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Snakes are observed in Tore-Supra, after injection of high velocity solid hydrogen or deuterium pellets ablated insider the q equals 1 surface. They are detected, immediately after the ablation, as oscillations on the line integrated densities of the central interferometer channels. The corresponding oscillations on the soft X-ray signals detach from the noise about 70 ms later. Snakes survive sawtooth crashes, but are nevertheless affected by them. Variations, during the about 500 ms long lifetime, of the snake radius r(s), of the rotation frequency and of the rotation direction are discussed, stressing the effects of the sawtooth crashes. In many snakes r(s)/r(q) equals 1 is of the order of 0.5. Since the snake has a m equals 1, n equals 1 helicity, this points out the existence of a flat or inverted safety factor profile, confirmed by calculation of the current profile using Spitzer's resistivity. Combined simulations of the snake oscillations on both interferometer and soft X-ray signals have indicated that, starting about 80 ms after the snake formation, the impurity (carbon) density inside the snake is much larger than outside it. Since a change of regime seems to appear about 80 ms after the snake formation on the soft X-ray, it seems plausible that impurity (carbon) accumulation takes place at this time. A stability criterion taking into account both impurity and bootstrap effects is presented, the result agrees with the model proposed by Wesson.

Additional information

Authors: PECQUET AL ET AL, CEN, CEA Cadarache, Saint-Paul-lez-Durance (FR)
Bibliographic Reference: Article: CEN Cadarache Laboratory Report (1996) EUR-CEA-FC-1579
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