Impact of the α parameter on the microstability of internal transport barriers

C. Bourdelle, G.T. Hoang, X. Litaudon, C.M. Roach, Tuomas Tala

    Research output: Contribution to journalArticleScientificpeer-review

    72 Citations (Scopus)


    In plasmas exhibiting an internal transport barrier (ITB), locally very high pressure gradient (∇P) is obtained. It induces high values of the magnetohydrodynamic α parameter (α = −q 2 βRP/P, with R the major radius, q the safety factor, P the pressure, ∇ the radial gradient and β the ratio between kinetic and magnetic pressure). Similarly to low or negative magnetic shear (s), high α reduces the curvature and ∇ B drifts driving curvature-type microinstabilities. Therefore, high values of α can stabilize part of the microturbulence, which leads to higher pressure gradient and to even higher α. This possibility for entering a positive feedback loop is very attractive to sustain ITBs in high performance plasmas. Indeed, α scales favourably with higher pressure and does not require any external momentum input. In this paper, after having discussed the α stabilization mechanism in detail, we report the experimental microstability analyses of ITBs from an international multi-machine database—the International Tokamak Physics Activity database, accessible on the webb. We show that α is indeed a relevant parameter of ITB physics that should be taken into account in interpretative and predictive one-dimensional transport codes.
    Original languageEnglish
    Pages (from-to)110 - 130
    Number of pages21
    JournalNuclear Fusion
    Issue number2
    Publication statusPublished - 2005
    MoE publication typeA1 Journal article-refereed


    • JET
    • plasma
    • fusion energy
    • fusion reactors
    • internal transport barriers


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