Numerical Modelling of Transport and Turbulence in Tokamak Edge Plasma with Divertor Configuration PDF Download

Author: Davide Galassi
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Category :
Languages : en
Pages : 0

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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: Camille Baudoin
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Category :
Languages : en
Pages : 0

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Fusion devices are a promising solution for a new source of energy. However, using fusion reaction to produce power within a magnetic confinement is a scientific and technological challenge as it requires a high confinement in the core plasma at the same time as a good control of plasma exhaust on the material walls. This work is motivated by the key problematic of power handling in fusion power plants necessary to avoid damaging the expensive plasma facing components (PFC). The understanding of the physics underlying the heat transport, and more specifically is a critical task for the engineering design of future Tokamak devices. In this context, it is mandatory to make reliable predictions of the power spreading in order to correctly size the future Tokamaks. This calls for a theoretical ground describing the way energy escapes the core plasma through the separatrix and deposits on the PFCs. Some theoretical and experimental studies attempt to achieve such a task, however no definitive conclusion have been drawn yet. To achieve this goal, numerical modelling is a necessary complement to experimental results. This PhD work has been dedicated to the study of the different aspects of the heat transport in the edge plasma using a numerical fluid approach. Special focus was devoted to two types of mechanisms suspected to play an important role in the heat transport: intermittent turbulence; the large-scale convective transport.

Author:
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Category : Cleveland (Ohio)
Languages : en
Pages : 21

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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: Clothilde Colin
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

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Book Description
The possibility to produce power by using magnetically confined fusion is a scientific and technological challenge. The perspective of ITER conveys strong signals to intensify modeling effort on magnetized fusion plasmas. The success of the fusion operation is conditioned by the quality of plasma confinement in the core of the reactor and by the control of plasma exhaust on the wall. Both phenomena are related to turbulent cross-field transport that is at the heart of the notion of magnetic confinement studies, particle and heat losses. The study of edge phenomena is therefore complicated by a particularly complex magnetic geometry.This calls for an improvement of our capacity to develop numerical tools able to reproduce turbulent transport properties reliable to predict particle and energy fluxes on the plasma facing components. This thesis introduces the TOKAM3X fluid model to simulate edge plasma turbulence. A special focus is made on the code Verification and the Validation. It is a necessary step before using a code as a predictive tool. Then new insights on physical properties of the edge plasma turbulence are explored. In particular, the poloidal asymmetries induced by turbulence and observed experimentally in the Low-Field-Side of the devices are investigated in details. Great care is dedicated to the reproduction of the MISTRAL base case which consists in changing the magnetic configuration and observing the impact on parallel flows in the poloidal plane. The simulations recover experimental measurements and provide new insights on the effect of the plasma-wall contact position location on the turbulent features, which were not accessible in experiments.

Author: A. Thyagaraja
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ISBN:
Category : Agricultural engineering
Languages : en
Pages :

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Author: David MacTaggart
Publisher: Springer
ISBN: 3030163431
Category : Technology & Engineering
Languages : en
Pages : 270

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The book presents an advanced but accessible overview of some of the most important sub-branches of magnetohydrodynamics (MHD): stability theory, magnetic topology, relaxation theory and magnetic reconnection. Although each of these subjects is often treated separately, in practical MHD applications they are normally inseparable. MHD is a highly active field of research.The book is written for advanced undergraduates, postgraduates and researchers working on MHD-related research in plasma physics and fluid dynamics.

Author: Brian LaBombard
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ISBN:
Category :
Languages : en
Pages : 20

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Particle transport in the edge plasma and scrape-off layer will play a key role in the performance and operation of a tokamak fusion reactor: setting the width of the scrape-off layer density profile and its impurity screening characteristics, regulating the energetic particle fluxes onto first-wall components and associated impurity generation rates, and determining the effectiveness of the divertor in receiving particle exhaust and controlling neutral pressures in the main-chamber. The processes which govern particle transport involve plasma turbulence, phenomena which can not yet be reliably computed from a first-principles numerical simulation. Thus, in order to project to a reactor-scale experiment, such as ITER, one must first develop an understanding of particle transport phenomena based on experimental measurements in existing plasma fusion devices. Over the past few years of research, a number of fundamental advances in the understanding of the cross-field particle transport physics have occurred, replacing crude, incorrect, and often misleading transport models such as the "constant diffusion coefficient" model with a more appropriate description of the phenomenon. It should be noted that this description applies to transport processes in the absence of ELM phenomenon, i.e., physics underlying the "background" plasma state. In this letter, we first review the experimental support for this understanding which is based extensively on data from L-mode discharges and from H-mode discharges at time intervals without ELMs. We then comment on its implications for ITER.

