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Two-dimensional microdensitometric mapping of intensity distributions of photographs of pellet ablation clouds reveal irregular structures which are interpreted as the first direct visualisation of pre-existing magnetic islands in tokamak plasmas. The erratic character of these features is compared to a theroretical model of magnetic turbulence transport. The experimental observations are based on the following facts: when H2 (D2) pellets are injected into tokamak plasmas, the ablation undergoes strong fluctuations which appear on time-integrated photographs as a series of luminous striations. On good photographs taken on the TFR tokamak (1986) there are cases where the curvature changes along one or more striation(s), strongly suggesting shapes of magnetic islands. Recent photographs of ablation clouds of D2 pellets injected into the Tore Supra tokamak TS also exhibit these features. Accepting the hypothesis that the striations follow the actual magnetic field lines, it is concluded that the magnetic islands are visualised directly. The thermal filamentation has been re-examined, taking into account the good confinement in residual islands and the temperature dependence of impurity radiation, and is shown to yield islands comparable to those described. The non-reproducibility of the observed structures can be understood with the help of a time-dependent dynamical model.

Additional information

Authors: DRAWIN H W, CEA, Département de Recherches sur la Fusion Contrôlée, Saint-Paul-lez-Durance (FR);DUBOIS M A, CEA, Département de Recherches sur la Fusion Contrôlée, Saint-Paul-lez-Durance (FR)
Bibliographic Reference: Paper presented: 18th European Conference of the EPS-Plasma Physics Division, Berlin (DE), June 3-7, 1991
Availability: Available from (1) as Paper EN 35933 ORA
Record Number: 199110385 / Last updated on: 1994-12-02
Original language: en
Available languages: en