Study of unsolved issues of D1 oscillation ("Sawtooth") via 2-D ECE 
Imaging System on TEXTOR

H. Park, E, Mazzucato, T. Munsat, N.C. Luhmann, Jr., C.W.   Domier,
M. van de Pol, T. Donne, I. Classen

 A novel 2-D Electron Cyclotron Emission Imaging (ECEI) system measuring
electron temperature fluctuations applied to the study of sawtooth crash
physics on TEXTOR plasmas. A 128-channel prototype imaging system, 
covering 8 cm (radial) by 16 cm (vertical), with high spatial (1 cm x 1 cm) 
and temporal (up to ~5 msec) resolution, employs large aperture optics to 
form a spatially resolved image of several cyclotron layers simultaneously. 
The ECEI system, which includes a 16-channel vertical array of antennas and 
wide-band transmission line, has provided behaviours of the electron 
temperature fluctuations similar to the "magnetic reconnection" process 
during crash time of D1 (sawtooth) oscillations, revealing details not 
accessible through conventional methods (1-D ECE and/or tomography). 
Long history of the theoretical and experimental study of the D1 
oscillation left remnants of
mysteries and unresolved issues such as role of the current sheet and
controversial reconnection time scale. Visualization of the electron
fluctuations associated with the sawtooth crash in 2-D elucidates the
understanding of the heat transfer process via magnetic reconnection.