Plasma rotation and finite Larmor radius losses in a reversed field pinch

A model is proposed to calculate the effect of particle and momentum losses due to unconfined ion orbits at the edge of the plasma in a reversed field pinch device. The value of the shearing rate can be obtained, requiring that the electromagnetic force due to the return current in the confined plasma is equilibrated by the reaction force due to direct ion momentum losses. A natural scaling of the shearing rate with the ion cyclotron frequency is obtained by assuming that this equilibrium is achieved through the effect of orbit squeezing caused by the finite value of the radial electric field gradient. The extension of the shear layer has been obtained requiring that the contribution of charge exchange viscosity to the perpendicular momentum balance is negligible. A shear layer width scaling as the Larmor radius is obtained. The scaling relations derived account for recent observations obtained in the reversed field experiment, RFX [G. Rostagni, Fusion Eng. Des. 25, 301 (1995)]. A quantitative comparison with experimental data indicates that impurities in low ionization stages can play an important role in the shear layer dynamics. (C) 1998 American Institute of Physics. [S1070-664X(98)03509-5]