H-mode and VH-mode Confinement Improvement in DIII-D: Investigations of Turbulence, Local Transport, and Active Control of the Shear in the E[times] B Flow PDF Download
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Languages : en
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The hypothesis of stabilization of turbulence by shear in the E[times] B drift speed successfully predicts the observed turbulence reduction and confinement improvement seen at the L to H transition. This same hypothesis is the best explanation to date for the further confinement improvement seen in the plasma core when the plasma goes from H-mode to VH-mode. Consequently, the most fundamental question for H-mode studies now is: how is the electric field E[sub r] formed? The radial force balance equation relates E[sub r] to the main ion pressure gradient[triangledown]P[sub i], poloidal rotation[nu][sub[theta]i], and toroidal rotation[nu][sub[phi]i]. In the plasma edge, direct measurements show[triangledown]P[sub i] and[nu][sub[theta]i] are the important terms at the L to H transition, with[triangledown]P[sub i] being the dominant, negative term throughout most of the H-mode. Since E[sub r] is observed to change prior to the change in[triangledown]P[sub i], the authors infer that main ion rotation, probably[nu][sub[theta]i], changes first, triggering the transition. E[sub r] is seen to change prior to the change in fluctuations, consistent with E[times] B shear causing the change in fluctuations and transport. In the plasma core, E[sub r] is primarily related to[nu][sub[phi]i]. There is a clear temporal and spatial correlation between the change in E[times] B shear and the region of local confinement improvement when the plasma goes from H-mode to VH-mode. Direct manipulation of[nu][sub[phi]i] and E[times] B shear using the drag produced by a non-axisymmetric magnetic perturbation has produced clear changes in local transport consistent with the E[times] B shear stabilization hypothesis. The implications of these results for theories of the L to H and H to VH transitions will be discussed. 83 refs., 5 figs.
![H-mode and VH-mode Confinement Improvement in DIII-D: Investigations of Turbulence, Local Transport, and Active Control of the Shear in the E[times] B Flow PDF](https://books.google.com/books/content/images/frontcover/k735jwEACAAJ?fife=w80-h100)
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Languages : en
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The hypothesis of stabilization of turbulence by shear in the E[times] B drift speed successfully predicts the observed turbulence reduction and confinement improvement seen at the L to H transition. This same hypothesis is the best explanation to date for the further confinement improvement seen in the plasma core when the plasma goes from H-mode to VH-mode. Consequently, the most fundamental question for H-mode studies now is: how is the electric field E[sub r] formed? The radial force balance equation relates E[sub r] to the main ion pressure gradient[triangledown]P[sub i], poloidal rotation[nu][sub[theta]i], and toroidal rotation[nu][sub[phi]i]. In the plasma edge, direct measurements show[triangledown]P[sub i] and[nu][sub[theta]i] are the important terms at the L to H transition, with[triangledown]P[sub i] being the dominant, negative term throughout most of the H-mode. Since E[sub r] is observed to change prior to the change in[triangledown]P[sub i], the authors infer that main ion rotation, probably[nu][sub[theta]i], changes first, triggering the transition. E[sub r] is seen to change prior to the change in fluctuations, consistent with E[times] B shear causing the change in fluctuations and transport. In the plasma core, E[sub r] is primarily related to[nu][sub[phi]i]. There is a clear temporal and spatial correlation between the change in E[times] B shear and the region of local confinement improvement when the plasma goes from H-mode to VH-mode. Direct manipulation of[nu][sub[phi]i] and E[times] B shear using the drag produced by a non-axisymmetric magnetic perturbation has produced clear changes in local transport consistent with the E[times] B shear stabilization hypothesis. The implications of these results for theories of the L to H and H to VH transitions will be discussed. 83 refs., 5 figs.
