Improved confinement and transport studies in the reversed field experiment (RFX)

This paper presents results obtained on the Reversed Field eXperiment, RFX [G. Rostagni, Fusion Engineering and Design 25, 301 (1995)], on the study o f local transport properties of the RFP configuration both in the standard regime and during improved confinement phases obtained with poloidal curren t drive. The plasma core is dominated by magnetic turbulence, as indicated by both energy and particle transport analysis. Diffusivity decreases towar d the edge and is reduced in the core at high Lundquist number. The edge re gion is apparently not affected by dynamo mode turbulence and determines gl obal confinement in standard operation. Particle and heat diffusivity in th is region show a favorable dependence with increasing density. The most int eresting feature of the edge region of RFX consists of a sheared radial ele ctric field with shearing rate comparable to that observed in tokamaks. Ind ications of turbulence reduction in the shear layer have been obtained. Enh anced energy confinement has been produced by pulsed poloidal current drive (PPCD). A clear reduction of the dynamo activity has been observed with a decrease of core thermal diffusivity to level comparable to the edge and gl obal confinement improvement up to a factor of 3. (C) 1999 American Institu te of Physics. [S1070-664X(99)90305-1].