Efficiency of plasma biasing by movable localized limiter in tokamak ISTTOK

The correlation between the efficiency of plasma biasing and the position of a localized limiter has been investigated. It has been found that two distinct zones of limiter position can be distinguished in ISTTOK plasma: Far-zone at r _lim > 7.2 cm, and Close-zone at r _lim < 7.2 cm. The strongest modifications of edge plasma potential take place for both positive and negative limiter biasing when it is positioned inside Close-zone. The transition between zones is correlated with effective screening of the wall by the limiter, being more efficient as it is immersed deeper into the plasma. The indicative difference between zones can be a difference of non-ambipolar fluxes flowed onto short-circuited limiter: mostly positive (ion) inside Far-zone, and mostly negative (electron) inside Close-zone.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.