Guiding-center models for edge plasmas and numerical simulations of isolated plasma filaments

Jens Madsen

    Research output: Book/ReportPh.D. thesisResearch

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    Abstract

    The work presented in this thesis falls into two categories: development of reduced dynamical models applicable to edge turbulence in magnetically confined fusion plasmas and numerical simulations of isolated plasma filaments in the scrape-off layer region investigating the influence of finite Larmor radius effects on the radial plasma transport. The coexistence of low-frequency fluctuations, having length scales comparable to the ion gyroradius, steep pressure gradients and strong E × Bflows in the edge region of fusion plasmas violates the standard gyrokinetic ordering. In this thesis two models are presented that overcome some of the difficulties associated with the development of reduced dynamical models applicable to the edge. Second order guiding-center coordinates are derived using the phasespace Lie transform method. Using a variational principle the corresponding Vlasov-Maxwell equations expressed in guiding-center coordinates are derived including a local energy theorem. The second order terms describe lowest order finite Larmor radius effects. This set of equations might be relevant for edge plasmas due to the capability of capturing strong E ×B - flows and lowest order finite Larmor radius effects self-consistently. Next, an extension of the existing gyrokinetic formalism with strong flows is presented. In this work the background electric fields is dynamical, whereas earlier contributions did only incorporate a stationary electric field. In an ordering relevant for edge plasma turbulence, fully electromagnetic second order gyrokinetic coordinates and the corresponding gyrokinetic Vlasov- Maxwell equations are derived, including a local energy theorem. By taking the polarization and magnetization densities in the drift kinetic limit, we present the gyrokinetic Vlasov-Maxwell equations in a more tractable form, which could be relevant for direct numerical simulations of edge plasma turbulence. Finally, an investigation of the influence of finite Larmor radius effects on the radial transport of isolated plasma filaments (blobs) in the scrape-off region of fusion plasmas is presented. We employ an isothermal electrostatic two-dimensional gyrofluid model to simulate the blob dynamics. The numerical simulations show that, to lowest order, the blob center of mass velocity scales as the acoustic speed times the square root of the ratio of the structure size to the gradient lengthscale of the magnetic field, in agreement with earlier results. However, when the ratio of the thermal ion gyroradius to the structure size exceeds 10 − 20 % the transport is dramatically enhanced. Having traveled its own initial size 15 times, blobs carry ∼ 80 % of their initial density above this threshold but only ∼ 20 % below. This observation demonstrate the importance of finite Larmor radius effects in plasma transport modeling.
    Original languageEnglish
    Place of PublicationRoskilde
    PublisherDanmarks Tekniske Universitet, Risø Nationallaboratoriet for Bæredygtig Energi
    Number of pages152
    ISBN (Print)978-87-550-3844-8
    Publication statusPublished - Sep 2010
    SeriesRisø-PhD
    Number65(EN)

    Keywords

    • Fusion energy
    • Risø-PhD-65(EN)
    • Risø-PhD-65
    • Risø-PhD-0065

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