An interpretation of fluctuation induced transport derived from electrostatic probe measurements

Fluctuation-induced particle fluxes (Γñφ̃) in the edge of Alcator C-Mod [Phys. Plasmas 1, 1511 (1994)] are inferred from a fast-scanning probe using standard analysis techniques. The magnitude and profile shape of Γñφ̃ is clearly inconsistent with fluxes inferred from global particle and power balance. These differences are difficult to reconcile if Γñφ̃ is interpreted as a measure of the particle flux in the unperturbed plasma. However, if Γñφ̃ is reinterpreted as the particle flux which must “fill-in” the presheath zone formed by the probe, these inconsistencies are eliminated. In this case, an effective diffusivity in the presheath zone (Dps) can be estimated from Γñφ̃. Dps is found to be in the range of diffusivities inferred from global particle balance (DSOL), indirectly supporting the hypothesis. However, the profile of Dps and its dependency on discharge conditions are markedly different than DSOL, implying that Dps is also not simply related to transport in the unperturbed plasma.