaks near outer mid-plane, while in the divertor region the amplitude is small. However, present simulations show that as the background plasma profiles become more poloidally non-uniform the amplitude of density fluctuations, {tilde n}{sub i}, starts peaking in the divertor. It is found that in the divertor region the amplitude of n{sub i} fluctuations grows approximately linearly with the local density of the background plasma, n{sub i0}, while the amplitude of T{sub e} and {phi} fluctuations is positively correlated with the local electron temperature, T{sub e0}. Correlation analysis shows that plasma turbulence is isolated by the x-points.
Author: Davide Galassi Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
Nuclear fusion could offer a new source of stable, non-CO2 emitting energy. Today, tokamaks offer the best performance by confining a high temperature plasma by means of a magnetic field. Two of the major technological challenges for the operation of tokamaks are the power extraction and the confinement of plasma over long periods. These issues are associated with the transport of particles and heat, which is determined by turbulence, from the central plasma to the edge zone. In this thesis, we model turbulence in the edge plasma. We study in particular the divertor configuration, in which the central plasma is isolated from the walls by means of an additional magnetic field. This complex magnetic geometry is simulated with the fluid turbulence code TOKAM3X, developed in collaboration between the IRFM at CEA and the M2P2 laboratory of the University of Aix-Marseille.A comparison with simulations in simplified geometry shows a similar intermittent nature of turbulence. Nevertheless, the amplitude of the fluctuations, which has a maximum at the equatorial plane, is greatly reduced near the X-point, where the field lines become purely toroidal, in agreement with the recent experimental data. The simulations in divertor configuration show a significantly higher confinement than in circular geometry. A partial inhibition of the radial transport of particles at the X-point contributes to this improvement. This mechanism is potentially important for understanding the transition from low confinement mode to high confinement mode, the intended operational mode for ITER.
Author: Publisher: ISBN: Category : Cleveland (Ohio) Languages : en Pages : 21
Book Description
Journal of a trip to a GAR encampment in Washington, DC. Very detailed description of his trip to the White House. Includes description of a day spent sight seeing in Cleveland, OH on the return trip to Michigan.
Author: Publisher: ISBN: Category : Languages : en Pages : 13
Book Description
The edge-plasma profiles and fluxes to the divertor and walls of a divertor tokamak with a magnetic X-point are simulated by coupling a 2D transport code (UEDGE) and a 3D turbulence code (BOUT). An relaxed iterative coupling scheme is used where each code is run on its characteristic time scale, resulting in a statistical steady state. Plasma variables of density, parallel velocity, and separate ion and electron temperatures are included, together with a fluid neutral model for recycling neutrals at material surfaces. Results for the DIII-D tokamak parameters show that the turbulence is preferentially excited in the outer radial region of the edge where magnetic curvature is destabilizing and that substantial plasma particle flux is transported to the main chamber walls. These results are qualitatively consistent with some experimental observations. The coupled transport/turbulence simulation technique provides a strategy to understanding edge-plasma physics in more detailed than previously available and to significantly enhance the realism of predictions of the performance of future devices.
Author: R. Cohen Publisher: ISBN: Category : Languages : en Pages : 16
Book Description
The edge-plasma profiles and fluxes to the divertor and walls of a divertor tokamak with a magnetic X-point are simulated by coupling a 2D transport code (UEDGE) and a 3D turbulence code (BOUT). An relaxed iterative coupling scheme is used where each code is run on its characteristic time scale, resulting in a statistical steady state. Plasma variables of density, parallel velocity, and separate ion and electron temperatures are included, together with a fluid neutral model for recycling neutrals at material surfaces. Results for the DIII-D tokamak parameters show that the turbulence is preferentially excited in the outer radial region of the edge where magnetic curvature is destabilizing and that substantial plasma particle flux is transported to the main chamber walls. These results are qualitatively consistent with some experimental observations. The coupled transport/turbulence simulation technique provides a strategy to understanding edge-plasma physics in more detailed than previously available and to significantly enhance the realism of predictions of the performance of future devices.
Author: Publisher: ISBN: Category : Languages : en Pages : 41
Book Description
The boundary plasma turbulence code BOUT models tokamak boundary-plasma turbulence in a realistic divertor geometry using modified Braginskii equations for plasma vorticity, density (ni), electron and ion temperature (T{sub e}; T{sub i}) and parallel momenta. The BOUT code solves for the plasma fluid equations in a three dimensional (3D) toroidal segment (or a toroidal wedge), including the region somewhat inside the separatrix and extending into the scrape-off layer; the private flux region is also included. In this paper, a description is given of the sophisticated physical models, innovative numerical algorithms, and modern software design used to simulate edge-plasmas in magnetic fusion energy devices. The BOUT code's unique capabilities and functionality are exemplified via simulations of the impact of plasma density on tokamak edge turbulence and blob dynamics.
Author: Ben Zhu Publisher: ISBN: Category : Languages : en Pages : 206
Book Description
A new global 3D two-fluid code, GDB, based on the drift-reduced Braginskii model has been developed and tested to study the turbulent transport across the entire tokamak edge region: from plasma sources in the inner core to plasma sinks in the outer-most scrape-off layer (SOL). In this code, profiles of plasma density, electron and ion temperature, electric potential, magnetic flux and parallel flow are evolved self-consistently. Milliseconds-long simulations are carried out in a shifted-circle magnetic configuration with realistic Alcator C-Mod tokamak inner wall limited (IWL) discharge parameters. The resistive ballooning instability is identified as the predominant driver of edge turbulence in the L-mode regime. Simulations show, in agreement with experimental observations, as the simulation moves towards density limit regime by increasing density, the turbulent transport is drastically enhanced and the plasma profiles are relaxed; on the other hand, as the simulation approaches to the H-mode regime by increasing temperature, the turbulent transport is suppressed and plasma profiles are steepened with a pedestal-like structure forming just inside of the separatrix. Radial transport level and turbulence statistics of these simulations also qualitatively match the experimental measurements. Spontaneous E x B rotation in the electron diamagnetic drift direction in the closed flux region are observed in all cases. It can be explained based on the steady state ion continuity relation [mathematical equation]. E x B rotation in the closed flux region is found mostly cancels the ion diamagnetic drift as H-mode-like regimes are approached, and exceeds it by a factor of two or more at lower temperatures due to parallel ion flows.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Propagating filaments of enhanced plasma density, or blobs, observed in 3D numerical simulations of a diverted, neutral-fueled tokamak are studied. Fluctuations of vorticity, electrical potential [phi], temperature T{sub e} and current density J{sub {parallel}} associated with the blobs have a dipole structure perpendicular to the magnetic field and propagate radially with large E ยท B drift velocities (> 1 km/s). The simulation results are consistent with a 3D blob dynamics model that incorporates increased parallel plasma resistivity (from neutral cooling of the X-point region), blob disconnection from the divertor sheath, X-point closure of the current loops, and collisional physics to sustain the [phi], T{sub e}, J{sub {parallel}} dipoles.
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Author: R. H. Cohen
Author: Davide Galassi
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Author: R. Cohen
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Author: Bijal Chandrakant Modi
Author: Ben Zhu
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