MHD activity as seen in soft x-ray radiation

Abstract

Tokamak plasmas are in a state of permanent reorganization due to strong MHD activity. Central MHD phenomena can be observed with pinhole cameras in which silicon wafers with additional foils record soft x-ray emission from the plasma along many chords with high temporal resolution. To separate the repetition time and the characteristic radius of various MHD phenomena such as sawteeth and, in H-mode, ELMs from these chord-integrated measurements, the method of singular-value decomposition is of great value. Moreover, the helical (1,1) MHD mode is analysed in great detail. In a zeroth approximation, the minimum and maximum values of the spatial eigenvectors are used to determine the radius of the q = 1 surface. In a next step, a model of the (1,1) mode is developed which evaluates the line-integrated signals in elliptical geometry. This serves for interpolation and thereby improves spatial resolution. Finally, the information is added into a fast equilibrium reconstruction code which leads to improved current profile identification.