Author: J. Watkins Publisher: ISBN: Category : Languages : en Pages : 2
Book Description
Far scrape-off layer (SOL) plasma parameters in DIII-D depend strongly on the discharge density and confinement regime. In L-mode, cross-field transport increases with the average discharge density and elevates the far SOL density, thus increasing plasma-wall contact. Far SOL density near the low field side (LFS) of the main chamber wall also increases with decreasing plasma current and with decreasing outer wall gap. In H-mode, between edge localized modes (ELMs), plasma-wall contact is weaker than in L-mode. During ELMs plasma fluxes to the LFS wall increase to, or above the L-mode levels. A large fraction of the net cross-field fluxes is convected through the SOL by large amplitude intermittent transport events. In high density L-mode and during ELMs in H-mode, intermittent events propagate all the way to the LFS wall and may cause sputtering.
Author: Publisher: ISBN: Category : Languages : en Pages : 48
Book Description
We are in the process of assembling a database of edge and divertor plasma parameters suitable for use in benchmarking tious 2D models of the scrape-off- layer (SOL) plasma. Also, we are using the Braams B2 code to derive transport coefficients for the edge plssma. In parallel, work is starting on an upgrade to the B2 code that includes padlel current flow and EXB drifts. These efforts are directed at increasing the confidence level of models of the tokamak edge plasma so that we can predict the effect of planned upgrades to DIII-D (e.g., the Advanced Divertor Program) and the performance of next generation machines such as CIT or ITER, where initial design studies show that plasma conditions at the divertor targets can have a large impact on the lifetime and cost of the machine. This report summarizes our recent progress in characterizing the DIII-D SOL plasma and in modeling these data with the the B2 code. Section I contains a brief description of the diagnostics available for characterizing the SOL plasma. In Section II we present our measurements of the SOL parameters for H-mode plasmas. This includes data showing how the divertor plasma parameters (n{sub e}(r), T{sub e}(r), and Q(r)) vary from ohmic to L-mode to H-mode, and power balance for quasi-stationary H-mode plasmas. Section III covers divertor-target heat-flux asymmetries for double and single null operation with forward and reversed toroidal field. In Section IV we show the scaling of L-mode parameters with neutral beam power, and Section V concludes with a summary of the results obtained from the Braams B2 SOL simulation code.
Author: Publisher: ISBN: Category : Languages : en Pages : 33
Book Description
In a magnetically diverted tokamak, the scrape-off layer (SOL) and divertor plasma provides separation between the first wall and the core plasma, intercepting impurities generated at the wall before they reach the core plasma. The divertor plasma can also serve to spread the heat and particle flux over a large area of divertor structure wall using impurity radiation and neutral charge exchange, thus reducing peak heat and particle fluxes at the divertor strike plate. Such a reduction will be required in the next generation of tokamaks, for without it, the divertor engineering requirements are very demanding. To successfully demonstrate a radiative divertor, a highly radiative condition with significant volume recombination must be achieved in the divertor, while maintaining a low impurity content in the core plasma. Divertor plasma properties are determined by a complex interaction of classical parallel transport, anomalous perpendicular transport, impurity transport and radiation, and plasma wall interaction. In this paper the authors describe a set of experiments on DIII-D designed to provide detailed two dimensional documentation of the divertor and SOL plasma. Measurements have been made in operating modes where the plasma is attached to the divertor strike plate and in highly radiating cases where the plasma is detached from the divertor strike plate. They also discuss the results of experiments designed to influence the distribution of impurities in the plasma using enhanced SOL plasma flow. Extensive modeling efforts will be described which are successfully reproducing attached plasma conditions and are helping to elucidate the important plasma and atomic physics involved in the detachment process.
Author: Publisher: ISBN: Category : Languages : en Pages : 11
Book Description
Far scrape-off layer (SOL) and near-wall plasma parameters in DIII-D depend strongly on the discharge parameters and confinement regime. In L-mode discharges cross-field transport increases with the average discharge density and flattens far SOL profiles, thus increasing plasma-wall contact. In H-mode between edge localized modes (ELMs), plasma-wall contact is generally weaker than in L-mode. During ELMs plasma fluxes to the wall increase to, or above the L-mode levels. Depending on the discharge conditions ELMs are responsible for 30-90% of the ion flux to the outboard chamber wall. Cross-field fluxes in far SOL are dominated by large amplitude intermittent transport events that may propagate all the way to the outer wall and cause sputtering. A Divertor Material Evaluation System (DiMES) probe containing samples of several ITER-relevant materials including carbon, beryllium and tungsten was exposed to a series of upper single null (USN) discharges as a proxy to measure the first wall erosion.
