Behavior of reflected extraordinary mode in the fundamental electron cyclotron heating and current drive in JT-60U

Unwanted X-mode behavior in the fundamental harmonic electron cyclotron (EC) wave launching from the low field side with an oblique toroidal injection angle is experimentally studied on JT-60U by changing the poloidal injection angle. It is found that there is a critical region in the poloidal injection angle to cause the rise in vacuum pressure at the NBI port adjacent to the ECRF antenna port. The typical EC beam power is about 1 MW and the pulse length is 3 s in this experiment. The ray-trace calculation indicates that the X-mode launched from the low field side is guided into the NBI port after reflection at the cut-off layer for the critical poloidal injection angle. The results suggest that the O-mode injection with quite high mode purity, that is, with a very low fraction of X-mode is required for the avoidance of the overheat of the in-vessel components due to the reflected X-mode in the case of the EC beam launching from the low field side, especially for long pulse operation