Edge Turbulence Measurements During the L- to H-mode Transition by Phase Contrast Imaging on DIII-D PDF Download
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Author: Miklos Porkolab
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Category :
Languages : en
Pages : 8
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Author: Miklos Porkolab
Publisher:
ISBN:
Category :
Languages : en
Pages : 8
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Book Description
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Category :
Languages : en
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A CO[sub 2]-laser imaging diagnostic measures the vertical line integrals of the density fluctuations at 16 radial locations within a 7.6-cm-wide region at the outer edge of the DIII-D tokamak. The phase-contrast technique provides a flat response to wave numbers in the range 1--16 cm[sup[minus]1] and is mostly sensitive to radial wave vectors. This last property ensures that fluctuations are detected essentially in the plasma frame, since the E x B drift associated with the radial electric field is mainly in the poloidal direction. The radial resolution is 0.5 cm, the data acquisition bandwidth is 1 MHz, and the density sensitivity is on the order of 10[sup 9] cm[sup[minus]3]. The DIII-D Phase Contrast Imaging (PCI) diagnostic has been employed in the study of plasma turbulence, particularly at the transition from L- to H-mode. Past work has shown that the average amplitude of the line-integrated fluctuations in the plasma is reduced at the onset of the H-mode, in agreement with measurements by other diagnostics in DIII-D and elsewhere. In this paper the authors report on measurements of turbulence carried out during a study of low-power L- to H-mode transitions. As the input power approaches the threshold for H-mode, the transition occurs on a progressively slower time scale. The new results are consistent with past observations in L-mode and in the fully developed H-mode; novel features emerge, however, in the transient phase immediately following the transition. The phenomenology of these events, which lack a theoretical explanation at present, will be discussed in the next section.
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Category : Power resources
Languages : en
Pages : 486
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Category : Aeronautics
Languages : en
Pages : 704
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Category :
Languages : en
Pages :
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A novel CO[sub 2] laser phase contrast imaging diagnostic has been developed for the DIII-D tokamak, where it is being employed to investigate density fluctuations at the outer edge of the plasma. This system generates 16-point, 1-D images of a 7.6 cm wide region in the radial direction, and is characterized by long wavelength (7.6 cm) and high frequency (100 MHz) capability, as well as excellent sensitivity ([rvec n] [approx-gt] 10[sup 9] cm[sup [minus]3]). The effects of vertical line integration have been studied in detail, both analytically and numerically with actual flux surface geometries generated by the EFITD magnetic equilibrium code. It is shown that in the present configuration the measurement is mostly sensitive to radial wave vectors. Experimental results on fluctuation suppression at the L- to H-mode transition and on the L-mode wave number spectrum are discussed briefly. Finally, future plans for extending the measurement to the core of the plasma and for investigating externally launched fast waves are presented.
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Category : Physics
Languages : en
Pages : 398
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General physics, atomic physics, molecular physics, and solid state physics.
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Category : Science
Languages : en
Pages : 1000
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Category : Dissertations, Academic
Languages : en
Pages : 836
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![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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Category :
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 : Fusion reactors
Languages : en
Pages : 808
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