Magnetic reconnection during sawtooth instability in ASDEX Upgrade

Sawtooth oscillations are internal periodic relaxation events in a tokamak that lead to a rapid redistribution (crash) of plasma core temperature and density. Two topics of sawtooth crash physics are studied. First, the question of whether magnetic reconnection during the crash has global (everywhere along the q = 1 helical magnetic line) or local (only at a particular toroidal location on the q = 1 helical magnetic line) nature is investigated. Numerical analysis of heat distribution during the crash phase shows that heat distributes itself helically along the torus faster than the temporal resolution of any existing electron cyclotron emission (ECE) diagnostics. It makes local and global (symmetric along the helical axis) magnetic reconnection indistinguishable for an observer. Only if the heat propagation of a local crash remains confined within a helical region, then the distinction between the local and global crashes can be measured. Statistical analysis of sawtooth crashes with the ECEI diagnostic is conducted in ASDEX Upgrade. The displacement of the heat within a finite helical region has not been observed. The statistical data supports global magnetic reconnection. Second, the radial velocity of the plasma core during the crash is studied with the electron cyclotron emission imaging (ECEI) diagnostic. These measurements introduce a novel approach for studying magnetic reconnection during sawteeth since the radial velocity characterises the rate of the reconnection. The measurements have been compared with nonlinear two-fluid simulations. The comparison reveals good qualitative and quantitative agreement, which indicates that two-fluid effects (inertia and pressure gradient of electrons) are sufficient for the correct prediction of the experimental results. Contrarily, the crash time of the Kadomtsev model, which is based on a single-fluid picture of magnetic reconnection, disagrees with the experimental results.