Floating potentials in the vicinity of biased flush-mounted probes

An array of surface-mounted probe triplets in the Alcator C-Mod divertor region [1, 2] demonstrates the influence of a biased probe tip on local plasma potential. For the experiments described here, each poloidally aligned triplet contains one probe which measures the local I-V characteristic with a bias sweep and two passive probes which sample the floating potential. The floating potential at the two passive tips varies as a function of the bias at the swept tip. This dependence is shown to vary according to the local plasma parameters. Variations in both the amplitude and width of the floating potential disturbance on electron temperature and density are resolved and compared with different mechanisms of cross-field current transport.