Observation of the inward propagation of spontaneous toroidal flow from the plasma boundary in LHD

Spontaneous generation of toroidal flow from the separatrix and its inward radial propagation in association with the change in the electron temperature gradient have been observed near the plasma boundary in the modulated electron cyclotron heating (MECH) experiment in the Large Helical Device (LHD). The observations presented in this paper provide strongly supports the hypothesis for the conversion of the poloidal flow to the toroidal one at the plasma peripheral region, exhibiting a clear delay response on MECH in the time derivation of the toroidal flow ((V ̇_ϕ≡∂V_ϕ)⁄∂t) to the poloidal one ((V ̇_θ≡∂V_θ)⁄∂t) at a normalized radius of ρ≈ 0.97. The ratio of order unity for |V ̇_ϕ⁄V ̇_θ | during a conversion phase from poloidal flow into toroidal one is consistent with that predicted by a quasi-stationary theoretical model based on the turbulent instability, regardless of its sign and spatial structure. The present work demonstrates a new dynamic response in the plasma momentum transport and represents a significant confirmation of its non-local nature