Enhanced dynamo contribution to the resistivity of a reversed field pinch due to non-uniform effective charge profiles

The effect of a non-uniform effective ion charge distribution on the dynamo contribution to the plasma resistivity of a reversed field pinch is studied, based on a stationary energy or helicity balance. The radial distribution of the impurity is obtained from the ETA-BETA II line emission profiles. It is found that the main impurity ions are concentrated in the outer region of the plasma and, consequently, the effective ion charge, Zeff, is close to one everywhere, except in a small slab close to the plasma border, where it can be as high as three to four. The loop voltage required to sustain the RFP configuration with such 'hollow' Zeff profiles has to balance a higher dissipation in the outer region; hence, the apparent resistivity on axis is considerably greater than in the case of uniform Zeff. This effect could also be present in other RFP devices and might very well bridge the gap that was observed previously between the average effective ion charge and the resistivity anomaly factor, particularly for the low plasma density operation. (author)