Spontaneous helical density snakes within MST tokamak discharges

Density snakes are steady m=1, n=1 helical density structures that form in the core of tokamak plasmas. Here we present studies of density snakes that form spontaneously in tokamak discharges (B_t = 0.13T) within the Madison Symmetric Torus (MST) . They are detected directly using a high-bandwidth 11-chord far-infrared interferometer. The helical structure rotates toroidally, yielding clear 1-2 kHz density fluctuations. Studies in other tokamaks, like JET, EAST, and Alcator C-mod, show snakes are associated with increased resistivity near the q=1 surface from either impurity accumulation or pellet ablation, observed primarily as fluctuations in soft x-ray measurements. Snakes in MST have unusually broad radial extent, consistent with a safety factor q=1 surface around r/a = 0.5. While snakes in other devices tend to coexist with the sawtooth cycle, those in MST appear to be strongly affected and often destroyed by such events. Simple analytical models are compared to interferometer and magnetic probe data to characterize the mode structure.