The quasi-coherent signature of enhanced D alpha H-mode in Alcator C-Mod

The steady-state H-mode regime found at moderate to high density in Alcator C-Mod, known as enhanced D-alpha (EDA) H-mode, appears to be maintained by a continuous quasi-coherent (QC) mode in the steep edge gradient region. L arge amplitude density and magnetic fluctuations with typical frequencies o f about 100 kHz are driven by the QC mode. These fluctuations are measured in the steep edge gradient region by inserting a fast-scanning probe contai ning two poloidally separated Langmuir probes and a poloidal field pick-up coil. As the probe approaches the plasma edge, clear magnetic fluctuations were measured within about 2 cm of the last-closed flux surface (LCFS). The mode amplitude falls off rapidly with distance from the plasma centre with an exponential decay length of k(r) approximate to 1.5 cm(-1), measured 10 cm above the outboard midplane. The root-mean-square amplitude of the fluc tuation extrapolated to the LCFS was (B) over tilde (theta) approximate to 5 G. The density fluctuations, on the other hand, were visible on the Langm uir probe only when it was within a few millimetres of the LCFS. The potent ial and density fluctuations were sufficiently in phase to enhance particle transport at the QC mode frequency. These results show that the QC signatu re of the EDA H-mode is an electromagnetic mode that appears to be responsi ble for the enhanced particle transport in the plasma edge.