Influence of Magnetic Geometry on Scrape-off Layer Profiles

C. J. Boswell, J. L. Terry, B. LaBombard, B. Lipschultz

M.I.T. Plasma Science and Fusion Center, 175 Albany St., Cambridge, MA 02139 USA

By observing the balanced and unbalanced double null discharges we are able to study the influence of magnetic geometry on the inner and outer scrape-off layer profiles. In a balanced double null discharge there exist two distinct regions of the scrape-off layer, a high-field side scrape-off layer (inner scrape-off layer) and a low-field side scrape-off layer (outer scrape-off layer). In an unbalanced double null configuration these two regions still exist outside the secondary separatrix. Between the primary and secondary separatrix there exists a region of the scrape-off layer in which the low- and high-field sides are magnetically connected (common scrape-off layer).

The D_a emission distributions on the inner and outer scrape-off layers are different. The D_a emission at the high-field side shows a strong peaking in its spatial distribution at the boundary between the inner scrape-off layer and the common scrape-off layer, with a width of ∼ 5 mm. The D_a emission distribution at the low-field side shows a broader distribution with a width of ∼ 1-2 cm and its peak occurring near the primary separatrix. When the discharge is double null there is no common scrape-off layer and the D_a emission distribution on the high-field side peaks at the separatrix and remains unchanged on the low-field side.

A one-dimensional physical space, two-dimensional velocity space kinetic neutral modeling code [1] reproduces the behavior of the observed D_a emission distributions and implies a reduced transport in the inner scrape-off layer. In this code plasma profiles are specified and taken to be typical temperature and density profiles as measured in the common and outer scrape-off layers. In the inner scrape-off layer the plasma density must have a rapid cross-field decay, with an e-folding length of ∼ 2-3 mm, beginning at the boundary with the common scrape-off layer. This is unlike the low-field side where no noticeable difference in cross-field e-folding length (∼ 2 cm) is measured at the boundary between the common and outer scrape-off layer.

[1] B.\ LaBombard, "KN1D: A 1-D Space, 2-D Velocity, Kinetic Transport Algorithm for Atomic and Molecular Hydrogen in an Ionizing Plasma," PSFC-RR-01-3.