The quasi-coherent signature of enhanced Dα H-modein Alcator C-Mod

The steady-state H-mode regime found at moderate to high density in AlcatorC-Mod, known as enhanced Dα (EDA) H-mode, appears to be maintainedby a continuous quasi-coherent (QC) mode in the steep edge gradient region.Large amplitude density and magnetic fluctuations with typical frequencies ofabout 100 kHz are driven by the QC mode. These fluctuations are measured inthe steep edge gradient region by inserting a fast-scanning probe containingtwo poloidally separated Langmuir probes and a poloidal field pick-up coil. Asthe probe approaches the plasma edge, clear magnetic fluctuations weremeasured within about 2 cm of the last-closed flux surface (LCFS). The modeamplitude falls off rapidly with distance from the plasma centre with anexponential decay length of kr≈1.5 cm-1, measured 10 cm abovethe outboard midplane. The root-mean-square amplitude of the fluctuationextrapolated to the LCFS was θ≈5 G. The densityfluctuations, on the other hand, were visible on the Langmuir probe only whenit was within a few millimetres of the LCFS. The potential and densityfluctuations were sufficiently in phase to enhance particle transport at theQC mode frequency. These results show that the QC signature of theEDA H-mode is an electromagnetic mode that appears to be responsible for theenhanced particle transport in the plasma edge.