Comments on Equilibrium Plasma Flows in the Limiter Shadow Region of Alcator C

Resistive MHD is used to examine bulk plasma flows in the limiter shadow region of Alcator C. Expressions for the Pfirsch-SchlUter perpendicular plasma flow velocities are obtained for a toroidal system in which the pressure profile depends on radius only. Data from Langmuir probe measurements in the shadow of the limiter is used to estimate the magnitude of this plasma convection. Because of the short density scrape-off lengths in Alcator C edge plasma (3 mm), the magnitude of the perpendicluar flow velocity can lead to significant poloidal and/or toroidal flow velocity components. If the primary contribution to the perpendicular flow velocity is from the poloidal component, then the magnitude of the poloidal flow can easily exceed that of a radial flow velocity estimated from Bohm diffusion. On the other hand, if toroidal flow velocity dominates the total perpendicular flow, then the toroidal velocity can be a significant fraction of the sound speed. As a result, self-consistent plasma density and temperature profiles can exhibit a poloidal asymmetry. Such a poloidal asymmetry may explain the preferred location of the "marfe" phenomena observed in the Alcator C tokamak. The scaling of the perpendicular flow velocity obtained is consistent with the observed combination of edge density, scrape-off length, and plasma current which precipitates a marfe. The poloidal component of the perpendicular flow velocity exhibits a stagnation point on the inside and outside midplane where this component changes sign. The direction of the poloidal flow is always towards the inside of the torus independent of toroidal field and plasma current directions. Since this perpendicular equilibrium allows for an arbitrary radial ambipolar E-field to be present, an E x B plasma rotation can be superimposed on these flows. In this case, the inner stagnation point can move to the upper or lower inside position depending on the toroidal magnetic field direction. If such a poloidal rotation on the order suggested by CO2 laser scattering data is included, the stagnation' point coincides with the observed upper inside marfe position for the normal toroidal field direction and is consistent with preliminary observations of the marfe moving to the lower inside when the toroidal field is reversed. Independent of the mechanism by which a marfe initiates, PfirschSchlUter cross field convection may act to scrape-off the edge plasma boundary and transport energy into the radiating marfe region. In this way, a convection/radiation power balance can be maintained, forcing the marfe boundary to be toriodally symmetric and locallized poloidally to the inside of the torus.