Effect of applied resonant magnetic fields on the measured MHD mode structure in a Tokamak plasma

An important problem in nuclear fusion research is to design an active control method capable of preventing the growth of tearing mode instabilities through the control of rotation or active feedback. In this project, the mode structure of m = 2 magnetohydrodynamic (MHD) instabilities has been measured on the High Beta Tokamak-Extended Pulse (HBT-EP) for kink stabilized discharges and for discharges with applied resonant magnetic fields. Soft X-ray emissivity as well as magnetic diagnostics have been used to measure the MHD mode structure. Magnetic diagnostics measure the external magnetic field perturbed by MHD instabilities.; In plasma discharges that are kink-mode stabilized by a nearby conducting wall, perturbation of the soft X-ray emissivity at the same radial location but different poloidal locations shows that the mode is predominantly m = 2. Tomographic reconstructions of the soft X-ray emissivity distribution show m = 2 island structures which indicates this is a tearing mode. The fluctuation component of the soft X-ray emissivity distribution observed to have a phase inversion radius. This observation is consistent with the existence of an island structure. The measured fluctuation of the poloidal magnetic field is observed to be in phase with the measured fluctuation of the soft X-ray emissivity at the outside edge of the plasma.; To study the effect of the applied resonant magnetic field on MHD instabilities, a set of modular saddle coils producing an m/n = 2/1 magnetic helicity has been installed on the HBT-EP. The applied resonant magnetic field with changing frequency modifies the toroidal rotation speed of the MHD mode in the tokamak.; Two configurations of saddle coil connection have been used. One configuration (single-phase connection) creates a stationary wave in the lab frame. Another configuration (double-phase connection) splits the saddle coil set into two parts and uses two power amplifiers to provide saddle coil current, and generate a traveling wave in the lab frame. Controlled acceleration and deceleration have been observed by programming the frequency of the applied saddle coil current. In this work, we measure these rotation control effects through the measured fluctuation of the soft X-ray emissivity.; The phase difference between the MHD mode structure determined by the soft X-ray tomography measurements and the MHD mode structure measured by a set of probes mounted on the inside surface of the conducting shells is a constant as a function of time in both the controlled acceleration and deceleration discharges. Perturbation of the soft X-ray emissivity at the same radial location but different poloidal locations clearly shows control of m = 2 mode rotation by application of externally applied oscillating resonant magnetic field. Tomographic reconstruction also shows this mode has an m = 2 island structure. (Abstract shortened by UMI.)...