Improved confinement events triggered by emissive electrode biasing on the tokamak ISTTOK

Edge polarization experiments were carried out on ISTTOK using an emissive electrode. The dependence of the bias current on the radial electric field, measured by a radial array of Langmuir probes, is in qualitative agreement with the expression derived by Stringer for the transitional region between plateau and collisional transport regimes (Stringer T.E. 1993 Nucl. Fusion33 1249). Improved particle confinement is clearly observed for negative bias associated with a large radial electric field. In some cases, a double-peaked profile of the radial electric field is observed just after biasing, evolving afterwards to a single-peaked profile as confinement improves. These types of profiles may be related to the multiple solutions of the non-linear equation for the radial electric field, recently put forward by Kasuya et al (2003 Nucl. Fusion43 244). Above a certain threshold of the bias current stronger improved confinement events are observed, during short periods, for both bias polarities.