Magnetic surfaces and striations during pellet ablation

At present, the striations observed on the Hα or Hβ emission signal of ablated pellets are not well understood. An attractive explanation of these striations would be a limitation of the ablation process due to the smaller amount of energy available on resonant magnetic surfaces. To evaluate the ability of the latter to produce the observed features, a model has been developed which takes into account the Maxwellian distribution of electrons, the proportion of trapped particles, the geometric effects linked with the shear and the finite lifetime of the filaments. In tokamaks, it is found that this model is unable to reproduce the observed striations for smooth q-profiles but that the general shape of the Hα (or Hβ) signal can be fitted by introducing local flattenings (e.g. small magnetic islands) around resonant surfaces. In low shear stellarators, striations (rather shallow) due to rational q-values can exist in the absence of magnetic islands, but only if a suprathermal electron tail is present.