Heterodyne ECE diagnostic in the mode detection and disruption avoidance at TEXTOR

The paper describes the detection of the m/n = 2/1 disruption precursor at TEXTOR. This frequently observed precursor is detected by realtime cross-correlation of two radially separated channels of the heterodyne electron cyclotron emission diagnostic. An electronic module developed to detect the precursor and its functionality is briefly described. A disruption statistic for TEXTOR is presented, from which the probability of the occurrence of the m/n = 2/1 mode is seen. A method to generate reproducible disruption precursors at TEXTOR is also presented. Experiments to avoid disruptions by inducing toroidal velocity shear by tangential neutral beam injection and mitigate disruptions by He-puffing are presented. In the experiments performed, up to 50% of the disruptions could be avoided. Furthermore, calculations of the precursor frequency and its evolution are described and compared with measurements. PACS numbers: 07.57.Pt, 52.30.Ex