Author: Publisher: ISBN: Category : Languages : en Pages : 14
Book Description
In this paper, the authors examine a denser and broader scrape-off layer (SOL) plasma, first seen in VH mode, in the DIII-D tokamak. The enhanced SOL appears in many types of double-null (DN) discharges and is not a property of VH-mode only. The DN enhanced SOL density and temperature profiles exhibit a 5--6 cm broad profile outside the separatrix. For DN and single-null (SN) boundary geometry with similar core plasma conditions, the enhanced SOL is only observed in high triangularity discharges. The origin of the enhanced SOL is, however, not yet understood.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Simulations of plasmas with a DIII-D shape indicate plasma drifts are important at power levels near the L- to H-mode plasma transition. In addition to enhancing plasma flows in the divertor region, drifts are found to play a key role in establishing highly sheared radial electric fields in the edge of the confined plasma, for the physics of the high confinement operating mode (H-mode). Measurements of the plasma structure in the vicinity of the X-point of DIII-D indicate the importance of drifts on plasma flow between the scrape-off layer (SOL) and closed field lines. The large electric fields provide large flows around the X-point, and these are conjectured to play a role in the transition from L- to H-mode confinement. These results indicate the relevance of modeling the edge and SOL plasmas of present tokamak devices using models which include E x B, (nabla)B, and pressure gradient drifts. The results of simulation of specific DIII-D discharges is reported in this paper. They start with discussion of the simulation of an Ohmic discharge in Section 2, including a study of the effect of varying several operational parameters. Simulation of a higher triangularity L-mode discharge is discussed in Section, and a summary is given in Section 4.
Author: S. Allen Publisher: ISBN: Category : Languages : en Pages : 51
Book Description
Trace {sup 13}CH{sub 4} injection experiments into the main scrape-off layer of low density L-mode and high-density H-mode plasmas have been performed in the DIII-D tokamak [Luxon{_}NF02] to mimic the transport and deposition of carbon arising from a main chamber sputtering source. These experiments indicated entrainment of the injected carbon in plasma flow in the main SOL, and transport toward the inner divertor. Ex-situ surface analysis showed enhanced {sup 13}C surface concentration at the corner formed by the divertor floor and the angled target plate of the inner divertor in L-mode; in H-mode, both at the corner and along the surface bounding the private flux region inboard of the outer strike point. Interpretative modeling was made consistent with these experimental results by imposing a parallel carbon ion flow in the main SOL toward the inner target, and a radial pinch toward the separatrix. Predictive modeling carried out to better understand the underlying plasma transport processes suggests that the deuterium flow in the main SOL is related to the degree of detachment of the inner divertor leg. These simulations show that carbon ions are entrained with the deuteron flow in the main SOL via frictional coupling, but higher charge state carbon ions may be suspended upstream of the inner divertor X-point region due to balance of the friction force and the ion temperature gradient.
Author: P.C Stangeby Publisher: CRC Press ISBN: 9780750305594 Category : Science Languages : en Pages : 738
Book Description
The Plasma Boundary of Magnetic Fusion Devices introduces the physics of the plasma boundary region, including plasma-surface interactions, with an emphasis on those occurring in magnetically confined fusion plasmas. The book covers plasma-surface interaction, Debye sheaths, sputtering, scrape-off layers, plasma impurities, recycling and control, 1D and 2D fluid and kinetic modeling of particle transport, plasma properties at the edge, diverter and limiter physics, and control of the plasma boundary. Divided into three parts, the book begins with Part 1, an introduction to the plasma boundary. The derivations are heuristic and worked problems help crystallize physical intuition, which is emphasized throughout. Part 2 provides an introduction to methods of modeling the plasma edge region and for interpreting computer code results. Part 3 presents a collection of essays on currently active research hot topics. With an extensive bibliography and index, this book is an invaluable first port-of-call for researchers interested in plasma-surface interactions.
Author: S. Allen Publisher: ISBN: Category : Languages : en Pages : 4
Book Description
High temporal and spatial resolution measurements in the boundary of the DIII-D tokamak show that edge localized modes (ELMs) are composed of fast bursts of hot, dense plasma that travel radially starting at the separatrix at {approx}450 m/s and rotate in the scrape off layer (SOL), convecting particles and energy to the SOL and walls. The temperature and density in the ELM plasma initially correspond to those at the top of the density pedestal but decay with radius in the SOL. The temperature decay length ({approx}1.2-1.5 cm) is much shorter than the density decay length ({approx}3-8 cm), which in turn decreases with increasing pedestal density. The local particle and energy flux at the wall during the bursts are 10-50% ({approx} 1-2 x 10{sup 21} m{sup -2} s{sup -1}) and 1-2% ({approx} 20-30 kW/m{sup 2}) respectively of the LCFS average fluxes, indicating that particles are transported radially much more efficiently than heat.
Author: J. Watkins
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Author: Cedric Kar-Wai Tsui
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Author: S. Allen
Author: P.C Stangeby
Author: S. Allen