Magnetic Turbulence in Tokamaks

-From a discussion of the disruption process, it is concluded that this process plausibly consists of the onset of a fine grain turbulence. This turbulence must be able to produce the large 1 aly values of the inductive electric field which are associated with the reorganization of the c at poloidal flux Y(r) and the current density Z(r) on the magnetic surfaces of radius r. It is then plausible that the turbulence belongs to a class of rippling modes in the presence of which the Ohm law takes the form 1 aly --a az =qZ-r K . c at r a r ( ar) a a z The anomalous term rK may explain the experimental values of -1. for magnetic a r ( ar) c at perturbations corresponding to a substantial radial ergodicity of the flux lines. The stability of the modes in the presence of such an ergodicity is accordingly considered. It is found that the modes may be unstable even in collisionless regime, the ergodicity playing a role similar to the resistivity to partially remove the M. H. D. constraint. 1. Discussion of the disruptive process. The disruptions associated with the surface q = 1 and the soft disruptions associated with the surface q = 2, as they are detected through soft X ray emission 11, 2, 31, consist of a sudden (50-200 ps) partial flattening of the temperature profile in a large domain on both sides of the resonant magnetic surface where the safety factor q = 1 or 2. They are generally preceded by the relatively slow onset of an oscillating structure which has been identified to magnetic islands in the case q = 2 [2,4]. Owing to the strong analogy of the X ray signal (in space and time) in the case q = 1 and q = 2, this interpretation is also plausible for the disruptions q = 1. Eventually the oscillating structure persists after the disruption. Near the end of the disruptions q = 2 a very strong and short negative voltage pulse around the major axis appears [5] . Such a negative spike is absent in the case q = 1. This is not surprising owing to the fact that the effect of the disruptions q = 1, while significant relatively far from the resonant magnetic surface, does not reach the plasma edge. The presence of the voltage spike in the case q = 2 means a sudden variation of the poloidal flux embrassed by the magnetic surface at the plasma edge. It is likely that the poloidal flux 2 n R Y ( r ) embrassed by the surface r (see Fig. 1) varies by a quantity of the same order in the domain affected by the disruptions. Assuming that this variaArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1977610