MHD activity in the ISX-B tokamak: experimental results and theoretical interpretation

The observed spectrum of magnetohydrodynamic (MHD) fluctuations in the Impurity Study Experiment (ISX-B) tokamak is clearly dominated by the n = 1 mode when the q = 1 surface is in the plasma. This fact agrees well with theoretical predictions based on three-dimensional (3-D) resistive MHD calculations. They show that the (m = 1; n = 1) mode is the dominant instability. It drives other n = 1 modes through toroidal coupling and n > 1 modes through nonlinear couplings. These theoretically predicted mode structures have been compared in detail with the experimentally measured wave forms (using arrays of soft x-ray detectors). The agreement is excellent. More detailed comparisons between theory and experiment have required careful reconstructions of the ISX-B equilibria. The equilibria so constructed have permitted a precise evaluation of the ideal MHD stability properties of ISX-B. The present results indicate that the high-beta ISX-B equilibria are marginally stable to finite-n ideal MHD modes