Author: Ben Zhu
Publisher:
ISBN:
Category :
Languages : en
Pages : 206

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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: Sean Bozkurt Ballinger
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

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Book Description
Tokamaks are currently being designed and built to achieve net positive unharnessed fusion energy, an important milestone on the path to electricity production. Experimental trends predict an additional challenge in these upcoming devices: a decrease in the area of the metal wall on which the plasma deposits significant heat flux, increasing the likelihood of melting damage. The heat deposition area is proportional to a parameter called the heat flux width, which decreases with increasing poloidal magnetic field and average plasma pressure. In devices designed to achieve physics breakeven such as ITER and SPARC, the heat flux width is predicted by some estimates to be less than 1 millimeter. It is therefore crucial to develop methods to more accurately predict the heat flux width and to mitigate large heat fluxes. Data from the Alcator C-Mod tokamak are particularly relevant in the effort to predict conditions in SPARC, as both are designed to use a higher magnetic field than other major tokamak experiments. Before this work, the relationship between the heat flux width and edge profiles of plasma density and temperature in C-Mod was unknown. Studies with plasma edge simulation codes were limited to a small number of discharges at a time, with many model settings being ad-hoc and difficult to evaluate for general applicability. Simulations of C-Mod had a much shorter outer divertor leg compared to SPARC, making it difficult to use detachment studies in C-Mod to speculate on detachment in SPARC. Finally, there was only a rough idea of edge plasma conditions in SPARC, and it was not known whether detachment would even be feasible. This thesis uses data from Alcator C-Mod and simulations with the UEDGE code to investigate heat flux width scalings, detachment, and advanced divertor concepts to inform the design of next-generation tokamaks that can pro duce significant fusion energy while remaining safe against heat flux damage. This thesis begins by augmenting a C-Mod heat flux width database (containing ~300 discharges) with midplane density and temperature profile data. Detailed analysis finds that the outer target heat flux width depends on the edge plasma pressure, but fails to find a clear dependence on edge gradients. The scaling of the heat flux width with the edge pressure varies by confinement mode and is used to confirm predictions of the heat flux width of 0.2-0.4 mm in SPARC and 0.4-0.6 mm in ITER H-mode scenarios. The UEDGE code is then used to simulate the edge of Alcator C-Mod plasmas. 75 discharges from the heat flux width database are successfully modeled in UEDGE using a fully automated process that matches experimental midplane density and temperature profiles. The resulting heat flux width in UEDGE is then compared to experimental measurements, and it is found that the UEDGE and experimental values are correlated but that UEDGE overestimates the heat flux width by an average factor of 1.8. The UEDGE-modeled discharges are modified to include single-particle drift effects and (separately) to remove flux limits. These changes do not significantly improve the UEDGE heat flux width match to experiment but demonstrate the capability of this framework to evaluate which settings in the UEDGE model improve agreement with experiment over the large range of edge plasma conditions included in the C-Mod database. One particular C-Mod attached H-mode discharge is then simulated in UEDGE, and a good match is achieved to experimental data at the midplane and outer target simultaneously with full drift effects included in the model. This discharge is also simulated with a ~2x longer outer divertor leg, an important component of advanced divertor concepts that could enable better high heat flux handling. Detachment is found to occur when a nitrogen impurity is introduced at a fixed fraction of 3.5% of the main ion density in the real C-Mod geometry, while with the longer leg, detachment occurs at a significantly lower fraction of 2.4% nitrogen. This bodes well for the SPARC design, which features a long outer leg. Finally, a full-power SPARC H-mode scenario is directly simulated with UEDGE. It is found that detachment is possible at the high heat fluxes and small heat flux width predicted for SPARC and that the heat flux at the targets can remain significantly reduced with a carbon impurity fraction around 1%. This value is not a prediction of the detachment threshold in SPARC due to the use of bifurcated attached and detached solutions obtained at low power, but is encouraging when compared to the detachment thresholds in C-Mod UEDGE simulations. This study confirms that detachment is a promising solution to mitigate high heat fluxes in the SPARC full-power scenario.

Author: R. Cohen
Publisher:
ISBN:
Category :
Languages : en
Pages : 16

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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.