Fast Imaging of Filaments in the X-Point Region of Alcator C-MOD

A rich variety of field-aligned fluctuations has been revealed using fast imaging of D$_{\mathrm{\alpha }}$ emission from Alcator C-Mod's lower X-point region. Field-aligned filamentary fluctuations are observed along the inner divertor leg, within the Private-Flux-Zone (PFZ), in the Scrape-Off Layer outside the outer divertor leg, and, under some conditions, at or above the X-point. The locations and dynamics of the filaments in these regions are strikingly complex in C-Mod. Changes in the filaments' generation appear to be ordered by plasma density and magnetic configuration. In a Lower Single Null with 0.12 \textless $n/n_{Greenwald}$ \textless 0.45 and Bx$\nabla $B directed down, filaments typically move up the inner divertor leg toward the X-point. Reversing the field direction results in the appearance of filaments outside of the outer divertor leg. With the divertor targets ``detached'', filaments inside the LCFS are seen. These studies were motivated by observations of filaments in the X-point and PFZ regions in MAST, and comparisons with those observations will be made.