Application of emissive probes for plasma potential measurements in fusiondevices

In experimental fusion devices, up to now, only cold probes were used to de termine the plasma potential in the scrape-off layer (SOL), and their float ing potential was assumed to be proportional to the plasma potential. Howev er, drifting electrons or beams shift the current-voltage characteristic of a cold probe by a voltage, which corresponds to the mean kinetic energy of the drifting electrons. This problem can be avoided by the use of electron emissive probes, since an electron emission current is independent of elec tron drifts in the surrounding plasma. In addition emissive probes are inse nsitive to electron temperature fluctuations in the plasma. We have used an arrangement of three emissive probes in the edge plasma region of ISTTOK ( Instituto Superior Tecnico tokamak) at Lisbon. The probes have been mounted in such a way that the tips are positioned on the same poloidal meridian b ut on different minor radii in the SOL. With this arrangement, the plasma p otential has been measured in the edge region of the ISTTOK, and first resu lts are presented in this contribution.