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The radial electric field E r at the edge of the RFX plasma changes sign, from negative (inward) to positive, when an m =0 perturbation is externally applied. Similar behaviour had been previously observed in presence of wall locked ( m =1) modes, whereby the much stronger plasma–wall interaction made data interpretation more complex. The findings are compatible with the presence of an m =0, n =1 magnetic island. While in ordinary situations the edge losses of RFX are interpreted as dominated by finite Larmor radius ion effects, inside the island associated with the external perturbation the diffusion of the electrons is enhanced and exceeds the ion loss flux, thus reversing E r .

The spectroscopic methods adopted to diagnose impurity and hydrogen influxes into the RFX plasma are described and the results of the observations are discussed. The impurity emission lines are currently monitored by means of narrow band interference filters and photodiode array equipped spectrometers, both absolutely calibrated. Influxes of carbon and oxygen are deduced from measurements of C II and O II lines and are related to the impurity content in the plasma core. Molecular band emission observations show a screening effect of the chemically sputtered products. The influx measurements are carried out over different viewing chords showing strong asymmetries in the plasma wall interactions, observed also by CCD cameras and related to strong magnetic field errors.