X-Ray Imaging of MHD Activity in the Text Tokamak

Soft x-ray emission from a tokamak plasma may he used to obtain information about the electron temperature, density and impurity content of the plasma. Since the plasma is optically thin to x-rays, any measurement of x-ray emission is the sum of x-rays emitted all along the line of sight of the detector. It is useful to find a way to unfold the local values of x-ray emission from such a set of chord-integrated measurements. In this work, we implement a tomographic technique to invert the signals from two arrays of x-ray detectors that yields a two dimensional picture of x-ray emission from a poloidal slice of the plasma. This imaging method is applied to studies of magnetohydrodynamic motion in two separate discharges. One type of discharge is prepared so that a large amplitude m = 2 island persists throughout the discharge without causing a disruption. X -ray imaging is used to directly determine the location and width of the island. In another type of discharge, impurity pellet injection occasionally results in the suppression of sawteeth and the formation of a persistent m = 1 mode with the absence of sawteeth. X-ray imaging is used to determine the size of the m = 1 perturbation. The x-ray emissivity profiles are found to peak in the center, in the absence of sawteeth, implying accumulation of impurities. The m = 1 may be described as an internal kink-tearing mode which grows and saturates, but sawteeth do not occur because increased impurity radiation keeps the central thermal energy lower than necessary to trigger a sawtooth crash.