The Fine-scale Structure of the Radial Electric Field in the Scrape-Off-Layer during ICRF Heating in Alcator C-Mod

Full text: By observing the radial structure in the poloidal dynamics of the SOL turbulence during the application of ICRF power (PRF > 0.3 MW), we find a fine-scale radial structure in the poloidal phase velocities (Vpol) of the broadband turbulence. The radial profiles are very different from typical profiles in Ohmic plasmas. Since Vpol(r) in the SOL is dominated by VExB , this structure implies that a fine-scale Er profile is formed in the presence of the ICRF. This profile extends to regions well separated toroidally from the ICRF antennas (∼ 2 m). The ||Vpol|| values in the far SOL imply an Er as large as 25 kV/m. The size-scale of the structure in this radial profile is much smaller than the fast wave perpendicular wavelength (∼ 10 cm). The observed velocity fields are consistent with the presence of potential structures arising as a consequence of sheath rectification of the ICRF waves, and potentials as large as 350 V are implied. Such Er profiles and potentials may help to explain the increased impurity content observed with ICRF heating, as a consequence of both enhanced sputtering and enhanced transport/penetration across the SOL. This effect will be important for impurity generation and SOL transport in regions well away from the antennas. Using 2D Gas-Puff-Imaging we find that, in the ∼ 3 cm region outside the separatrix, the steady-state dominant propagation direction for Vpol reverses up to three times; i.e., in some configurations, Vpol(r) varies from downward (Er > 0) in the ∼ 1 cm outside the separatrix, and then alternates from upward (Er < 0), to downward (Er > 0), to upward (Er < 0) in the next ∼ 2 cm. The local maxima in radial profiles of the potential occur on the field-lines just grazing active antennas. Thus the fine scale structure is a consequence of different antennas mapping to different SOL radii at the GPI view. The dependence of the implied potentials upon launched power follows the theoretically predicted trend (∼ PRF1/2). However, the potential structures are found to be significantly broadened compared to the basic theoretical expectation, having a radial width that is ∼ 5 x δe, where where the expected width, δe, is the skin depth for RF waves in the C-Mod SOL. The observed radial width also exhibits a power-dependence. (author)