Structure and dynamics of sawteeth crashes in ASDEX Upgrade

The crash phase of the sawteeth in ASDEX Upgrade tokamak [Herrmann et al., Fusion Sci. Technol. 44(3), 569 (2003)] is investigated in detail in this paper by means of soft x-ray (SXR) and electron cyclotron emission (ECE) diagnostics. Analysis of precursor and postcursor (1,1) modes shows that the crash does not affect the position of the resonant surface q=1. Our experimental results suggest that sawtooth crash models should contain two ingredients to be consistent with experimental observations: (1) the (1,1) mode structure should survive the crash and (2) the flux changes should be small to preserve the position of the q=1 surface close to its original location. Detailed structure of the reconnection point was investigated with ECE imaging diagnostic. It is shown that reconnection starts locally. The expelled core is hot which is consistent with SXR tomography results. The observed results can be explained in the framework of a stochastic model. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3529363]