Characterization and stability of m=1, n=1 density snakes in Madison Symmetric Torus tokamak plasmas

Steady, helical perturbations known as "density snakes" with poloidal and toroidal mode numbers m=1, n=1 have been studied in several tokamak experiments. These three-dimensional helical states are interesting due to their stability and persistence, including their coexistence with the sawtooth cycle. Presented here are studies of density snakes in tokamak plasmas in the Madison Symmetric Torus (MST) device. They are diagnosed using an 11-chord interferometer, internal and edge magnetic coils, and impurity ion spectroscopy. Their properties are compared to those of prior experiments, highlighting some interesting differences: The density perturbation associated with the snake is larger, the structure occupies a broader span of minor radius, and the snakes are less stable than in other devices. These features are consistent with the relatively low edge safety factor, q(a) ≥ 2.2, magnetic equilibria used in the experiments, which moves the q=1 resonant surface outward in radius. The helical structure and distribution of snake events is characterized, with emphasis on whether they are best described as ideal or resistive kink modes. Finally, an analysis of their perturbation or destruction during sawtooth crashes is given.