Observation of turbulence spreading into edge stochastic magnetic layer caused by MHD mode excitation in LHD*

The theoretical models of turbulence spreading have been proposed since 1990’s [1,2]. Experimentally, however, direct observation of the turbulence spreading is difficult, because it requires to distinguish the fluctuations excited locally from those propagated remotely from somewhere else. While there are several examples [3,4], in this contribution we present new observation where the spreading is induced by MHD instability.

T_e and n_e profiles are flat in the stochastic layer due to magnetic field braiding effect, and there is, therefore, no drive for turbulence. On the other hand, steep pressure gradient is developed at the confinement boundary, which provides energy source to excite turbulence. The density fluctuation profiles were measured by 2D PCI diagnostic [5]. The measured frequency and wavenumber ranges are 10 kHz < f < 500kHz and 0.1 mm^-1 < k_perp < 0.8 mm^-1, respectively. In the edge region of interest, k_perp*rho_s < 1. The measured fluctuation is, therefore, due to ion scale turbulence. The density fluctuation is localized at the steep pressure gradient region at the confinement boundary. However, once a coherent MHD mode is excited due to the pressure gradient with ~ 4kHz and m/n=3/3, the density fluctuation begins to spread into the edge stochastic magnetic layer. The density fluctuation in the stochastic layer increases with increasing magnetic fluctuation of the MHD mode. It is noted that the MHD mode frequency is outside of the frequency range of the density fluctuation measurements. The results clearly show an evidence of turbulence spreading induced by the magnetic fluctuation, and suggest nonlinear coupling between the MHD instability and density fluctuation to trigger the turbulence spreading. With the turbulence spreading, the divertor heat load profile is widened, and the peak heat load decreases. The results demonstrate a possible scheme to mitigate divertor heat load with turbulence control in the edge region.