Blob/hole formation and zonal-flow generation in the edge plasma of the JET tokamak

Abstract

The first experimental evidence showing the connection between blob/hole formation and zonal-flow generation was obtained in the edge plasma of the JET tokamak. Holes as well as blobs are observed to be born in the edge shear layer, where zonal-flows shear off meso-scale coherent structures, leading to disconnection of positive and negative pressure perturbations. The newly formed blobs transport azimuthal momentum up the gradient of the azimuthal flow and drive the zonal-flow shear while moving outwards. During this process energy is transferred from the meso-scale coherent structures to the zonal flows via the turbulent Reynolds stress, resulting in nonlinear saturation of edge turbulence and suppression of meso-scale fluctuations. These findings carry significant implications for the mechanism of structure formation in magnetically confined plasma turbulence.

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

“…In both cases, |I sat | increases rapidly at τ < 0, and then, |I sat | gradually decreases at τ > 0. This tendency is in agreement with tokamaks [2]. The time duration that the averaged I sat exceeds the offset level is ∼ 8 µs, which is much shorter than that reported in many tokamaks, such as Tore Supra [22], Alcator C-Mod [22], MAST [22], T-10 [12], JT-60U [9], and JET [2].…”

“…3 in Ref. [2], the normalized radial gradient of I sat in JET tokamak was 80-110 m −1 near the separatrix where the skewness was also almost zero. Therefore, the edge of the LHD divertor leg has steep normalized gradient as well as near the separatrix in tokamaks.…”

“…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.…”

“…To obtain the typical shapes of the positive and negative spikes, the conditional averaging method [2,9,12,22] was employed. Before applying the conditional averaging method, we used the moving normalization technique defined by the following equation: Figure 7 shows the conditional averaged shapes of I sat at 1.182 m < z < 1.186 m and 1.19 m < z < 1.194 m, which correspond to the regions where the skewness is negative and positive, respectively.…”

“…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.…”