New Insights to the Sawtooth Oscillation (“m/n = 1/1 mode”) in Hot Plasmas based on High Resolution 2-D Images of Te Fluctuations

Abstract

* partners in the Trilateral Euregio ClusterTwo dimensional (2-D) images of electron temperature fluctuations with high temporal and spatial resolution have been employed to study the sawtooth oscillation (m/n=1/1 mode) in Toroidal EXperiment for Technology Oriented Research (TEXTOR) tokamak plasmas. 2-D imaging data revealed new physics which were not available in previous studies based on the 1-D electron temperature measurement and X-ray tomography. Review of the physics of the sawtooth oscillation is given by comparative studies with prominent theoretical models suggest that a new physics paradigm is needed to describe the reconnection physics of the sawtooth oscillation. The new insights are: A pressure driven instability (not a ballooning mode) leads to the "X-point" reconnection process. The reconnection process is identified as a random 3-D local reconnection process with a helical structure. The reconnection time scale is similar for different types of sawtooth oscillation ("kink" and "tearing" type) and is significantly faster than the resistive time scale. Heat flow from the core to the outside of the inversion radius during the reconnection process is highly collective rather than stochastic.

Non-monotonic growth rates of sawtooth precursors evidenced with a new method on ASDEX Upgrade

“…Several diagnostics can be used to monitor a sawtooth crash precursor, and some interesting results have been produced thanks to ECE [18][19][20][21] or 2D ECE imaging [18,22,23], thanks to reflectometry via 2D density rotational tomography [24], or thanks to polarimetric measurements, which give access to line-integrated electron density and perturbed magnetic field [9], from which central current density can be derived [25]. Magnetic measurements or interferometry can also be used.…”