Poloidally asymmetric plasma response with ECH deposition near q=1 in TCV

AbstractA small component of ECCD exists when heating off-axis with no toroidal injection due to the field helicity. The sign of the ECCD will change depending on the absorption location in a poloidal plane, the beam injection relative to ▿Bvac, and the direction of the toroidal magnetic field. The importance of the ECCD component can be confirmed by the behavior of the sawteeth when the ECH deposition crosses the q=1 surface. The sawtooth period is very sensitive to the local shear at q=1 and it is shown that difference in sawtooth period when heating above and below the midplane are consistent with the presence of a small ECCD component. Experiments have also shown that the sawtooth period is strongly determined by the absorbed ECH power near the q=1 surface. In particular significant changes in the sawtooth period are observed as the absorption location is varied on a scale smaller than the beam spot size. A narrow region of rapidly changing confinement is shown to exist near the q=1 surface to be located outside the q=1 surface by a factor of 1.3 in radii. The region can be used as a method for measuring the beam deposition width in the plasma.