Comparison of limiter and emissive electrode bias on the Tokamak ISTTOK

Biasing experiments have been performed on the tokamak ISTTOK with both a moving limiter and an emissive electrode. We have observed that large currents (>15 A) can be drawn at negative applied voltage by both localized limiter and emissive electrode bias, leading to significant modifications in the edge plasma potential profile and to an improvement in particle confinement. However, compared with the localized limiter, the emissive electrode has the advantage of perturbing significantly less the plasma column. Furthermore, its use leads to the formation of stronger radial electric fields and consequently to a much larger improvement in particle confinement. (author)