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

Abstract

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 × 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 × 10 3 s −1 and the convective particle flux induced by the ejective blobs can lead to about 58% loss of radial particle flux.

Convective transport by intermittent blob-filaments: Comparison of theory and experiment

“…54 Measurements of blob outward transport using conditional sampling are reported in many experiments (e.g., Refs. 18,19,24,125,126,144, and 169 while evidence for holes moving inward can be found in Refs. 126, 129, 130, 144, 158, 159, 169, and 180).…”

“…Because the density is positive definite, sufficiently large perturbations are necessarily positive and their statistics are necessarily non-Gaussian. The significance of these blob-like structures was demonstrated by measurements which showed that radially convecting blobs (or "intermittent plasma objects") caused an order unity fraction of the particle flux in the scrape-offlayers, e.g., in DIII-D, 18 TEXTOR, 125 HL-2A, 126 MAST, 127 among other devices. This implies that the physics of blobs is important for an understanding of the heat and particle fluxes to the wall of magnetic fusion devices.…”

“…Wavelet analysis has been used to evaluate the blob size, lifetime, local intermittency measure, packing fraction, linear density of structures, and bicoherence in various devices. 22,126,144,150,152,164,171,177,178,180,189,191,218 Statistical analyses have searched for self-organized criticality or 'avalanche'-like behavior using techniques such as the waiting time distribution, self-similarity, and Hurst exponents. 125,151,173,187,226,237 The fractal structure was investigated in HYBTOK-II, T-10, LHD, and NAGDIS-II, 178,238,239 and the singular value (or proper or biorthogonal) decomposition technique was used to identify blob structures.…”

Blob/Hole Generation in the Divertor Leg of the Large Helical Device

“…In tokamaks, the change from negative to positive skewness is observed near the LFS separatrix, where the blobs and holes are thought to be generated [1][2][3][4]. Below, we focus on the statistics of the I sat fluctuation at 1.182 m ≤ z ≤ 1.194 m where the skewness changes from negative to positive along the z axis.…”

“…Plasma blobs and holes are generated near the separatrix [1][2][3][4] where steep density and temperature gradients exist. The blobs form field-aligned structures in the scrape-off layer (SOL) [5,6], and they are transported to the first wall across the magnetic field.…”