Off-Axis Sawteeth and Double-Tearing Reconnection in Reversed Magnetic Shear Plasmas in TFTR

Off-axis sawteeth are often observed in reversed magnetic shear plasmas when the minimum safety factor $q$ is near or below 2. Fluctuations with $m/n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2/1$ ( $m$ and $n$ are the poloidal and toroidal mode numbers) appear before and after the crashes. Detailed comparison has been made between the measured ${T}_{e}$ profile evaluation during the crash and a nonlinear numerical magnetohydrodynamics simulation. The good agreement between observation and simulation indicates that the off-axis sawteeth are due to a double-tearing magnetic reconnection process.