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

A small component of ECCD exists when heating off-axis with no toroidal inj ection due to the field helicity. The sign of the ECCD will change dependin g on the absorption location in a poloidal plane, the beam injection relati ve to delB(vac), and the direction of the toroidal magnetic field. The impo rtance of the ECCD component can be confirmed by the behavior of the sawtee th 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 w ith the presence of a small ECCD component. Experiments have also shown tha t 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 t he 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 f actor of 1.3 in radii. The region can be used as a method for measuring the beam deposition width in the plasma. (C) 2001 Elsevier Science B.V. All ri ghts reserved.