Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2000.Includes bibliographical references (p. 175-181).Magnetohydrodynamic (MHD) activity has been analyzed primarily using electron cyclotron emission (ECE) diagnostics on Alcator C-Mod tokamak. The main results are that i) two MHD instabilities have been identified during current ramp-up discharges (resistive 'multiple' tearing mode and ideal interchange mode) and ii) a new approach to diagnose edge localized modes (ELMs) using ECE diagnostics was explored. Both MHD modes were accompanied by hollow pressure and current profiles. The associated q-profiles were also hollow with q0 >> 1, where q0 is the safety factor on the magnetic axis. In both cases, the electron temperature fluctuations observed on ECE diagnostics agreed reasonably well with the perturbed pressure fluctuations predicted in a resistive linear stability code (MARS). For the resistive 'multiple' tearing mode , the MHD fluctuations were peaked near the outer q=3 rational surface but had several other resonant layers, which affected the plasma globally. The predicted growth time was ~0.44 msec, which is within the typical range of tearing mode evolution times. For the ideal interchange mode, the MHD fluctuations were highly localized near the inner q=5 rational surface. According to ideal MHD stability theory, the q = 5 surface was found to be ideally unstable because of the reversed pressure gradient (dp/dr > 0) and q > 1 with moderate shear. When kinetic effects were added, the ideally unstable mode was finite ion Larmor radius (FLR) stabilized. However, considering that 1) electrons are collisional, 2) ions are collisionless, and 3) the thermal ion transit frequency is comparable to the ion diamagnetic drift frequency, ion Landau damping was found to be strong enough to drive a kinetic Mercier instability. As a result, a FLR modified kinetic Mercier instability has been identified, possibly for the first time since the Mercier criterion was formulated forty years ago. During 'Type III' ELMs, rather unusual signal changes were observed on two ECE diagnostics; signal drops of second harmonic X-mode on one diagnostic and signal spikes of fundamental harmonic 0-mode on another. These were explained in terms of refraction effects and found to be useful to infer the associated geometrical dimensions. For this investigation, a new ray tracing code, which can accommodate poloidal variations, has been developed. As a result, an ELM has been modeled successfully as a poloidally elongated density loss. Observations are consistent with the following dimensions; radial width of the affected region ([delta] r) ~ 1 - 3 cm, poloidal elongation ~1.5 (equivalent to a poloidal wave number ... minimum density 0.5 x I020m-3 at the mid plane ~~ 1cm inside the last closed flux surface (LCFS). This knowledge helps to assess the influence of the particle loss on the main plasma. Considering that ELMs challenge present diagnostic capabilities in terms of spatiotemporal resolution, such indirect measurement opens the door to improved physical understanding of ELMs. In particular, it is the first to reveal the poloidal structure of an ELM.by Yongkyoon In.Ph.D.