Numerical and experimental study of the redistribution of energetic and impurity ions by sawteeth in ASDEX Upgrade: Paper

In the non-linear phase of a sawtooth, the complete reconnection of field lines around the q = 1 flux surface often occurs resulting in a radial displacement of the plasma core. A complete time-dependent electromagnetic model of this type of reconnection has been developed and implemented in the EBdyna_go code. This contribution aims at studying the behaviour of ions, both impurity and fast particles, in the pattern of reconnecting field lines during sawtoothing plasma experiments in the ASDEX Upgrade tokamak by using the newly developed numerical framework. Simulations of full reconnection with tungsten impurity that include the centrifugal force are achieved and recover the soft x-ray measurements. Based on this full-reconnection description of the sawtooth, a simple tool dedicated to estimate the duration of the reconnection is introduced. This work then studies the redistribution of fast ions during several experimentally observed sawteeth. In some cases of sawteeth at ASDEX Upgrade, full reconnection is not always observed or expected so the code gives an upper estimate of the actual experimental redistribution. The results of detailed simulations of the crashes are compared with measurements from various diagnostics such as collective Thomson scattering and fast-ion D-alpha (FIDA) spectroscopy, including FIDA tomography. A convincing qualitative agreement is found in different parts of velocity space.