Changes in density fluctuations associated with confinement transitions close to a rational edge rotational transform in the W7-AS stellarator

S, Zoletnik, Nils Plesner Basse, Mark Saffman, Winnie Edith Svendsen, M Endler, M, Hirsch, A, Werner, C Fuchs

    Research output: Contribution to journalJournal articleResearchpeer-review

    Abstract

    At certain values of the edge rotational transform t(a), the confinement quality of plasmas in the Wendelstein 7-AS (W7-AS) stellarator is found to react very sensitively to small modifications of the edge rotational transform t(a). As t(a) can be reproducibly changed, either by external fields or by a small plasma current, these transitions offer a precise way to systematically analyse differences in plasma turbulence between bad and good confinement cases. This paper presents results of the study of electron density fluctuations associated with confinement changes. Wavenumber and frequency spectra and radial profiles are compared., A slow and reproducible transition is induced by a small plasma current and the sequence of events leading to bad confinement is investigated. The laser scattering core plasma density fluctuation measurements are complemented by edge beam emission spectroscopy results and magnetic fluctuation measurements with Mirnov coils. Clear correspondence between plasma fluctuations and confinement degradation is observed: the weight of larger structures increases, fluctuations increase in the plasma core, the poloidal flow velocity decreases and regions of mode-like activity move radially in the plasma. These changes occur gradually and controllably if the, magnetic configuration is steered by a small plasma current.
    Original languageEnglish
    JournalPlasma Physics and Controlled Fusion
    Volume44
    Issue number8
    Pages (from-to)1581-1607
    ISSN0741-3335
    DOIs
    Publication statusPublished - 2002

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