Stationary magnetic pertubation diagnostic on TFTR (abstract)

Stationary magnetic perturbations (SMPs) in tokamaks may consist of (1) static magnetic field errors, (2) dynamic magnetic field errors, and (3) stationary MHD modes (or locked mode). The diagnostic aimed to detect the latter two classes of SMPs on the TFTR tokamak has two pairs of sensors, each consisting of a saddle loop for measuring predominantly radial field and auxiliary loops for measuring predominantly toroidal and vertical fields. Members of each pair are placed 180 degree apart toroidally. Predominantly n=1 SMPs are found in various discharges including ohmic discharges. Abrupt changes in SMP signals precede a ''tail-end'' disruption, suggesting a causality relationship. Variations in the SMP signals are also coincident with severe performance degradation of super shots and pellet injected discharges. An analysis of individual and paired signals appears to indicate that these SMPs are dynamic field errors as well as stationary MHD modes