Study of edge plasma properties comparing operation in hydrogen and heliumin RFX

The properties of the edge plasma in the reversed field pinch RFX have been investigated by comparing the operation in helium with those normally perf ormed in hydrogen. It has been found that a spontaneous velocity shear laye r takes place in the edge region also in helium discharges. The edge struct ure of hydrogen and helium discharges have been interpreted using a momentu m balance equation, which takes into account anomalous viscosity and fricti on with neutrals. The electrostatic turbulence properties are also compared : it is found that electrostatic turbulence drives most of the particle los ses and a small fraction of the energy losses also for the He discharges. T he modifications of the mean profiles, including the E x B velocity, during PPCD are briefly discussed and compared with the results obtained in hydro gen. (C) 2001 Elsevier Science B.V. All rights reserved.