Edge plasma investigation on the reversed field pinch ETA BETA II

The boundary region of the ETA BETA II RFP experiment has been investigated by movable instrumented limiters equipped with Langmuir probes, heat flux probes, and surface collectors. Edge plasma temperature and density are of the order of 10 eV and 0.5−1 × 1019 m−3. The heat deposition on the movable limiter is asymmetric between the electron and the ion drift side, and a power flux of the order of 300 MW/m2 is measured on the electron drift side. This high value has been explained by the presence of current carrying electrons coming from the hotter region of the plasma, through a stochastic region extending into the plasma. Surface collectors have been inserted in the plasma and analyzed by RBS and ERD techniques. The implanted impurities are mainly oxygen and metallic elements of the liner, both showing a quite linear build up as a function of the exposure time, whereas the amount of trapped deuterium tends to a stationary value after a few discharges. The deuterium flux incident on the wall has been inferred, the impurity yield estimated, and the generation mechanisms discussed. The deposition radial profiles give information on the screening capability of the boundary region. The estimate of the transmitted impurity flux appears in agreement with what is expected in terms of the known impurity content in the discharge. The stochastic behaviour of the magnetic field in a wide portion of the plasma near the wall is suggested to improve an uniform thermal load, and a more effective screening of heavy impurities.