C. J. BOSWELL, J. L. TERRY, B. LIPSCHULTZ, B. LABOMBARD - PSFC, MIT
Alcator C-Mod currently employs a CCD camera, filtered for deuterium Balmer-alpha emission, that views the inner wall tangentially. With this view it has been observed that a radially narrow band of deuterium emission occurs, away from the inner wall, for most diverted discharges. This narrow emission region is expected to occur in the scrape-off layer where there are steep gradients in the electron temperature and density profiles. The emission band is seen to exist at or near the separatrix closest to the inner wall, regardless of which null the separatrix is associated with. This implies that steep profile gradients occur at the inboard-most separatrix. When dynamically scanning the the diverted discharge from a lower single null to an upper single null the separatrices associated with the lower and upper x-point, near the inner wall, switch their relation in major radius. During the scan the emission band at the high-field side of the plasma remained on the inboard-most separatrix, while the deuterium Balmer-alpha emission band on the low-field side remained at or near the primary separatrix regardless of whether the plasma was lower or upper null.