Plasma Physics Seminar ( Phys 769)

Hyeon K. Park, Princeton Plasma Physics Laboratory, Princeton University, Princeton, NJ

Self-organized temperature redistribution of m=1 mode (sawtooth) oscillation on TEXTOR*

A novel 2-D Electron Cyclotron Emission Imaging (ECEI) system [1] for measuring electron temperature fluctuations has been applied to study the physics of the sawtooth oscillation in TEXTOR. 2-D images with high spatial resolution [128 pixels covering 8 cm (radial) x 16 cm (vertical)], and high temporal resolution (up to ~5 microsec) have revealed the details of 2-D images of the electron temperature fluctuations during the precursor phase and the crash time of m=1 (sawtooth) oscillations, with a level of detail that is not accessible through conventional methods (1-D ECE and/or tomography). A new paradigm for the m=1 oscillation has been developed based on the evolution of the 2-D electron temperature image with the accumulated knowledge from previous experimental results [2] on TEXTOR. The evolution of the 2-D image clearly illustrates that the crash can occur in almost every direction along the q~1 surface (high and low field sides) and the heat transported out of the q~1 surface follows the magnetic pitch and symmetrically accumulates outside of the inversion radius before initiation of the diffusive process. These observations suggest that the sawtooth crash is likely toroidally global, but poloidally localized. The new reconnection model is based on the local magnetic field from the current carrying m/n=1/1 layer which has a crescent shape hugging the m=1 mode outside the inversion radius. The physical dimensions of this layer are similar to the observed "current sheet" [2]. The shearless m/n=1/1 layer due to the poloidal fields induced by the current sheet delocalize a wider radial area (mixing zone) so that the reconnection process and heat transport can be enhanced. Heat transport from the m=1 mode can be accelerated when the reconnection process reaches the outermost surface of the m=1 mode along the magnetic pitch of the q=1 surface. The observed enhanced fluctuation level of the Te (magnetic surface) near the crash time could be a cause of the magnetic reconnection and other possibilities like stochastic magnetic islands and fractals are equally important candidates.