EQUILIBRE ET STABILITE D'UN PLASMA TOROIDAL

The problem of the evolution and the behavior of a plasma inside its vessel in interaction with the exterior circuits has been analyzed. In order to study the effect of the deformation of the plasma cross section which is experimentally observed, the equilibrium of the plasma, from which "inner " parameters can be determined in terms of the "outer " given ones, is first developped to all orders in the small parameter S = — — « ̂ 1, through it appears that higher order terms in £ 3 and above are unimportant unless specific cases. There, the equilibrium parameters are given to 0( € 2). The stability of this equilibrium is then analysed. It is shown that, in general, MHD "internal " stability concerning the plasma itself can be disantangled from the stability of the plasma tube as a whole in interaction whith the other circuits, the plasma being stable with respect to the ÎIHD "internal " stability but often at the bordeline. The stability of the plasma tube is then studied. The problem reduces to a "canonical " one on a finite dimensional system. Local stability condition is first given, expressing a