Intermittent Bursts in the Boundary Plasma of HT-7

Abstract

Intermittent characteristics of turbulence induced by coherent structures (blobs) was clearly observed in the ion saturation current signal, density fluctuation, particle flux and heat flux in HT-7. It is obvious that ion saturation current signal has deviated from Gaussian distribution and the skewness (S) and flatness (K) of signal increase radially outwards in the scrape-off layer (SOL). Using conditional analysis (CA), asymmetric character of the intermittent bursts are demonstrated. Owing to the radial propagation of the coherent structures, the particle density profile in SOL is non-exponential and flat outwards from the last close flux surface (LCFS). It is found around LCFS that the large burst fluctuations (above 2.5 rms) are responsible for about 50% of the total particle transport. Burst events move radially outwards with Eθ×B velocity, and the blob size can be calculated as δr ∼ Vrτc. Our experiment shows that the blob size, life time and drift velocity experienced a pronounced decorrelation in the shear layer. The electrostatic Reynolds stress components become very strong and show a radially steep gradient in the proximity of the shear layer. These experimental findings may imply that the coherent structures are titled by the developed shear flow in the E × B shear layer.