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(THP1/20) Triggering Mechanisms for Transport Barriers

J. A. Heikkinen¹⁾, O. Dumbrajs²⁾, S. Karttunen¹⁾, T. Kiviniemi²⁾, T. Kurki-Suonio²⁾, M. Mantsinen²⁾, K. Rantamäki¹⁾, S. Saarelma²⁾, R. Salomaa²⁾, S. Sipilä²⁾, T. Tala^1)
 
1) VTT Chemical Technology, Espoo, Finland
²⁾ Helsinki University of Technology, Espoo, Finland

Abstract.  The radial shear $\omega_{E\times B}^{}$ of the E×B flow is evaluated with the Monte Carlo orbit following code ASCOT at the onset of the L-H transition and internal transport barriers (ITB) in JET, TFTR, ASDEX Upgrade, TEXTOR, and FT-2 tokamaks. Systematically, a large shear (sufficient for turbulence suppression) is found for local parameters close to the experimental threshold conditions at the barrier location. For L-H transition in JET and ASDEX Upgrade, the large shear is obtained by increasing the edge ion temperature. For TEXTOR, the radial electric field and the electrode current bifurcate at a threshold electrode voltage. In a JET database study, toroidal rotation is found to be dominant in triggering the JET ITB, and an empirical s- $\omega_{E\times B}^{}$ fit is found for the transition threshold. For TFTR and FT-2, in which toroidal rotation does not play a role, ASCOT predicts a significant $\omega_{E\times B}^{}$ shear for the ITB conditions. The ripple-induced transport is not found to be important here.

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