Statistical characterization of blob turbulence across the separatrix in HL-2A tokamak

Blob statistical characteristics across the separatrix of HL-2A tokamak plasma have been studied using a reciprocating Langmuir five-probe array. The radial profile of inverse pressure gradient scale length has a maximum just inside the last closed flux surface (LCFS), where the skewness is close to zero. Conditional average reveals that the density holes and blobs are produced just inside the LCFS and they propagate in opposite directions. The poloidal velocity of blobs changes its sign when it is across the separatrix, which is consistent with E x B drift flow. The dramatic change in phase shift between density and potential fluctuations across the separatrix suggests the distinct properties of turbulence when the magnetic field line changes from a closed to an open one. The dependence of a weak three-wave interaction in terms of wavelet bicoherence on strong time-asymmetry blobs is observed for the first time. Moreover, the effective blob generation rate is estimated as 8.0 x 103 s-1 and the convective particle flux induced by the ejective blobs can lead to about 58% loss of radial particle flux.