Persistent density perturbations at rational-q surfaces following pellet injection in the Joint European Torus

In the Joint European Torus the ablation of injected pellets produces a striking resonance effect when the pellets reach surfaces with q values 1 and (3/2. Subsequently, structures with mode numbers m=1,n=1 and m=3,n=2 are observed with the soft-x-ray cameras for up to 2 s as compact snakelike perturbations. These structures, which persist through several sawtooth collapses, give information on the radii of the q=1 and q=(3/2 surfaces and the q-profile evolution. The observations can be explained by the formation of magnetic islands.