OBSERVATION OF MAGNETIC-FIELD PERTURBATIONS DURING SAWTOOTH ACTIVITY IN TOKAMAK PLASMAS

Sawtooth activity is a prominent example of a global plasma instabilit y which is observed in virtually all tokamak devices. Despite numerous experimental and theoretical investigations, the phenomenon is still barely understood. As far as experimental effort is concerned, much at tention has been paid to soft x-ray emission from the plasma and to it s analysis in terms of two-dimensional contour plots, because it is th ought to reflect the shape and temporal behaviour of magnetic flux sur faces during a sawtooth cycle. Recently, more direct methods of detect ing sawtooth-reIated changes in the magnetic field structure have beco me available and have added new facets to the general picture. In this paper, some observations made on the Julich tokamak TEXTOR by means o f a Faraday rotation diagnostic technique will be reported. First, in correlation with the sawtooth collapse a localized periodic perturbati on of the magnetic field with principal mode numbers m = 1 and n = 0 h as been detected which, in the presence of an m = n = 1 island, may gi ve rise to magnetic field line stochastization and thereby contribute significantly to a rapid expulsion of electronic energy from the plasm a core region. Second, the so-called precursor oscillations prior to a sawtooth crash have been investigated and estimates have been obtaine d for the growth rate and width of a magnetic island forming immediate ly before the collapse.