Mirror Langmuir Probe measurements of fluctuation-induced heat and particle transport in the Alcator C-Mod boundary plasma

A ``Mirror Langmuir Probe'' technique [1] has recently been implemented on a four-electrode scanning Langmuir/Mach probe, allowing simultaneous measurements of electron temperature, density and plasma potential fluctuations at high bandwidth (1 MHz) at four separate spatial locations. This unique capability allows cross-field profiles of fluctuation-induced heat and particle transport fluxes to be recorded in the C-Mod boundary plasma for the first time. Wavenumber and frequency spectra of $\tilde{T}_e$, $\tilde{n}$ and $\tilde{\Phi}$ and their relative phase shifts can also be computed, providing key information on transport dynamics (drift-wave vs. interchange). Higher-order correlations, such as Reynolds stress components, may also be evaluated. By varying current, toroidal field, and line-averaged density, we have begun to explore the scaling of fluctuation-induced transport and its relationship to the empirical scalings of the heat flux widths seen in C-Mod L and H-mode plasmas. Of particular interest are the flux-gradient relationships near the last-closed flux surface and their connections to the ``critical gradient transport'' behavior that has been reported previously from observations of time-averaged $T_e$, $n$ profiles.\\[4pt] [1] Rev. Sci. Instrum. {\bf 78} (2007) 073501.