Control of Disruptive Instabilities

Available from UMI in association with The British Library. Requires signed TDF. In tokamak devices, at critical values of discharge parameters (n_ e, q(a), beta) the plasma can suddenly be terminated. The process is called a disruption. It is a major limitation in the operation of tokamaks, not only because of the limitation it imposes on the operation parameter space, but also due to the severe thermal and electromechanical loadings on the vessel. These difficulties and implications for fusion reactors have attracted increasing attention and a variety of approaches in the attempt to avoid, reduce or overcome the problem have been investigated. The growth of a magnetic perturbation is believed to be responsible for the disruptive process, and previous experiments have examined the effect on this perturbation of magnetic feedback. In DITE experiments have been done to extend this work by using a more sophisticated feedback loop. The detector-coils and feedback saddle-coils (configured to treat the m = 21, n = 1 structure which is dominant in disruption precursors) were mounted inside the vacuum vessel and fast programmable loop-gain and loop-phase controllers were used. Open-loop experiments contained studies of mode locking and plasma response to applied (2,1) fields. The feedback work explored the effect on disruption precursors over a large area in parameter space and was the first to address in detail the effect of feedback on disruptions. Both open-loop and feedback experiments were conducted on Ohmic discharges and discharges with lower hybrid current drive (LHCD). The experiments have demonstrated disruption precursor control in both types of discharge. Disruptions were studied in Ohmic plasmas. They were postponed and the density limit was extended.