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(EX/P1-10) Electrode and Limiter Biasing Experiments on the Tokamak ISTTOK

C.G. Silva1), H. Figueiredo1), J.A.C. Cabral1), I. Nedzelsky1), C.A.F. Varandas1)
1) Centro Fusao Nuclear, Instituto Superior Tecnico, Lisboa, Portugal

Abstract.  The aim of this contribution is to present a comparison between limiter and electrode biasing. Limiter biasing has been investigated on the tokamak ISTTOK, with both constant and alternating (50 Hz) bias voltages. However, the modifications on Er due to limiter biasing are, in general, limited to the SOL. Electrode bias has the advantage of forcing the electric field in the edge plasma. For positive limiter bias, an increase (30%) in the average plasma density is observed, although, without a significant increase the particle confinement. In spite of the lower density, particle confinement is higher for negative bias due to low turbulent transport. For electrode bias changes in the plasma parameters are more significant. For positive bias, a substantial increase (>60%) in the average plasma density is observed, without significant changes in the edge density, leading to steeper profiles. The ratio n/H$\scriptstyle \alpha$ also increases significantly (>25%), indicating an improvement in gross particle confinement. The plasma potential profile is strongly modified as both the edge Er and its shear increase significantly. Good correlation between confinement modifications and ExB shear has been found.

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IAEA 2003