New insights on boundary plasma turbulence and the quasi-coherent mode in Alcator C-Mod using a Mirror Langmuir Probe

A new Mirror Langmuir Probe diagnostic, combined with a double-coil scanning magnetic probe, is used to interrogate Alcator C-Mod's quasi-coherent mode (QCM) with unprecedented detail. In ohmic EDA H-modes, the QCM is found to reside in a region of positive radial electric field, with a radial width (similar to 3 mm) that spans open and closed field line regions. Large amplitude, in-phase sinusoidal bursts (similar to 100 kHz) in density, electron temperature, and plasma potential are observed, with potential lagging density by similar to 16 degrees, producing an outward radial transport velocity of similar to 10 m/s. Mode propagation corresponds to the sum of local E x B and electron diamagnetic drift velocities. Poloidal magnetic field fluctuations project to current filaments carrying peak current densities of similar to 25 A/cm(2). An evaluation of parallel electron force balance (Ohm's law) over a fluctuation cycle indicates a significant electromotive component. Interchange drive is also a contributor in the current continuity (vorticity) equation. Thus, the QCM is primarily a separatrix-spanning electron drift-wave with interchange and electromagnetic contributions. (C) 2014 AIP Publishing LLC.