The LT-4 Tokamak. II. MHD Activity

Abstract

An extensive experimental program has been carried out on the MHD activity occurring in the LT-4 plasma during ohmic heating discharges, under.a wide range of reproducible conditions, for 3·6 > q(a) > 2·5. Perturbations are studied by using large arrays of individually monitored magnetic coils, distributed both poloidally and toroidally, and two toroidally separated soft X-ray poloidal arrays. The results show that the behaviour can be broadly classified into four distinct regimes within each of which the observable MHD phenomena are remarkably reproducible, and whose boundaries can be very well described by the single parameter q(a). Although most of the observations conform with those on other tokamaks, some appear inconsistent with the usual interpretation, for example, the assumption of rigid toroidal rotation. Further, while internal m/ n = 1/1 and m = 0 sawteeth activity at the q = 1 surface are clearly defined, the magnetic signals usually ascribed to a 2/1 mode when q( a) .;; 3 are clearly shown to have a predominant 3/1 structure, while activity at the q = 2 surface appears relatively weak