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 plasmas confined in a reversed field pinch configuration is reported, based on measurements made on the RFX experiment. The radial width of the 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 losses as in tokamaks. Because of the spatial structure of the plasma potential, a naturally occurring double velocity shear layer has been identified at the edge with a shear value comparable to that of tokamaks and stellarators.