Possible Break-down of Up–Down Symmetry for Propagation of Parallel Velocity Gradient Driven Mode near Edge Transport Barrier

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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.