Effects of flush-mounted probe bias on local turbulent fluctuations

Recent measurements on Alcator C-Mod explore the reliability of fluctuation measurements made with surface-mounted probe triplets in the divertor region. The normal configuration of each triplet consists of one angled probe operating as a swept tip and two nearby flush tips measuring floating potential fluctuations. In this mode, changes in the floating potential fluctuation levels and spectra have been observed which correlate to the level of current collected by the swept tip. The disturbance amplitude falls off with distance from the source probe, as well as with increasing electron density. The observed fluctuation changes are likely related to the low frequency plasma potential changes induced by the neighboring probe bias. An understanding of these observations is essential to interpretation of fluctuation measurements in any high-density divertor region.