Emissive probe measurements of plasma potential fluctuations in the edge plasma regions of tokamaks

The plasma potential ${\Phi }_{\mathrm{pl}}$ and its fluctuations ${\mathrm{\Phi ̃}}_{\mathrm{pl}}$ were measured by electron emissive probes in the edge plasma regions of two fusion experiments: the Instituto Superior Técnico Tokamak (ISTTOK) (Lisbon, Portugal), and the Czech Academy of Sciences Torus (CASTOR) tokamak (Prague, Czech Republic). Into ISTTOK, three emissive probes were inserted outside the last closed flux surface (LCFS) on different minor radii. In CASTOR, two emissive probes, poloidally separated, and two cold cylindrical probes, mounted on the same shaft, were used, which could be radially shifted outside and inside the LCFS. The advantages of a sufficiently emissive probe are that in principle ${\Phi }_{\mathrm{pl}}$ and ${\mathrm{\Phi ̃}}_{\mathrm{pl}}$ can be measured directly, without being affected by electron temperature fluctuations or drifting electrons.