Formation of a density blob and its dynamics in the edge and the scrape-off layer of a tokamak plasma

Abstract

Formation of a density blob and its motion in the edge and scrape-off layer (SOL) of a tokamak plasma have been simulated using two-dimensional, two-field, fluid model equations. The simulation results show that density blobs form in the edge or in the edge-to-SOL transition region where the poloidal velocity shear is maximum. From the numerical data, a condition for density blob formation has been obtained. Dynamics of the detached blob in the edge and SOL regions has been studied. It is observed that not all the blobs that form in the edge or edge-to-SOL transition region are capable of ejection deep into the SOL. A condition for their ejection is also discussed. Radial particle transport associated with the blobs in the SOL has been calculated. It is found that about 60% of the total radial particle flux is carried out by these blobs.

Turbulent transport and the plasma edge

“…Initial models were restricted to the SOL and accounted for sheath dominated parallel losses only [48][49][50]. Recently, 2D simulations with the parallel losses being due to parallel expansion of an originally poloidally localised structure and encompassing a fueling edge region in addition to the SOL, have had tremendous success in reproducing detailed properties of the SOL, as transport statistics.…”

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

“…Nevertheless, as noted above, blob formation and subsequent radial transport is routinely seen in experiments 18,19,21,125,130,134,135,202 and in turbulence simulations that allow full profile modification. 40,41,50,55,59,62 The scaling of the blob radial velocity with blob size and other parameters depends on the current path through the blob (Sec. V) but in most cases the predictions for the magnitude of the radial velocity are in order-of-magnitude agreement with experiments, v x $ 0:01 À 0:1 c s .…”

Generation of blobs and holes in the edge of the ASDEX Upgrade tokamak

“…While the radial drift of the blobs could be clarified (at least for toroidal devices) to be due to the self-generated E×B drift caused by the charge separation by curvature drifts [17], the generation of blobs remains blurred. An interchange mechanism active at the separatrix has been concluded from fluid simulations [18][19][20][21]. In the basic toroidal experiment TORPEX, without closed field lines, an interchange instability was found to be responsible for blob generation [15,16], while in the linear experiments VINETA and CSDX, the blobs emerge out of quasi-coherent drift waves [13,14].…”