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(EX/C4-4) Experimental Evidence of Fluctuations and Flows near Marginal Stability in the Plasma Boundary Region in Fusion Plasmas

C. Hidalgo¹⁾, B. Gonçalves²⁾, M.A. Pedrosa¹⁾, C. Silva²⁾, K. Erents³⁾, G.F. Matthews^3)
 
1) Laboratorio Nacional de Fusion, Euratom-Ciemat, Madrid, Spain
²⁾Associação EURATOM/IST, Centro de Fusão Nuclear, Lisbon, Portugal
³⁾ Euratom/UKAEA, Abingdon, United Kingdom

Abstract.  Electrostatic turbulence has been investigated in the plasma boundary region in the JET tokamak and in the TJ-II stellarator. In both devices the naturally occurring velocity shear layer organizes itself to reach a condition in which the radial gradient in the poloidal phase velocity of fluctuations is comparable to the inverse of the correlation time of fluctuations. This result suggests that there is no continuous increase of the ExB flow when approaching the critical power threshold for the transition to improved confinement regimes and that ExB sheared flows organized themselves to be close to marginal stability. The statistical properties of ExB turbulent transport are correlated with the radial structure of fluctuations both in JET and TJ-II devices. The expected value of ExB turbulent transport events increases strongly as the gradient increases above its most probable value. These results are consistent with the paradigm of turbulent transport self-regulated via fluctuations near marginal stability.

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