The difference in H-mode confinement between co- and co plus counter-neutral beam heating in the JFT-2M tokamak

The improvement in H-mode confinement in the JFT-2M-tokamak shows the difference between CO- and counter-neutral beam (CTR-NB) heating. The improvement in the CO is larger than that in the CO + CTR or CTR H-mode. The reason for tau(CO) > tau(CO + CTR) is considered not to be the effect of the sheared toroidal rotation, dv(phi)/dr, in the core. v(phi) at the last closed flux surface shows a finite value (about 20-40 km s(-1) and the direction is dependent on the beam injection angle) in a divertor H-mode. The finite toroidal rotation at the edge plays a role in the observed tau(CO) > tau(CO + CTR). Since the radial electric field formed by the poloidal rotation is negative, the electric field shear at the edge is increased by the positive electric field of v(phi)(CO) x B-theta. In contrast, the electric field from v(phi)(CTR) x B-theta is negative, which reduces the electric field shear. Therefore, the difference in the improvement between the CO and CO + CTR H-mode, which comes from the difference in the pedestal height, may be related to the electric field shear at the edge.