Influence of Magnetic Geometry on Scrape-off Layer Profiles

Using a filtered and absolutely calibrated visible imaging camera tangentially view-ing the inner wall of the Alcator C-Mod tokamak, it was observed that the Dα emission profile peaked at the secondary separatrix. A one-dimensional physical space, two-dimensional velocity kinetic neutral code [4] was used to simulate the neutrals in the region near the inner wall. The plasma profiles were required as inputs to the code and measured values from the low-field-side scrape-off layer were used by keeping the values constant on a magnetic flux surface. For the model to reproduce the observed measurements a sharp decay in the plasma density, with a scale length of sim 3mm, at the secondary separatrix was artificially introduced. The model and the observations were in good agreement and indicate the emission profile has two scale lengths, 1) on the high-field-side of the emission peak due to the plasma density profile and 2) on the low-field-side of the emission peak due to an effective ionization mean-free-path on neutrals originating at the secondary separatrix. 1