PLASMA POTENTIAL WELL AND VELOCITY SHEAR-LAYER AT THE EDGE OF REVERSED-FIELD PINCH PLASMAS

The first evidence of an electrostatic potential well at the edge of p lasmas confined in a reversed field pinch configuration is reported, b ased on measurements made on the RFX experiment. The radial width of t he well decreases with plasma current, whereas the potential drop is a few times the electron temperature at the edge, almost independent of plasma current. The resulting radial electric field points inward at the edge and this behavior has been related to finite Larmor radius lo sses as in tokamaks. Because of the spatial structure of the plasma po tential, a naturally occurring double velocity shear value has been id entified al the edge with a shear value comparable to that of tokamaks and stellarators.