A tokamak edge phenomenon, dubbed the 'marfe' (for multifaceted asymmetric radiation from the edge), is described. This phenomenon, observed in medium- to high-density Alcator C discharges, is characterized by greatly increased radiation, density and density fluctuations, and decreased temperature in a relatively small volume at the inner major radius edge of the plasma. The marfe appears to be confined to minor radii greater than or of the order of that of the limiter. The affected region is typically above the midplane, extending poloidally for about 30° and toroidally for 360°. The temperature and density of the core plasma are unaffected by the marfe. A simple transport model is used to show that the marfe is the manifestation of a thermal instability, with impurity radiation being the main energy loss mechanism out of the marfe volume. A density threshold nm for marfe onset is observed; nm is found to be an increasing function of plasma current and a decreasing function of intrinsic low-Z impurity levels. Detailed observations from spectroscopy, bolometry, Langmuir probe measurements, interferometry and CO2 scattering are presented.
Investigation of multifaceted asymmetric radiation from the edge (MARFE) with impurity injection from the upper divertor on the Experimental Advanced Superconducting Tokamak