Possible break-down of up-down symmetry for propagation of parallel velocity gradient driven mode near edge transport barrier

The propagation direction of parallel velocity gradient driven mode in the edge of H-mode plasmas is investigated. We study the possibility to observe a preferential propagation (either up-ward or down-ward) direction, simultaneously in the low-field as well as high-field sides of torus. The condition to observe such an up-down preferential propagation is derived. The proximity of the barrier to the separatrix is found to be the key for such preferential propagations. This result provides a hypothesis, which will be tested by future experiments, and a step forward to understand the broken up-down symmetry in the H-mode plasmas.