New and more accurate techniques for finding poloidal and toroidal mode numbers of MHD oscillations from Mirnov coils are presented. It is found that the data for the MHD mode structure are best fit by a model which can accommodate toroidal effects on the mode geometry. The experimentally observed toroidal effects are compared with theoretical expectations. Examples of other physical MHD phenomena occurring during startup on TFTR are also illustrated. An example of toroidally coupled 4,1 and 3,1 modes is presented. An efficient algorithm is described that computes the best fit to Mirnov coil phase data by determining both the m, n numbers and a free toroidal correction parameter (λ', where λ' = 0 → cylindrical). The possibility of the mode being poloidally asymmetric owing to toroidal effects and an off-centre location of the mode justify leaving the λ' parameter free. Since the optimal value of λ' can be obtained automatically with a computer, the need for 'guesswork' is eliminated.
Non-resonant global mode in LHD partial collapse with net toroidal current