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The internal magnetic structure of a tokamak is as little understood as that of astrophysical plasmas. The current profile itself has not been measured, and small-scale magnetic turbulence is undetectable using customary methods. A new experimental method was applied, based on the analysis of H(alpha) banding observed during pellet injection. The profile of the safety factor q can be determined very accurately, and chains of small-scale magnetic islands are directly observed, which may explain the anomalous transport of electron energy. The existence of a shear plateau in the proximity of the surface q = 1, where internal disruptions occur, was also confirmed. The object of investigation is a magnetic, geometrically plane structure with two resonant disturbances, which therefore give rise to two islands. By introducing an anharmonic term into the expression of these disturbances it was possible to vary the angle at point X of their separatrices. The stochasticity threshold was shown to be greatly reduced. The investigation was repeated with non-monotonous shear, presenting inflexion points on the resonances and in toroidal geometry. The results will be of relevance to the study of internal disruptions in tokamaks, and possibly to that of pre-eruptive configurations on the sun.

Additional information

Authors: SABOT R ET AL., CEA, CEN Cadarache, Saint-Paul-lez-Durance (FR);HENNEQUIN P, PMI, Ecole Polytechnique, Palaiseau (FR)
Bibliographic Reference: Paper presented: Troisième Congrès de la Société Française de Physique - Plasma, Nancy (FR), September 16-18, 1992
Availability: Available from (1) as Paper FR 37070 ORA
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