Edge temperature and density measurements with a thermal helium beam in the RFX reversed field pinch

The electron temperature and density at the edge plasma of the reversed field pinch experiment RFX are measured from the intensity ratio of selected spectral lines emitted by an atomic thermal helium beam injected into the plasma discharge. This technique, already performed in several Tokamaks, has been, for the first time, applied to a reversed field pinch to extend the edge density and temperature measurements to high-current discharges where intrusive probes cannot withstand the high-power fluxes. The temperature at the edge increases with the plasma current and decreases with the plasma average electron density. These edge values have been used to analyse the behaviour of the impurity influxes. The influence of the edge temperature and density on the efficiency of carbon penetration has been studied by means of a Monte Carlo simulation of the plasma behaviour at the edge.