Influence of electrode and limiter biasing on the ISTTOK plasma

In this contribution, limiter and electrode biasing experiments are compared, in particular with regard to their effects on the edge plasma parameters. For a positive electrode bias (EB), the plasma potential profile is strongly modified in the region between the electrode and the limiter. Both the edge Er and its shear increase significantly (values of Er larger than 10 kV m−1 have been measured). The modification in the edge parameters is accompanied by a substantial increase in the average plasma density and an improvement in the gross particle confinement. For moderate/high plasma densities no steady-state discharges have been achieved with EB since a degradation in confinement is observed a few milliseconds after the bias is applied. For negative bias (−250 < Vbias < 0 V), no significant modification in either the global or the edge parameters is observed since the electrode current drawn is too low to modify the plasma parameters. The general discharge behaviour for positive limiter biasing is similar to that observed with EB. However, the larger increase in radiation losses and the smaller modification of the edge electric field observed when bias is applied leads to almost no modification in particle confinement and to a stronger and faster degradation in confinement a few milliseconds later.