Snake-like density perturbations in JET

A large, very localized pressure perturbation has been observed in JET which is due to the formation of a small region on the q=1 surface with greatly increased density, which persists for a surprisingly long time. This effect, called the snake, is seen after the injection of D 2 pellets, at the onset of sawtoothing and after high power neutral injection. Many detailed properties of the snakes are found, principally from soft X-ray measurements, and impurities are found to play a significant role. The snake is used as a diagnostic of the q=1 surface through the sawtooth cycle. The possible mechanisms which could sustain the snake are discussed