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Category : Science
Languages : en
Pages : 1130
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Author: אגודת ויסוקי (בני ברק, ישראל)
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Category : Judaism
Languages : en
Pages : 20
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Languages : en
Pages :
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Category :
Languages : en
Pages : 6
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A regime of improved H-mode energy confinement, VH-mode, is obtained in the DIII-D tokamak with adequate vessel conditioning. The improved confinement in VH-mode is consistent with the extension of the region of high E x B velocity shear turbulence suppression zone further in from the plasma boundary. The energy confinement enhancement in VH-mode can be limited by ELMs, localized momentum transfer events, or operation at high heating power or low q. Energy confinement enhancement improves with increasing triangularity of the plasma cross section and is independent of elongation. The termination of VH-mode is associated with an edge localized kink mode which is destabilized by both the large edge pressure gradient and edge current density.
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Languages : en
Pages : 10
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A major portion of the DIII-D program includes studies of the L-H transition, of the VH-mode, of particle transport and control and of the power-handling capability of a diverter. Significant progress has been made in all of these areas and the purpose of this paper is to summarize the major results obtained during the last two years. An increased understanding of the origin of improved confinement in H-mode and in VH-mode discharges has been obtained, good impurity control has been achieved in several operating scenarios, studies of helium transport provide encouraging results from the point of view of reactor design, an actively pumped diverter chamber has controlled the density in H-mode discharges and a radiative diverter is a promising technique for controlling the heat flux from the main plasma.
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Languages : en
Pages : 4
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Languages : en
Pages : 7
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Core turbulence fluctuation levels have been suppressed in DIII-D discharges with weak or negative magnetic shear (NCS) near the magnetic axis. In some weak magnetic shear discharges the ion thermal transport has been reduced to neoclassical levels throughout the whole plasma. The cause of the transport reduction is investigated by calculating the stability of toroidal drift waves, i.e., ion temperature gradient modes (ITG) and trapped electron modes (TE), with a comprehensive gyrokinetic linear stability code. It is found that the ITG modes and TE modes are stabilized by ExB velocity shear. The ExB velocity shear is primarily responsible for the spontaneous growth of a region of suppressed ion thermal transport. Surprisingly, the negative magnetic shear and Shafranov shift are only weak stabilizing influences for the ITG and TE modes in the DIII-D cases studied. Negative magnetic shear does eliminate the ideal magnetohydrodynamic ballooning mode instability which is a necessary access criteria for these improved core confinement regimes. Dilution of the thermal ions by fast ions from the heating beams and hot ions compared to electrons are found to be important stabilizing influences in the core.
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Languages : en
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Author: L. ZENG
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Category :
Languages : en
Pages : 7
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OAK-B135 The quiescent H-mode (QH-mode) is an ELM-free and stationary state mode of operation discovered on DIII-D. This mode achieves H-mode levels of confinement and pedestal pressure while maintaining constant density and radiated power. The elimination of edge localized modes (ELMs) and their large divertor loads while maintaining good confinement and good density control is of interest to next generation tokamaks. This paper reports on the correlations found between selected parameters in a QH-mode database developed from several hundred DIII-D counter injected discharges. Time traces of key plasma parameters from a QH-mode discharge are shown. On DIII-D the negative going plasma current (a) indicates that the beam injection direction is counter to the plasma current direction, a common feature of all QH-modes. The D{sub {alpha}} time behavior (c) shows that soon after high powered beam heating (b) is applied, the discharge makes a transition to ELMing H-mode, then the ELMs disappear, indicating the start of the QH period that lasts for the remainder of the high power beam heating (3.5 s). Previously published work showing density and temperature profiles indicates that long-pulse, high-triangularity QH discharges develop an internal transport barrier in combination with the QH edge barrier. These discharges are known as quiescent, double-barrier discharges (QDB). The H-factor (d) and stored energy (c) rise then saturate at a constant level and the measured axial and minimum safety factors remain above 1.0 for the entire QH duration. During QDB operation the performance of the plasma can be very good, with {beta}{sub N}*H{sub 89L} product reaching 7 for> 10 energy confinement times. These discharges show promise that a stationary state can be achieved.
