On the relaxation physics of plasma potential and poloidal flows in plasmas: experiments in tokamak (Castor) and stellarator (TJ-II)

Decay times of plasma flows and plasma profiles in the plasma boundary region have been measured after a sudden biasing switch-off in experiments on the CASTOR tokamak and TJII stellarator. A biased electrode in CASTOR and biased limiters in TJ-II have been used to polarize the edge plasma. The edge plasma potential and flows have been characterized by means of Langmuir and Mach probes. Potential profiles and poloidal flows can be well fitted by a exponential decay time in the range of 10-30 μs when the electrode biasing is turn off in the CASTOR tokamak. TJ-II experiments have shown evidence of two different relaxation times: the rapid time scale is in the order of 50 μms and the slow time scale (about 10 ms) is linked to the evolution in the plasma density. The obtained experimental results can shed some light on quantifying the importance of neoclassical mechanisms when compared with anomalous ones in the study of the damping of the radial electric fields and flows in fusion plasmas. (orig.)